# How to reverse three-phase motor – Complete command!

Do you know how to reverse the direction of rotation of three-phase motors? How to make the inversion automatically? The World of Electrical shows in this article how to mount a reversal of electric motors in an automatic and timed way.

The electric motors are devices capable of converting electrical energy into mechanical energy. They can be differentiated by several characteristics, but a determining factor is their power supply, as there are motors that operate in direct current (DC), alternating current (AC) and some that can operate both in direct current and in the current. alternating. In addition, there are single-phase motors and three-phase motors.

# Three-phase motor rotation inversion

To perform the automatic inversion of the direction of rotation of a three-phase motor, we must understand how the reversal occurs in a three-phase motor. To reverse a three-phase motor, that is, turn the motor in the opposite direction to the previous one, just invert the phases that supply the motor.

In the image below we have a load diagram with the representation of the supply phases (R, S and T,) the DJM motor circuit breaker for protection and the K1 contactor to control the motor M start. In our control diagram, the motor has phases R, S and T feeding the poles in the U, V and W motor. In this case, the motor rotates clockwise.

However, if we invert the S phase with the T phase, we will have the inversion of the motor rotation direction. In this case, the engine starts to rotate anticlockwise.

How the direction reversal of three-phase motors is made.

# Automatic three-phase motor rotation inversion

In this image below we have the complete diagram that we will use during the article, we will show and explain each part of the automatic three-phase motor rotation inversion diagram.

Automatic three-phase motor rotation inversion diagram.

To perform the inversion automatically, we will use a second contactor K2, but in this case, the phases will be inverted. Phase R enters pole 1, phase S enters pole 5 and phase T enters pole 3. And at the output pole, 2 of the contactor is connected to the U of the motor, 4 to V and 6 to W.

Now we have contactor K1 controlling the clockwise rotation, and K2 controlling the clockwise rotation.

## Mounting the Electric Charge Command

So now we are going to connect the charging cables to our electrical control. First, we connect the 3 power cables to the motor circuit breaker input. It is important to use cables of different colors and washers to avoid mixing the cables of electrical control.

Then connect the motor circuit breaker output cables to the load contacts of contactor K1. At the load input of contactor K1, we will derive three cables for the load contacts of contactor K2, however, we invert the S phase with the T phase.

From the output of the load contacts of contactor K2, we will connect the cables to the output of the load contacts of contactor K1, connecting outputs 2, 4 and 6 of K1 to outputs 2, 4 and 6 of K2. And from the output of the load contacts of contactor K1, we will derive the wires that will be connected to the motor U, V and W contacts. Okay, the loading part is already finished, now we will understand how the command capable of reversing automatically and timely works.

## Command Part of the Diagram

In the control part of our diagram we have the DJ, which is the circuit breaker for protection of the control, the S1 which is the selector switch to start our motor, T1 is the timer that will control the time of change between the directions of rotation, and contactors K1 and K2, remembering that each contactor controls a direction of rotation of the motor, as we saw in the load connection.

When the selector switch has its contact closed, both T1 and K1 are supplied. And with the K1 energized, our engine starts to turn clockwise, and at the same moment, the timer starts to count the time.

A very important detail in this command is that contactor K1 can never work together with contactor K2 if there is no short circuit because in K2 the S and T phases are inverted. So we need to make sure that K1 and K2 never work at the same time.

For this reason, we use what in commands we call magnetic interlocking, which is to use a normally closed K2 contact on the K1 supply line and a normally closed K1 contact on the K2 supply line.

In this way, when K1 is working, the closed contact of K1 on the K2 line opens, and this prevents K2 from working at the same time as K1. The same is true when K2 is in operation, preventing K1 from functioning.

The closed contact of T1 goes on the K1 line and an open contact of T1 on the K2 line. When the time programmed in our timer is reached, the timer contacts are reversed, that is, the open closes and the closed opens. At this point, contactor K1 is switched off and contactor K2 is switched on, the motor reversing.

## Mounting the Electric Control Unit

Now that we have explained the command part, let’s do the assembly. To connect our remote, it is necessary to disconnect two cables from the input of the motor circuit breaker and connect to the input of the protective circuit breaker of the remote.

From the circuit breaker output, we connect a cable that will be derived between the A2 of the timer, the A2 of the contactor K1 and A2 of the contactor K2. From the other pole of the circuit breaker, we connect to the open contact of switch S1. From the output of switch S1, we connect to the A1 of the timer and then we proceed to contact 15 of our timer, which is the common timer contact.

Timer contact 16 connects with contact 21 of K2, which is the input of the normally closed contact that we use for the magnetic interlock. From closed contact 22 of K2, we connect a cable that goes to contact A1 of K1. We now have K1 feeding done.

For the supply of K2, we will supply output 18 of the timer which is the open contact, going to contact 21 which is the closed contact of contactor K1. Then, from output 22 of the closed contact of K1, we connect to contact A1 of contactor K2.

Our connection is ready and now just set the timer before energizing. In this timer model that we are using, we can set the time in seconds, minutes or hours. In addition, we can define whether the contact changes will be delayed on energization, pulse on energization, impulse prolongation or cyclical time. There is also an option with one or two sets of reversible contacts.

These 4 energizing functions of the relay mean that it can be used in several different types of electrical machines. This cyclical use, for example, is widely used in bread making machines, which need to beat the dough by turning the motor in both directions from time to time. It can also be used in timed lighting systems, generator starts, irrigation systems, timer motor drives, and others.