4.2 Duty types 4.2.1 Duty type S1 – Continuous running duty Operation at a constant load maintained for sufficient time to allow the machine to reach thermal equilibrium, see Figure 1.
Declaration of duty
The purchaser is responsible for declaring the duty, which can be specified in one of three ways: a) numerically, if the load is constant or varies predictably; b) as a time sequence graph illustrating the variable quantities; or c) by choosing a duty type from S1 to S10 that is at least as demanding as the anticipated duty.
The duty type shall be designated by the appropriate abbreviation, specified in 4.2, written after the value of the load
An expression for the cyclic duration factor is given in the relevant duty type figure
The purchaser typically cannot supply the moment of inertia of the motor (J M) or the relative thermal life expectancy (TL), as detailed in Annex A; these values are supplied by the manufacturer.
Where the purchaser does not declare a duty, the manufacturer shall assume that duty type S1 (continuous running duty) applies.
Duty types
4.2.1 Duty type S1 – Continuous running duty
Operation at a constant load maintained for sufficient time to allow the machine to reach thermal equilibrium, see Figure 1
The appropriate abbreviation is S1 t t t Θmax
P V electrical losses Θ temperature Θ max maximum temperature attained t time
Figure 1 – Continuous running duty – Duty type S1
4.2.2 Duty type S2 – Short-time duty
Operating a machine at a constant load for a limited time, which is shorter than the duration needed to achieve thermal equilibrium, should be followed by a sufficient de-energized rest period This rest allows the machine temperatures to stabilize within 2 K of the coolant temperature, as illustrated in Figure 2.
The appropriate abbreviation is S2, followed by an indication of the duration of the duty, Example: S2 60 min Δt P
P V electrical losses Θ temperature Θ max maximum temperature attained t time Δ t P operation time at constant load
Figure 2 – Short-time duty – Duty type S2
4.2.3 Duty type S3 – Intermittent periodic duty
NOTE Periodic duty implies that thermal equilibrium is not reached during the time on load
A series of uniform duty cycles consists of periods of operation under constant load followed by intervals of de-energization and rest, as illustrated in Figure 3 In this configuration, the starting current remains low enough to prevent a significant increase in temperature.
The appropriate abbreviation is S3, followed by the cyclic duration factor
P V electrical losses Θ temperature Θ max maximum temperature attained t time
T C time of one load cycle Δ t P operation time at constant load Δ t R time de-energized and at rest
Figure 3 – Intermittent periodic duty – Duty type S3
4.2.4 Duty type S4 – Intermittent periodic duty with starting
NOTE Periodic duty implies that thermal equilibrium is not reached during the time on load
A sequence of identical duty cycles, each cycle including a significant starting time, a time of operation at constant load and a time de-energized and at rest, see Figure 4
The correct abbreviation is S4, which is accompanied by the cyclic duration factor, the moment of inertia of the motor (J_M), and the moment of inertia of the load (J_ext), both referenced to the motor shaft.
P V electrical losses T C time of one load cycle Θ temperature Δ t D starting/accelerating time Θ max maximum temperature attained Δ t P operation time at constant load Δ t R time de-energized and at rest Cyclic duration factor = ( Δ t D + Δ t P )/T C
Figure 4 – Intermittent periodic duty with starting – Duty type S4
4.2.5 Duty type S5 – Intermittent periodic duty with electric braking
NOTE Periodic duty implies that thermal equilibrium is not reached during the time on load
A series of identical duty cycles is defined, with each cycle comprising a starting time, a period of operation under constant load, a duration of electric braking, and a resting phase when de-energized, as illustrated in Figure 5.
The correct abbreviation is S5, which is accompanied by the cyclic duration factor, the moment of inertia of the motor (J_M), and the moment of inertia of the load (J_ext), both referenced to the motor shaft.
Example: S5 25 % J M = 0,15 kg × m 2 J ext = 0,7 kg × m 2 Δt F Θmax Θ t t t
P load T C time of one load cycle
P V electrical losses Δ t D starting/accelerating time Θ temperature Δ t P operation time at constant load Θ max maximum temperature attained Δ t F time of electric braking t time Δ t R time de-energized and at rest
Figure 5 – Intermittent periodic duty with electric braking – Duty type S5
4.2.6 Duty type S6 – Continuous operation periodic duty
NOTE Periodic duty implies that thermal equilibrium is not reached during the time on load
A series of identical duty cycles is characterized by alternating periods of operation under constant load and periods of operation without load, with no intervals of de-energization or rest, as illustrated in Figure 6.
The appropriate abbreviation is S6, followed by the cyclic duration factor
P V electrical losses T C time of one load cycle Θ temperature Δ t P operation time at constant load Θ max maximum temperature attained Δ t V operation time at no-load
Figure 6 – Continuous operation periodic duty – Duty type S6
4.2.7 Duty type S7 – Continuous operation periodic duty with electric braking
NOTE Periodic duty implies that thermal equilibrium is not reached during the time on load
The article describes a series of identical duty cycles, where each cycle includes a starting time, a period of operation under constant load, and a phase of electric braking Notably, there is no interval of de-energization or rest during these cycles, as illustrated in Figure 7.
The appropriate abbreviation is S7, followed by the moment of inertia of the motor (J M ) and the moment of inertia of the load (J ext ), both referred to the motor shaft
P V electrical losses T C time of one load cycle Θ temperature Δ t D starting/accelerating time Θ max maximum temperature attained Δ t P operation time at constant load
Cyclic duration factor = 1 Δ t F time of electric braking
Figure 7 – Continuous operation periodic duty with electric braking – Duty type S7
4.2.8 Duty type S8 – Continuous operation periodic duty with related load/speed changes
NOTE Periodic duty implies that thermal equilibrium is not reached during the time on load
A sequence of identical duty cycles involves a period of operation at a constant load, maintaining a predetermined rotational speed, followed by additional periods at varying constant loads that correspond to different rotational speeds, which can be achieved by altering the number of poles in induction motors Notably, there is no interval of de-energization or rest during this process.
The correct abbreviation is S8, which includes the motor's moment of inertia (J M) and the load's moment of inertia (J ext), both referenced to the motor shaft Additionally, it encompasses the load, speed, and cyclic duration factor for each speed condition.
