Power Control 417
of starter provides starting, stopping, and overload protection similar to
that of a magnetic contactor. However, manual starters must be mounted
near the motor that is being controlled. Remote-control operation is not
possible, as it would be with a magnetic contactor. This is due to the small
control current that is required by the magnetic contactor. Magnetic con-
tactors also provide a low-voltage protection by the dropout of the con-
tacts when a low-voltage level occurs. Manual starters remain closed until
they are manually turned off. They are usually limited to small sizes.Motor Overload Protection
Both manual and magnetic starters can have overload protection
contained in their enclosures. It is common practice to place either bime-
tallic or melting-alloy overload relays in series with the motor branch cir-
cuit power lines. These devices are commonly called heaters. An overload
protective relay is selected according to the current rating of the motor cir-
cuit to which it is connected. An identification number is used on overload
protective devices. This number is used to determine the current that will
cause the overload device to “trip” or open the branch circuit. Some typi-
cal overload protection or heater tables are given in Tables 15-2 and 15-3.
Table 15-2 is used for melting-alloy-type devices. The Table Number
(26142) is selected according to the type and size of controller, the size ofTable 15-1. Sizes of Magnetic Contactors
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Maximum Horsepower Rating of Load
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Ampere Rating Single-Phase Three-Phase
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NEMA Size Open Enclosed 115 V 230 V 115 V 200 V 230 V 460 V
00 10 9 0.33 1 0.75 1.5 1.5 2
0 20 18 1 2 2 3 3 5
1 30 27 2 3 3 7.5 7.5 10
2 50 45 3 7.5 7.5 10 15 25
3 100 90 7.5 15 15 25 30 50
4 150 135 — — 25 40 50 100
5 300 270 — — — 75 100 200
6 600 540 — — — 150 200 400
7 900 810 — — — — 300 600
8 1350 1215 — — — — 450 900
9 2500 2250 — — — — 800 1600
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