Control for Three Pumps

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Objectives:

• Analyze a motor control circuit.

• List the steps of operation in a control circuit.

• Connect this circuit in the laboratory.

LABORATORY EXERCISE

Name _______ Date _____

Materials Required

Three-phase power supply Control transformer 3 motor starters with normally open auxiliary contacts 6 toggle switches to simulate auto-man switches and float switches 8-pin control relay and 8-pin socket 3 three-phase motors or equivalent motor loads 1 normally open and 1 normally closed push button

Ill. 1 Analyzing the circuit.

One of the primary duties of an industrial electrician is to troubleshoot existing control circuits.

To troubleshoot a circuit, the electrician must understand what the circuit's designed to do and how it accomplishes it. To analyze a control circuit, start by listing the major components. Next, determine the basic function of each component. Finally, determine what occurs during the circuit operation.

To illustrate this procedure, the circuit previously discussed in Unit 36 will be analyzed. The hydraulic press circuit's shown in Ill. 1. In order to facilitate circuit analysis, wire numbers have been placed beside the components. The first step will be to list the major components in the circuit.

1. Normally closed stop push button

2. Normally open push button used to start the hydraulic pump

3. Two normally open push buttons used as run buttons

4. Normally open push button used for the reset button

5. Normally open pressure switch

6. Two normally open limit switches

7. Two solenoid valves

8. Three 8-pin control relays (CR2, UPLSCR, and DNCR)

9. Three 11-pin control relays (CR1, DNLSCR, and UPCR)

10. Control transformer

11. Green pilot light The next step in the process is to give a brief description of the function of each listed component:

1. (Normally closed stop push button)-Used to stop the operation of the hydraulic pump motor.

2. (Normally open push button used to start the hydraulic pump)-Starts the hydraulic pump.

3. (Two normally open push buttons used as run buttons)-Both push buttons must be held down to start the action of the press.

4. (Normally open push button used for the reset button)-Resets the press to the topmost position.

5. (Normally open pressure switch)-Determines whether or not there is enough hydraulic pressure to operate the press.

6. (Two normally open limit switches)-Determine when the press is at the top of its stroke and when it's at the bottom of its stroke.

7. (Two solenoid valves)-The up solenoid valve opens on energize to permit hydraulic fluid to move the press upward. The down solenoid valve opens on energize to permit hydraulic fluid to move the press downward.

8. (Three 8-pin control relays [CR2, UPLSCR, and DNCR])-Part of the control circuit.

9. (Three 11-pin control relays [CR1, DNLSCR, and UPCR])-Part of the control circuit.

10. (Control transformer)-Reduces the value of the line voltage to the voltage needed to operate the control circuit.

11. (Green pilot light)-Indicates there is enough hydraulic pressure to operate the pump.

The final step is to analyze the operation of the circuit. To analyze circuit operation, trace the current paths each time a change is made in the circuit. Start by pressing the pump start button.

1. When the pump start button is pressed, a circuit's completed to the coil of starter HP.

2. When coil HP energizes, all HP contacts change position. The three load contacts close to connect the pump motor to the line. The HP auxiliary contact located between wire points 11 and 12 closes to maintain the circuit after the pump start button is released, and the HP auxiliary contact located between wire numbers 10 and 15 closes to provide power to the rest of the circuit.

3. After the hydraulic pump starts, the hydraulic pressure in the system increases and closes the pressure switch.

4. When the pressure switch closes, a current path is provided to the green pilot light to indicate that there is sufficient hydraulic pressure to operate the press. A current path also exists through the normally open held closed up limit switch to control relay coil UPLSCR.

5. When UPLSCR relay energizes, both UPLSCR contacts open. The UPLSCR contact located between wire numbers 18 and 19 opens to break a current path to CR1 coil.

UPLSCR contact located between wire numbers 27 and 29 opens to break the current path to coil UPCR.

6. Both run push buttons must be held down to provide a current path through the normally closed CR2 contact located between wire numbers 18 and 19 to the coil of CR1 relay.

7. When CR1 relay coil energizes, the CR1 contact located between wire numbers 18 and 20 closes to provide a path to CR2 coil in the event that the DNLSCR contact should close. The CR1 contact located between wire numbers 16 and 24 closes to provide a current path to the down control relay (DNCR). The CR1 contact located between wire numbers 28 and 27 closes to provide an eventual current path to the up control relay (UPCR).

8. When DNCR coil energizes, the DNCR contact located between wire numbers 29 and 30 opens to provide interlock with the up control relay. The DNCR contact between wire numbers 16 and 32 closes and provides a current path to the down solenoid valve.

9. When the down solenoid valve energizes, the press begins its downward stroke. This causes the normally open held closed up limit switch to open and de-energize the coil of the up limit switch control relay (UPLSCR).

10. Both UPLSCR contacts reclose.

11. When the press reaches the bottom of its stroke, the down limit switch located between wire numbers 16 and 23 closes to provide a current path to the coil of the down limit switch control relay (DNLSCR).

