Figure 1-1.-Schematic of controller with emergency run push button.

Figure 1-2.-Schematic of controller with reset-emergency run

Figure 1-1.-Schematic of controller with emergency run push

lever or rod.

button.

short circuits in motor controllers is obtained through

emergency run operation, the operator must hold down

this push button and press the START button to start the

other devices. To protect against these short circuits,

motor. While the emergency run push button is

circuit breakers are installed in the power supply system,

depressed, the motor cannot be stopped by opening the

thereby protecting the controller, motor, and cables.

overload relay contact.

Short-circuit protection is provided in controllers where

A REST-EMERGENCY RUN lever is shown in

it is not otherwise provided by the power distribution

figure 1-2. As long as the lever or rod is held down, the

system or where two or more motors are supplied power

overload relay contact is closed. The start button must

be momentarily closed to start the motor. Figure 1-3

shows a START-EMERGENCY RUN pushbutton. The

motor starts when the button is pushed, and it continues

to run without overload protection as long as it is held

down. For this reason, push buttons that are marked

start-emergency run should not be kept closed for more

than a second or two unless the emergency run operation

is desired.

Manual controllers also may be provided with an

emergency run feature. The usual means is a start-

emergency run push button or lever, which keeps the

main contactor coil energized despite the tripping action

of the overload relay mechanism.

SHORT-CIRCUIT PROTECTION

Overload relays and contractors are usually not

designed to protect motors from currents greater than

about six times normal rated current of ac motors or four

Figure 1-3-Schematic of controller with start-emergency run

times normal rated current of dc motors. Since short-

push button.

circuited currents are much higher, protection against

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