Example: S8 J M = 0,5 kg × m 2 J ext = 6 kg × m 2 16 kW 740 min − 1 30 %
P V electrical losses T C time of one load cycle Θ temperature Δ t D starting/accelerating time Θ max maximum temperature attained Δ t P operation time at constant load (P1, P2, P3) n speed Δ t F time of electric braking (F1, F2)
Figure 8 – Continuous operation periodic duty with related load/speed changes – Duty type S8
4.2.9 Duty type S9 – Duty with non-periodic load and speed variations
The duty involves non-periodic variations in load and speed within the allowable operating range, often featuring frequent overloads that can significantly surpass the reference load.
For this duty type, a constant load appropriately selected and based on duty type S1 is taken as the reference value ("P ref " in Figure 9) for the overload concept t t t t Θ
P ref reference load Δ t D starting/accelerating time
P V electrical losses Δ t P operation time at constant load Θ temperature Δ t F time of electric braking Θ max maximum temperature attained Δ t R time de-energized and at rest n speed Δ t S time under overload
Figure 9 – Duty with non-periodic load and speed variations – Duty type S9
4.2.10 Duty type S10 – Duty with discrete constant loads and speeds
A duty cycle is defined by a set of distinct load values, and potentially speed, where each load/speed combination is sustained long enough for the machine to achieve thermal equilibrium Notably, the minimum load in a duty cycle can be zero, indicating a no-load condition or when the machine is de-energized and at rest.
The correct abbreviation is S10, which is accompanied by the per unit quantities p/Δt for the specific load and its duration, as well as the per unit quantity TL representing the relative thermal life expectancy of the insulation system The reference for thermal life expectancy is based on continuous running duty and the permissible temperature rise limits for duty type S1 When the system is de-energized and at rest, the load is denoted by the letter r.
The TL value must be rounded to the nearest multiple of 0.05, with guidance on its importance and calculation provided in Annex A.
For this duty type a constant load appropriately selected and based on duty type S1 shall be taken as the reference value (‘P ref ’ in Figure 10) for the discrete loads
The discrete load values typically represent equivalent loading derived from time integration It is not essential for each load cycle to be identical; rather, each load within a cycle should be sustained long enough to achieve thermal equilibrium Additionally, each load cycle must be integrable to ensure a consistent relative thermal life expectancy.
P i constant load within a load cycle t i time of a constant load within a cycle
P ref reference load based on duty type S1 T C time of one load cycle
P V electrical losses, denoted as ΔΘi, represent the difference in temperature rise of the winding at various loads during a single cycle compared to the temperature rise at the reference load based on duty cycle S1 This analysis considers the reference temperature Θref at the specified load and the operational speed n.
Figure 10 – Duty with discrete constant loads – Duty type S10
Assignment of rating
The manufacturer is responsible for assigning the rating as outlined in section 3.2, choosing from the classes defined in sections 5.2.1 to 5.2.6 The selected class designation must be indicated following the rated output In the absence of a specified designation, the rating defaults to continuous running duty.
When accessory components like reactors and capacitors are integrated by the manufacturer into a machine, the rated values should be based on the supply terminals of the entire system.
NOTE This does not apply to power transformers connected between the machine and the supply
Special considerations are required when assigning ratings to machines fed from or supplying static converters IEC 60034-17 gives guidance for the case of cage induction motors covered in IEC 60034-12.
Classes of rating
5.2.1 Rating for continuous running duty
A rating at which the machine may be operated for an unlimited period, while complying with the requirements of this standard
This class of rating corresponds to duty type S1 and is designated as for the duty type S1
5.2.2 Rating for short-time duty
A rating at which the machine may be operated for a limited period, starting at ambient temperature, while complying with the requirements of this standard
This class of rating corresponds to duty type S2 and is designated as for the duty type S2
A rating at which the machine may be operated on duty cycles, while complying with the requirements of this standard
This class of rating corresponds to one of the periodic duty types S3 to S8 and is designated as for the corresponding duty type
Unless otherwise specified, the duration of a duty cycle shall be 10 min and the cyclic duration factor shall be one of the following values:
5.2.4 Rating for non-periodic duty
A rating at which the machine may be operated non-periodically while complying with the requirements of this standard
This class of rating corresponds to the non-periodic duty type S9 and is designated as for the duty type S9
5.2.5 Rating for duty with discrete constant loads and speeds
The machine can operate continuously under duty type S10, adhering to standard requirements, with a maximum permissible load that accounts for all components, including insulation systems and bearings, while respecting thermal limits Unless stated otherwise in relevant IEC standards, this maximum load should not exceed 1.15 times the load value for duty type S1 The minimum load can be zero, allowing for no-load operation or when the machine is de-energized and at rest Additional considerations for this rating class are detailed in Annex A.
This class of rating corresponds to the duty type S10 and is designated as for the duty type S10
NOTE Other relevant IEC standards may specify the maximum load in terms of limiting winding temperature (or temperature rise) instead of per unit load based on duty type S1
The rating for testing indicates the constant load at which a machine can operate until thermal equilibrium is achieved, leading to a stator winding temperature rise equivalent to the average temperature increase during one load cycle of the designated duty type.
NOTE The determination of an equivalent rating should take account of the varying load, speed and cooling of the duty cycle
This class of rating, if applied, is designated 'equ'.
Selection of a class of rating
A machine manufactured for general purpose shall have a rating for continuous running duty and be capable of performing duty type S1
If the duty has not been specified by the purchaser, duty type S1 applies and the rating assigned shall be a rating for continuous running duty
When a machine is intended to have a rating for short-time duty, the rating shall be based on duty type S2, see 4.2.2
For machines designed to handle varying loads, including periods of no-load or de-energized states, the appropriate rating should be classified as a periodic duty rating This rating must be based on one of the duty types ranging from S3 to S8, as outlined in sections 4.2.3 to 4.2.8.
When a machine is intended non-periodically to supply variable loads at variable speeds, including overloads, the rating shall be a rating for non-periodic duty based on duty type S9, see 4.2.9
For machines designed to provide discrete constant loads, including periods of overload or no-load (when de-energized and at rest), the appropriate rating should be based on duty type S10, as outlined in section 4.2.10.
Allocation of outputs to class of rating
In the determination of the rating:
For duty types S1 to S8, the specified value(s) of the constant load(s) shall be the rated output(s), see 4.2.1 to 4.2.8
For duty types S9 and S10, the reference value of the load based on duty type S1 shall be taken as the rated output, see 4.2.9 and 4.2.10.
