Volt meter reading 760volts in 3phase dol starter overloads

[Sparky42]

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?

 

[Pete999]

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?

 

[Sparky42]

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

 

[Pete999]

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

Can you post a diagram of the starter

 

[Bobster]

Where exactly are you measuring these voltages? An what is the reference point.

 

[Pete999]

Where exactly are you measuring these voltages? An what is the reference point.

Excellent question, should have thought of that myself Doooo

 

[rapparee]

He says he's testing between

1) a1 and l1

2) earth and a1 on a contactor.



I wonder which phase is feeding a1.

Is a1 energised when he is testing?

 

[Andy-1960]

Is the conveyor being controlled via a VSD?

 

[James]

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.

 

[Sparky42]

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?

 

[rapparee]

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.

 

[Sparky42]

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

 

[rapparee]

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.

 

[Sparky42]

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.

 

[rapparee]

Sorry.

I was editing my post when you were answering the first version