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GOALS:
- Define the term jogging.
- State the purpose of jogging.
- State the difference between jogging and inching.
- Describe the operation of a jogging control circuit using control relays.
- Describe the operation of a jogging control circuit using a selector switch.
- Connect a jogging circuit.
The definition of jogging or inching as described by NEMA is "the quickly repeated closure of a circuit to start a motor from rest for the purpose of accomplishing small movements of the driven machine." The term jogging actually means to start a motor with short jabs of power at full voltage. The term inching means to start a motor with short jabs of power at reduced volt age. Although the two terms mean different things, they are often used interchangeably because both are accomplished by preventing the holding contacts from sealing the circuit.
Jogging Circuits
Ill. 1 The stop button prevents the holding contact from sealing the
circuit.
Ill. 2 Double acting push buttons are used to provide jogging control.
Various jogging circuits are presented in this section.
As with many other types of control circuits, there are different ways in which jogging can be accomplished, but basically, jogging is accomplished by preventing the holding contact from sealing the circuit around the start push button when the motor starter energizes. It should also be noted that jogging circuits require special motor starters rated for jogging duty.
One of the simplest jogging circuits is shown in Ill. 1. This circuit's basically a start-stop push button control circuit that has been reconnected so that the START button is in parallel with both the STOP button and holding contact. To jog the motor, simply hold down the STOP button and jog the circuit by pressing the START button. To run the motor, release the STOP button and press the START button. If the motor is in operation, the STOP button will break the circuit to the holding contact and de-energize M coil.
Double Acting Push Buttons
Jogging can also be accomplished using a double acting push button. Two circuits of that type are shown in Ill. 2. The normally closed section of the JOG push button is connected in such a manner that when the button is pushed it will defeat the holding contact and prevent it from sealing the circuit. The normally open section of the JOG button completes a circuit to energize the coil of the motor starter. When the button is released, the normally open section breaks the circuit to M coil before the normally closed section reconnects to the circuit. This permits the starter to re open the holding contacts before the normally closed section of the JOG button reconnects.
Although this circuit's sometimes used for jogging, it does have a severe problem. The action of either of these two circuits depends on the normally open M auxiliary contact (holding contact), which is used to seal the circuit, being open before the normally closed section of the JOG button makes connection. Since push buttons employ a spring to return the contacts to their normal position, if a person's finger should slip off the JOG button, it's possible for the spring to re establish connection with the normally closed contacts before the holding contact has time to reopen. This would cause the motor to continue running instead of stopping. In some cases, this could become a significant safety hazard.
Using a Control Relay
The addition of a control relay to the jog circuit eliminates the problem of the holding contacts making connection before the normally closed section of the jog push button reconnects. Two circuits that em ploy a control relay to provide jogging are shown in Ill. 3. In both of these circuits, the control relay, not M starter, provides the auxiliary holding contacts.
The JOG push button energizes the coil of M motor starter but does not energize the coil of control relay CR. The START push button is used to energize the coil of CR relay. When energized, CR relay contacts provide connection to M coil. The use of control relays in a jogging circuit's very popular because of the simplicity and safety offered.
A jogging circuit for a forward-reverse control is shown in Ill. 4. Note that a control relay is used to provide jogging in either direction. When the for ward jog push button is pressed, the normally open section makes connection and provides power to F coil.
This causes F load contacts to close and connect the motor to the power line. The normally open F auxiliary contact closes, also, but the normally closed section of the forward jog button is now open, preventing coil CR from being energized. Since CR contact remains open, the circuit to F coil cannot be sealed by the normally open F auxiliary contact.
If the forward start button is pressed, a circuit's completed to F coil, causing all F contacts to change position. The normally open F auxiliary contact closes and provides a path through the normally closed section of both jog buttons to CR coil. This causes CR auxiliary contact to close and provide a current path through the now closed F auxiliary contact to F coil, sealing the circuit when the forward push button is re leased. The reverse jog button and reverse start button operate the same way. Note also that normally closed F and R auxiliary contacts are used to provide interlocking for the forward-reverse control.
Jogging Controlled by a Selector Switch
A selector switch can also be employed to provide jogging. The switch is used to break the connection to the holding contacts (Ill. 5). In this circuit, a single-pole single-throw toggle switch is used. When the switch is in the ON position, connection is made to the holding contacts. If the switch is in the OFF position, the holding contacts cannot seal the circuit when the START button is released. Note that the START button acts as both the start and jog button for this circuit. A selector switch can be used to provide the same basic type of control (Ill. 6).
Inching Controls
As stated previously, jogging and inching are very similar in that both are accomplished by providing short jabs of power to a motor to help position certain pieces of machinery. Inching, however, is accomplished by providing a reduced amount of power to the motor.
Transformers can be used to reduce the amount of volt age applied to the motor during inching, or reactors or resistors can be connected in series with the motor to reduce the current supplied by the power line. In the circuit shown in Ill. 7, resistors are connected in series with the motor during inching. Notice that inching control requires the use of a separate contactor because the power supplied to the motor must be separate from full line voltage.
Ill. 3 Control relays provide jogging control.
Ill. 4 Jogging using a control relay on a forward-reverse control.
Ill. 5 A single pole single throw toggle switch provides jog or run control.
Ill. 6 A selector switch provides run-jog control.
Ill. 7 Resistors are used to reduce power to the motor.
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. 8. 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. 9. 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.
Ill. 8 The jog button is connected incorrectly.
Ill. 9 Another incorrect connection for the jog button.