Figure 1-4.-Across-line, 3-phase controller.

but the circuit breaker rating is too high to protect each

motor separately.

Short-circuit protection for control circuits is

provided by fuses in the controller enclosure, which

provides protection for remote push buttons and

pressure switches.

FULL-FIELD PROTECTION

Full-field protection is required in the controller for

a dc motor when a shunt field rheostat or a resistor is

used to weaken the motor field and obtain motor speeds

more than 150 percent of the speed at rated field current.

Full-field protection is provided automatically by a relay

that shunts out the shunt field rheostat for the initial

acceleration of the motor, and then cuts it into the motor

field circuit. In this way, the motor first accelerates to

100 percent or full-field speed, and then further

accelerates to the weakened-field speed determined by

the rheostat settings.

The controller for an anchor windlass motor

provides stepback protection by automatically cutting

back motor speed to relieve the motor of excessive load.

Figure 1-4.-Across-line, 3-phase controller.

LOW-VOLTAGE RELEASE (LVR)

contactor coil, M to

When the coil is energized, it

When the supply voltage is reduced or lost

closes line contacts

and

which connect the

altogether, an LVR controller disconnects the motor

full-line voltage to the motor. The line contactor

from the power supply, keeps it disconnected until the

auxiliary contact, MA, also closes and completes a hold-

supply voltage returns to normal, and then automatically

ing circuit for energizing the coil circuit after the start

restarts the motor. This type of controller is equipped

push button has been released.

with a maintaining master switch.

LOW-VOLTAGE PROTECTION (LVP)

When the supply voltage to an LVP controller is

reduced or lost, the motor is disconnected from the line.

Upon restoration of power, the motor will not start until

you manually depress the start push button.

MAGNETIC ACROSS-LINE

CONTROLLERS

A typical 3-phase, across-line controller is shown in

figure 1-4. Figure 1-5 shows a small cubical contactor

for a 5-horsepower motor. All contractors are similar in

appearance, but they vary in size.

An elementary or schematic diagram of a magnetic

controller is shown in figure 1-2. The motor is started

by pushing tie strut button. The action completes the

circuit from

through the control fuse, stop button,

Figure 1-5.-Contactor for a 5-horsepower motor.

start button, the overload relay contacts, OL, and the

1-6