Electrical Distribution Systems
Motor controllers range from a simple toggle switch to a complex system using
solenoids, relays, and timers. The basic functions of a motor controller are to
control and protect the operation of a motor.
STATE the function of motor controllers.
STATE three protective features (overloads) that may
be incorporated into a motor controller.
Given a simplified drawing of a motor controller,
DESCRIBE the operation of that motor controller.
Motor controllers range from a simple toggle switch to a complex system using solenoids, relays,
and timers. The basic functions of a motor controller are to control and protect the operation of
a motor. This includes starting and stopping the motor, and protecting the motor from
overcurrent, undervoltage, and overheating conditions that would cause damage to the motor.
There are two basic categories of motor controllers: the manual controller and the magnetic
A manual controller, illustrated by Figure 9, is a controller whose contact assembly is operated
by mechanical linkage from a toggle-type handle or a pushbutton arrangement. The controller
is operated by hand.
The manual controller is provided with thermal and direct-acting overload units to protect the
motor from overload conditions. The manual controller is basically an "ON-OFF" switch with
Manual controllers are normally used on small loads such as machine tools, fans, blowers,
pumps, and compressors. These types of controllers are simple, and they provide quiet operation.
The contacts are closed simply by moving the handle to the "ON" position or pushing the
START button. They will remain closed until the handle is moved to the "OFF" position or the
STOP button is pushed. The contacts will also open if the thermal overload trips.
Manual controllers do NOT provide low voltage protection or low voltage release. When power
fails, the manual controller contacts remain closed, and the motor will restart when power is
restored. This feature is highly desirable for small loads because operator action is not needed
to restart the small loads in a facility; however, it is undesirable for larger loads because it could
cause a hazard to equipment and personnel.