Jogging Control Circuits

Jogging is the process of briefly energizing a motor for the purposes of partial rotation or small movement, such an action could be used to align a shaft with a coupling or to briefly move an applied load. Jogging is a fleeting act and does not use a holding circuit. A holding circuit is utilized in a three-wire control circuit and maintains the last asserted state, Let's discuss the operation of a regular three-wire control circuit since it's a central component of this lesson. 

Our ladder logic's first rung is made up of a maintained contact normally closed emergency stop, a momentary contact normally closed stop push button, a momentary contact normally open start push button, a contactor coil, and a normally closed overload auxiliary contact. The second rung includes a normally open-holding auxiliary (aux) contact associated with the contactor connected in parallel with the momentary contact normally open start push button.

If an operator pressed the start button, the momentary normally open start switch would close, energizing the contactor coil through the normally close emergency stop, the normally closed stop, the now closed start and the normally closed overload contact. The primary contactor contacts close when the coil of the contactor is energized. The motor then experiences an inrush current and starts rotating. Once the motor reaches its rated speed, the inrush current subsides and levels out at the full load rated current.

The momentary start push button would return to its typically open, deactivated state if an operator were to release it. The primary contacts remain closed, and the motor continues to spin as expected because the now-closed aux holding contact keeps the contactor coil in an electrified condition as seen below:

The holding circuit serves to maintain the most recently asserted state, but to stop the motor, a user must press the normally closed stop push button. Pressing the stop button de-energizes the contactor coil and returns the associated contacts to their de-energized state. The auxiliary holding contact opens, removing the path parallel to the start push button, and the contactor primary contacts open, allowing the motor to spin freely until it comes to a stop.

The motor starts when the operator presses and releases the start button. The motor pauses and is prepared to start again when an operator presses and releases stop. However, if a user were to hit the maintained emergency stop after spotting a dangerous situation, the motor would stop and the system would be disabled. Importantly, because the emergency stop is maintained rather than momentary, the system would stay disabled until the emergency stop is reset. The start button also won't activate the contactor coil, so the contactor's primary contacts won't close despite repeated attempts. The system can now start the motor, but not before the emergency stop has been reset and put back in the closed position.

 

The normally closed overload contact would de-energize the contactor coil if this motor were to experience a high inertial load while operating, or if the three-phase AC system lost a phase, or if the rotor were to become stuck due to a misaligned shaft or bad bearings. Only after the overload cool and reset can the motor be restarted. 

It should be noted that the motor is immediately and directly connected to full voltage upon closure of the contactor primary contact. Such an event is characterized by a significant surge of current known as inrush, which can be up to six times the normal full load current. Direct online motor starters like this one must obviously be rated to carry this current. As a motor ramps up to rated speed, growing counter electromotive force, often known as C EMF, opposes the applied voltage. As a result, inrush current, albeit high and undesirable from a distribution standpoint, is typically brief.

 

Since the holding auxiliary (aux) contact in parallel to the momentary contact's normally open start push button maintains a current path when the contactor coil is energized, the only thing standing in the way of us constructing a jogging circuit is the removal of the auxiliary holding contact. As you can see below:

The momentary contact start push button energizes the contactor coil only when an operator is in act actively pressing it. When the contactor coil is energized the contactor contacts would close and the motor would experience inrush current then being rotating, the moment the operator releases the start push button the spring returns to its deactivated open state, the contactor coil would then be de-energized and the primary contacts would open and the motor would free spin to a halt. Without the aux normally open holding contact an operator could briefly energize the motor to inch the shaft or apply load along until it reaches the desired location.   

Only when an operator is actively pressing the momentary contact start push button will it energize the contactor coil. When the contactor coil is energized, the contactor contacts close, causing the motor to experience inrush current and begin rotating.  When the start push button is released, the spring returns to its deactivated open state, the contactor coil de-energizes, the primary contacts open, and the motor free spins to a stop. 

A jogging circuit is a circuit that incorporates momentary switches without a holding contact in parallel, in contrast, a three-wire control circuit with a holding contact maintains the last asserted state. The functionality of both maintained run and momentary jog circuits can be created by incorporating a maintained contact selector switch in the second rung, the diagrams below show the contact is closed in the run position and open the jog position, the selector switch effectively enabling the holding contact in the run position and disabling it in the jog position. Let's take a closer look at this run, jog circuit below, making use of a selector switch. 

If the operator were to press the start button with the selector switch in the run position, the momentary contact normally open start switch would close, and the coil of the contactor would be powered through the normally closed emergency stop, normally closed stop, the now closed start, and normally closed overload contact. The normally open auxiliary contact in parallel to the push button closes when the contactor's coil energizes, and the primary contactor contacts close as well. This causes the motor to experience an inrush current as it starts to rotate. Once the motor reaches its rated speed, the inrush current subsides and levels out at the full load rated current.