Rated output
The rated output is the output at the terminals and shall be expressed in watts (W)
The rated output is the apparent power at the terminals and shall be expressed in volt- amperes (VA) together with the power factor
The rated power factor for synchronous generators shall be 0,8 lagging (over-excited), unless otherwise specified by the purchaser
The rated output is the mechanical power available at the shaft and shall be expressed in watts (W)
In certain countries, the mechanical power output of motors is commonly measured in horsepower, where 1 horsepower (h.p.) is equivalent to 745.7 watts, and 1 metric horsepower (ch or cheval) has a different value.
The rated output is the reactive power at the terminals and shall be expressed in volt-amperes reactive (var) in leading (under-excited) and lagging (over-excited) conditions.
Rated voltage
For direct current (d.c.) generators designed to function within a limited voltage range, the rated output and current should be applicable at the maximum voltage of that range, unless stated otherwise.
A.C generators designed for operation within a limited voltage range must have their rated output and power factor applicable at any voltage within that range, unless specified otherwise.
Co-ordination of voltages and outputs
Building machines for every voltage rating is impractical Typically, for a.c machines, preferred voltage ratings above 1 kV are determined by design and manufacturing factors, as outlined in Table 1.
Minimum rated output kW (or kVA)
Machines with more than one rating
For machines with more than one rating, the machine shall comply with this standard in all respects at each rating
For multi-speed motors, a rating shall be assigned for each speed
When a rated quantity, such as output, voltage, or speed, can take on multiple values or vary continuously within specified limits, the rating must be clearly indicated at these values or limits This requirement excludes voltage and frequency variations during operation, as outlined in section 7.3, and does not apply to star-delta connections used for starting.
General
Unless otherwise specified, machines shall be suitable for the following site operating conditions For site operating conditions deviating from those values, corrections are given in Clause 8.
Altitude
The altitude shall not exceed 1 000 m above sea-level.
Maximum ambient air temperature
The ambient air temperature shall not exceed 40 °C.
Minimum ambient air temperature
The ambient air temperature shall not be less than −15 °C for any machine
The ambient air temperature must be at least 0 °C for machines with specific characteristics, including those with a rated output exceeding 3,300 kW (or kVA) at 1,000 min⁻¹, a rated output below 600 W (or VA), the presence of a commutator, a sleeve bearing, or when water is used as a primary or secondary coolant.
Water coolant temperature
For the reference water coolant temperature see Table 4 For other water coolant temperatures see Table 9 The water coolant temperature shall not be less than +5 °C.
Storage and transport
When temperatures lower than specified in 6.4 are expected during transportation, storage, or after installation, the purchaser shall inform the manufacturer and specify the expected minimum temperature.
Purity of hydrogen coolant
Hydrogen cooled machines shall be capable of operating at rated output under rated conditions with a coolant containing not less than 95 % hydrogen by volume
NOTE For safety reasons, the hydrogen content should at all times be maintained at 90 % or more, it being assumed that the other gas in the mixture is air
To calculate efficiency per IEC 60034-2 standards, the gaseous mixture must consist of 98% hydrogen and 2% air by volume, adhering to the specified pressure and temperature of the re-cooled gas, unless otherwise agreed Windage losses should be determined based on the corresponding density.
Electrical supply
For three-phase alternating current (a.c.) machines operating at 50 Hz or 60 Hz and designed for direct connection to distribution or utilization systems, the rated voltages must be based on the nominal voltages specified in IEC 60038.
NOTE For large high-voltage a.c machines, the voltages may be selected for optimum performance
For a.c motors supplied from static converters these restrictions on voltage, frequency and waveform do not apply In this case, the rated voltages shall be selected by agreement.
Form and symmetry of voltages and currents
AC motors designed for fixed frequency power supplies, whether sourced from a local generator or a supply network, must be capable of operating on a supply voltage with a harmonic voltage factor (HVF) that does not exceed specified limits.
– 0,02 for single-phase motors and three-phase motors, including synchronous motors but excluding motors of design N (see IEC 60034-12), unless the manufacturer declares otherwise
The HVF shall be computed by using the following formula:
= n where u n is the ratio of the harmonic voltage U n to the rated voltage U N ; n is the order of harmonic (not divisible by three in the case of three-phase a.c motors); k = 13
Three-phase a.c motors must be compatible with a three-phase voltage system that limits the negative-sequence component to a maximum of 1% of the positive-sequence component for extended durations, or 1.5% for short periods not exceeding a few minutes, while also ensuring that the zero-sequence component does not exceed 1% of the positive-sequence component.
When the limiting values of the high-voltage factor (HVF) and the negative-sequence and zero-sequence components are present simultaneously at rated load, it is crucial that this does not cause harmful temperatures in the motor It is advised that the resulting temperature rise should not exceed approximately 10 K, in accordance with the specified limits of this standard.
In areas with significant single-phase loads, such as induction furnaces, and in rural regions with mixed industrial and domestic systems, power supplies may experience distortion that exceeds acceptable limits Consequently, special arrangements will be required to address these issues.
7.2.1.2 AC motors supplied from static converters have to tolerate higher harmonic contents of the supply voltage; see IEC 60034-17 for the case of cage motors within the scope of IEC 60034-12
When the supply voltage is highly non-sinusoidal, such as that produced by static converters, both the root mean square (r.m.s.) value of the total waveform and the fundamental component are crucial for assessing the performance of an alternating current (a.c.) machine.
Three-phase a.c generators must be capable of powering circuits that, when connected to a balanced and sinusoidal voltage system, produce currents with a harmonic current factor (HCF) not exceeding 0.05 Additionally, the system's currents should ensure that neither the negative-sequence nor the zero-sequence components exceed 5% of the positive-sequence component.
The HCF shall be computed by using the following formula:
2 n 2 where i n is the ratio of the harmonic current I n to the rated current I N ; n is the order of harmonic; k = 13
When deformation and imbalance limits are reached simultaneously under rated load, it is crucial that this does not cause harmful temperatures in the generator It is advised that any resulting temperature rise should not exceed approximately 10 K, in accordance with the specified standards.
Three-phase synchronous machines must operate continuously on an unbalanced system without exceeding the rated current for any phase The negative-sequence current component (I₂) to rated current (Iₙ) ratio must remain within the limits specified in Table 2 Additionally, under fault conditions, the product of the squared ratio (I₂/Iₙ)² and time (t) must also comply with the values outlined in Table 2.
