Forward reverse motor control circuit

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The direction of rotation of an industrial three-phase alternating current motor is determined by the applied phase sequence. Let's say a motor rotates clockwise when the phase sequence visible at the motor terminals is L1, L2, and L3. If any two leads are switched, the motor will spin counterclockwise; typically, L1 is exchanged with L3. However, the applied phase sequence would be the reverse of the preceding one, and the motor would revolve in the other direction, whether L3 was to be swapped for L2 or L2 was to be swapped for L1. 

A manual reversing motor starter can change the direction of rotation with the help of a drum or a cam switch (figure 1.1) ; it is made up of contacts and an overload manual that must be activated by a user at the point of use. The motor is protected from sustained overload situations by the overload, which establishes or breaks an electrical connection to start or stop the motor.

Figure 1: Manual reversing motor starter

Figure 1.1: Drum Switch

The motor starter must be used to make or break connections, not the drum switch, even though this method is suitable for some applications. The drum switch's only function is to switch the applied phase sequence.

Drum switches often aren't designed to make inrush current or break full load current, thus an operator needs to be trained in how to utilize them properly. Magnetic reversing motor starters are used since a manual motor starter's application is limited to conditions requiring practically continual human monitoring because, as the name implies, it must be manually controlled by an operator at the point of usage.

A magnetic starter uses paired Contactors to selectively energize a motor rather than a drum switch in this case the manual motor serves to disconnect the magnetic reversing motor starter for repair and services. One of the contactors is designated the F or forward contactor and is connected such the applied phase sequence L1, L2, and L3 is seen by the motor when the forward primary contacts close, let's say that this would see the motor rotate in the clockwise direction, the other contactor is designated the R or the reversing contactor and is connected such to the applied phase sequence seen by the motor is L3, L2, L1 as shown in figure 2.

Figure 2: Magnetic reversing motor starter using paired contactors (Power Circuit)

The motor would turn counterclockwise when the reversing contactor closed. It should be noted that the overload functions as both a forward and a reversing contactor; when either contactor is closed, the elements of the overload in series can sense incoming current. An overload in either direction is equally important.

Importantly, the simultaneous closure of the forward and reversing contactor is something that should never ever happen. Not only does it not make sense to spin a motor both clockwise and counterclockwise, but simultaneous closure of the forward and reversing contactors also results in a phase-to-phase event where phase 1 and phase 3 collide with no current controlling element in between. In this lesson, we'll go over three common interlock techniques used to prevent this.

The pilot ladder logic diagram for a magnetic reversing motor starter (figure 3) features an emergency stop in series with a normally closed stop push button the remainder of rung 1 and spilling over into rung 2 is a traditional three-wire control circuit used to control the forward contactor. Note the momentary normally open forward push button has a holding contact labeled F associated with the forward contactor in parallel to it in rung 2, rung 3, and spilling over into run 4 is another traditional 3-wire control circuit used to control the reversing contactor. Note the momentary normally open reverse push button has a holding contact labeled R associated with a reversing contactor in parallel to it in rung 4.

Figure 3: The pilot ladder logic diagram for a magnetic reversing motor starter (Control Circuit)

The motor is protected from persistent overloads in both forward and reverse modes by the typically closed overload pilot contact in rung 1. Let's go through the ladder logic diagram to see how this magnetic reversing motor starter operates. If the forward push button were pressed, the momentary normally opened forward switch would close, energizing the forward coil via the normally closed emergency stop, the normally closed stop, the now closed forward button and the normally closed overload contact as shown in figure 4.

Figure 4: The pilot ladder logic diagram for a magnetic reversing motor starter when the forward push-button was pressed 

The F holding contact, which is connected to a forward push button and is normally open, closes when the forward contactor's coil is energized, as are the forward contactor primary contacts.

Figure 5: Forward contactor primary contacts in their closed state 

Take note that the applied phase sequence as provided by the forward contactor is L1, L2, L3. The motor experiences an inrush current and starts rotating clockwise; however, once the motor reaches the rated speed, the inrush current subsides and levels out to the full load-rated current. If the operator were to release the momentary contact forward button, the spring return would return the contact to its normally open deactivated state. 

The forward contactor coil's electrified condition is maintained by the now closed F auxiliary contact, which implies that the forward primary contactor contacts remain closed and the motor continues to rotate clockwise as shown in figure 6.

Figure 6: Forward contactor coil's electrified condition is maintained by the now closed F auxiliary contact

The forward contactor coil is de-energized by the now open stop push button, the associated contacts return to their de-energized state, the forward auxiliary contact opens, blocking the path parallel to the forward push button, the forward contactor primary contacts open, and the motor free spins to a stop.

