What is jogging control?

Jogging has been defined by National Electrical Manufacturers Association (NEMA) as for the purpose to accomplish small movements of the operated machine, the quickly repeated closure of a network or circuit is required to start a motor. Across-the-line-starters are referred to by using the term "Jogging" and the reduced voltage starters are referred to using the term "Inching". Both these terms are used interchangeably. In a motor control circuit, multiple ways and methods to achieve a proper jog. The motor starter coil might be sealed with the hold circuit to prevent this we interrupt the holding contact in all of these methods from the start-stop buttons.

Jog circuits

Jog circuits are the type of motor control circuits that are commonly used to start the motor or jog the motor which is used for precise positioning of the motor controlling conveyor belts. An overheating of the power contacts might occur due to repetitive inrush currents when any starter is subjected to jog a motor. The starter of the motor should be increased in size and horsepower rating if the motor jogs for more than five times. There are many circuit designs through which we can achieve jog functions. The common feature for jog circuits is that they can disable the holding contact of a three-wire circuit. A switch or a push-button is generally put in series with the holding contact to achieve this.

Jog circuit with selector switch

This is the most basic type of jog circuit with a single-pole, single-throw (SPST) switch which is connected in series with the holding contact. The current behaves the same as in the three-wire circuit because the SPST switch does not provide any opposition to the current flow. A "start" or "run" button is normally acted by a normally-open push button. There is an open in series contact with the two-three holding contacts if the SPST switch gets opened by chance from a normally-closed position and effectively it removes them from the circuit. The normally open push button acts as a jog button and the motor is energized as long as the operator is pressing the button without the holding contact.

Jog circuit with control relay

Control relays are also used in the jogging circuit and behave just similar to motor starters but lack power contacts and overload protection. These control relays are the types of loads that must be connected in parallel with the starter of the motor so that rated voltage is developed in the relay coils. The current must flow from line 1 to line 2 and help to energize the one load in their way in any of the schematic diagrams. We know that when a switch is open it offers infinite resistance and when it is closed it offers zero resistance, so the current must be limited to prevent the short circuits in some devices.

Jogging circuit operation

A switch is connected in parallel with the seal-in of the pilot device. The control circuit controls the load connected to the pilot device when the switch is closed. The seal-in contacts energize the coil when we immediately press the start button and it sends power through the start push button. The seal-in contacts no longer need to be held down because now it maintains the power to the coil. The coil of the starter of the motor can be de-energized in the following ways-

• The normally-closed contacts of the overload circuit will open breaking it so that the motor will go into overload.
• The stop button is then pressed to remove power to cause the coil to get de-energized from the seal-in contact.
• Another way to stop the control circuit is to turn the switch to the jog position. The coil will be de-energized immediately.
• Only for the amount of time, the start push button is pressed the coil can only be energized at that time.

Interlocking control

The kind of motor control circuits where a second motor does not start unless the first motor is started or the third motor does not start until the second motor is started is called interlocking. There is a kind of sealing or interlocking known as mechanical interlocking. Between the forward and reverse contactors at the factory, a mechanical interlocking device is assembled. They lock out the contractor at the beginning of the stroke with another contractor to prevent short circuits and burnouts. The mechanical interlock prevents the accidental closing of the coil when the forward contactor coil is energized and which is closed using the forward push button. The majority of the reversing starters have mechanically joined interlocks that use one or both of the following methods that is "push-button interlock" and "auxiliary contact interlock". The method of push-button interlock helps in preventing both the coils of the starter from getting energized. The method of auxiliary contact interlock consists of normally closed auxiliary contacts which are present in the reversing starter's forward and reverse contactors. The wiring is done by following the above theory. All plug-in or toggle-controlled fixed switch disconnectors and compact circuit breakers can be also interlocked.

Types of interlocking

Mechanical interlocking

When the lever frames are connected with the wire to the signals and the points, the mechanical interlocking is achieved. This type of mechanical interlocking consists of locking frames, rock bars, plungers, etcetera. A few steps of this process will be done manually and the major process of this interlocking is the lever movement.

Electrical interlocking

All the processes are carried out automatically and electrically here. Only an electrical circuit is used for this type of interlocking process and there would not be any mechanical means to achieve this type of interlocking. In this process relays and switches are mostly used.

How to select an interlocking device?

According to the conditions and modes to be used the devices are selected. The hazards that the device might handle should also be considered along with it when selecting an interlocking device. The probability of the device failure must be considered while selecting. Also, the access time and stopping time must be considered.

Context and Applications

This topic is significant in the professional exam for undergraduate, Graduate, and Post-graduate courses.

• Bachelors in Electrical Engineering
• Masters in Electrical Engineering

Practice Problems

1. What does a jogging circuit do?

1. Enable the seal-in circuit
2. Both enable the seal-in circuit and disable the start circuit
3. Both disable the seal-in circuit and activate the start circuit
4. Disable the short jabs circuit

Answer: Option c

Explanation: The jog circuit disables both the seal-in circuit and activates the start circuit.

2. What is the main purpose of jogging control?

1. To accomplish small movements of the operated machines
2. To accomplish large movements of the operated machine
3. Both a and b
4. None of these

Answer: Option a

Explanation: Jogging has been defined by NEMA as for the purpose to accomplish small movements of the operated machine.

3. What does a jog circuit with control relays lacks?

1. Underload protection
2. Power contacts and overload protection
3. Both a and b
4. None of these

Answer: Option b

Explanation: Control relays are also used in the jogging circuit and behave just similar to motor starters but also lack power contacts and overload protection.

4. How the control loads must be connected with motor starters?

1. In parallel
2. In series
3. Both a and b
4. None of these

Answer: Option a

Explanation: The control loads must be connected in parallel with motor starters so that the rated voltage is developed.

5. What does a mechanical interlock prevent?

1. The accidental closing of the coil
2. The accidental opening of the coil
3. Both a and b
4. None of these

Answer: Option a

Explanation: The mechanical interlock prevents the accidental closing of the coil when the forward contactor coil is energized and which is closed using the forward push button.

Related Concepts

• Railway interlocking
• Breath alcohol ignition interlock device
• Lockout-tagout