Figure 1-16.-A dc controller with shunt field rheostat.

If the motor becomes overloaded, the excessive

When contacts LC1 and LC2 close, motor-starting

current flows through the series field (SE), the armature

current through the overload coil (OL) (at the top right of

(A), the series relay coil (SR), the starting resistor (R),

fig. 1-15) will open the overload contacts (OL) (at the

and the overload relay coil (OL). At the same time, the

bottom of fig. 1-15), disconnecting the motor from the line.

shunt field winding (SH), is connected across the line

If the main contactor drops out because of an

and establishes normal shunt field strength. Contacts

excessive drop in line voltage or a power failure, the

LC3 close and prepare the circuit for the accelerating

motor will remain disconnected from the line until an

contactor coil (AC). Contacts LC4 close the holding

operator restarts it with the start pushbutton. This pre-

circuit for the line contactor coil (LC).

vents automatic restarting of equipment when normal

The motor armature current flowing through the

power is restored.

series relay coil causes its armature to pull in, opening

Speed Control

the normally closed contacts (SR). As the motor speed

picks up, the armature current drawn from the line

Figure 1-16 illustrates a rheostat that is added to the

decreases. At approximately 110 percent of normal

basic controller ciruit to obtain varying speed.

running current, the series relay current is not strong

If resistance is added in series with the shunt field

enough to hold the armature in; therefore, it drops out

the field will be weakened and the motor will speed up.

and closes its contacts (SR). These contacts are in series

If the amount of resistance in series is decreased, the

with the accelerating relay coil (AC) and cause it to pick

field strength will increase, and the motor will slow

up its armature, closing contacts AC1 and AC2.

down.

Auxiliary contacts (AC1) on the accelerating relay

Contacts FA (fig. 1-16) are closed during the

keep the circuit to the relay coil closed while the main

acceleration period, providing fill shunt field strength.

contacts (AC2) short out the starting resistor and the

After the motor has accelerated to the across-the-line

series relay coil. The motor is then connected directly

position, contacts FA open, placing the rheostat in the

across the line, and the connection is maintained until

the STOP button is pressed.

shunt field circuit to provide full field protection.

Figure 1-16.-A dc controller with shunt field rheostat.

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