Table 2 − Unbalanced operating conditions for synchronous machines
Item Machine type Maximum I 2 / I N value for continuous operation Maximum ( I 2 / I N ) 2 × t in seconds for operation under fault conditions
1 Indirect cooled windings motors 0,1 20 generators 0,08 20 synchronous condensers 0,1 20
2 Direct cooled (inner cooled) stator and/or field windings motors 0,08 15 generators 0,05 15 synchronous condensers 0,08 15
3 Indirect cooled rotor windings air-cooled 0,1 15 hydrogen-cooled 0,1 10
4 Direct cooled (inner cooled) rotor windings
> 1 250 ≤ 1 600 MVA 0,05 5 a For these machines, the value of I 2 /I N is calculated as follows:
I b For these machines, the value of (I 2 /I N ) 2 × t , in seconds, is calculated as follows:
(I 2 /I N ) 2 × t = 8 – 0,005 45 (S N – 350) where in the two footnotes, S N is the rated apparent power in MVA
7.2.4 DC motors supplied from static power converters
When a d.c motor is powered by a static power converter, the pulsating voltage and current can negatively impact its performance This leads to increased losses and a rise in temperature, making commutation more challenging compared to a d.c motor powered by a pure d.c source.
Motors with a rated output exceeding 5 kW, designed for use with a static power converter, must be specifically engineered for the designated supply Additionally, motor manufacturers may require the inclusion of external inductance to minimize supply undulation.
The static power converter supply shall be characterized by means of an identification code, as follows:
CCC is the identification code for converter connection according to IEC 60971;
U aN is represented by three or four digits that denote the rated alternating voltage at the converter's input terminals, measured in volts (V) The frequency, denoted by f, is indicated by two digits representing the rated input frequency in hertz (Hz).
L consists of one, two or three digits indicating the series inductance to be added externally to the motor armature circuit, in mH If this is zero, it is omitted
Motors with a rated output of up to 5 kW can be compatible with any static power converter, regardless of whether external inductance is used It is essential that the motor's rated form factor is not exceeded and that the insulation level of the motor armature circuit is suitable for the rated alternating voltage at the static power converter's input terminals.
The output current of the static power converter is assumed to have minimal undulation, ensuring that the current ripple factor remains below 0.1 under rated conditions.
Voltage and frequency variations during operation
A.C machines designed for operation on a fixed frequency power supply from an a.c generator, whether local or through a supply network, experience voltage and frequency variations classified into two categories: zone A and zone B, as illustrated in the accompanying figure.
11 for generators and synchronous condensers, and Figure 12 for motors
For d.c machines, when directly connected to a normally constant d.c bus, zones A and B apply only to the voltages
A machine must continuously perform its primary function within zone A, as outlined in Table 3, although it is not required to fully meet performance standards at rated voltage and frequency, as indicated by the rating point in Figures 11 and 12 Additionally, temperature increases may exceed those observed at rated voltage and frequency.
A machine must effectively perform its primary function within zone B, although it may show greater performance deviations at rated voltage and frequency compared to zone A Additionally, temperature increases in zone B are likely to exceed those observed in zone A It is advisable to avoid prolonged operation at the edge of zone B.
In practical applications, machines may occasionally need to operate outside zone A, but these excursions should be limited in value, duration, and frequency To mitigate potential issues, corrective measures, such as reducing output, should be implemented promptly Taking such actions can help prevent a decrease in machine lifespan due to temperature effects.
The temperature-rise limits outlined in this standard are applicable at the rating point but may be exceeded as the operating point deviates from it In extreme conditions at the boundaries of zone A, temperature rises can exceed the specified limits by approximately 10 K.
An a.c motor can only start at the lower voltage limit if its starting torque is sufficiently aligned with the load's counter-torque, although this is not mandated by the clause For details on the starting performance of design N motors, refer to IEC 60034-12.
NOTE 4 For machines covered by IEC 60034-3, different voltage and frequency limits apply
Table 3 − Primary functions of machines
Item Machine type Primary function
1 AC generator, excluding item 5 Rated apparent power (kVA), at rated power factor where this is separately controllable
2 AC motor, excluding items 3 and 5 Rated torque (Nm)
3 Synchronous motor, excluding item 5 Rated torque (Nm), the excitation maintaining either rated field current or rated power factor, where this is separately controllable
Rated apparent power (kVA) within the zone applicable to a generator, see Figure 11, unless otherwise agreed
5 Turbine-type machine, with rated output ≥ 10 MVA
6 DC generator Rated output (kW)
7 DC motor Rated torque (Nm), the excitation of a shunt motor maintaining rated speed, where this is separately controllable
Y axis voltage p.u 2 zone B (outside zone A)
Figure 11 – Voltage and frequency limits for generators
Figure 12 – Voltage and frequency limits for motors
Three-phase a.c machines operating on unearthed systems
Three-phase a.c machines must be designed for continuous operation with the neutral grounded or close to earth potential They should also function on unearthed systems with one line at earth potential for brief periods, such as during normal fault clearance For continuous or extended operation under these conditions, a machine with appropriate insulation levels is necessary.
If the winding does not have the same insulation at the line and neutral ends, this shall be stated by the manufacturer
Consulting the machine manufacturer is essential before proceeding with the earthing or interconnection of the machine's neutral points This precaution is necessary due to the potential dangers posed by zero-sequence components of currents across various frequencies under certain operating conditions, as well as the risk of mechanical damage to the windings during line-to-neutral fault situations.
Voltage (peak and gradient) withstand levels
For a.c motors the manufacturer shall declare a limiting value for the peak voltage and for the voltage gradient in continuous operation
For cage induction motors within the scope of IEC 60034-12, see also IEC 60034-17
For high-voltage a.c motors, see also IEC 60034-15
For creepage and clearance distances of bare live copper, see IEC 60664-1
Thermal class
A thermal class in accordance with IEC 60085 shall be assigned to the insulation systems used in machines
It is the responsibility of the manufacturer of the machine to interpret the results obtained by thermal endurance testing according to the appropriate part of IEC 60034-18
NOTE 1 The thermal class of a new insulation system should not be assumed to be directly related to the thermal capability of the individual materials used in it
NOTE 2 The continued use of an existing insulation system is acceptable where it has been proved by satisfactory service experience.
Conditions for thermal tests
During the thermal testing of an AC motor, the high voltage factor (HVF) of the supply must not exceed 0.015, and the negative-sequence voltage component should remain below 0.5% of the positive-sequence component, with the zero-sequence component's influence being disregarded.
According to the agreement, the negative-sequence component of the current system can be measured in place of the negative-sequence component of the voltage system It is stipulated that the negative-sequence component of the current must not exceed 2.5% of the positive-sequence component.
