Motor control transformers are designed to reduce sup- ply voltages to motor control circuits. Most AC commer- cial and industrial motors are operated from three-phase AC supply systems in the 208- to-600-V range. However, the control systems for these motors generally operate at 120 V. The major disadvantage to higher-voltage control schemes is that these higher voltages can be much more lethal than 120 V. Additionally, on a higher-voltage con- trol system tied directly to the supply lines, when a short circuit occurs in the control circuit a line fuse will blow or a circuit breaker will trip, but may not do so right away.
In some cases light-duty contacts, such as those in stop buttons or relay contacts, can weld together before the protective device trips or blows.
Step-down control transformers are installed when the control circuit components are not rated for the line voltage. Figure 3-27 shows the typical connection for a step-down motor control transformer. The primary side
PART 3 Transformer Connections and Systems
Transformer Polarity
Transformer polarity refers to the relative direction or polarity of the induced voltage between the high-voltage and low-voltage terminals of a transformer. An under- standing of transformer polarity markings is essential in making three-phase and single-phase transformer connec- tions. Knowledge of polarity is also required to connect potential and current transformers to power metering and protective relays.
On power transformers, the high-voltage winding leads are marked H1 and H2 and the low-voltage winding leads are marked X1 and X2 (Figure 3-25). By convention, H1 and X1 have the same polarity, which means that when H1 is instantaneously positive, X1 also is instantaneously positive. These markings are used in establishing the proper terminal connections when single-phase trans- formers are connected in parallel, series, and three-phase configurations.
In practice, the four terminals on a single-phase trans- former are mounted in a standard way so the transformer has either additive or subtractive polarity. Whether the polarity is additive or subtractive depends on the loca- tion of the H and X terminals. A transformer is said to have additive polarity when terminal H1 is diagonally opposite terminal X1. Similarly, a transformer has sub- tractive polarity when terminal H1 is adjacent to termi- nal X1. Figure 3-26 illustrates additive and subtractive transformer terminal markings along with a test circuit that can be used to verify markings. Also shown is a battery-operated transformer polarity checker that can perform the same test.
H1 H2
X1 X2
High-voltage winding
Low-voltage winding
24 V 240 V
Figure 3-25 Transformer polarity markings.
Photo courtesy Rockwell Automation, www.rockwellautomation.com.
Figure 3-26 Additive and subtractive transformer terminal markings.
Photo courtesy Tesco, www.tesco-advent.com.
120 V
H1 H2
X2 X1
Test circuit Additive polarity
12 V
132 V
Voltmeter reading
⫽ 120 V ⫹ 12 V
⫽ 132 V
Voltmeter reading
⫽ 120 V ⫺ 12 V
⫽ 108 V 120 V
H1 H2
X2 X1
Test circuit Subtractive polarity
12 V
108 V H1
X1 X2
H2
H1
X2 X1
H2
Battery operated transformer polarity checker
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54 Chapter 3 Motor Transformers and Distribution Systems
(H1 and H2) of the control transformer will be the line voltage, while the secondary voltage (X1 and X2) will be the voltage required for the control components.
Single-, dual-, and multitap primary control transform- ers are available. The versatile dual- and multitap primary transformers allow reduced control power from a variety of voltage sources to meet a wide array of applications.
Figure 3-28 shows the connections for a typical dual primary transformer used to step 240 or 480 V down to 120 V. The primary connections on the transformer are identified as H1, H2, H3, and H4. The transformer coil between H1 and H2 and the one between H3 and H4 are rated for 240 V each. The low-voltage secondary connec- tions on the transformer, X1 and X2, can have 120 V from either a 480- or 240-V line. If the transformer is to be used to step 480 V down to 120 V, the primary windings are connected in series by a jumper wire or metal link. When the transformer is to be used to step 240 V down to 120 V, the two primary windings must be connected in parallel with each other.
