Changing the Logic of an On-Delay Timer to an Off-Delay Timer

Objectives:

• Discuss the difference in logic between on- and off-delay timers.

• Draw a schematic diagram of a circuit that will change the logic of an on-delay timer into the logic of an off-delay timer.

• Connect an on-delay timer circuit that will operate with the logic of an off-delay timer.

Ill. 1 Basic circuit to change the logic of an on-delay timer into an off-delay timer.

Ill. 2 Starter 1M energizes immediately, but the timer does not start timing.

Some manufacturers purchase on-delay timers only. The reason for this is that most timing circuits require the logic of an on-delay timer. If it should become necessary to construct a circuit with the logic of an off-delay timer, it's a relatively simple matter to build a circuit using an on-delay timer that will operate with the same logic as an off-delay timer. A circuit of this type is shown in Ill. 1. The basic idea is to cause the timer to start operating when a control component is turned off instead of on. Control relay CR is used to perform this function.

In the circuit shown in Ill. 1, starter 1M is to energize immediately when switch S1 closes. When switch S1 opens, starter 1M should remain energized for some period of time before de-energizing. This is the logic of an off-delay timer. This logic can be accomplished by using an on-delay timer and the circuit shown in Ill. 1. When switch S1 closes, CR coil energizes and all CR contacts change position ( Ill. 2). The normally closed CR contact connected in series with TR coil opens to prevent the timer energizing.

The normally open CR contact connected in series with starter coil 1M closes and energizes the coil. This causes both 1M auxiliary contacts to close. Starter 1M is now energized, but the timer has not started its time sequence.

When switch S1 is reopened, CR coil de-energizes and all CR contacts return to their normal position ( Ill. 3). When the CR contact connected in series with starter coil 1M reopens, a current path is maintained though the now closed 1M auxiliary contact connected in parallel with the open CR contact. When the CR contact connected in series with timer coil TR closed, it provided a path to coil TR and the timer began its time sequence.

At the end of the timing sequence, timed contact TR opens and de-energizes coil 1M, causing all 1M contacts to return to their normal position ( Ill. 4). The auxiliary 1M contact connected in series with timer coil TR opens and de-energizes coil TR. This causes contact TR to reclose and the circuit's back to the beginning state shown in Ill. 1.

Ill. 3 Switch S1 opens and starts the timer.

Ill. 4 Starter 1M de-energizes when timer contact TR opens.

Ill. 5 Off-delay timer circuit using a pneumatic timer.

Changing an Existing Schematic

The circuit shown in Ill. 5 is an off-delay timer circuit for the control of two motors.

It is assumed that the timer used in this circuit's a pneumatic timer. This circuit was discussed in the previous unit. Both motors start when the start button is pressed. When the stop button is pressed, motor #2 stops operating immediately, but motor #1 continues to operate for a period of 10 seconds. Now assume that it's necessary to change the circuit logic to permit an on-delay timer to be used.

Notice in the circuit in Ill. 5 that timer coil TR is energized or de-energized at the same time as starter coil 2M. In the amended circuit, starter 2M will control the starting of on-delay timer TR ( Ill. 6). A set of 1M auxiliary contacts prevents coil TR from being energized until starter 1M has been energized. To understand the operation of the circuit, trace the logic through each step of operation. Assume that the start button is pushed and coil CR energizes. This causes all CR contacts to close and connect starters 1M and 2M to the line ( Ill. 7). Both 1M auxiliary contacts close, but the normally closed 2M auxiliary contact connected in series with TR coil opens and prevents it from starting its time sequence.

When the stop button is pressed, CR coil de-energizes and all CR contacts return to their normal position ( Ill. 8). Motor starter 1M remains energized because of the closed 1M auxiliary contact connected in parallel with the CR contact. When starter 2M de-energizes, the normally closed auxiliary contact connected in series with timer coil TR recloses and on delay timer TR begins its timing sequence.

After a delay of 10 seconds, timed contact TR opens and disconnects starter coil 1M from the line ( Ill. 9). This stops the operation of motor #1 and returns all 1M auxiliary contacts to their normal position. When timer TR de-energizes, timed contact TR returns to its normally closed position and the circuit's back to its original state as shown in Ill. 6.

LABORATORY EXERCISE

Ill. 6 Modifying the circuit for an on-delay timer.

Ill. 7 Both motors start at the same time.

Ill. 8 Starter 2M de-energizes; timer TR starts its time sequence.

Ill. 9 Timed contact de-energizes starter 1M.

Materials Required

Three-phase power source Control transformer 2 three-phase motors or equivalent motor load 2 three-phase motor starters with at least two normally open and one normally closed auxiliary contacts 8-pin or 11-pin on-delay timer with appropriate socket 11-pin control relay with 11-pin socket

Connecting the Circuit

1. Using the circuit shown in Ill. 6, place pin numbers beside the control components that mount into tube sockets. These components will probably be the control relay and the timer. Be sure to place pin numbers beside contacts as well as coils. Circle the pin numbers to distinguish them from wire numbers.

2. Place wire numbers on the schematic diagram.

3. Connect the control part of the circuit.

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

5. If the control part of the circuit operates properly, turn off the power and connect the motors or equivalent motor loads.

6. Turn on the power and test the entire circuit for proper operation.

7. Turn off the power and disconnect the circuit.

QUIZ:

1. Why do some companies purchase only on-delay timers?

Ill. 10 Motor 1 stops operating before motor 2.

2. Refer to the circuit shown in Ill. 10. This circuit assumes the use of a pneumatic off-delay timer. It is also assumed that the timer is set for a delay of 10 seconds.

Describe the operation of this circuit when the start push button is pressed.

3. Assume that the circuit in Ill. 10 is in operation. Describe the action of the circuit when the stop button is pressed.

4. The circuit shown in Ill. 10 employs a pneumatic off-delay timer. Redraw the circuit in the space provided in Ill. 11 to use an electronic on-delay timer.

Make certain that the logic of the circuit's the same.

5. After your instructor has approved the redrawn circuit, connect the circuit in the laboratory.

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

7. Turn off the power and return the components to their proper place.