Laboratory manual for Electronics: Motor Control: Off-Delay Timers

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Objectives:

• Discuss the operation of an off-delay timer.

• Draw the NEMA contact symbols used to represent both normally open and normally closed off-delay contacts.

• Discuss the difference in operation between pneumatic and electronic timers.

• Connect a circuit in the laboratory employing an off-delay timer.

The logic of an off-delay timer is as follows: When the coil is energized, the timed contacts change position immediately. When the coil is de-energized, the timed contacts remain in their energized position for some period of time before changing back to their normal position.

Ill. 1 shows the standard NEMA contact symbols used to represent an off-delay timer.

Notice that the arrow points in the direction the contact will move after the time delay period.

The arrow indicates that the normally open contact will delay reopening and that the normally closed contact will delay reclosing. Like on-delay timers, some off-delay timers will contain instantaneous contacts as well as timed contacts, and some won't.

Helpful Hint

When the coil is energized, the timed contacts change position immediately. When the coil is de-energized, the timed contacts remain in their energized position for some period of time before changing back to their normal position.

Ill. 1 NEMA standard symbols for off-delay contacts.

Ill. 2 Basic operation of an off-delay timer.

Example Circuit #1

The circuit shown in Ill. 2 illustrates the logic of an off-delay timer. It will be assumed that the timer has been set for a delay of 5 seconds. When switch S1 closes, TR coil energizes. This causes the normally open TR contacts to close immediately and turn on the lamp. When switch S1 opens, TR coil will de-energize, but the TR contacts will remain closed for 5 seconds before they reopen. Notice that the time delay period does not start until the coil is de-energized.

Example Circuit #2

In the second example it's assumed that timer TR has been set for a delay of 10 seconds.

Two motors start when the start button is pressed. When the stop button is pressed, motor #1 stops operating immediately, but motor #2 continues to run for 10 seconds ( Ill. 3). In this circuit the coil of the off-delay timer has been placed in parallel with motor starter 1M, permitting the action of the timer to be controlled by the first motor starter.

Example Circuit #3

Now assume that the logic of the previous circuit's to be changed so that when the start but ton is pressed both motors still start at the same time, but when the stop button is pressed, motor #2 must stop operating immediately and motor #1 continues to run for 10 seconds.

In this circuit the action of the timer must be controlled by the operation of starter 2M instead of starter 1M ( Ill. 4). In the circuit shown in Ill. 4, a control relay is used to energize both motor starters at the same time. Notice that timer coil TR energizes at the same time as starter 2M, causing the normally open TR contacts to close around the CR contact connected in series with coil 1M. When the stop button is pressed, coil CR de-energizes and all CR contacts open. Power is maintained to starter 1M, however, by the now closed TR contacts. When the CR contact connected in series with coils 2M and TR opens, these coils de-energize, causing motor #2 to stop operating and starting the time sequence for the off-delay timer. After a delay of 10 seconds, TR contacts reopen and de-energize coil 1M, stopping the operation of motor #1.

Ill. 5 Connection diagram for a Dayton model 6A855 timer.

Ill. 3 Off-delay motor circuit using pneumatic timer.

Ill. 4 Motor 1 stops after motor 2.

Using Electronic Timers

In the circuits shown in Ill. 3 and Ill. 4, it was assumed that the off-delay timers were of the pneumatic type. It is common practice to develop circuit logic assuming that the timers are of the pneumatic type. The reason for this is that the action of a pneumatic timer is controlled by the coil being energized or de-energized. The action of the timer is dependent on air pressure, not an electric circuit. This, however, is generally not the case when using solid state time delay relays. Solid-state timers that can be used as off-delay timers are generally designed to be plugged into an 11-pin tube socket. The pin connection for a Dayton model 6A855 timer is shown in Ill. 5. Although this is by no means the only type of electronic timer available, it's typical of many.

Notice in Ill. 5 that power is connected to pins 2 and 10. When this timer is used in the on-delay mode, there is no problem with the application of power because the time sequence starts when the timer is energized. When power is removed, the timer de-energizes and the contacts return to their normal state immediately.

An off-delay timer, however, does not start the timing sequence until the timer is de-energized.

Since this timer depends on an electronic circuit to operate the timing mechanism, power must be connected to the timer at all times. Therefore, some means other than disconnecting the power must be used to start the timing circuit. This particular timer uses pins 5 and 6 to start the operation. The diagram in Ill. 5 uses a start switch to illustrate this operation. When pins 5 and 6 are shorted together, it has the effect of energizing the coil of an off-delay timer and all contacts change position immediately. The timer will remain in this state as long as pins 5 and 6 are short circuited together. When the short circuit between pins 5 and 6 is removed, it has the effect of de-energizing the coil of a pneumatic off-delay timer and the timing sequence will start. At the end of the time period, the contacts will return to their normal position.

