Operation; Momentary Start-Stop Circuit; Hand-Off-Auto Start-Stop Circuit - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

OPERATION

Three basic types of motor control circuits are discussed in
the following. This topic is only intended to illustrate general
principles. There are many variations for each of these of circuits.

MOMENTARY START-STOP CIRCUIT

Figure 35 illustrates a momentary push-button start-stop
circuit. Both the START and the STOP buttons are spring loaded
and return to the positions shown after pressing momentarily.
Pressing the START button completes a circuit energizing starter
solenoid M. Contacts 1M through 3M start the motor and
contact 4M forms a holding circuit across the START button
allowing it to be released. Pressing the STOP button opens the
holding circuit, drops out the starter coil M, and stops the motor.
For digital control N.C. and N.O. momentarily actuated relay
contacts under computer control are added to the start and stop
contacts circuit. The N.C. contact is in series with the stop
contact and the N.O. contact is in parallel with the start contact.
An overload in any of the motor coils causes the associated
overloads OL to heat up and open the OL contacts, opening the
holding circuit, and stopping the motor. Overloads are thermal
heaters which allow brief periods of significant overload but
their response is too slow to protect the electrical system from
a short circuit. The circuit shown includes a separate manually
operated, fused (F), line disconnect, for short circuit protection.
FUSED
DISCONNECT
1M
L1
F
2M
L2
F
3M
L3
F
TRANSFORMER
F
STOP
Fig. 35. Momentary Push-button Start-Stop Circuit.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
STARTER
OL
T1
MOTOR
OL
T2
OL
T3
START
OL
M
4M
C2538
ELECTRIC CONTROL FUNDAMENTALS

HAND-OFF-AUTO START-STOP CIRCUIT

The starter switch in Figure 36 has three positions: HAND,
OFF, and AUTO. The HAND position energizes starter solenoid
M and starts the motor. The OFF position deenergizes starter
solenoid M and stops the motor. The AUTO position allows the
motor to be turned on or off as called for by an operating control
device (interlock) such as a thermostat, pressure controller, or
relay contact. This is the preferred starter circuit when the motor
load is under automatic control.
FUSED
DISCONNECT
L1
F
L2
F
L3
F
TRANSFORMER
F
OPERATING CONTROL
DEVICE
Fig. 36. Hand-Off-Auto Start-Stop Circuit.
115
STARTER
1M OL
T1
OL
2M
T2
OL
3M
T3
HAND
OL
OFF M
AUTO
C2539
MOTOR