8.3.2 Temperature of machine before test
If the temperature of a winding is to be determined from the increase of resistance, the initial winding temperature shall not differ from the coolant by more than 2 K
For short-time rating tests (duty type S2), it is essential that the machine's initial temperature during the thermal test is maintained within 5 K of the coolant temperature.
A machine may be tested at any convenient value of coolant temperature See Table 11 (for indirect cooled windings) or Table 14 (for direct cooled windings)
8.3.4 Measurement of coolant temperature during test
The coolant temperature during testing should be determined by averaging the readings from temperature detectors taken at equal intervals during the last quarter of the test duration To minimize errors caused by the time lag in temperature changes of large machines in response to coolant temperature variations, it is essential to implement all reasonable precautions to reduce these fluctuations.
8.3.4.1 Open machines or closed machines without heat exchangers (cooled by surrounding ambient air or gas)
Ambient air or gas temperature must be measured using multiple detectors strategically positioned at various points around the machine, specifically at heights of 1 to 2 meters To ensure accurate readings, each detector should be shielded from radiant heat and drafts.
8.3.4.2 Machines cooled by air or gas from a remote source through ventilation ducts and machines with separately mounted heat exchangers
The temperature of the primary coolant shall be measured where it enters the machine
8.3.4.3 Closed machines with machine-mounted or internal heat exchangers
The temperature of the primary coolant shall be measured where it enters the machine The temperature of the secondary coolant shall be measured where it enters the heat exchanger.
Temperature rise of a part of a machine
The temperature rise, denoted as Δθ, refers to the difference between the temperature of a specific machine part, measured using the method outlined in section 8.5, and the coolant temperature, which is measured according to section 8.3.4.
To ensure accurate comparison with temperature rise limits (refer to Tables 7 or 8) or temperature limits (see Table 12), it is essential to measure the temperature right before shutting down the machine at the conclusion of the thermal test, as outlined in section 8.7.
When this is not possible, for example, when using the direct measurement of resistance method, see 8.6.2.3
For machines evaluated under actual periodic duty (duty types S3 to S8), the temperature recorded at the conclusion of the test should be noted as the temperature at the midpoint of the rise period that results in the highest heating during the final operational cycle.
Methods of measurement of temperature
Three methods of measuring the temperature of windings and other parts are recognized:
− embedded temperature detector (ETD) method;
Different methods shall not be used as a check upon one another
For indirect testing see IEC 60034-29
The temperature of the windings is determined from the increase of the resistance of the windings
8.5.3 Embedded temperature detector (ETD) method
Temperature is measured using built-in detectors, such as resistance thermometers, thermocouples, or semiconductor negative coefficient detectors, strategically placed in areas of the machine that become inaccessible after construction.
Temperature measurement in completed machines is conducted using thermometers on accessible surfaces This includes various types of thermometers such as bulb-thermometers, non-embedded thermocouples, and resistance thermometers In environments with strong or fluctuating magnetic fields, alcohol thermometers are recommended over mercury thermometers for safety and accuracy.
Determination of winding temperature
In general, for measuring the temperature of the windings of a machine, the resistance method in accordance with 8.5.1 shall be applied (but see also 8.6.2.3.3)
For a.c stator windings of machines having a rated output of 5 000 kW (or kVA) or more the ETD method shall be used
For alternating current (a.c.) machines with a rated output between 200 kW (or kVA) and 5,000 kW (or kVA), manufacturers must select either the resistance method or the ETD method, unless an alternative agreement is made.
For alternating current (a.c.) machines with a rated output of 200 kW (or kVA) or less, manufacturers must select either the direct measurement version or the superposition version of the resistance method, unless an alternative agreement is made.
For machines with a rated output of 600 W (or VA) or less, temperature measurement can be conducted using thermometers when the windings are non-uniform or when complex connections are present The temperature rise limits specified in Table 7, item 1d for the resistance method must be adhered to.
The thermometer method is essential in situations where the resistance method is impractical for measuring temperature rise, particularly with low-resistance commutating coils and compensating windings It is also applicable to single layer windings, whether rotating or stationary, and is commonly used during routine tests on mass-produced machines.
For a.c stator windings having only one coil-side per slot, the ETD method shall not be used for verifying compliance with this standard: the resistance method shall be used
For accurate temperature monitoring of windings in service, an embedded detector at the bottom of the slot is ineffective as it primarily measures the iron core temperature A detector positioned between the coil and the wedge provides a closer approximation of the winding temperature, making it more suitable for service checks Since this temperature may be relatively low, a thermal test should be conducted to establish the relationship between it and the temperature measured using the resistance method.
For other windings having one coil-side per slot and for end windings, the ETD method shall not be used for verifying compliance with this standard
The resistance method and the thermometer method are commonly used for measuring the windings of armatures with commutators and field windings While the resistance method is generally preferred, the ETD method can be utilized for stationary field windings in d.c machines that have multiple layers.
One of the following methods shall be used:
• direct measurement at the beginning and the end of the test, using an instrument having a suitable range;
• measurement by d.c current/voltage in d.c windings, by measuring the current in and the voltage across the winding, using instruments having suitable ranges;
• measurement by d.c current/voltage in a.c windings, by injecting direct current into the winding when de-energized
The temperature rise, θ2 – θa, may be obtained from the equation:
The initial resistance measurement is taken at the winding's cold temperature, denoted as θ1 (°C) At the conclusion of the thermal test, the winding's temperature is recorded as θ2 (°C), while the coolant's temperature at the same point is represented as θa (°C).
R 1 is the resistance of the winding at temperature θ1 (cold);
R 2 is the resistance of the winding at the end of the thermal test; k is the reciprocal of the temperature coefficient of resistance at 0 °C of the conductor material
For aluminium k = 225 unless specified otherwise
For practical purposes, the following alternative formula may be found convenient:
To accurately measure temperatures at the conclusion of a thermal test using the direct measurement resistance method, a swift shutdown is essential This process necessitates a well-organized procedure and a sufficient number of personnel.
If the initial resistance reading is obtained within the time interval specified in Table 5, that reading shall be accepted for the temperature measurement
Rated output ( P N ) kW or kVA
Time interval after switching off power s
If resistance readings cannot be obtained within the time frame specified in Table 5, they should be taken as soon as possible, but no later than twice the specified interval Additional readings should be recorded approximately every minute until a clear decline from the maximum value is observed These readings should be plotted over time, ideally using a semi-logarithmic scale with temperature on the logarithmic axis, and extrapolated to the appropriate interval in Table 5 for the machine's rated output The resulting temperature value will be considered the shutdown temperature In cases where successive measurements indicate rising temperatures post-shutdown, the highest recorded value should be used.
