Now that we have had a look at some basic interlocking of an electrical circuit, let’s take a look at how you can put this to use. The circuit below is that of a three phase forward reverse. In this circuit we have gone for the very basic interlocking on the electrical only and there are some additional things we can do with this circuit. For now however, we will focus on the circuit as it is.
Before looking at the circuit, you will know that to change direction of a three phase motor you need to swap any two phases. To explain that better, you have the red phase connected to U1, the white phase to V1 and the blue phase to W1. You need to change the direction of the motor so what you will do is connect the red phase to W1, the white phase to V1 and the blue phase to U1
If we look at the power circuit below, you will note that is exactly what this circuit will do for you by simply changing from one contactor to the other.
In order for us to go from one direction to the other, we will need the control circuit set up to give us that flexibility. Look at the circuit below and notice that I have used a single stop, forward and reverse to control the motor. In the forward line we have a normally closed contact of the reverse contactor and in the reverse line, a normally closed contact of the forward contactor. That means if the motor is running forward, you will not be able to start it in reverse until you have stopped it first. Have a play with the circuit and if there is something that does not make sense, let me know and I will explain more
Please note that in some applications you may find the contactors being mechanically interlocked as well. That merely means that you do not just rely on the electrical interlock for the changeover. What I will do in a future segment is to include something else into this circuit sometimes known as “plugging control” and you would find this sort of application on overhead cranes as an example.
For now though, have a look and see if it all makes sense to you. Remember, always work safe, your life depends on it