12. All DNLSCR contacts change position. The DNLSCR contact located between wire numbers 20 and 21 closes to provide a current path through the now closed CR1 contact to the coil of CR2 relay. The DNLSCR contact located between wire numbers 24 and 25 opens and breaks the current path to DNCR relay. The DNLSCR contact located between wire numbers 16 and 27 closes to provide a current path to UPCR relay when the DNCR contact located between 29 and 30 recloses.

13. When CR2 coil energizes, the normally closed CR2 contact located between wires 18 and 19 opens to prevent a maintained current path to CR1 when the UPLSCR contact reopens. The normally open CR2 contact located between 18 and 21 closes to maintain a current path to the coil of CR2 in the event that CR1 or DNLSCR contacts should open.

14. When the DNCR relay coil de-energizes, the DNCR contact located between wires 29 and 30 recloses to permit coil UPCR to be energized. The DNCR contact located between 16 and 32 reopens to break the current path to the down solenoid valve.

15. When the UPCR coil energizes, the normally closed UPCR contact located between wires 25 and 26 opens to provide interlock with the DNCR relay coil. The UPCR contact located between 16 and 28 closes to maintain a circuit through the now closed CR1 contact to the coil of UPCR. The UPCR contact located between 16 and 31 closes and provides a current path to the up solenoid valve.

16. When the up solenoid valve opens, hydraulic fluid causes the press to begin its upward stroke.

17. When the press starts upward, the down limit switch reopens and de-energizes the coil of DNLSCR relay.

18. When coil DNLSCR de-energizes, the DNLSCR contact located between wires 20 and 21 reopens, but a current path is maintained by the now closed CR2 contact. The DNLSCR contact located between 24 and 25 recloses, but the current path to DNCR coil remains broken by the UPCR contact located between 25 and 26. The DNLSCR con tact located between wires 16 and 27 reopens, but a current path is maintained by the now closed UPCR and CR1 contacts.

19. When the press reaches the top of its stroke, the up limit switch again closes and provides a current path to the coil of UPLSCR relay.

20. The UPLSCR contact located between wires 18 and 19 opens to break the current path to CR1 coil. The UPLSCR contact located between wires 27 and 29 opens to break the current path to the coil of UPCR. 21. When CR1 coil de-energizes, all CR1 contacts return to their normal position. The CR1 contact between wires 18 and 20 reopens, CR1 contact between wires 16 and 24 reopens to prevent a current path from being established to the DNCR relay coil, and CR1 contact between wires 27 and 28 reopens.

22. When coil UPCR de-energizes, its contacts return to their normal position. The UPCR contacts located between wires 16 and 28 reopen, and the UPCR contact located between wires 16 and 31 reopens to break the circuit to the up solenoid.

23. Before the circuit can be restarted, the current path to relay CR2 must be broken by releasing one or both of the run push buttons. This will return all contacts back to their original state.

24. In the event the press should be stopped in the middle of its stroke, the up limit switch will be open and coil UPLSCR will be de-energized. The DNCR coil will also be de-energized. If the reset button is pressed and held, a circuit will be completed through the normally closed DNLSCR and DNCR contacts to the coil of UPCR. This will cause the up solenoid valve to energize and return the press to its up position.

Determining What the Circuit Does

The circuit in this experiment is intended to operate three pumps. The pumps are used to pump water from a sump to a roof storage tank. The water in the storage tank is used for cooling throughout the plant. After the water has been used for cooling, it returns to the sump to be recooled. Three float switches are used to detect the water level in the storage tank.

As the water is drained out of the tank, the level drops and the float switches turn on the pumps to pump water from the sump back to the storage tank ( Ill. 2).

List the Components

List the major components in the control circuit shown in Ill. 3.

Describe the Components

Give a brief description of the function of the components in this circuit.

Ill. 2 Roof-mounted tank for plant cooling system.

Ill. 3 Control circuit for three pumps.

Describing the Circuit Operation

Describe the operation of the circuit. Assume that in the normal state the roof storage tank is filled with water, and all the auto-off-man switches are set in the auto position. Also assume that the three motor starters control the operation of the three pumps, although the pumps are not shown on the schematic.

QUIZ:

To answer the following questions, refer to the circuit shown in Ill. 3.

1. Assume that all three pumps are operating. What would be the action of the circuit if the auto-off-man switch of pump #2 were to be switched to the off position?

2. Assume that the auto-off-man switch of pump #3 is set in the manual position. What will be the operation of the circuit if float switch FS1 closes?

3. Assume that the roof storage tank empties completely, but none of the pumps have started. Which of the following could not cause this condition?

a. The emergency stop button has been pushed and the control relay is de-energized.

b. The auto-off-man switch of pump #1 has been set in the off position.

c. The auto-off-man switch of pump #1 has been set in the manual position.

d. 1M coil is open.

4. Assume that all three pumps are in operation and OL3 contact opens. Will this affect the operation of the other two pumps?

5. Assume that FS2 float switch is defective. If the water level drops enough to close float switch FS3, will pump #3 start running?

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