Laboratory manual for Electronics: Motor Control: Jogging Controls

Objectives:

• Describe the difference between inching and jogging circuits.

• Discuss different jogging control circuits.

• Draw a schematic diagram of a jogging circuit.

• Discuss the connection of an 8-pin control relay.

• Connect a jogging circuit in the laboratory using double-acting push buttons.

• Connect a jogging circuit in the laboratory using an 8-pin control relay.

Ill. 1 Inching control circuit.

Jogging or inching control is used to help position objects by permitting the motor to be momentarily connected to power. Jogging and inching are very similar and the terms are often used synonymously. Both involve starting a motor with short jabs of power. The difference between jogging and inching is that when a motor is jogged, it's started with short jabs of power at full voltage. When a motor is inched, it's started with short jabs at reduced power.

Inching circuits require the use of two contactors, one to run the motor at full power and the other to start the motor at reduced power (Ill. 1). The run contactor is generally a motor starter that contains an overload relay while the inching contactor does not. In the circuit shown in Ill. 1, if the inch push button is pressed, a circuit's completed to S contactor coil causing all S contacts to close. This connects the motor to the line through a set of series resistors used to reduce power to the motor. Note that there is no S holding contact in parallel with the inch push button. When the push button is released, S contactor de-energizes and all S contacts reopen and disconnect the motor from the power line. If the run push button is pressed, M contactor energizes and connects the motor directly to the power line. Note the normally open M auxiliary contact connected in parallel with the run push button to maintain the circuit when the button is released.

Ill. 2 Run-jog controls using a single-pole switch.

Ill. 3 Jogging control using a double-acting push button.

Other Jogging Circuits

Like most control circuits, jog circuits can be connected in different ways. One method is shown in Ill. 2. In this circuit a simple single-pole switch is inserted in series with the normally open M auxiliary contact connected in parallel with the start button. When the switch is open, it's in the jog position and prevents M holding contact from providing a complete path to M coil. When the start button is pushed, M coil will energize and connect the motor to the power line. When the start button is released, M coil will de-energize and disconnect the motor from the line. If the switch is closed, it's in the run position and permits the holding contact to complete a circuit around the start button.

Another method of constructing a run-jog control is shown in Ill. 3. This circuit employs a double-acting push button as the jog button. The normally closed section of the jog push button is connected in series with the normally open M auxiliary holding contact.

If the jog button is pressed, the normally closed section of the button opens to disconnect the holding contacts before the normally open section of the button closes. Although M auxiliary contact closes when M coil energizes, the now open jog button prevents it from completing a circuit to the coil. When the jog button is released, the normally open section reopens and breaks contact before the normally closed section can reclose.

Although a double-acting push button can be used to construct a run-jog circuit, it's not generally done because there is a possibility that the normally closed section of the jog button could reclose before the normally open section reopens. This could cause the holding contacts to lock the circuit in the run position causing an accident. To prevent this possibility, a control relay is often employed (Ill. 4). In the circuit shown in Ill. 4, if the jog push button is pressed, M contactor energizes and connects the motor to the line. When the jog button is released, M coil de-energizes and disconnects the motor from the line.

When the run push button is pressed, CR relay energizes and closes both CR contacts. The CR contacts connected in parallel with the run button close to maintain the circuit to CR coil, and the CR contacts connected in parallel with the jog button close and complete a circuit to M coil.

Ill. 4 Run-jog control using a control relay.

LABORATORY EXERCISE

Name ___ Date _

Materials Required:

Three-phase power supply Three-phase motor starter 1 three-phase motor or equivalent motor load 3 double-acting push buttons (N.O./N.C. on each button) 1 8-pin tube socket 1 8-pin control relay 1 single-pole switch Control transformer

Connecting Jogging Circuits

In this experiment four different jog circuits will be connected in the laboratory. Three of these circuits are illustrated in ills 30-2, 30-3, and 30-4. The fourth circuit will be designed by the student in accord with given circuit parameters.

