Sequence Starting and Stopping for Three Motors

Objectives :

• Discuss the step-by-step procedure for designing a circuit.

• Change a circuit designed with pneumatic timers into a circuit to use electronic timers.

• Connect the circuit in the laboratory.

• Troubleshoot the circuit.

LABORATORY EXERCISE

Name ________________ Date ______

Materials Required

Three-phase power supply Control transformer 2 8-pin control relays and 8-pin sockets 3 three-phase motor starters 4 electronic timers ( Dayton model 6A855 or equivalent) and 11-pin sockets 3 three-phase motors or equivalent motor loads

In this experiment a circuit will be designed and connected. The requirements of the circuit are as follows:

1. Three motors are to start in sequence from motor #1 to motor #3.

2. There is to be a time delay of 3 seconds between the starting of each motor.

3. When the stop button is pressed, the motors are to stop in sequence from motor #3 to motor #1.

4. There is to be a time delay of 3 seconds between the stopping of each motor.

5. An overload on any motor will stop all motors.

When designing a control circuit, satisfy one requirement at a time. This may at times lead to an unforeseen dead end, but don't let these dead ends concern you. When they happen, back up and redesign around them. In this example the first part of the circuit's to start three motors in sequence from motor #1 to motor #3 with a 3-second delay between the starting of each motor. This is also the time to satisfy the requirement that an overload on any motor will stop all motors. The first part of the circuit can be satisfied by the circuit shown in Ill. 1. (Note: In this experiment the motor connections won't be shown because of space limitations. It is to be assumed that the motor starters are controlling three phase motors. It is also assumed that all timers are set for a delay of 3 seconds.) When the start button is pressed, coils 1M and TR1 energize. Starter 1M starts motor #1 immediately, and timer TR1 starts its time sequence of 3 seconds. After a delay of 3 seconds, timed contact TR1 closes and energizes coils 2M and TR2. Starter 2M starts motor #2 and timer TR2 begins its 3-second timing sequence. After a delay of 3 seconds, timed contact TR2 closes and energizes motor #3. The motors have been started in sequence from #1 to #3 with a delay of 3 seconds between the starting of each motor. This satisfies the first part of the circuit logic.

The next requirement is that the circuit stop in sequence from motor #3 to motor #1. To fulfill this requirement, power must be maintained to starters 2M and 1M after the stop button has been pushed. In the circuit shown in Ill. 1, this isn't possible. Since all coils are connected after the M auxiliary holding contact, power will be disconnected from all coils when the stop button is pressed and the holding contact opens. This circuit has proven to be a dead end. There is no way to fulfill the second requirement with the circuit connected in this manner. Therefore, the circuit must be amended in such a manner that it won't only start in sequence from motor #1 to motor #3 with a 3-second time delay between the starting of each motor but also be able to maintain power after the start button is pressed.

This amendment is shown in Ill. 2.

To modify the circuit so that power can be maintained to coils 2M and 1M, a control relay has been added to the circuit. Contact 1CR2 prevents power from being applied to coils 1M and TR1 until the start button is pressed.

Ill. 1 The motors start in sequence from 1 to 3.

Ill. 2 A control relay is added to the circuit

Designing the Second Part of the Circuit

The second part of the circuit states that the motors must stop in sequence from motor #3 to motor #1. Don't try to solve all the logic at once. Solve each problem as it arises. The first problem is to stop motor #3. In the circuit shown in Ill. 2, when the stop button is pressed, coil 1CR will de-energize. This will cause contact 1CR2 to open and de-energize coils 1M and TR1. Contact TR1 will open immediately and de-energize coils 2M and TR2, causing contact TR2 to open immediately and de-energize coil 3M. Notice that coil 3M does de-energize when the stop button is pressed, but so does everything else. The circuit requirement states that there is to be a 3-second time delay between the stopping of motor #3 and motor #2.

Therefore, an off-delay timer will be added to maintain connection to coil 2M after coil 3M has de-energized ( Ill. 3).

The same basic problem exists with motor #1. In the present circuit, motor #1 will turn off immediately when the stop button is pressed. To help satisfy the second part of the problem, another off-delay relay must be added to maintain a circuit to motor #1 for a period of 3 seconds after motor #2 has turned off. This addition is shown in Ill. 4.

Motors #2 and #1 will now continue to operate after the stop button is pressed, but so will motor #3. In the present design, none of the motors will turn off when the stop button is pressed. To understand this condition, trace the logic step-by-step. When the start button is pressed, coil 1CR energizes and closes all 1CR contacts. When contact 1CR2 closes, coils 1M and TR1 energize. After a period of 3 seconds, timed contact TR1 closes and energizes coils 2M, TR2, and TR4. Timed contact TR4 closes immediately to bypass contact 1CR2.

