I thought it would be useful to have a look at another application of an interlocking. This particular application is one that you would most likely have come across without paying too much attention to it. So, what is it?
Enter the Star/Delta Starter. The circuit is not that complicated at all. In fact, I have seen more errors on the motor connection than on the control circuit. We all know the motors have all the windings marked as: U, V, W for the one side and U1, V1 and W1 for the other side of the winding. Then there are the manufacturers that put a bit more thought into it and mark them as U1, V1, W1, for the start of the windings and U2, V2 and W2, for the end of the windings. Either way it does not matter how they are marked. What is important to us is that we connect them the right way.
Here is a simple method I used when doing my apprenticeship. I used A1, B1 and C1 for the start of the windings and A2, B2 and C2 for the end. Now when having to connect the motor in delta, it is simple to remember, ABC CAB. That means that when the motor is connected in delta, we will have the same phase connected to A1 and C2,  the next phase to B1 and A2 and the third phase to C1 and B2. Let’s have a quick look at the connections on a sketch
You will note that I drew a dotted line to show the connections between the windings. If the motor is to be controlled via a star/delta starter, that line would not be there. You will have 6 leads from your panel to the motor. The 3 leads off the main contactor go to where we have L1, 2 and 3 on the sketch and the 3 leads from your star and delta contactors go to the C2, A2 and B2 connections. The key thing here is to remember that if you had L1 or the red phase connected to A1, make sure that the lead onto C2 is also on the same phase and the same applies for the others!
Below is the diagram for the circuit as I wire it. You will note once again, I have only shown electrical interlocking between the Star and Delta Contactors. You may want to include mechanical interlocking as well and that is fine. It does not alter the control circuit in any way
To be able to see the drawings, please click on it to open
We will do this a bit different. Follow the circuit and see if you understand it. If you have any questions, please send them to me on: frans@sparkyhelp.com and I will gladly answer them
Till next time, remember to put safety first and production second!