Sometimes called “inching,” jogging is the term given to the momentary energization of a motor only so long as an operator is pressing a button.
A jog circuit is a circuit that allows an operator to either start the motor or “jog” the motor and are commonly used for motors controlling conveyor belts to allow for precise positioning of materials.
Any motor starter that is used to jog a motor will be subjected to repetitive inrush currents, which can cause overheating of the power contacts. If a motor is expected to be jogged more than five times in a minute, the motor starter should be increased in size and horsepower rating for this more severe operating condition.
To achieve a jog function, there are several common circuit designs, each with their own advantages and disadvantages. A common feature that all jog circuits have is that they have some method of disabling the holding contact used in the three-wire circuit. This is usually accomplished by putting some component in series with the holding contact, such as a switch or momentary pushbutton.
Jog Circuit with Selector Switch
The most basic of the jog circuits, this is essentially a three-wire circuit with an SPST (single-pole, single-throw) switch connected in series with the holding contact.
In the closed position, the SPST switch offers no opposition to the flow of current and the circuit behaves the same as a standard three-wire circuit. The normally open push button is acting as a “start” or “run” button.
If the SPST switch is opened, then it has introduced an open in series with the 2-3 holding contacts, effectively removing them from the circuit. Without the holding contact, the motor starter will only be energized as long as an operator is pressing the normally open pushbutton, which acts as a “jog” button in this position.
- The main advantage of this circuit is the ease of installation and the cheapness of equipment.
- The main disadvantage is that you must change the position of your selector switch to change the function of your button.
Dangerous Jog Circuit
The next jog circuit we will look at uses a four-contact momentary pushbutton as our “Jog” button. This button has one set of normally closed contacts which are wired in series with the 2-3 holding contact, and one set of normally open contacts which are in series with only the stop button and the motor starter.
In normal operation, the stop and start buttons provide their standard functions in a three-wire circuit, as the 2-3 holding contact is in series with the normally closed contacts of the jog button. If the jog button is pressed, the normally closed contacts will open and the normally open contacts will close, providing a path for current to energize the motor starter.
When the motor starter is energized, all contacts associated with it will change their state, including the 2-3 holding contact, but because the jog button is being depressed the holding contact cannot maintain power to the starter. Once the jog button is released the motor comes to a stop.
This circuit is sometimes referred to as the “dangerous jog circuit” for reasons which may appear obvious now. If the jog button’s normally closed contacts are able to return to their normal condition before the motor starter’s armature has had a chance to drop out, then the coil will remain energized and the motor will continue to run. This is dangerous because if an operator pushes a jog button, expecting the motor to stop when they release the button, and the motor continues to run, it could introduce a hazard to a person caught off guard. In short, we never want machines to surprise people.
- This circuit has the advantage of being simple to install and has separate buttons dedicated to starting and jogging the motor.
- The main disadvantage is the hazard introduced by the quick return of the jog button to its normal state.
Jog Circuit with Control Relay
A more sophisticated jogging circuit uses a control relay as shown in the figure above. Control relays behave just like motor starters but lack overload protection and power contacts. Control relays are loads that must be connected in parallel with the motor starter to ensure they get their rated voltage.
In any schematic diagram, the current must find its way from Line 1 to Line 2 and energize only one load along the way. Switches offer either infinite resistance when they are open or zero resistance when they are closed, so some device must limit the current to prevent short circuits. Notice that the current that passes through the control relay does not pass through the motor starter. This means that they will both get their rated value of voltage and pull in their armatures.
As a rule, we NEVER connect loads in series.
Under normal conditions, if the start button is pressed, current will be able to complete the circuit and energize the control relay. Once the relay is energized, the two normally open contacts associated with it will change their state and close. This will provide a path for current to energize the motor starter, closing the 2-3 holding contacts and running the motor.
The circuit will continue to operate as a standard three-wire circuit providing low-voltage protection (LVP) until either the stop button is pressed, or an overload occurs.
If the jog button is pressed while the motor is running, there will be no change to the circuit.
If, however, the jog button is pressed while the motor is not running, it will provide a path for current to energize the motor starter. The current will not be able to energize the control relay, and so once the jog button is released, the starter drops out and the motor comes to a stop.
- The advantage of this circuit lies in its safety and reliability. Having two separate buttons for “run” and “jog” functions increases ease of control for the operator.
- The disadvantage is the additional cost and installation time associated with the control relay and additional current drawn in the control circuit due to the second coil.
Reference
Basic Motor Control by Aaron Lee and Chad Flinn is used under a CC BY 4.0 Licence.