Connecting Circuit 1

1. Refer to the schematic diagram in Ill. 2. Place wire numbers beside the components following the procedure discussed in previous experiments.

2. Using the components shown in Ill. 5, place corresponding wire numbers beside the components.

3. Connect the circuit by following the wire numbers in the schematic diagram in Ill. 2.

4. Turn on the power and test the circuit for proper operation. The motor should jog when the switch is open and run when the switch is closed.

5. Turn off the power and disconnect the circuit.

Connecting the Second Run-Jog Circuit

1. Using the schematic shown in Ill. 3, place wire numbers beside the components.

2. Place corresponding wire numbers beside the components shown in Ill. 6.

3. Connect the circuit using the schematic diagram in Ill. 3.

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

5. Turn off the power and disconnect the circuit.

Connecting the Third Run-Jog Circuit

The third run-jog circuit involves the use of a control relay. In this circuit, an 8-pin control relay will be used. Eight-pin relays are designed to fit into an 8-pin tube socket. Therefore, the socket is the device to which connection is made, not the relay itself. Eight-pin relays commonly have coils with different voltage ratings such as 12 VDC, 24 VDC, 24 VAC, and 120 VAC, so make certain that the coil of the relay you use is rated for the circuit control voltage. Most 8-pin relays contain two single-pole, double-throw contacts. A diagram showing the standard pin connection for 8-pin relays with two sets of contacts is shown in Ill. 7.

Connecting the Tube Socket

When making connections to tube sockets, it's generally helpful to place the proper relay pin numbers beside the component on the schematic diagram. To distinguish pin numbers from wire numbers, pin numbers will be circled. The schematic in Ill. 4 is shown in Ill. 8 with the addition of relay pin numbers. The connection diagram in Ill. 7 shows that the relay coil is connected to pins 2 and 7. Note that CR relay coil in Ill. 8 has a circled 2 and 7 placed beside it.

The connection diagram also indicates that the relay contains two sets of normally open contacts. One set is connected to pins 1 and 3, and the other set is connected to pins 8 and 6.

Note in the schematic of Ill. 8 that one of the normally open CR contacts has the circled numbers 1 and 3 beside it and the other normally open CR contact has the circled numbers 8 and 6 beside it.

1. Using the drawing in Ill. 8, place wire numbers on the schematic.

2. Using the wire numbers placed on the schematic diagram in Ill. 8, place corresponding wire numbers beside the proper components shown in Ill. 9.

3. Connect the circuit shown in Ill. 8.

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

5. Turn off the power and disconnect the circuit.

Ill. 5 Components needed to connect circuit 1.

Ill. 6 Components needed to connect the second run-jog circuit.

Ill. 7 Standard diagram for an 8-pin control relay.

Ill. 8 Adding pin numbers aids in connecting the circuit.

Ill. 9 Components for circuit.

Ill. 10 Jog button has been connected incorrectly.

QUIZ:

1. Explain the difference between inching and jogging.

2. What is the main purpose of jogging?

3. Refer to the circuit shown in Ill. 10. In this circuit, the jog button has been connected incorrectly. The normally closed section has been connected in parallel with the run push button and the normally open section has been connected in series with the holding contacts. Explain how this circuit operates.

4. Refer to the circuit shown in Ill. 11. In this circuit the jog push button has again been connected incorrectly. The normally closed section of the button has been connected in series with the normally open run push button and the normally open section of the jog button is connecting in parallel with the holding contacts. Explain how this circuit operates.

5. In the space provided in Ill. 12, design a run-jog circuit to the following specifications:

a. The circuit contains two push buttons, a normally closed stop button and a normally open start button.

b. When the start button is pressed, the motor will run normally. When the stop button is pressed, the motor will stop.

c. If the stop button is manually held in, however, the motor can be jogged by pressing the start button.

d. The circuit contains a control transformer, motor, and three-phase motor starter with at least one normally open auxiliary contact.

6. After your instructor has approved the new circuit design, connect the circuit in the laboratory.

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 place.

Ill. 11 Push button has been connected incorrectly.