The control transformer secondary can be grounded or ungrounded. Where grounding is provided, the X2 side of the circuit common to the coils must be grounded at the control transformer. This will ensure that an accidental ground in the control circuit will not start the motor, or make the stop button or control inoperative. An additional requirement for all control transformers is that they be
480 V L1
H4 H1
H2 H3 H2 H3 H1
240 V 240 V
L2
X1 X2
120 V
240 V L1
H4
240 V 240 V
L2
X1 X2
120 V
Parallel connection for 240 V Series connection for 480 V
Figure 3-28 Typical dual-voltage 480-V and 240-V transformer connections.
Photo courtesy Siemens, www.siemens.com.
Figure 3-27 Motor control transformer wiring.
Photo courtesy of Superior Panels, www.superiorpanels.com.
M OL L1 T1
T3 OL M
L2 T2
Control transformer
H1 H2
X1 X2
L3
Control options Breaker or fused disconnect
Motor
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PART 3 Transformer Connections and Systems 55
L1
CB M OL
L2
X1
FU1 FU2
FU3 X2 L3
M Stop GND
M OL Start
Motor
Figure 3-29 Fuse protection for both the primary and secondary of the transformer and the correct ground connec- tion for a grounded control system.
Photo courtesy SolaHD, www.solahd.com.
480 V AC
480 V AC Line A Phase A
480 V AC
Phase C 480 V AC
Phase B 480 V AC Iline Iphase
Line B
Line C
Three- phase motor load
Figure 3-31 Three-phase, three-wire delta transformer connection supplying power to a three-phase motor load.
protected by fuses or circuit breakers. Depending on the installation, this protection can be placed on the primary, secondary, or both sides of the transformer. Figure 3-29 shows fuse protection for both the primary and secondary of the transformer and the correct ground connection for a grounded control system. The fuses must be properly sized for the control circuit. Section 430.72 of the Code lists requirements for the protection of transformers used in motor control circuits.
Three-Phase Transformers
Large amounts of power are generated and transmitted using high-voltage three-phase systems . Transmission voltages may be stepped down several times before they reach the motor load. This transformation is accom- plished using three-phase wye - or delta -connected transformers or a combination of the two. Figure 3-30 illustrates some of the common three-phase wye and delta transformer connections. The connections are named after the way the windings are connected inside the transformer. Polarity markings are fixed on any transformer and the connections are made in accordance with them.
The transformers supplying motor loads can be con- nected on the load (secondary) side either in delta or in wye configuration. Two types of secondary distribution systems commonly used are the three-phase three-wire system and three-phase four-wire system. In both, the secondary voltages are the same for all three phases. The three-phase three-wire delta system is used for balanced loads and consists of three transformer windings connected end to end. Figure 3-31 shows a typical three-phase, three-wire delta transformer
Figure 3-30 Common wye and delta transformer connections.
Wye-wye three-phase transformer connection
Delta-delta three-phase transformer connection
Delta-wye three-phase transformer connection X1 X1
L o a d
L o a d L o a d X1
X2 X2 X2 H1
H1
H1 H2 H2 H2 L1
L2
L3
Primary Secondary
X1
X1 X1
X2
X2 X2
H1
H1
H1 H2
H2
H2 L1
L2 L3
Primary Secondary
H1
H1
H1 H2
H2
H2 L1
L2 L3
Primary
X1 X1
X1 X2 X2 X2
Secondary
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56 Chapter 3 Motor Transformers and Distribution Systems connection supplying power to a three-phase motor load. For a delta-connected transformer:
• The phase voltage ( E phase ) of the transformer sec- ondary is always the same as the line voltage ( E line ) of the load.
• The line current ( I line ) of the load is equal to the phase current ( I phase ) of the transformer secondary multiplied by 1.73.
kVA (transformer) = _______________ I line × E line × √ __3 1,000 • The constant 1.73 is the square root of 3 and is used
because the transformer phase windings are 120 electrical degrees apart.