After a delay of 3 seconds, timed contact TR2 closes and energizes coils 3M and TR3.

Timed contact TR3 closes immediately and bypasses contact TR1. When the stop button is pressed, coil 1CR de-energizes and all 1CR contacts open, but a circuit's maintained to coils 1M and TR1 by contact TR4. This prevents timed contact TR1 from opening to de-energize coils 2M, TR2, and TR4, which in turn prevents timed contact TR2 from opening to de-energize coils 3M and TR3. To overcome this problem, two more contacts controlled by relay 1CR will be added to the circuit ( Ill. 5). The circuit will now operate in accord with all the stated requirements.

Ill. 3 Timer TR3 prevents motor 2 from stopping.

Ill. 4 Off-delay timer TR4 prevents motor 1 from stopping.

Modifying the Circuit

The circuit in Ill. 5 was designed with the assumption that all the timers are of the pneumatic type. When this circuit's connected in the laboratory, 8-pin control relays and electronic timers will be used. The circuit will be amended to accommodate these components.

The first change to be made concerns the control relays. Notice that the circuit requires the use of four normally open contacts controlled by coil 1CR. Since 8-pin control relays have only two normally open contacts, it will be necessary to add a second control relay, 2CR. The coil of relay 2CR will be connected in parallel with 1CR, which will permit both to operate at the same time ( Ill. 6). Timers TR1 and TR2 are on-delay timers and don't require an adjustment in the circuit logic to operate. Timers TR3 and TR4, however, are off-delay timers and do require changing the circuit. The coils must be connected to power at all times. Assuming the use of a Dayton timer model 6A855, power would connect to pins 2 and 10. Starter 3M will be used to control the action of timer TR3 by connecting a 3M normally open auxiliary contact to pins 5 and 6 of timer TR3 (Ill. 7). Starter 2M will control the action of timer TR4 by connecting a 2M normally open auxiliary contact to pins 5 and 6 of that timer. The circuit's now complete and ready for connection in the laboratory.

Ill. 5 Control relay contacts are added to permit the circuit to turn off.

Ill. 6 Adding a control relay to the circuit.

Ill. 7 Changing pneumatic timers for electronic timers.

Connecting the Circuit

1. Using the circuit shown in Ill. 7, place pin numbers beside the proper components. Circle the pin numbers to distinguish them from wire numbers.

2. Place wire numbers on the schematic.

3. Connect the control circuit in the laboratory.

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

5. Turn off the power and connect the motor loads to starters 1M, 2M, and 3M.

6. Turn on the power and test the complete circuit.

7. Turn off the power.

8. Disconnect the circuit and return the components to their proper places.

QUIZ:

Refer to the circuit in Ill. 7 to answer the following questions. It is assumed that all timers are set for a delay of 3 seconds.

1. When the start button is pressed, motor #1 starts operating immediately. Three seconds later motor #2 starts, but motor #3 never starts. When the stop button is pressed, motor #2 stops operating immediately. After a delay of 3 seconds, motor #1 stops running. Which of the following could not cause this condition?

a. TR3 coil is open.

b. 3M coil is open.

c. TR2 coil is open.

d. 2CR coil is open.

2. When the start button is pressed, motor #1 starts operating immediately. Motor #2 does not start operating after 3 seconds, but after a delay of 6 seconds motor #3 starts operating.

When the stop button is pushed, motors #3 and #1 stop operating immediately. Which of the following could cause this condition?

a. 2CR coil is open.

b. TR1 coil is open.

c. TR3 coil is open.

d. 2M coil is open.

3. When the start button is pressed, all three motors start normally with a 3-second delay between the starting of each motor. When the stop button is pressed, motor #3 stops operating immediately. After a delay of 3 seconds, both motors #2 and #1 stop operating at the same time. Which of the following could cause this problem?

a. Timer TR1 is defective.

b. Timer TR2 is defective.

c. Timer TR3 is defective.

d. Timer TR4 is defective.

4. When the start button is pressed, nothing happens. None of the motors start. Which of the following could not cause this problem?

a. Overload contact OL1 is open.

b. 1CR relay coil is open.

c. 2CR relay coil is open.

d. The stop button is open.

5. When the start button is pressed, motor #1 does not start, but after a delay of 3 seconds motor #2 starts, and 3 seconds later motor #3 starts. When the stop button is pressed, motor #3 stops running immediately and after a delay of 3 seconds motor #2 stops running.

Which of the following could cause this problem?

a. Starter coil 1M is open.

b. TR1 timer coil is open.

c. Timer TR4 is defective.

d. 1CR coil is open.