Laboratory manual for Electronics: Motor Control: Sequence Control

Objectives:

• Define sequence control.

• Discuss methods of obtaining sequence control.

• Connect a control circuit for three motors that must be started in a predetermined sequence.

LABORATORY EXERCISE:

Name ____ Date _____

Materials Required:

Three-phase power supply Control transformer 3 motor starters containing at least three load contacts and two normally open auxiliary contacts 3 squirrel cage motors or three simulated motor loads 4 double-acting push buttons (N.O./N.C. on each button)

Sequence control forces a circuit to operate in a predetermined manner. In this experiment three motors are to be started in sequence from 1 to 3. The requirements for the circuit are as follows:

1. The motors must start in sequence from #1 to #3. E.g., motor #1 must be started before motor #2 can be started, and motor #2 must start before motor #3 can be started. Motor #2 cannot start before motor #1, and motor #3 cannot start before motor #2.

2. Each motor is started by a separate push button.

3. One stop button will stop all motors.

4. An overload on any motor will stop all three motors.

As a general rule, there is more than one way to design a circuit that will meet the specified requirements, just as there is generally more than one road that can be taken to reach a destination. One design that will meet the requirements is shown in Ill. 1.

Since the logic of the circuit's of primary interest, the load contacts and motors are not shown. In this circuit, push button #1 must be pressed before power can be provided to push button #2. When motor starter #1 energizes, the normally open auxiliary contact 1M closes, providing power to coil 1M and to push button #2. Motor starter #2 can now be started by pressing push button #2. Once motor starter #2 energizes, auxiliary contact 2M closes and provides power to coil 2M and push button #3. If the stop button should be pressed or any overload contact open, power will be interrupted to all starters.

Ill. 1 First example of starting the motors in sequence.

Ill. 2 A second circuit for sequence control

A Second Circuit for Sequence Control

A second method of providing sequence control is shown in Ill. 2. In this circuit, normally open auxiliary contacts located on motor starters 1M and 2M are used to ensure that the three motors start in the proper sequence. A normally open 1M auxiliary contact connected in series with starter coil 2M prevents motor #2 from starting before motor #1, and a normally open 2M auxiliary contact connected in series with coil 3M prevents motor #3 from starting before motor #2. If the stop button should be pressed or if any overload contact should open, power will be interrupted to all starters.

Developing a Wiring Diagram

The schematic shown in Ill. 2 is shown with the motors in Ill. 3. A drawing of the components needed to connect this circuit's shown in Ill. 4. The schematic diagram shown in Ill. 3 is shown with wire numbers in Ill. 5. The components with corresponding wire numbers are shown in Ill. 6.

Ill. 3 Sequence control with motors.

Ill. 4 Components needed to connect the circuit.

Ill. 5 Numbering the schematic.

Ill. 6 Numbering the components.

Connecting the Circuit

1. Using the materials listed at the beginning of this experiment, connect the circuit shown in Ill. 5. Follow the number sequence shown.

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

3. Turn off the power and disconnect the circuit.

4. Using the schematic diagram shown in Ill. 1, add wire numbers to the schematic.

5. Place these wire numbers beside the proper components shown in Ill. 4.

6. Connect the circuit shown in Ill. 1 by following the wire numbers placed on the schematic.

7. Turn on the power and test the circuit for proper operation.

8. Turn off the power and disconnect the circuit. Return the components to their proper places.

QUIZ:

1. What is the purpose of sequence control?

2. Refer to the schematic diagram in Ill. 5. Assume that the 1M contact located between wire numbers 29 and 30 had been connected normally closed instead of normally open. How would this circuit operate?

3. Assume that all three motors shown in Ill. 5 are running. Now assume that the stop button is pressed and motors 1 and 2 stop running, but motor 3 continues to operate. Which of the following could cause this problem?

a. Stop button is shorted.

b. 2M contact between wire numbers 31 and 32 is hung closed.

c. The 3M load contacts are welded shut.

d. The normally open 3M contact between wire numbers 23 and 31 is hung closed.

4. Referring to Ill. 5, assume that the normally open 2M contact located between wire numbers 23 and 29 is welded closed. Also assume that none of the motors are running. What would happen if:

a. The #2 push button were to be pressed before the #1 push button?

b. The #1 push button were to be pressed first?

5. In the control circuit shown in Ill. 2, if an overload occurs on any motor, all three motors will stop running. In the space provided in Ill. 7, redesign the circuit so that the motors must still start in sequence from 1 to 3, but an overload on any motor will stop only that motor. If an overload should occur on motor 1, for example, motors 2 and 3 would continue to operate.