LABORATORY EXERCISE

Name ___ Date __

Materials Required:

Three-phase power supply Control transformer 2 double-acting push buttons (N.O./N.C. on each button) 2 three-phase motor starters with at least one normally open auxiliary contact Dayton Solid-State Timer-model 6A855 or equivalent 11-pin control relay and two 11-pin sockets 2 three-phase motors or equivalent motor loads

Amending Circuit #1

The circuit in Ill. 3 has been amended in Ill. 6 to accommodate the use of an electronic timer. Notice in this circuit that power is connected to pins 2 and 10 of the timer at all times. Since the action of the timer in the original circuit's that the coil of the timer operates at the same time as starter coil 1M, an auxiliary contact on starter 1M will be used to control the action of timer TR. When the start button is pressed, coil 1M energizes and all 1M contacts close. This connects motor #1 to the line, the 1M contact in parallel with the start button seals the circuit, and the normally open 1M contact connected to pins 5 and 6 of the timer closes and starts the operation of the timer. When timer pins 5 and 6 become shorted, the timed contact connected in series with 2M coil closes and energizes starter 2M.

When the stop button is pressed, coil 1M de-energizes and all 1M contacts return to their normal position, stopping the operation of motor #1. When the 1M contacts connected to timer pins 5 and 6 reopen, the timing sequence of the timer begins. After a delay of 10 seconds, timed contact TR reopens and disconnects starter coil 2M from the circuit.

This stops the operation of motor #2.

Ill. 6 Amending the first circuit for an electronic timer.

Ill. 7 Amending circuit 2 for an electronic timer.

Amending Circuit #2

Circuit #2 will be amended in much the same way as circuit #1. The timer must have power connected to it at all times ( Ill. 7). Notice in this circuit that the action of the timer is controlled by starter 2M instead of 1M. When coil 2M energizes, a set of normally open 2M contacts closes and shorts pins 5 and 6 of the timer. When coil 2M de-energizes, the 2M auxiliary contacts reopen and start the time sequence of timer TR.

Circuit #2 assumes the use of an 11-pin control relay instead of an 8 pin. An 11-pin control relay contains three sets of contacts instead of two. Ill. 8 shows the connection diagram for most 11-pin control relays. Notice that normally open contacts are located on pins 1 and 3, 6 and 7, and 9 and 11. The coil pins are 2 and 10. Pin numbers have been placed beside the components in Ill. 7.

Connecting the First Circuit

1. Place wire numbers on the schematic shown in Ill. 6.

2. Using an 11-pin tube socket, connect the control part of the circuit in Ill. 6.

3. Set the electronic timer to operate as an off-delay timer and set the time delay for 10 seconds.

4. Plug the timer into the tube socket and turn on the power.

5. Test the control part of the circuit for proper operation.

6. If the control portion of the circuit operated properly, connect the motors or equivalent motor loads and test the entire circuit for proper operation.

7. Turn off the power and disconnect the circuit.

Connecting the Second Circuit

1. Place wire numbers on the schematic diagram shown in Ill. 7.

2. Using two 11-pin tube sockets, connect the control part of the circuit.

3. Set the electronic timer to operate as an off-delay timer and set the time delay for 10 seconds.

4. Plug the timer and control relay into the tube sockets and turn on the power.

5. Test the control part of the circuit for proper operation.

6. If the control portion of the circuit operated properly, connect the motors or equivalent motor loads and test the entire circuit for proper operation.

7. Turn off the power and disconnect the circuit.

8. Return the components to their proper location.

QUIZ:

1. Describe the operation of an off-delay timer.

2. Why is it common practice to develop circuit logic assuming all timers are of the pneumatic type?

3. Refer to the schematic diagram shown in Ill. 6. Assume that starter coil 2M is open. Describe the action of the circuit when the start button is pressed and when the stop button is pressed.

4. Refer to the circuit shown in Ill. 7. Assume that when the start button is pressed, motor #1 starts operating immediately, but motor #2 does not start. When the stop button is pressed, motor #1 stops operating immediately. Which of the following could cause this condition?

a. 1M coil is open.

b. 2M coil is open.

c. Timer TR isn't operating.

d. CR coil is open.

5. Refer to the circuit shown in Ill. 7. When the start button is pressed, both motors #1 and #2 start operating immediately. When the stop button is pressed, motor

#2 stops operating immediately, but motor #1 remains running and does not turn off after the time delay period has expired. Which of the following could cause this condition?

a. CR contacts are shorted together.

b. 2M auxiliary contacts connected to pins 5 and 6 of the timer did not close.

c. 2M auxiliary contacts connected to pins 5 and 6 of the timer are shorted.

d. The stop button is shorted.

6. Refer to the circuit shown in Ill. 7. Assume that timer TR is set for a delay of 10 seconds. Now assume that timer TR is changed from an off-delay timer to an on-delay timer. Explain the operation of the circuit.

7. Using the space provided in Ill. 9, modify the circuit in Ill. 7 to operate as follows:

a. When the start button is pressed, motor #1 starts running immediately. After a delay of 10 seconds, motor #2 begins running. Both motors remain operating until the stop button is pressed or an overload occurs.

b. When the stop button is pressed, motor #2 stops operating immediately, but motor #1 continues to operate for a period of 10 seconds before stopping.

c. An overload on either motor will stop both motors immediately.

d. Assume the use of electronic timers in final design.

8. After your instructor has approved the modification, connect your circuit in the laboratory.

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

10. Turn off the power, disconnect the circuit, and return the components to their proper location.

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