Volt meter reading 760volts in 3phase dol starter overloads

[Megawatt]

The

We're in the UK, our voltages are different, and we don't use delta supplies. Our normal service except for new, very large installations is 230/400V 4-wire Y. So anything over about 430V is abnormal.

Let's look at the possibilities.

1) The high voltage does not exist. The voltages being shown are not the actual voltage present, due to interference from a VFD or similar, causing the instruments to malfunction.

2) The voltage actually exists, and is coming from the supply. This doesn't make sense. If you lose the neutral on a 230/400V supply, 230V single-phase circuits can receive anywhere between 0 and 400V. 3-phase circuits will still have 400V between each phase and another, but the neutral could be anywhere from 0 to 400V from each of them. Nowhere does the voltage rise above 400V.

3) The voltage is being produced within the panel / system. I don't think it's been explicitly stated what the control voltage of these contactors is. Do some / all of the coils operate at 230 or 400V? If there is a control transformer in the panel, a fault on the secondary that puts it in contact with a line of the primary, could add their voltages. It would be possible to have say 510V from one 110V control wire to one line wire, if the other side of the control supply is in contact with another line. There are some very unlikely scenarios that we could think up, where isolated devices that can behave like transformers could add to make higher voltages still.

But all of this is just conjecture if we can't see the circuit!

the voltage in the states for a house is normally 120/240. When you lose your neutral the 120 will go up to 190 which will fry clocks, TVs or anything plugged into to the sockets which we call receDo y’all use ground rods to bond your service.

 

[Megawatt]

Bond

The

the voltage in the states for a house is normally 120/240. When you lose your neutral the 120 will go up to 190 which will fry clocks, TVs or anything plugged into to the sockets which we call receDo y’all use ground rods to bond your service.

Bonding the neutral is the most important thing you can do and we call sockets receptacles in the states

 

[davesparks]

Bond

Bonding the neutral is the most important thing you can do and we call sockets receptacles in the states

No we don't install earth rods to connect to neutral here, our distribution systems are reliably connected to earth at the substation.

 

[Bobster]

No we don't install earth rods to connect to neutral here, our distribution systems are reliably connected to earth at the substation.

Well, we do. Just not in public houses.

 

[davesparks]

Well, we do. Just not in public houses.

Good point, I should have been a bit clearer about that one

 

[Megawatt]

Im

Well, we do. Just not in public houses.

learning something new on how you do things in the UK. It’s like a different world with all the different voltages and standards.

 

[Pete999]

So when the contactor is off and the machine is isolated.

What is the continuity between

L1 Common and L1 Load.

L2 Common and L2 Load.

L3 Common and L3 Load.

Also when the coil is de-energising does it completely release, or is it mechanically stuck half way. Does a crack with a screwdriver properly release it?


The 110v is a waste of time. It sounds like you are testing at 2 points on the same phase. Unless the control circuit to pull in the contactor is 110v. I just can't tell from here.

You need to always test the lives with one prong on phase and the other on the earth bar.

Don't do much in the way of Safe isolation then in your place of work?

 

[rapparee]

Don't do much in the way of Safe isolation then in your place of work?

 

[marconi]

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.

 

[David Prosser]

I suspect Marconi may well of hit the nail on the head.

If it not that then I did have similar voltages to earth but across all three phases not between phases though (well higher closer to 620v) a few years back. In a factory with a guaranteed(ish) supply. It had two independent HV supplies that would switch automatically to the back up if the normal supply failed. They were supposedly interlocked on both HV and LV sides of the sub station.

At the time the fault presented itself, both HV circuit breakers were closed therefore both transformers were live. The LV side was still open on the second supply but because of the interaction between the earthing and neutral it gave strange readings between Live and earth. An analogue meter would jump up then slowly drop back to virtually nothing. It disappeared after the HV breaker was opened. Not by me I will add !