The other commonly used three-phase distribution is the three-phase, four-wire system. Figure 3-32 shows a typical wye-connected three-phase, four-wire distribu- tion system. The three phases connect at a common point, which is called the neutral. Because of this, none of the windings are affected by the other windings. Therefore, the wye three-phase, four-wire system is used for unbalanced loads. The phases are 120 electrical degrees apart; how- ever, they have a common point. For a Wye transformer connected transformer:
• The phase-to-phase voltage is equal to the phase-to- neutral voltage multiplied by 1.73.
• The line current is equal to the phase current.
kVA (transformer) = I line× E line× √ __
_______________ 3 1,000 • Common arrangements are 480Y/277 V and
208Y/120 V.
Line A
Ground Phase C
Phase B Phase A
Neutral
Iline
Iphase Line B
Line C Neutral Single phase Three phase
Available voltages
120 V 277 V
208 V 480 V
Figure 3-32 Wye-connected, three-phase, four-wire distri- bution system.
The delta-to-wye configuration is the most commonly used three-phase transformer connection. A typical delta- to-wye voltage transformation is illustrated in Figure 3-33.
The secondary provides a neutral point for supplying line-to-neutral power to single-phase loads. The neutral point is also grounded for safety reasons. Three-phase loads are supplied at 208 V, while the voltage for single- phase loads is 208 V or 120 V. When the transformer sec- ondary supplies large amounts of unbalanced loads, the delta primary winding provides a better current balance for the primary source.
The autotransformer , shown in Figure 3-34, is a trans- former consisting of a single winding with electrical connection points called taps . Each tap corresponds to a different voltage so that effectively a portion of the same inductor acts as part of both the primary and secondary winding. There is no electrical isolation between the input and output circuits, unlike the traditional two-winding transformer. The ratio of secondary to primary voltages is equal to the ratio of the number of turns of the tap they connect to. For example, connecting at the 50 percent tap (middle) and bottom of the autotransformer output will halve the input voltage. Because it requires both fewer windings and a smaller core, an autotransformer for some power applications is typically lighter and less costly than a two-winding transformer. A variable autotransformer is one in which the output connection is made through a sliding brush. Variable autotransformers are widely used where adjustable AC voltages are required.
An autotransformer motor starter, such as shown in Figure 3-35, reduces inrush motor current by using a three-coil autotransformer in the line just ahead of the motor to step down the voltage applied to the motor ter- minals. By reducing the voltage, the current drawn from the line is reduced during start-up. During the starting period, the motor is connected to the reduced-voltage taps on the autotransformer. Once the motor has accelerated, it is automatically connected to full-line voltage.
Instrument Transformers
Instrument transformers are small transformers used in conjunction with instruments such as ammeters, voltme- ters, power meters, and relays used for protective pur- poses (Figure 3-36). These transformers step down the voltage or current of a circuit to a low value that can be effectively and safely used for the operation of instru- ments. Instrument transformers also provide insulation between the instrument and the high voltage of the power circuit.
A potential (voltage) transformer operates on the same principle as a standard power transformer. The
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PART 3 Transformer Connections and Systems 57 Figure 3-33 Typical delta-to-wye, three-phase, four-wire transformer configuration.
L1
L2 C
A B
L3 Primary
L1 L2 N
L3 Secondary
T1 T2 T3
208-V 3 motor load
208-V 1 motor load
208-V 1 motor load
120-V lighting loads
3 1 1
H1 A
H2
X2 X1 X2 X1 X2 X1
H1 H2 H1 H2
120 V 120 V
120 V
2,400 V 2,400 V
2,400 V B
C
A B C N (Neutral)
Figure 3-34 Autotransformer.
Photo courtesy Superior Electric, www.superiorelectric.com.
Secondary
Variable autotransformer
Primary
50% tap L1
L2
Figure 3-35 Autotransformer motor starter.
Photo courtesy Rockwell Automation. www.rockwellautomation.com.
L1 L2 L3
T1 T2 T3
Start
Motor
main difference is that the capacity of a potential trans- former is relatively small compared to power transform- ers. Potential transformers have typical power ratings of from 100 VA to 500 VA. The secondary low-voltage side
is usually wound for 120 V, which makes it possible to use standard instruments with potential coil ratings of 120 V.
