Fault Finding

Came across a different motor the other day and thought this may be useful. Even though I decided to throw this in under Fault Finding, it is more a motor connection topic. Reason for me putting it here is that I happened to walk in when the motor in question was being tested.

On starting the motor, it made a very load “growling” noise which drew my attention straight away. When asking what was happening, I got told the motor is drawing in excess of 80A. The full load current of the motor should be 9A. This prompted me to do a quick test on the motor as the sound I heard told me we had one of the following issues:

  • Reverse Winding
  • Open circuit on one winding

Reading the manufacturers plate told me we had a different issue as the motor windings are made up of 6 different windings. These motors were designed for different voltage applications. We were applying 415V to it and it became clear to me that there must be a wiring issue. The windings were labelled as: U1, U2, U3,U4; V1, V2, V3, V4; W1, W2, W3, W4

Opening the terminal box, I found 6 terminals which means that certain windings were bridged. Let’s have a look at the description on the nameplate and then we will look at the terminal box and how it was to be connected.


Looking at this sketch, we will note the bridge between ends 2 and 3 of each set. This means that effectively, we have created one winding per set which is made up of two windings in series. So that leaves us with the following configuration:

Winding one:               U1 to U4

Winding two:               V1 to V4

Winding three:            W1 to W4

That means for us to connect this motor in Delta as per the manufacturer’s instructions for a 415V supply, we have to connect the following together:

U1 to W4

V1 to U4

W1 to V4

Thing to remember here is not to let the numbers throw you. Stick to the basic delta configuration and you will see it is quite easy. On a normal motor, the number 4’s would all be 2. That is the only difference once we have bridged the 2’s and 3’s

Great, we have figured out how to connect the motor and then we look at the terminal box to find the following: 12 ends and only 6 connector studs! Scary? Not really. Go back to your connection diagram and proceed as follows:

Connect U2 and U3 together on one stud, then V2 and V3 on the next stud and lastly, W2 and W3 on the third stud. We now have 6 wires left to go onto 3 studs. Look at the delta connection and follow it to give you the following:

U1 and W4 together on one stud and connect one phase from your contactor to this same stud

V1 and U4 together on the next stud with the next phase from your contactor to it

W1 and V4 on the last stud with the third phase from the contactor. Job done! Your motor is now connected in Delta and ready to be started

By the way, we did that and tested the motor. Running at 8.3A and quiet as it should be.

As usual, I hope you find this useful and remember to always work safe, test before you touch and do your installation test before you turn switches on!

Have a great week and for those who have not done it yet, check out the Manual for Electricians Volume 1 on: www.lulu.com/spotlight/SparkyHelp

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