coil above or below its rated voltage?
10. Which contactor coil rating refers to the amount of voltage below which the magnetic field becomes too weak to maintain the contacts in their closed position?
11. Explain why the inrush current to an AC contactor coil is much higher than its normal operating current.
12. Explain how a shading coil prevents an AC contac- tor from chattering.
13. Why are AC contactor assemblies made of lami- nated steel?
14. In what way can misalignment of the armature and core of an AC contactor cause a contactor coil to run hot?
15. Why do manufacturers recommend that discolored silver contacts not be filed?
16. Why do contactors require some form of arc suppression?
17. Does the severity of contact arcing increase or decrease with each of the following changes?
a. A decrease in the voltage level
b. Use of an AC rather than a DC power source c. Change of load from resistive type to inductive type d. An increase in the speed of contact separation 18. Why is it harder to extinguish an arc on contacts
passing direct current than on contacts passing alternating current?
19. Compare the design features of AC and DC contactors.
20. What is the function of an arc chute?
21. Explain the operation of the blowout coil used in DC contactors.
22. List six things to check as part of routine preventive maintenance for large contactors.
23. a. Explain the main advantage of using a vacuum contactor.
b. List three common switching applications for vacuum contactors.
PART 2 Contactor Ratings, Enclosures, and Solid-State Types
NEMA Ratings
The National Electric Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) maintain guidelines for contactors. The NEMA standards for contactors differ from those of the IEC and it is impor- tant to understand these differences.
A philosophy of the NEMA standards is to provide electrical interchangeability among manufacturers for a given NEMA size. Because the customer often orders a contactor by the current, motor horsepower, and voltage ratings, and may not know the application or duty cycle planned for the load, the NEMA contactor is designed by convention with sufficient reserve capacity to assure per- formance over a broad band of applications.
The continuous current rating and horsepower at the rated voltages categorize NEMA size ratings. NEMA con- tactor size guides for AC and DC contactors are shown in Figure 6-21. Because copper contacts are used on some contactors, the current rating for each size is an 8-hour open rating—the contactor must be operated at least once every 8 hours to prevent copper oxide from forming on the tips and causing excessive contact heating. For contactors with silver to silver-alloy contacts, the 8-hour rating is equivalent to a continuous rating. The NEMA current rating is for each main contact individually and not the contactor as a whole.
As an example, a Size 00 three-pole AC contactor rated at 9 A can be used for switching three separate 9-A loads simultaneously. Additional ratings for total horsepower are also listed. When selecting always ensure that the contactor ratings exceed the load to be controlled. NEMA contactor sizes are normally available in a variety of coil voltages.
As the NEMA size number classification increases, so does the current capacity and physical size of the
60 Hz AC contactor NEMA ratings 600 volts max NEMA
size
00 9
0 18
1 27
2 45
3 90
4 135
5 270
6 540
7 810
8 1215
9 2250
Continuous amps
DC contactor NEMA ratings 600 volts max NEMA
size
1 25
2 50
3 100
4 150
5 300
6 600
7 900
8 1350
9 2500
Continuous amps NEMA size 0
NEMA size 2
Figure 6-21 NEMA contactor size guide.
Photos courtesy Siemens, www.siemens.com.
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146 Chapter 6 Contactors and Motor Starters E X A M P L E 6 - 1
Problem: Use the table in Figure 6-21 to determine the NEMA size of an AC contactor required for a 480-V heating element load with a continuous current rating of 80 A.
Solution: According to the table, a size 2 contactor is rated for 45 A, while a size 3 is rated for 90 A. Since the load falls between these two values, the larger-size contactor must be used. The voltage requirement is sat- isfied because the controller can be used for all volt- ages up to 600 V.
Magnetic contactors are also rated for the type of load to be utilized or for actual applications. Load uti- lization categories include:
• Nonlinear loads such as tungsten lamps for light- ing (large hot-to-cold resistance ratio, typically 10:1 or higher; current and voltage in phase).
• Resistive loads such as heating elements for furnaces and ovens (constant resistance; current and voltage in phase).
• Inductive loads such as industrial motors and transformers (low initial resistance until the transformer becomes magnetized or the motor reaches full speed; current lags behind voltage).
• Capacitive loads such as industrial capacitors for power factor correction (low initial resistance as capacitor charges; current leads voltage).
IEC Ratings
IEC contactors, compared to NEMA devices, generally are physically downsized to provide higher ratings in a smaller package (Figure 6-22). On average, IEC devices are 30 to 70 percent smaller than their NEMA counterparts. IEC contac- tors are not defined by standard sizes, unlike NEMA contac- tors. Instead, the IEC rating indicates that a manufacturer or laboratory has evaluated the contactor to meet the require- ments of a number of defined “applications.” With knowledge of the application you can choose the appropriate contactor by defining the correct utilization category. This makes it pos- sible to reduce contactor size, and therefore cost. The IEC rating system is broken down into different “utilization cat- egories” that define the value of the current that the contactor must make, maintain, and break. The following category defi- nitions are the most commonly used for IEC contactors:
1 L1
T1 2
3 L2
T2 4
5 L3
T3 6
13 21
A2 14 A1
22 31 43
32 44 Terminal markings
Figure 6-22 IEC type contactor.