The primary side is designed to be connected in parallel with the circuit to be monitored.
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58 Chapter 3 Motor Transformers and Distribution Systems A current transformer is a transformer that has its primary connected in series with the line conductor.
The conductor passes through the center of the trans- former, as illustrated in Figure 3-37, and constitutes one primary turn. A current transformer supplies the instrument and/or protective device with a small cur- rent that is proportional to the main current. The sec- ondary winding consisting of many turns is designed to produce a standard 5 A when its rated current is flowing in the primary. The secondary circuit of a cur- rent transformer should never be opened when there is current in the primary winding. If the secondary is not loaded, this transformer acts to step up the voltage to a dangerous level, because of the high turns ratio.
Therefore, a current transformer should always have its secondary shorted when not connected to an exter- nal load.
PART 3 Review Questions
1. Explain the way in which the high-voltage and low- voltage leads of a single-phase power transformer are identified.
2. A polarity test is being made on the transformer shown in Figure 3-38.
a. What type of polarity is indicated?
b. What is the value of the voltage across the secondary winding?
Figure 3-37 Current transformer.
Photo courtesy ABB, www.abb.com.
Secondary Primary
Current transformer
Ammeter
Secondary
Primary
Motor Voltmeter
Potential transformer
Current transformer
Ammeter
Protective relay
Motor
Figure 3-36 Instrument transformers.
Photos courtesy Hammond Manufacturing, www.hammondmfg.com.
Figure 3-38 Circuit for review question 2.
H1 100 V
90 V
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Discussion Topics 59
TROUBLESHOOTING SCENARIOS 1. The control transformer for an across-the-line
three-phase motor starter is tested and found to have an open in the secondary winding. Discuss what would occur if an attempt where made to tem- porarily operate the control system directly from two of the three-phase supply lines.
2. The two primary windings of a dual-primary con- trol transformer (240 V or 480 V) are to be con- nected in parallel to step the line voltage of 240 V down to a control voltage of 120 V. Assuming the two primary windings are incorrectly connected in series instead of parallel what effect would this have on the control circuit?
DISCUSSION TOPICS AND CRITICAL THINKING QUESTIONS 1. Discuss how electric power might be distributed
within a small commercial or industrial site.
2. Research the specifications for a typical four-wire electrical power panelboard capable of feeding
single-phase and three-phase loads. Include in your findings:
• All electrical specifications • Internal bus layout
• Connections for single-phase and three-phase loads c. Redraw the diagram with the unmarked leads of
the transformer correctly labeled.
3. The control circuit for a three-phase 480-V motor is normally operated at what voltage? Why?
4. A 240/480-V dual-primary control transformer is to be operated from a 480-V three-phase system. How would the two primary windings be connected rela- tive to each other? Why?
5. For the motor control circuit of Figure 3-39, assume the circuit is incorrectly grounded at X1 instead of
correctly as shown at X2. With this incorrect con- nection, explain how the control circuit would oper- ate if point 2 of the stop or start push button were to become accidentally grounded.
6. What are the two basic types of three-phase trans- former configurations?
7. The phase-to-neutral voltage of a wye-connected, three-phase, four-wire distribution system is rated for 277 V. What would its phase-to-phase rating be?
8. Why is it necessary to apply the constant 1.73 ( √ __
3 ) in three-phase circuit calculations?
9. Explain the basic difference between the primary and secondary circuits of a standard voltage trans- former and an autotransformer.
10. How are autotransformers used to reduce the start- ing current for large three-phase motors?
11. Give two examples of the way in which instrument transformers are used.
12. Compare the primary connection of a potential transformer with that of a current transformer.
13. What important safety precaution should be followed when operating current transformers in live circuits?
14. The current rating of the primary winding of a current transformer is 100 A and its secondary rating is 5 A. An ammeter connected across the secondary indicates 4 A.
What is the value of the current flow in the primary?
Figure 3-39 Circuit for review question 5.