If a resistance reading cannot be made until after twice the time interval specified in Table 5, this method of correction shall only be used by agreement
8.6.2.3.4 Windings with one coil-side per slot
For machines with a single coil-side per slot, the direct measurement resistance method is applicable if the machine stops within the time frame outlined in Table 5 However, if the machine requires more than 90 seconds to come to a complete stop after power is turned off, the superposition method can be utilized, provided there is prior agreement.
The detectors shall be suitably distributed throughout the winding and the number of detectors installed shall be not less than six
Detectors should be strategically positioned at locations where the highest temperatures are expected, ensuring they are adequately shielded from direct contact with the primary coolant while prioritizing safety.
The highest reading from the ETD elements shall be used to determine the temperature of the winding
NOTE ETD elements or their connections may fail and give incorrect readings Therefore, if one or more readings are shown to be erratic, after investigation they should be eliminated
8.6.3.2 Two or more coil-sides per slot
The detectors shall be located between the insulated coil-sides within the slot in positions at which the highest temperatures are likely to occur
8.6.3.3 One coil-side per slot
The detectors shall be located between the wedge and the outside of the winding insulation in positions at which the highest temperatures are likely to occur, but see also 8.6.1
Temperature detectors must be positioned between two adjacent coil-sides within the end windings, specifically in areas where the highest temperatures are expected Each detector's sensing point should be in direct contact with the coil-side surface and sufficiently shielded from coolant effects, as referenced in section 8.6.1.
To ensure accurate temperature monitoring, thermometers should be strategically positioned at locations where the highest temperatures are expected, such as near the core iron in the end windings These thermometers must be adequately shielded from direct contact with the primary coolant while maintaining effective thermal contact with the winding or other machine components.
The highest reading from any thermometer shall be taken to be the temperature of the winding or other part of the machine.
Duration of thermal tests
8.7.1 Rating for continuous running duty
The test shall be continued until thermal equilibrium has been reached
8.7.2 Rating for short-time duty
The duration of the test shall be the time given in the rating
The manufacturer's assigned rating for equivalent loading should be applied until thermal equilibrium is achieved If an actual duty test is conducted, the specified load cycle must be maintained until nearly identical temperature cycles are observed The criterion for this is that the temperature plot's gradient between corresponding points of successive duty cycles should be less than 2 K per hour Additionally, measurements should be taken at reasonable intervals over time if necessary.
8.7.4 Ratings for non-periodic duty and for duty with discrete constant loads
The rating for equivalent loading assigned by the manufacturer (see 5.2.6) shall be applied until thermal equilibrium has been reached.
Determination of the thermal equivalent time constant for machines of duty
The thermal equivalent time constant, applicable under normal operating conditions with ventilation, can be approximated by analyzing the cooling curve as outlined in section 8.6.2.3 This time constant value is essential for determining the temperature profile.
The cooling time of the machine after being disconnected from the supply is 1.44 times, or equivalently 1/ln(2) times, the duration required to reach half of the full load temperature rise.
Measurement of bearing temperature
Either the thermometer method or the ETD method may be used
The measuring point shall be as near as possible to one of the two locations specified in Table 6
Type of bearing Measuring point Location of measuring point
Ball or roller A In the bearing housing and not more than 10 mm a from the outer ring of the bearing b
B Outer surface of the bearing housing as close as possible to the outer ring of the bearing
Sleeve A In the pressure zone of the bearing shell c and not more than
10 mm a from the oil-film gap b
In the bearing shell, the distance is measured to the nearest point of the ETD or thermometer bulb For an 'inside out' machine, point A is located in the stationary part, no more than 10 mm from the inner ring, while point B is positioned on the outer surface of the stationary part, as close as possible to the inner ring The bearing shell supports the bearing material and is secured within the housing, with the pressure zone being the section of the circumference that bears the combined weight of the rotor and radial loads.
To ensure accurate temperature measurements, it is essential to minimize the thermal resistance between the temperature detector and the object being measured This can be achieved by filling air gaps with thermally conductive paste.
Temperature differences exist between measuring points A and B, as well as between these points and the hottest point of the bearing, influenced by factors such as bearing size For sleeve bearings with pressed-in bushings and ball and roller bearings with an inside diameter of up to 150 mm, the temperature difference between points A and B is negligible However, for larger bearings, this temperature difference is approximately 15 K.
Limits of temperature and of temperature rise
Limits for operation are defined based on site conditions outlined in Clause 6 and for continuous running duty at reference ratings The article also details how to adjust these limits when operating under different conditions and ratings Additionally, it provides guidelines for modifying limits during thermal testing when the test site conditions vary from the operating site.
The limits are stated relative to the reference coolant specified in Table 4
A rule is given to allow for the purity of hydrogen coolant
Temperature rises under reference conditions shall not exceed the limits given in Table 7 (air coolant) or Table 8 (hydrogen coolant) as appropriate
For operating conditions beyond continuous running duty and for rated voltages exceeding 12,000 V, adjustments to the limits must be made as specified in Table 9 Additionally, refer to Table 10 for the coolant temperature limits assumed in Table 9.
In the case of thermometer readings made in accordance with 8.6.1, the limit of temperature rise shall be according to Table 7
For indirectly air-cooled windings, if the test site conditions differ from those at the operating site, the adjusted limits specified in Table 11 must be applied at the test site.
If the adjusted limits in Table 11 result in temperatures at the test site deemed excessive by the manufacturer, an agreement must be reached regarding the testing procedure and limits.
Adjustments for windings indirectly cooled by hydrogen are not provided at the test site, as it is highly improbable that they will undergo testing at rated load outside of the operating site.
Table 7 outlines the permissible temperature rise limits for windings that are indirectly cooled by air, categorized by thermal classes: 130 (B), 155 (F), and 180 (H) The measurement methods include the use of a thermometer (Th), resistance (R), and an embedded temperature detector (ETD).
