Laboratory manual for Electronics: Motor Control: Forward-Reverse Control

Objectives:

• Discuss cautions that must be observed in reversing circuits.

• Explain how to reverse a three-phase motor.

• Discuss interlocking methods.

• Connect a forward-reverse motor control circuit.

The direction of rotation of any three-phase motor can be reversed by changing any two motor T leads. Since the motor is connected to the power line regardless of which direction it operates, a separate contactor is needed for each direction. Since only one motor is in operation, however, only one overload relay is needed to protect the motor. True reversing controllers contain two separate contactors and one overload relay built into one unit.

Ill. 1 Forward-reverse control with interlock.

Ill. 2 Motor operating in the forward direction.

Ill. 3 Contacts to illustrate circuit logic.

Ill. 4 Motor operating in the reverse direction.

Interlocking

Interlocking prevents some action from taking place until some other action has been performed. In the case of reversing starters, interlocking is used to prevent both contactors from being energized at the same time. This would result in two of the three-phase lines being shorted together. Interlocking forces one contactor to be de-energized before the other one can be energized.

Most reversing controllers contain mechanical interlocks as well as electrical interlocks.

Mechanical interlocking is accomplished by using the contactors to operate a mechanical lever that prevents the other contactor from closing while the other is energized.

Electrical interlocking is accomplished by connecting the normally closed auxiliary contacts on one contactor in series with the coil of the other contactor (Ill. 1). Assume that the forward push button is pressed and F coil energizes. This causes all F contacts to change position. The three F load contacts close and connect the motor to the line. The normally open F auxiliary contact closes to maintain the circuit when the forward push button is released, and the normally closed F auxiliary contact connected in series with R coil opens (Ill. 2).

(Note: Ill. 2 illustrates the circuit as it's when the forward starter has been energized.

Schematics of this type are used throughout this laboratory manual to help students under stand how relay logic operates. This can lead to confusion, however, because contacts that are connected normally open will be shown closed and normally closed contacts will be shown open.

To help avoid confusion, normally open contacts that are closed during the stage the circuit's in at that moment will use double lines to indicate the contact is now closed. Contacts that are normally closed, but open at that stage of circuit operation, will show a line at the edges of the contact, but the contact will be open in the middle, as shown in Ill. 3.) If the opposite direction of rotation is desired, the stop button must be pressed first. If the reverse push button were to be pressed first, the now open F auxiliary contact connected in series with R coil would prevent a complete circuit from being established. Once the stop button has been pressed, however, F coil de-energizes and all F contacts return to their normal position. The reverse push button can now be pressed to energize R coil (Ill. 4).

When R coil energizes, all R contacts change position. The three R load contacts close and connect the motor to the line. Notice, however, that two of the motor T leads are connected to different lines. The normally closed R auxiliary contact opens to prevent the possibility of F coil being energized until R coil is de-energized.

LABORATORY EXERCISE

Name __ Date _

Materials Required:

Three-phase power supply

Control transformer

One of the following:

1. A three-phase reversing starter

2. Two three-phase contactors with at least one normally open and one normally closed auxiliary contact on each contactor; one three-phase overload relay or three single-phase overload relays

Three-phase squirrel cage motor or simulated motor load 3 double-acting push buttons (N.O./N.C. on each button)

Developing a Wiring Diagram

The same basic procedure will be used to develop a wiring diagram from the schematic as was followed in the previous experiments. The components needed to construct this circuit are shown in Ill. 5. In this example it will be assumed that two contactors and a separate three-phase overload relay will be used.

The first step is to place wire numbers on the schematic diagram. A suggested numbering sequence is shown in Ill. 6. The next step is to place the wire numbers beside the corresponding components of the wiring diagram (Ill. 7).

Wiring the Circuit

1. Using the components listed at the beginning of this unit, connect a forward-reverse control circuit with interlocks. Connect the control section of the circuit before connecting the load section. This will permit the control circuit to be tested without the possibility of shorting two of the three-phase lines together.

2. After checking with the instructor, turn on the power and test the control section of the circuit for proper operation.

3. Turn off the power and complete the wiring by connecting the load portion of the circuit.

4. Turn on the power and test the motor for proper operation.

5. Turn off the power and disconnect the circuit. Return the components to their proper place.

Ill. 5 Components needed to construct a reversing circuit.

Ill. 6 Placing wire numbers on the schematic.

Ill. 7 Placing corresponding wire numbers on the components.

Ill. 8 F and R normally open auxiliary contacts are connected incorrectly.

Ill. 9 The position of the holding contacts has been changed.

QUIZ:

1. How can the direction of rotation of a three-phase motor be changed?

2. What is interlocking?

3. Referring to the schematic shown in Ill. 1, how would the circuit operate if the normally closed R contact connected in series with F coil were to be connected normally open?

4. What would be the danger, if any, if the circuit were to be wired as stated in review question 3?

5. How would the circuit operate if the normally closed auxiliary contacts were to be connected so that F contact was connected in series with F coil and R contact was connected in series with R coil (Ill. 8)?

6. Assume that the circuit shown in Ill. 1 were to be connected as shown in Ill. 9. In what way would the operation of the circuit be different, if at all?