L1
CB M OL
L2
X1
FU1 FU2
FU3 X2 L3
M Stop GND
M OL Start
Motor
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60
4
Motor Control Devices
Control devices are components that govern the power delivered to an electrical load. Motor con- trol systems make use of a wide variety of control devices. The motor control devices introduced in this chapter range from simple pushbutton switches to more complex solid-state sensors.
The terms and practical applications presented here illustrate how selection of a control device depends on the specific application.
Chapter Objectives
This chapter will help you to:
1. Identify manually operated switches com- monly found in motor control circuits and explain their operation.
2. Identify mechanically operated switches commonly found in motor control circuits and explain their operation.
3. Identify different types of sensors and explain how they detect and measure the presence of something.
4. Describe the operating characteristics of a relay, solenoid, solenoid valve, stepper motor, and brushless DC motor.
PART 1 Manually Operated Switches
Primary and Pilot Control Devices A control device is a component that governs the power delivered to an electrical load. All components used in motor control circuits may be classed as either primary control devices or pilot control devices. A primary control device , such as a motor contactor, starter, or controller,
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PART 1 Manually Operated Switches 61 DPST—Double pole, single throw
DPDT—Double pole, double throw
Electrical ratings for switches are expressed in terms of the maximum interrupting voltage and current they can safely handle. AC and DC contact current ratings are not the same for a given switch. The AC current rating will be higher than its DC rating for an equivalent amount of volt- age. The reason for this is that AC current is at zero level twice during each cycle, which reduces the likelihood of an electric arc forming across the contacts. Also, higher decaying voltages are generated in DC circuits that con- tain inductive type load devices. Switch voltage and cur- rent ratings represent maximum values and may be used in circuits with voltages and currents below these levels but never above.
Pushbutton Switches
Pushbutton switches are commonly used in motor control applications to start and stop motors, as well as to control and override process functions. A push button operates by pressing a button that opens or closes contacts. Figure 4-3 shows commonly used types of pushbutton symbols and switching action. Abbreviations N.O. (normally open) and N.C. (normally closed) represent the state of the switch contacts when the switch is not activated. The N.O. push button makes a circuit when it is pressed and returns to its open position when the button is released. The N.C. push button opens the circuit when it is pressed and returns to the closed position when the button is released.
With a break-make push button, the top section con- tacts are N.C. and the bottom section contacts are N.O.
When the button is pressed, the bottom contacts are closed connects the load to the line. A pilot control device , such
as a relay or switch contact, is used to activate the primary control device. Pilot-duty devices should not be used to switch horsepower loads unless they are specifically rated to do so. Contacts selected for both primary and pilot con- trol devices must be capable of handling the voltage and current to be switched. Figure 4-1 shows a typical motor control circuit that includes both primary and pilot control devices. In the application shown, the closing of the toggle switch contact completes the circuit to energize contactor coil M. This in turn closes the contacts of the contactor to complete the main power circuit to the motor.
Toggle Switches
A manually operated switch is one that is controlled by hand. The toggle switches illustrated in Figure 4-2 are examples of manually operated switches. A toggle switch uses a mechanical lever mechanism to implement a posi- tive snap action for switching of electrical contacts. This type of switching or contact arrangement is specified by the appropriate abbreviation as follows:
SPST—Single pole, single throw SPDT—Single pole, double throw
Contactor
L1 L2
M Toggle
switch
Pilot control device
Primary control device
Motor
Figure 4-1 Primary and pilot control devices.
Photo courtesy Rockwell Automation, www.rockwellautomation.com.
Double pole, single throw
Double pole, double throw Single pole,
single throw
Single pole, double throw
DPST DPDT
SPST SPDT
Figure 4-2 Toggle switches.
N.O. (normally open) pushbutton
N.C. (normally closed) pushbutton
NEMA symbol IEC
symbol NEMA
symbol IEC
symbol
Start/stop motor control circuit M M
Start Stop
Holding contact
Starter coil
L1 L2
OL
Figure 4-3 Pushbutton symbols and switching action.
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