The article discusses the specifications for AC windings of machines based on their output ratings For machines with outputs of 5000 kW (or kVA) or more, the ratings are 80-85 a, 105-110 a, and 125-130 a For those with outputs above 200 kW (or kVA) but less than 5000 kW, the ratings are 80-90 a, 105-115 a, and 125-135 a Machines with outputs of 200 kW (or kVA) or less, excluding specific categories, have ratings of 80, 105, and 125 Additionally, AC windings with rated outputs of less than 600 W (or VA) are rated at 85, 110, and 130 Self-cooled AC windings without a fan (IC 40) or with encapsulated windings also follow the same ratings The article further details armature windings with commutators rated at 70-80, 85-105, and 105-125, as well as field windings for AC and DC machines, which are rated at 70-80, 85-105, and 105-125 Lastly, it specifies field windings of synchronous machines with cylindrical rotors and DC excitation embedded in slots, excluding synchronous induction motors.
Insulated stationary field windings of direct current (DC) machines with multiple layers typically range from 70 to 135 Low-resistance field windings for both alternating current (AC) and DC machines with multiple layers, as well as compensating windings for DC machines, have values between 80 and 125 Single-layer windings of AC and DC machines with exposed bare or varnished metal surfaces range from 90 to 135 For adjustments related to high-voltage AC windings, refer to item 4 of Table 9 The application of the superposition test method to windings of machines rated at 200 kW (or kVA) or less, with thermal classes 130 (B) and 155 (F), may allow for a temperature rise limit exceeding the resistance method by 5 K This also includes multiple layer windings, provided that the underlayers are in contact with the circulating primary coolant.
Table 8 − Limits of temperature rise of windings indirectly cooled by hydrogen
1 AC windings of machines having outputs of 5 000 kW (or kVA) or more or having a core length of 1 m or more Absolute hydrogen pressure b) ≤ 150 kPa (1,5 bar)
2a AC windings of machines having outputs of less than 5 000 kW
(or kVA), or having a core length of less than 1 m
2b DC field windings of a.c and d.c machines other than those in items 3 and 4
3 DC field windings of machines having cylindrical rotors 85 – 105 –
4a Low-resistance field windings of more than one layer and compensating windings
4b Single-layer windings with exposed bare or varnished metal surfaces c
For high-voltage a.c windings, refer to item 4 of Table 9 for necessary adjustments Notably, the temperature rise limit is influenced by hydrogen pressure Additionally, this includes multi-layer field windings, ensuring that each under layer is in contact with the circulating primary coolant.
Table 9 – Adjustments to limits of temperature rise at the operating site of indirect cooled windings to take account of non-reference operating conditions and ratings
Item Operation conditions or rating Adjustment to limit of temperature rise ( Δ θ ) in
1a Maximum temperature of ambient air or of the cooling gas at inlet to the machine ( θ c ) and for altitudes of up to 1 000 m
The difference between the thermal class and the observable temperature limit, which is the sum of the reference cold coolant inlet temperature of 40 °C and the temperature rise limit as specified in Tables 7 and 8, must be less than or equal to a certain value.
For a higher altitude replace 40 °C with the value given in Table 10
0 °C ≤ θ c ≤ 40 °C Increased by the amount by which the coolant temperature is less than 40 °C
Item Operation conditions or rating Adjustment to limit of temperature rise ( Δ θ ) in
1b Maximum temperature of ambient air or of the cooling gas at the inlet to the machine ( θ c ) and for altitudes of up to 1000 m
If the difference between the thermal class and the observable temperature limit, which is the sum of the reference cold coolant inlet temperature of 40 °C and the temperature rise limit from Tables 7 and 8, exceeds 5 K, then specific considerations must be taken into account.
For a higher altitude replace 40 °C with the value given in Table 10
0 °C ≤ θ c ≤ 40 °C Increased by the amount by which the coolant temperature is less than 40 °C, but this amount is reduced by the factor
1 lim.tmp with lim.tmp = limit of temperature rise according to
Tables 7 or 8 at 40 °C cold coolant temperature
1c 40 °C < θ c ≤ 60 °C Reduced by the amount by which the coolant temperature exceeds 40 °C 1d θ c < 0 °C or θ c > 60 °C By agreement
The maximum temperature of the water at the inlet to water-cooled heat exchangers, as well as the maximum temperature of ambient water for submersible machines with surface cooling or machines utilizing water jacket cooling, is denoted as \( \theta_w \).
Increased by 15 K and by the difference between
Increased by 15 K and reduced by the difference between θ w and 25 °C
Altitude (H) - power plant generator specific
1 000 m < H ≤ 4 000 m and maximum ambient air temperature not specified
H > 4000 m according specification of the purchaser
It is assumed that the decreased cooling effect due to altitude is balanced by a reduction in maximum ambient temperature, ensuring that the total temperature remains below 40 °C, in accordance with the temperature increases outlined in Tables 7 and 8.
The performance of power plant generators is influenced by altitude, as it affects air pressure However, if the absolute coolant pressure remains constant, no adjustments to the generator's capability are necessary, regardless of the altitude.
4 Rated stator winding voltage (U N ) 12 kV < U N ≤ 24 kV
U N > 24 kV Δ θ for embedded temperature detectors (ETD) shall be reduced by 1 K for each 1 kV (or part thereof) from 12 kV up to and including 24 kV
5 b Rating for short-time duty (S2), with rated output less than 5 000 kW (kVA) Increased by 10 K
6 b Rating for non-periodic duty (S9) Δ θ may be exceeded for short periods during the operation of the machine
The 7 b rating for duty with discrete loads (S10) allows for temporary exceedance of Δθ during machine operation This is based on the assumption that for every 100 meters of altitude increase, the ambient temperature decreases by 1% of the limiting rises.
1 000 m, the maximum ambient air temperature at the operating site can be as shown in Table 10 b For air-cooled windings only
Table 10 – Assumed maximum ambient temperature
Temperatures under reference conditions shall not exceed the limits given in Table 12
For other operating site conditions the limits shall be adjusted according to Table 13
If conditions at the test site differ from those at the operating site, the adjusted limits given in Table 14 shall apply at the test site
Routine tests
Routine tests are conducted as factory tests and can only be performed on machines assembled at the manufacturer's facility The machine does not need to be fully assembled, as it can be missing non-essential components for testing Additionally, routine tests typically do not require the machine to be coupled, with the exception of the open-circuit test for synchronous machines.
The minimum test schedule for machines with a rated output of 20 MW (MVA) or less is detailed in Table 15 For machines rated above 200 kW (kVA), additional routine tests may be conducted It is important to note that the term "synchronous machines" encompasses permanent magnet machines as well.