Figure 7: Forward contactor coil is de-energized by the now open stop push button

Figure 8

Figure 9: Forward contactor primary contacts open

In a similar manner, pressing the reverse push button would cause the momentarily normally opened reverse switch to close. The reverse coil would then be energized via the normally closed emergency stop, the normally closed stop, the now closed reverse button, and the normally closed overload auxiliary contact. When the coil of the reverse contactor energizes its associated contacts change states, the normally open  R auxiliary contact in parallel with the reverse button closes and the reverse contactor primary contacts closed.

The motor encounters an inrush current and starts turning counterclockwise; after the motor reaches its rated speed, the inrush current declines, and levels off at the full load-rated current. Take note that the applied phase sequence as provided by the reversing contactor is L3, L2, L1. The momentary contact reverse button must be released for the spring return to return to its typically open, deactivated position.

Figure 10: Pressing the reverse push button

 Figure 11

The reverse contactor primary contacts remain closed and the motor continues to rotate counterclockwise because, as shown in Figure 12, the now-closed R contact keeps the reverse contactor coil in an energized condition.

Figure 12: R contact maintains the energized state of the reverse contactor coil 

The normally closed stop button must be depressed once more to stop the motor (figure 13). The deactivated stop de-energizes the reverse contactor coil, returning the associated contacts to their de-energized state. The R holding contact then opens, obliterating the path parallel to the reverse push button. Finally, the reverse contactor primary contacts open (figure 14), causing the motor to come to a complete stop.

Figure 13: The stop button de-energizes the circuit 

Figure 14: Reverse contactor primary contacts open

This reversing motor starter returns to the ready condition once the stop button returns to its typically closed deactivated position. Observe that even in its naturally deactivated closed state, the maintained contact emergency stop does not, in any way, shape, or form, interfere with the system's operation. When an operator presses and releases forward, the motor spins clockwise when an operator presses and releases stop, the motor stops, ready to start another cycle when the operator presses and releases reverse, the motor spins counterclockwise when an operator presses and releases stop, the motor stops, ready to initiate yet another start cycle. 

However, if an operator were to spot a dangerous situation and hit the maintained emergency stop (figure 15). Importantly, because the emergency stop is maintained rather than momentary, the system will remain disabled until the emergency stop is reset. Neither the forward nor reverse button will energize either contactor coil, and as a result, the primary contacts will remain open despite repeated attempts to do so.

Figure 15: The emergency stop button disables the system 

Similarly, the normally closed overload contact serves to protect the motor from sustained overload conditions in both forward and reverse mode. Only when the overload has cooled and reset will the operator allow to start the motor, as shown in figure 16: If the motor sustains sustained overload, the normally closed overload contact would open and de-energize either contactor coil regardless of rotational direction.

Figure 16: Overload contact de-energizes the system

A magnetic reversing motor starter functions exactly like two regular three-wire circuits in the same ladder logic diagram: one for the forward mode serving the forward contactor, which is wired with the applied phase sequences L1, L2, and L3, and the other for the reverse mode serving the reverse contactor, which is wired with the applied phase sequences L3, L2, and L1. The emergency stop, stop, and normally closed overload auxiliary contact serves both the forward and reverse modes.

Regarding our earlier warning concerning the forward and reverse contactors being closed simultaneously, the truth is that nothing is currently in place to prohibit an operator from pressing the reverse button while the motor is spinning clockwise (figure 17).

Figure 17: Pressing the reverse button while the motor is spinning clockwise 

When the operator presses reverse, the reverse coil is also activated and the reverse contactor is closed. Phases 1 and 3 collide (Figure 18) and either one or both of the contactors explode while the forward contactor is closed, with the applied phase sequence being L1, L2, and L3. 

Figure 18: Phases 1 and 3 collide

Due to this, magnetic reversing starters are required to include an interlock. By "interlock," we mean a mechanism that prevents the simultaneous closure of the forward and reverse contactors.

 

Interlocks are generally made possible via three methods mechanical, electrical, and push-button, occasionally, more than one method is utilized. 

Let's begin with the most basic interlocking technique, which is the mechanical interlocking of a magnetic reversing motor. A mechanical interlock is a type of interlocking that stops one contactor's physical movement if the other contactor's physical movement is engaged.

A slanted dashed line between the interlock contactor coils is frequently used to schematically denote a mechanical interlock, as shown in figure 19

Figure 19 

A plastic wedge or plunger often inserted between the contactors is used to physically pair the forward and reverse contactors for a mechanical interlock (figure 20 - 20.1).

Figure 20

Figure 20.1

or sometimes the mechanical is an entirely separate device that must slip between the two contactors (Figure 21).

Figure 21

When the forward contactor coil is energized (figure 22) the forward contact carrier is dragged into the coil and pushes the plastic mechanical interlock into the path of the reversing contactor (figure 22.1). 

