Volt meter reading 760volts in 3phase dol starter overloads

TJ Anderson · Sep 13, 2019

marconi said:
The scribbled wiring diagram is the best I can put together and get photo'd for attachment at this time of the evening. Take a look at it which is my take on what Sparky42 has described as the two contactor and control wiring. All contactor coils are 400V. Both motors are three phase 3 wire and star connected. The first motor to start M1 is controlled DOL by the top contactor C1. The second motor M2 is started DOL when C1 closes. The pilot lamp indicates when the first motor M1 is running.

I reckon you may have welded closed L1 contacts on contactor C1. This means you still have control of M1 but L1 out of C1 remains energised at L1 line voltage when the C1 opens. With L1 at 400V there is a current path through the lower pair of windings of M1 and then through the pilot lamp to N. The lower pair of M1's windings interact with the third vertical winding to create a step-up transformer. The voltage on the L2 out terminal of C1 is thereby raised to a higher potential than L1 - what potential with respect to N (550V)and also L1 (760V) in of C1 depends on the voltage drops across the lower two windings and the pilot lamp, all in series.

Thus there is an energising ac emf derived from L1 now applied to the control circuitry of C2 which causes C2's coil to remain magnetised when the original closing emf derived from L2 out of C2 disappears. It requires a smaller current to keep the contactor closed than it does to close if first of all - you can work out why for yourself. This reasoning explains why removing the N to the pilot lamp or control cabinet caused C2 to open after it has been first closed by C1. And why the pilot lamp lamp burns much brighter.

I have faintly shown on the left the way the three windings of M1 act as a step up auto-transformer.

Or something like this explanation..

So, I would check contactor C1 thoroughly and I reckon you will find L1's contacts are welded closed or there is significant tracking happening even when its contacts are open.