For d.c machines, depending on size and design, a commutation test under load may be performed as a routine test
Table 15 – Minimum schedule of routine tests
(including synchronous induction motors) a Motors Generators
DC machines with separate or shunt excitation
1 Resistance of windings (cold) Yes Yes Yes
2 No-load losses and current Yes – –
3a No-load losses at unity power factor b
3b No-load excitation current at rated voltage by open-circuit test b
4 Excitation current at rated speed and rated armature voltage
5 Open circuit secondary induced voltage at standstill (wound rotor) c
6a Direction of rotation Yes Yes – Yes
The withstand voltage test, as outlined in IEV 411-33-04, is applicable under section 9.2, with the exception of permanent magnet machines For safety reasons, this test can be conducted at a reduced voltage It is important to note that tests 3a and 3b are mutually exclusive, meaning only one of these tests is necessary.
Withstand voltage test
A specified test voltage must be applied between the windings under test and the machine's frame, ensuring that the core and non-tested windings are connected to the frame This test is to be conducted only on a new, fully assembled machine under conditions that mimic normal operation, either at the manufacturer's facility or after installation on-site Additionally, the withstand voltage test should be performed immediately following a thermal test.
For polyphase machines rated above 1 kV, when both ends of each phase are individually accessible, the test voltage must be applied between each phase and the frame During this process, the core, along with the other phases and windings not under test, should be connected to the frame.
The test voltage must generally be of power frequency and closely resemble a sine wave, with the final voltage value specified in Table 16 For machines rated at 6 kV or higher, if power frequency equipment is unavailable, a d.c test may be conducted at a voltage 1.7 times the r.m.s value listed in Table 16, subject to agreement.
NOTE It is recognized that, during a d.c test, the surface potential distribution along the end winding insulation and the ageing mechanisms are different from those occurring during an a.c test
The test should start at a voltage not exceeding 50% of the full test voltage, which must then be gradually increased to the full value in increments of no more than 5% The transition from half to full voltage should take at least 10 seconds Once the full test voltage is reached, it must be sustained for 1 minute, as specified in Table 16, without any failures occurring during this duration, in accordance with IEC 60060-1.
During the routine testing of quantity produced machines up to 200 kW (or kVA) and rated for
U N ≤ 1 kV, the 1 min test may be replaced by a test of 1 s at 120 % of the test voltage specified in Table 16
The acceptance withstand voltage test conducted on the windings at full voltage should not be repeated However, if the purchaser requests a second test after additional drying, the test voltage must be set at 80% of the voltage indicated in Table 16.
To establish the test voltage for d.c motors powered by static power converters, use the greater value between the motor's direct voltage and the r.m.s phase-to-phase rating of the alternating voltage at the static power converter's input terminals.
Completely rewound windings shall be tested at the full test voltage for new machines
When conducting withstand voltage tests for partially rewound windings or overhauled machines, it is essential to follow a specific procedure For partially rewound windings, testing should occur at 75% of the test voltage designated for new machines, ensuring the old winding is thoroughly cleaned and dried beforehand In the case of overhauled machines, after proper cleaning and drying, the test voltage should be set to 1.5 times the rated voltage, with a minimum of 1,000 V for rated voltages of 100 V or higher, and a minimum of 500 V for rated voltages below 100 V.
Item Machine or part Test voltage (r.m.s.)
1 Insulated windings of rotating machines of rated output less than 1 kW (or kVA) and of rated voltage less than 100 V with the exception of those in items 4 to 8
2 Insulated windings of rotating machines of rated output less than 10 000 kW (or kVA) with the exception of those in item 1 and items 4 to 8 b
1 000 V + twice the rated voltage with a minimum of
3 Insulated windings of rotating machines of rated output 10 000 kW (or kVA) or more with the exception of those in items 4 to 8 b
1 000 V + twice the rated voltage Subject to agreement
4 Separately excited field windings of d.c machines
1 000 V + twice the maximum rated circuit voltage with a minimum of 1 500 V
5 Field windings of synchronous generators, synchronous motors and synchronous condensers
- up to, and including 500 V, Ten times the rated field voltage with a minimum of
- above 500 V 4 000 V + twice the rated field voltage
Item Machine or part Test voltage (r.m.s.)
5b) When a machine is intended to be started with the field winding short-circuited or connected across a resistance of value less than ten times the resistance of the winding
Ten times the rated field voltage with a minimum of
When starting the machine, it is essential to connect the field winding across a resistance that is at least ten times greater than the winding's resistance, or to initiate the process with the field windings in an open circuit, regardless of whether a field-dividing switch is used.
The voltage across the terminals of the field winding can reach up to 1,000 V, in addition to twice the maximum value of the root mean square (r.m.s.) voltage that may occur under specified starting conditions This applies to both the entire field winding and any individual section in the case of a sectionalized field winding.
6 Secondary (usually rotor) windings of induction motors or synchronous induction motors if not permanently short-circuited (e.g if intended for rheostatic starting)
6a) For non-reversing motors or motors reversible from standstill only
1 000 V + twice the open-circuit standstill voltage as measured between slip-rings or secondary terminals with rated voltage applied to the primary windings
6b) For motors to be reversed or braked by reversing the primary supply while the motor is running
1 000 V + four times the open-circuit standstill secondary voltage as defined in item 6a)
7 Exciters (except as below) As for the windings to which they are connected
Exception 1: exciters of synchronous motors
(including synchronous induction motors) if connected to earth or disconnected from the field windings during starting
1 000 V + twice the rated exciter voltage, with a minimum of 1 500 V
Exception 2: separately excited field windings of exciters (see item 4)
8 Electrically interconnected machines and apparatus
To minimize redundancy, it is advisable to avoid repeating the tests outlined in items 1 to 7 However, if testing is necessary for a group of machines and apparatus that have already successfully passed their withstand voltage tests, the applied test voltage for the interconnected setup should be set at 80% of the lowest test voltage suitable for any individual component within the arrangement.
9 Devices that are in physical contact with windings, for example, temperature detectors, shall be tested to the machine frame
During the withstand test on the machine, all devices in physical contact with the winding shall be connected to the machine frame
For two-phase windings with a common terminal, the highest r.m.s voltage between any two terminals during operation must be used in calculations Withstand tests for machines with graded insulation require mutual agreement The voltage between field winding terminals under specified starting conditions can be measured at a reduced supply voltage, which should then be adjusted according to the ratio of the specified starting voltage to the test voltage When multiple machines are electrically connected, the maximum voltage in relation to earth must be considered.