Photo courtesy Automation Direct, www.automationdirect.com.
contactor. Larger contacts are needed to carry and break the higher currents, and heavier mechanisms are required to open and close the contacts.
AC CATEGORIES
AC-1: This applies to all AC loads where the power factor is at least 0.95. These are primarily noninduc- tive or slightly inductive loads.
AC-3: This category applies to squirrel-cage motors where the breaking of the power contacts would occur while the motor is running. On closing, the contactor experiences an inrush, which is 5 to 8 times the nominal motor current, and at this instant, the voltage at the ter- minals is approximately 20 percent of the line voltage.
AC-4: This applies to the starting and breaking of a squirrel-cage motor during an inch or plug reverse. On energization, the contactor closes on an inrush current approximately 5 to 8 times the nominal current. On deenergization, the contactor breaks the same magni- tude of nominal current at a voltage that can be equal to the supply voltage.
DC CATEGORIES
DC-1: This applies to all DC loads where the time con- stant ( L / R ) is less than or equal to 1 millisecond. These are primarily noninductive or slightly inductive loads.
DC-2: This applies to the breaking of shunt motors while they are running. On closing, the contactor makes the inrush current around 2.5 times the nominal rated current.
DC-3: This applies to the starting and breaking of a shunt motor during inching or plugging. The time con- stant is less than or equal to 2 ms. On energization, the contactor sees current similar to that in category DC-2.
On deenergization, the contactor will break around 2.5 times the starting current at a voltage that may be higher than the line voltage. This would occur when the speed of the motor is low because the back emf is low.
DC-5: This applies to the starting and breaking of a series motor during inching or plugging. The time constant is less than or equal to 7.5 ms. On energiza- tion, the contactor sees about 2.5 times the nominal full-load current. On deenergization, the contactor
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PART 2 Contactor Ratings, Enclosures, and Solid-State Types 147 Although the enclosures are designed to provide pro- tection in a variety of situations, the internal wiring and physical construction of the device remains the same.
Consult the National Electrical Code (NEC) and local codes to determine the proper selection of an enclosure for a particular application.
The IEC provides a system for specifying the enclosures of electrical equipment on the basis of the degree of protec- tion provided by the enclosure. Unlike NEMA, IEC does not specify degrees of protection for environmental condi- tions such as corrosion, rust, icing, oil, and coolants. For this reason, IEC enclosure classification designations cannot be exactly equated with NEMA. The table at top of page 148 provides a guide for converting from NEMA enclosure type numbers to IEC enclosure classification designations.
The NEMA types meet or exceed the test requirements for the associated IEC classifications; for this reason the table should not be used to convert from IEC classifications to NEMA types and the NEMA to IEC conversion should be verified by test.
Solid-State Contactor
Solid-state switching refers to interruption of power by nonmechanical electronic means. Figure 6-24 shows a single-pole AC solid-state contactor that uses electronic switching. In contrast to a magnetic contactor, an elec- tronic contactor is absolutely silent, and its “contacts”
never wear out. Static contactors are recommended in applications that require a high switching frequency, such as heating circuits, dryers, single- and three-pole motors, and other industrial applications.
breaks the same amount of current at a voltage that can be equal to the line voltage.
Contactor Enclosures
Enclosed magnetic contactors must be housed in an approved enclosure based on the environment in which they must oper- ate to provide mechanical and electrical protection. Electri- cal codes mandate the type of enclosure to use. More severe environments require more substantial enclosures. Severe environmental factors to be considered include:
• Exposure to damaging fumes.
• Operation in damp places.
• Exposure to excessive dust.
• Subject to vibration, shocks, and tilting.
• Subject to high ambient air temperature.
There are two general types of NEMA enclosures:
nonhazardous-location enclosures and hazardous-location enclosures. Nonhazardous-location enclosures are further subdivided into the following categories:
• General-purpose (least costly)
• Watertight
• Oiltight
• Dust-tight
Hazardous-location enclosures are extremely costly, but they are necessary in some applications. Hazardous- location, explosion-proof enclosures involve forged or cast material and special seals with precision-fit toler- ances. The explosion-proof enclosures are constructed so that an explosion inside will not escape the enclosure. If an internal explosion were to blow open the enclosure, a general-area explosion and fire could ensue. Hazardous- location enclosures are classified into two categories:
• Gaseous vapors (acetylene, hydrogen, gasoline, etc.).
• Combustible dusts (metal dust, coal dust, grain dust, etc.).
All industrial electrical and electronic enclosures must conform to standards published by NEMA to meet the needs of location conditions. Figure 6-23 shows typical NEMA enclosure types including:
NEMA Type 1 —general-purpose type, which is the least costly, and used in a location where unusual service conditions do not exist.
NEMA Type 4 and 4X— Watertight and dust-tight.
NEMA Type 12 —Provides a degree of protection from noncorrosive dripping liquids, falling dirt, and dust.
