Motor Control Circuits; Step Controllers; Application; Equipment - Honeywell AUTOMATIC CONTROL Engineering Manual

ELECTRIC CONTROL FUNDAMENTALS
A 135-ohm manual potentiometer provides up to a 50 percent
minimum-position opening, and a 270-ohm manual potentiom-
eter provides up to a 100 percent minimum-position opening.

STEP CONTROLLERS

A step controller consists of a series of switches operated
sequentially by cams on a crankshaft. Figure 34 illustrates a
Series 90 step controller used to stage electric heating elements
or compressors. The step controller crankshaft is positioned
by a Series 90 actuator through an interconnecting linkage.
When heat is called for at the thermostat, the actuator turns the
electric heat elements on in sequence. When less heat is
required, the electric heat elements turn off in reverse sequence.
If power is interrupted, a spring return in the actuator cycles
the step controller to the no-heat position. As power resumes,
the loads recycle in sequence to the level called for by the
thermostat. The recycle feature assures that the starting loads
will not overload the power line.
In some Series 90 step controllers, the recycle feature is
accomplished with a relay rather than a spring-return actuator.
On resumption of power, after an interruption, the relay
deenergizes the loads, drives the controller to the no-heat
position, and then permits the step controller to recycle to the
position called for by the thermostat.

MOTOR CONTROL CIRCUITS

APPLICATION

Motor control circuits are used to:
1. Start and stop motors.
2. Provide electric overload and low-voltage protection.
3. Provide interlocks for human safety, equipment protection,
and integration with the temperature control system.

EQUIPMENT

STARTERS

The starter is a motor load switching device having one or
more load switching contacts and optional, auxiliary, pilot-duty
contacts. All contacts are closed by a solenoid which pulls in
an armature. Starters are provided to control high current
and/or voltage for single and multiple phase motors from a
single, low-current and/or voltage contact or switch.
ENGINEERING MANUAL OF AUTOMATION CONTROL
THERMOSTAT
135
W R B
W R B
LINKAGE
T1
T2
TRANSFORMER
Fig. 34. Typical Step Controller System.
Step controllers can also be actuated by Series 60 floating
controllers.
A starter also contains thermal overloads in series with the
load contacts. In the event of prolonged excess current draw
through any of the load contacts, the overload contact opens,
deenergizing the solenoid and stopping the motor. After the
overload has cooled and/or the problem has been corrected,
the overload reset button can restart the motor.
Starters can also contain a transformer to provide reduced
voltage to power the starter control circuit. In addition, they
can contain a manual hand-off-auto switch, a push-button
start/stop switch, and auxiliary switching contacts.

CONTACTORS AND RELAYS

Relays are load switching devices similar to starters but
without thermal overloads. Contactors are heavy-duty relays.
These devices switch electric heaters or other equipment that
have independent safety and overload protection.
114
CAM OPERATED
SWITCHES
SPRING-RETURN
ACTUATOR
STEP
CONTROLLER
CRANK
1 2 3 4
SHAFT
HEATING
ELEMENTS
LINE
VOLTAGE
C2535