Discuss Volt meter reading 760volts in 3phase dol starter overloads in the Industrial Electrician Talk area at ElectriciansForums.net

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Hi

I have been called out to a fault on a grain dryer where the motor interlocks are not disconnecting. Previous conveyors. Upon investigation I have found one starter that does not disconnect the preceding motor but when tripped I am getting 550v to earth from a1 and wait for it 760volts to phase from a1! This has baffled me as there's only 410v incoming.

Thoughts?
 
Hi

I have been called out to a fault on a grain dryer where the motor interlocks are not disconnecting. Previous conveyors. Upon investigation I have found one starter that does not disconnect the preceding motor but when tripped I am getting 550v to earth from a1 and wait for it 760volts to phase from a1! This has baffled me as there's only 410v incoming.

Thoughts?
What instrument are you measuring these voltages with?
 
My fluke volt meter, merger multifunction tester and my fluke clamp meter on voltage function. I thought at first it could be my volt meter but nope it's the same on all meters
 
Where exactly are you measuring these voltages? An what is the reference point.
Excellent question, should have thought of that myself Doooo
 
without a diagram of the circuit arrangement it is difficult to advise.

however one thing for consideration is some types of conveyors, especially long ones with plastic belts can generate some remarkable voltages due to static.

I would normally be testing the volt drop across each contact of the contactor to check for damaged contacts.
zero volts is the magic number and I would be worried about the state of the contacts if measuring anything over 2V

the voltages across the windings
a1 - b1
b1 - c1
c1 - a1
should all be in the region of 400v
assuming a star connection as it is DOL
if you can measure the voltage between the star connection and each of the phases, then each should be in the region of 240V and they should be close to equal.
any major difference is indicating a fault with a winding.
 
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Thanks everyone for your replies.

This is what I have found so far.

The the wire in question which feeds the coil on the contactor is fed from l2 on the outgoing side of another contactor as an interlock. So if the first motor fails then this contacted should de-energise what I have found is that when the first contacted is stopped by either the stop button or overload I am reading 110v from each outgoing phase to incoming phases there is no leakage through the contactor when tested dead??

Any thoughts?
 
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.
 
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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?
When the contactor is off common to load reads 110v across all phases

So l1 load to l1 common
L1 load to l2 common
L3 load to l3 common

L2load to l1 common and so on all read

110volts until I turn the main isolator off.

Does this mean I could have a broken/failing neutral on the incoming side??

I have tested insulation resistance of neutral to earth on whole building and reads 8meg ohms but incoming reads 0.00ohms.

They have recently had a new meter fitted and new transformer or am I clutching at straws seeing I've been here for two days now

Thanks
 
When the contactor is off common to load reads 110v across all phases

So l1 load to l1 common
L1 load to l2 common
L3 load to l3 common

L2load to l1 common and so on all read

110volts until I turn the main isolator off.

Does this mean I could have a broken/failing neutral on the incoming side??

I have tested insulation resistance of neutral to earth on whole building and reads 8meg ohms but incoming reads 0.00ohms.

They have recently had a new meter fitted and new transformer or am I clutching at straws seeing I've been here for two days now

Thanks

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.
 
I will add it's a TT System and the earth stake resistance is 16ohms

With a Ze of 0.25.

I can't fathom out that ive got no earth faults on the system itself but once I turn on any load I get these voltages.

Once contactor has been energised and then stopped the voltage to earth reads 550volts!!!! Unless the neutral is disconnected from the supply.
 
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.
It's not though it's across all phases.

I've even disconnected the outgoing wires that are connected to the motor and they still read 50volts to phase and they are not even connected anymore.
 
Just so you don't think I'm making this up.

This is l2 outgoing to earth from the contactor after the contactor has been de-energised
 

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I will add it's a TT System and the earth stake resistance is 16ohms

With a Ze of 0.25.

I can't fathom out that ive got no earth faults on the system itself but once I turn on any load I get these voltages.

Once contactor has been energised and then stopped the voltage to earth reads 550volts!!!! Unless the neutral is disconnected from the supply.
If the neutral is disconnected you would know about it.

The whole building would be "haunted" lights would be flickering off and on etc.

But you have to confirm the basics.

Have you got 3 phases, neutral and earth up to the machines isolator?

Have you got three phases going to the top of the contactor etc
 
yes I have. I've done a the normal tests.

all voltages are equal from all phases to earth, to neutral and to phase.

Only time I get these voltages is after a load has been applied and that's why I am thinking neutral failure.
 
It's not though it's across all phases.

I've even disconnected the outgoing wires that are connected to the motor and they still read 50volts to phase and they are not even connected anymore.


So a disconnected motor sitting on its own is producing 50v.

I assume you are testing to earth?
 

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