FORWARD AND REVERSE STARTER CONTROL

FORWARD AND REVERSE CONTROL

The direction of rotation of any three-phase motor can be reversed by changing or swapping any two phases to the motor (see below image). Now that we know a motor can be reversed by swapping any two phases it won’t be economical to always physically connect and disconnect phases to reverse the motor,
that can be time consuming in a busy industry .To solve this problem we will have to control the motor using the forward button and the reverse button, this can be accomplished by using two contactors (forward contactor and reverse contactor) interlocked (see the next section on interlocking).

 

INTERLOCKING

Interlocking of contactors means connecting two or more contactors in such a way that the action which take place in a 1st contactor prevent an action from taking place in another contactor until that 1st contactor action has been performed or stopped. In short we can say Interlocking prevent some action from starting until some other action has been performed or stopped. In the forward and reverse control we don’t want the forward contactor and reverse contactor to be both energized at the same time, to prevent that we are going to interlock both contactors

There are three ways of interlocking namely Mechanical interlock, Electrical interlock (double acting push buttons method) and Electrical (series auxiliary method).

For now we are going to concentrate only in the Electrical series auxiliary method, for others we will look into them in future.

Electrical interlocking series auxiliary method is accomplished by connecting the normally closed auxiliary contacts of the other contactor on the other contactor in series with the coil(see figure 1 below) :

FIGURE 1

 Lets imagine that we have a contactor KM1 and KM2 we connect KM2 normally closed auxiliary contact in series with KM1 contactor coil then we do the same to KM2 contactor, now if we energize KM1 contactor the normally open contact connected in series with KM2 will open thus disabling KM2 from energizing by cutting the power path to it(see figure 2), if we de-energize KM1 and energize KM2 the same will happen to KM2 contact at KM1 coil(see figure 3).

FIGURE 2.  KM1 energized

FIGURE 3.  KM2 energized

Below is a simple forward and reverse circuit for three phase motor

Figure 4

COMPONENTS

F1-Thermal Overload Relay,       

KM1-Main Forward Contactor

KM2-Main Reverse Contactor

3phase Induction Motor

S1-Stop push button (NC)

S2-Forward Start push button (NO)

S3-Reverse Start push button (NO)

CB1-3phase circuit breaker

CB2-Single phase circuit breaker

OPERATION

If the forward push button S2 is to be pressed first, the KM1 NC auxiliary contact connected in series with KM2 contactor coil will open to prevent the KM2 circuit from being on if by mistake reverse push button S3 is pressed, the KM1 self holding contact will hold and the KM1 coil will energize, the motor will start running in the forward direction, note that the KM1 contactor will be on at the power circuit and KM2 will be off.

When the STOP button S1 has been pressed, KM1 coil de-energizes and all its contacts return to their normal position (including the one connected in series with KM2).

The reverse push button S3 is pressed to energize KM2 coil. When KM2 coil energizes, all KM2 contacts change position, the KM2 NC contact connected in series with KM1 also change position by opening to prevent the possibility of KM1 coil to energize while KM2 is on by cutting the power path to KM1. Note now KM2 contactor is on at the power circuit and has connected the motor to different phases as compared to KM1 or simply has “swapped” the phases, which cause the motor to run in reverse direction.

INDICATORS

Below there is an extra circuit same as above but now with indicators, the bulb Z1 is connected in series with the normally open contact of KM1, Z2 bulb is connected in series with the normally open contact of KM2  and the siren SR1 is connected in series with the normally open contact of the Thermal overload relay F1.

 

 When KM1 is energized all the contacts of KM1 change position including the Normally Open contact of KM1 connected in series with Z1 ,that makes Z1 to light to indicate that the motor is running in forward direction.

Similarly when KM2  is energized all the contacts of KM2 change position including the Normally Open contact of KM2 connected in series with Z2 ,that makes Z2 to light to indicate that the motor is running in reverse direction.

Lets now assume that the there is a fault/overload in the motor then the thermal overload relay F1 trips or break the circuit, when that happens all the contacts of F1 will change position including the normally open contact 97, 98 .That will complete the circuit to siren SR1 then the siren SR1 will sound to indicate that the fault has occurred

NOTE that we did not add the contact numbers for the normally open indicators of Z1 and Z2, you can use any reserve normally open contact of the corresponding contactor.