If an operator were to release the momentary contact start push button the spring return would return it to its deactivated normally open state, note via selector in the closed run position and the now closed contactor holding contact maintains the energize state of the contactor coil this means the primary contactor contacts stays closed and the motor continues spinning, and that's the purpose of the selector switch in the run position keeps the contactor holding auxiliary contact enabled and maintains the last state, the emergency stop, stop and overload contact function as previously. 

In contrast with the selector switch now in the jog position the contactor holding auxiliary contact is essentially switched out of the circuit. 

When an operator presses the start push button it energizes the contactor coil. When the contactor coil energized the primary contactor contacts closed the motor would experience an inrush current and begin rotating, the moment the operator released the start push button the spring return returns to its deactivated open state because the parallel path afforded by the holding contact is no longer available due to the open selector switch in the jog position the contactor coil would be de-energized and the primary contactor contacts would be open the motor is brought to a rapid halt having accomplished a very small movement as intended, emergency stop, stop and overload contact function as previously. 

We have developed a motor starter with two modes—a run mode that preserves the most recently asserted state and a jog mode that enables a user to momentarily power a motor—by including a selector switch that selectively enables or disables the holding contact.

As was previously noted, jogging or inching circuits require contacts rated for this usage because closing the primary contactor contacts instantly and immediately connects the motor to full voltage. This event is marked by a significant inrush current surge. Normally, when a motor reaches its rated speed, the inrush current would decrease. However, because jogging events are transient in nature, contactors used in jogging applications must be rated for this use. Additionally, since the current that is considered, a jogging event is many times that of typical operating conditions, motors that often jog may also encounter premature overload conditions. 

A different run-and-jog circuit arrangement is possible; for example, a run-and-jog circuit that implements the run-and-jog function as intended using a control relay and its associated contacts to selectively enable or disable the holding contact

The first rung of the ladder logic diagram includes the maintain contact normally closed emergency stop, the momentary normally closed stop push button, the momentary normally open run push button, the normally closed side of the mechanically interlocked momentary contact push button labelled jog, the coil of the control relay (CR), and the normally closed overload contact. One of the control relay's normally open contacts, CR(a), makes up the second rung.

The third rung consists of the normally open side of the mechanically interlock momentary contact push button labeled jog and the contactor coil. Finally, the fourth rung contains another of the normally open contacts CR(b) associated with the control relay. 

Notice CR(a) in rung two is not a holding contact for the contactor coil, but rather the control relay (CR) coil, if the control relay coil held in the energized state, this means the contact CR(b) will also be held in its activate closed state and the motor will continue to run as intended, if the control relay coil remains de-energize the holding contact will be disabled and the motor will jog as intended. Let's walk through the ladder logic diagram and see how the this circuit works.

 

The coil of the control relay (CR) would be energized if an operator were to push the momentary contact normally open run button, through the normally closed emergency stop, the normally closed stop, and the now closed run, as well as the normally closed side of the mechanically interlock momentary contact jog push button and the normally closed overload contact.

When the coil of control relay (CR) is energized its associated contacts change states, CR(a) contact on rung two closes as does CR(b) in rung four (4) via an now closed CR(b) contact the contactor coil is energized and the primary contacts closed and the motor would experience inrush current and begin rotating.

Once the motor reaches its rated speed the inrush current would subsides and levels out a full load rated current, via the now closed CR(a) holding contact the coil of control relay (CR) is held in the energized state and an operator can release the run push button. 

By energizing the control relay coil this circuit enables a holding circuit provided by contact CR(a) while the control relay has been held in the energized state the CR(b) contact keeps the contactor coil energized and the motor continues to run as intended. The emergency stop, stop and overload contact function as previously discussed. 

Since this is a mechanically interlocked push button package, imagine what would happen if a user pressed and held the jog button while the motor was running. The normally closed side would open and the normally open side would close. By opening the path in rung one, the control relay's (CR) coil is de-energized, and the associated contacts returned to their de-energized state. CR(a) opens and removes the holding circuit; CR(b) opens.

However, as long as the operator actively pressing the jog button the contactor coil remains energized and the motor continues spinning, as soon as the operator release the momentary contact jog push button the contactor coil would be de-energize the primary contactor contacts would open and the the spring applied electrically release brakes would be engaged by the spring and the motor would be brought to a rapid halt, while the motor is running the jog button in effect acts as a delayed stop and that only when the jog button is release does the motor actually stop, for this button may be labeled jog stop for circuits employing this methodology.  

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