Magnetic Across-the-Line Controllers
A typical across-the-line, 3-phase controller is
shown in figure 9-18. Figure 9-19 shows the main
contactor assembly. All contactor assemblies are
similar in appearance but vary in size. You start
the motor by pushing the START button. This
action completes the circuit that will energize the
main contactor assembly, closing the main
contacts connecting the full-line voltage to the
motor.
The motor will continue to run until the
contactor coil is de-energized. It can be de-
energized by the STOP push button, failure of
the line voltage, or tripping of the overload relay
(OL).
Two-Speed Control
Figure 9-19.--Contactor assembly.
An ac induction motor for two-speed
button closes the LS contactor assembly,
operation may have a single winding or two
connecting the low-speed windings to the full-line
separate windings, one for each speed. Pressing
voltage. The two main contactor assemblies are
the HS push button closes the HS contactor
mechanically interlocked. This prevents both CBs
assembly, connecting the high-speed windings to
from being closed at the same time. The speed
the full-line voltage. Pressing the LS push
can be changed without stopping the motor. The
contactor coil is de-energized and energized in the
same manner as the magnetic controller.
Low-Voltage Protection
With low-voltage protection (LVP), when the
supply voltage is reduced or lost, you must restart
the controller manually. The master switch is
usually a momentary switch.
Low-Voltage Release
When the supply voltage is reduced or lost
altogether, a low-voltage release (LVR) controller
disconnects the motor from the power supply. It
keeps it disconnected until the supply voltage
returns to normal. Then it automatically restarts
the motor. This type of controller must be
equipped with a maintaining master switch.
Low-Voltage Release Effect
Most small manual across-the-line controllers
have a property known as low-voltage release
effect (LVRE). These controllers are switched on
to start a motor. When voltage is lost, the motor
stops. When voltage returns, since the switch is
closed, the motor will restart. Although not truly
an LVR controller, since it does restart the motor,
it is known as an LVRE.
Figure 9-18.--Across-the-line, 3-phase controller.
9-21
