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
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.
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.