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Momentary-contact push buttons provide LVP with
The motor will continue to run until the contactor
manual restart in the circuit shown in figure 1-6. If either
coil is de-energized by the stop push button, failure of
the For R operating coil is de-energized, the contactor
the line voltage, or tripping of the overload relay, OL.
will not reclose and start the motor when voltage is
restored unless either the forward or reverse pushbutton
is pressed. The circuit arrangement of the pushbuttons
provides an electrical interlock that prevents the
The rotation of a three-phase induction motor is
energizing of both coils at the same time.
reversed by interchanging any two of the three leads to
the motor. The connections for an ac reversing controller
Speed Control
are shown in figure 1-6. The stop, reverse, and forward
push-button controls are all momentary-contact
When you desire to operate an ac motor at different
switches. Note the connections to the reverse and for-
speeds, you must use a controller with a circuit as shown
ward switch contacts. (Their contacts close or open
in figure 1-7.
momentarily, then return to their original closed or
opened condition.)
An ac induction motor designed for two-speed
operation may have either a single set of windings or
If the forward pushbutton is pressed (solid to dotted
two separate sets of windings, one for each speed.
position), coil F will be energized and will close its
Figure 1-7 is a schematic diagram of the ac controller
holding contacts,
These contacts will remain closed
for a two-speed, two-winding induction motor. The low-
as long as coil F is energized. When the coil is energized,
speed winding is connected to terminals
it also closes line contacts F1, F2, and F3, which apply
The high-speed winding is connected to terminals
full-line voltage to the motor. The motor then runs in a
Overload protection is provided by the
forward direction.
low-speed overload (LOL) coils and contacts for the
If either the stop button or the reverse button is
low-speed winding and the high-speed overload (HOL)
pressed, the circuit to the F contactor coil is broken, and
contacts and coils for the high-speed winding. The LOL
the coil releases and opens line contacts F1, F2, F3, and
and HOL contacts are connected in series in the
maintaining contact
maintaining circuit, and both contacts must be closed
before the motor will operate at either speed.
If the reverse pushbutton is pressed (solid to dotted
position), coil R is energized and closes, holding con-
The control push buttons are the momentary-
and line contacts R1, R2, and R3. Note that
contact type. Pressing the high-speed push button closes
contacts R1 and R3 reverse the connections of lines 1
the high-speed contactor by energizing coil HM. The
and 3 to motor terminals T1 and T3. This causes the
motor rotor to rotate in the reverse direction. The F and
R contactors are mechanically interlocked to prevent
troth being closed at the same time.
Figure 1-6-Reversing ac controller.
Figure 1-7.-Two-speed, ac controller.

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