Figure 22

Figure: 22.1

If the reversing contactor coil is also energized (figure 23) with the forward contact carrier and armature already dragged into its coil the engaged mechanical interlock prevents the physical movement of the reversing contact carrier,

Figure 23

The same thing happens if the reversing contactor coil is engaged first (figure 24) the movement of the contact carrier disallows the physical movement of the forward contact carrier (figure 24.1), whichever coil is energized first is the one and only state that is continually asserted and disallows the ladder.

Figure 24

Figure 24.1

Short version: don't hire operators foolish enough to think that spinning a motor both clockwise and counter-clockwise at the same time is a good idea. If an operator were to simultaneously press the forward and reverse buttons, whichever contact carrier got the jump on the other would be the one and only asserted state on predicting which one gets the jump is a bit of a gamble.

Notice mechanical interlock does not prevent the opposite coil from being energized it simply prevents physical movement of the opposite contact carrier, the opposite coil, therefore, might experience a premature burnout if this condition were to continue since inrush current when the armature is not pulled into the coil is substantially higher than ordinary ceiling current when the armature is inside the coil.

If an obstinate operator pressed and released reverse then pressed and held forward, the forward coil would be expected to experience an early death.

Figure 25

It is, for this reason, we'll step up our game to incorporate an additional level of protection including electrical interlocks. Contactors ordinarily feature an associated auxiliary pilot contact used for holding purposes, if a system necessitates inclusion one can also attach an auxiliary contact block to expand the utility of the contactor, when the coil of the contactor is energized the contacts in the auxiliary contact block also change states. Consider the inclusion of one of the normally closed contacts associated with a forward contactor F1 in the path of the reversing contactor likewise consider the inclusion of one of the normally closed auxiliary contacts associated with the reversing contactor R1 in the path of the forward contactor coil as seen in figure 26 

Figure 26

The normally closed contact in rung three opens when the forward contactor is powered, and the other associated contacts also change states as shown in figure 27 and figure 27.1

Figure 27

Figure 27.1

The now-open F1 contact prevents the reverse contactor coil from being energized, so closing the reverse push button has no effect in the forward states as shown in figure 28

Figure 28

Similarly, to this, when the reversing contactor coil is powered, the contacts that are associated with it change states, which causes the normally closed R1 contact in rung one to open as shown in figure 29 and figure 29.1.

Figure 29

Figure 29.1

The now-open R1 contact prevents the forward coil from being energized, therefore closing the forward button has no effect while the system is in the reverse mode as shown in figure 30.

Figure 30

Therefore, the presence of electrical interlocks made possible by an auxiliary contact block stops the coil of the forward and reversing contactor from being energized at the same time.

Noting that nothing prevents an operator from manually pushing the reversing contact carrier in the forward mode, consider a scenario in which one of the auxiliary contact blocks is damaged, improperly wired, or improperly connected with the associated contactor. The contactors may be electrically interlocked but are not mechanically interlocked.

Consider another scenario in which one of the contact carriers is jammed or stuck in the closed position. The simultaneous closure of both contactors would constitute a phase-to-phase event. For this reason, a mechanical interlock can also be added to this system. Figure 31 shows a pilot schematic for a magnetic reversing motor starter that is both mechanically and electrically interlocked

Figure 31

Contrary to the electrical interlock, which prevents the opposite coil from being energized but does prevent the opposite contact carrier's physical movement, the mechanical interlock prevents the opposite contact carrier's physical movement but does not prevent the opposite coil from being energized. The complimentary double layer of protection makes sure that the contactors for the forward and reverse are never closed at the same time.

The reverse button is useless in forward mode, and the motor keeps spinning in a clockwise direction.

The forward button is useless in reverse mode, and the motor still rotates counterclockwise. In both modes, the motor can be stopped using the stop button. If the application required it, a spring-applied electrical released brake can be added to the motor such that when de-energized the motor is brought to a rapid stop rather than free-spinning to a halt

Finally, take a look at a third popular method of interlocking a magnetic reversing motor starter called push button interlocking (figure 32). This method uses mechanically interlocked forward and reverse buttons so that when one mode is selected, the opposite mode is deselected. If you want to think of it this way, a mechanical interlock allows physical movement of the opposite contact carrier, an electrical interlock prevents the opposite coil from being energized, and a push-button interlock disallows simultaneous assertion of both states 

Figure 32

Keep in mind that push button interlocking is only meant to be used in conjunction with mechanical and electrical interlocking and not to completely replace either of these methods.

Observe that the normally closed of the mechanically interlocked forward push button acts just like a stop button for the reverse circuit, similarly, notice that the normally closed side of the mechanically interlocked reverse push button acts just like a stop push button for the forward circuit. If you pay attention to how this system behaves in comparison to our prior description of reversing magnetic motor starters utilizing mechanical and electrical interlocks, you'll notice something very different.

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