NEMA Type 7 and 9 —Designed for use in hazard- ous locations.
NEMA type 1
NEMA type 12 NEMA type 7 and 9 NEMA type 4 and 4X
Figure 6-23 Typical contactor enclosure types.
This material and associated copyrights are proprietary to, and used with the permission of, Schneider Electric.
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148 Chapter 6 Contactors and Motor Starters
current applications: the disk (also known as puck type), stud mount, and module. Flexible-lead stud-mounted SCRs have a gate wire, a flexible cathode lead, and a smaller cath- ode lead that is used only for control purposes. The heat generated by the SCR must be dissipated; thus all contac- tors have some means to cool the SCR. Typically an alumi- num heat sink, with fins to increase the surface area, is used to dissipate this energy to air.
The most common high-power switching semiconduc- tor used in solid-state contactors is the silicon controlled rectifier (SCR). An SCR is a three-terminal semiconductor device (anode, cathode, and gate) that acts like the power contact of a magnetic contactor. A gate signal, instead of an electromagnetic coil, is used to turn the device on, allowing current to pass from cathode to anode. Figure 6-25 shows three types of SCR construction styles designed for higher-
V1
V2
Figure 6-24 Single-pole solid-state contactor.
Anode Cathode
Gate SCR symbol
Module type installed in a heat sink
Stud type Disk, or puck, type
Anode stud
Cathode control lead
Gate lead
Cathode power lead
Figure 6-25 Silicon controlled rectifier (SCR) switching semiconductor.
Disk and stud type photos courtesy Vishay Intertechnology, www.vishay.com. Module type photo courtesy Control Concepts, Inc., www.ccipower.com.
NEMA enclosure type number 1
2 3 3R 3S 4 and 4X
5 6 and 6P 12 and 12K
13
IP10 IP11 IP54 IP14 IP54 IP56 IP52 IP67 IP52 IP54
IEC enclosure designation
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PART 2 Contactor Ratings, Enclosures, and Solid-State Types 149
Off
On Anode
Cathode Gate
DC SCR
⫹
⫺
Figure 6-26 SCR testing circuit.
Dual SCR module
G1 SCR2
Load SCR1
L1 L2
G2
Figure 6-27 SCR connection for single-phase contactor.
Photo courtesy Digi-Key Corporation, www.digikey.com.
The SCR, like a contact, is in either the on state (closed contact) or the off state (open contact). SCRs are normally off switches that can be triggered on by a small current pulse into the gate electrode. Once turned on (or trig- gered ), the component then stays in the conducting state even when the gate on signal is removed. It returns to the off (blocking) state only if the anode-to-cathode current falls below a certain minimum or if the direction of the current is reversed. In this respect, the SCR is analogous to a latched contactor circuit—once the SCR is triggered, it will stay on until its current decreases to zero.
The SCR testing circuit shown in Figure 6-26 is practi- cal both as a diagnostic tool for checking suspected SCRs and as an aid to understanding how they operate. The operation of the circuit can be summarized as follows:
• A DC voltage source is used for powering the cir- cuit, and two pushbutton switches are used to latch and unlatch the SCR, respectively.
• Momentarily closing the on push button connects the gate to the anode, allowing current to flow from the negative terminal of the battery, through the cathode-gate junction, through the switch, through the bulb, and back to the battery.
• This gate current should cause the SCR to latch on, allowing current to go directly from cathode to anode without further triggering through the gate.
• Momentarily opening the normally closed off push button interrupts the current flow to the SCR and bulb. The light turns off and remains off until the SCR is triggered back into conduction.
• If the bulb lights at all times, this is an indication that the SCR is shorted.
• If the bulb fails to light when the SCR is triggered into operation, this is an indication that the SCR is faulted open.
Since an SCR passes current in one direction only, two SCRs are necessary to switch single-phase AC power. The two SCRs are connected inverse-parallel (back-to-back), as shown in Figure 6-27: one to pass current during the
positive half-cycle and the other during the negative half- cycle. Half the current is carried by each SCR, and sinu- soidal AC current flows through the resistive load R when gates G1 and G2 are fired at 0 degrees and 180 degrees of the input, respectively.
Inductive loads and voltage transients are both seen as problem areas in solid-state AC contactor control because they could falsely trigger an SCR into conduction. For this reason, for driving an inductive load, a snubber circuit is used to improve the switching behavior of the SCR. Fig- ure 6-28 shows an electronic contactor, with a simple RC snubber circuit used to control an inductive transformer
RC snubber module
Transformer load Inverse-parallel
SCRs 230 V AC
R C
Figure 6-28 SCR snubber circuit.
Photo courtesy Enerpro, www.enerpro-inc.com.
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150 Chapter 6 Contactors and Motor Starters
By electrically switching an SCR on at the AC sine wave zero crossing point, it remains on through the half cycle of the sine wave and turns off at the next zero crossing. In this scheme, known as zero-fi red control , the SCR is turned on at or nearly at the zero crossing point so that no current is being switched under load. The result is virtually no power line disturbances or EMI generation.