Reversing

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

Reversing

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

and

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

and

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

tacts

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.

1-7