Checking DC Resistance of a Transformer

This week I was at a plant when a machine blew one of its main 100A fuses. I was helping troubleshoot this problem. I started by ohming the lugs on the main disconnect to ground and measured 0.3 ohms. We turned off all circuit breakers to start isolating the source of the short to ground and narrowed it down to a 60Kva 3 phase transformer. The transformer had a 480V delta primary and a 400V Wye secondary. The neutral on the secondary was grounded. The primary connections ohmed 0.3 to the secondary's neutral.

After disconnecting the transformer it still showed the 0.3 ohms between any of the connections on primary and secondary. Because of this I concluded the transformer was bad and had a break down in insulation between a primary and secondary winding.

We went to the stock room and found another transformer that was the same, but just larger. I made the same resistance measurements between primary and secondary and it also showed 0.3 ohms. Because of this I backed off my conclusion of the bad transformer and had them wire up only the primary side so we could power it up and check the voltage on the secondary.

Well, it turned out the main fuse blew again and the transformer started to catch fire in the first winding. The maintenance men then took a transformer from the stock room and installed it. That resolved the problem and the machine was back running.

So my question is, how could I measure any kind of resistance between the primary and secondary of a 3 phase transformer? Would that mean the primary 480VAC has a path to ground since the secondary neutral is grounded?

I followed the connections on the transformer to see if it was internally wired as an autotranformer, but could not see any such connections. Also the electrical schematic showed it as a typical transformer with the primary and secondary isolated.

This also brought to my mind of how to properly check such as transformer when both the bad one and the one in the stock room checked the same when using a standard meter to measure resistance?

To add to the post, attached is a page from a schematic showing one of these transformers.

This is not the exact same machine, but a machine built by the same manufacturer that used the same brand and style transformer except a slightly different capacity. As you can see, it does show the transformer as an isolated primary and secondary.

That is definitely weird. Are you sure the primary side didn't have a connection of its own somewhere to ground or neutral?

Archie said:

To add to the post, attached is a page from a schematic showing one of these transformers.

This is not the exact same machine, but a machine built by the same manufacturer that used the same brand and style transformer except a slightly different capacity. As you can see, it does show the transformer as an isolated primary and secondary.

Click to expand...

Archie said:

So my question is, how could I measure any kind of resistance between the primary and secondary of a 3 phase transformer?

Click to expand...

You're just talented!

Skidood said:

That is definitely weird. Are you sure the primary side didn't have a connection of its own somewhere to ground or neutral?

Click to expand...

When the transformer was pulled out of the machine, I closely followed the wires to the point they entered the coils and saw nothing where the primary connected to anything other than the coils.

The design was almost exactly like this one, so it let me see very well the connections until they entered the coils:

https://www.frei.de/en/products/pow...se-transformers/products/series-dtb/dtbl.html

I am so baffled by this that I am now checking to see if the maintenance guy that pulled the new transformer from the stock had taken the exact same one I measured. I am thinking of the possibility that the new one I measured was also defective and they by chance grabbed a different new one other than that one I measured.

With a DC Ohmmeter, you would be hard pressed to detect the difference between a normal coiled wire that constitutes a healthy transformer winding and one that has a turn to turn short internally. That is when ohming between terminals on the same coil.

I made the same resistance measurements between primary and secondary and it also showed 0.3 ohms.

Click to expand...

This is not normal unless this is some special type of transformer different than your link and schematic (like an autotransformer). I'd be inclined to take that ohm measurement on the working new transformer if given the opportunity to shut down and disconnect it...

It's an AutoTransformer which is a single coil with different taps for secondary, therefore resistance will be close to zero. You should measure between lead and its insulation for short.

BachPhi said:

It's an AutoTransformer which is a single coil with different taps for secondary, therefore resistance will be close to zero. You should measure between lead and its insulation for short.

Click to expand...

This was my first suspicion so I tried following the wires until they went into the winding which is where I was unable to see any more connections. I started leaning away from this because of it being a delta primary and a wye secondary, which I was able to trace the wires to confirm.

According to your diagram, there's a ground on the primary (delta) side. If the ground on the secondary (wye) side were connected to it, I would expect to see a low resistance between the primary or secondary to ground.

Looking at the picture in your link, could the two grounds be connected in the terminal strip on top of the transformer?

PapaSmiff said:

According to your diagram, there's a ground on the primary (delta) side. If the ground on the secondary (wye) side were connected to it, I would expect to see a low resistance between the primary or secondary to ground.

Looking at the picture in your link, could the two grounds be connected in the terminal strip on top of the transformer?

Click to expand...

There is no ground on the primary side. The transformer had 7 terminals, 3 for primary and 4 for secondary. The main supply at this plant is an ungrounded 480V delta system.

I wish I would've taken pictures of the transformer. Maybe when I go back I will see if they kept the bad transformer so I can take pictures and check it again.

The label on the transformer had this information:

ELMA Wr. Neustadt
Typ: IZ 1626
Pri: 480V/74,4A
Sek: 400V/86,6A

I tried to find some information on that part number in hopes that it would tell me something I am missing, but I have only been able to find things that are close but not exactly it.

Well, for what it's worth, I would have troubleshot it the same way you did.

I thought I knew transformers but I'm puzzled by this one.

Wait a minute. Your link indicates it is a double star (wye-wye) hookup. If there's only one connection to ground, could the primary side ground be internally connected to the secondary side ground?

PapaSmiff said:

Wait a minute. Your link indicates it is a double star (wye-wye) hookup. If there's only one connection to ground, could the primary side ground be internally connected to the secondary side ground?

Click to expand...

The link I posted wasn't an exact match, I posted it for a reference to show the physical appearance. I was unable to find anything based on the tag information. The physical appearance was practically exact except for the blue color, even down to the lifting eye hooks. But I am not 100% sure about the internal wiring.

A wye primary with an internal ground could explain a low measured resistance between the primary and secondary windings. I was also told this wasn't the first transformer to go bad on this machine. So now I am wondering whether it is a wye primary with an internal ground which would not be the proper transformer for this plant's delta electrical system.

look closely in the link.
two options listed, 1) star/grounded star
2) auto transformer.
i bet you have the second option.
to test the winding you better use a variac ,volmeter & ampermeter.

energize each phase with the variac raising the voltage slowly,
while measuring the current.

a healthy winding would let it rise up to nominal voltage with only small magnetizing current.
a shorted winding will consume a much larger current even at low voltage.

use a single phase variac and test one winding at a time
1-2,2-3,3-1,
can be done on primary side or secondary side, it's the same
only different voltage.

also megger the winding to the metal core.

an ohmmeter is useless with this size of transformer.
to test the dc resistance of a winding this size a spacial
micro-ohmmeter is used with over voltage protections because of the high kickback voltage produced when disconnecting
the test source from the winding.

Archie said:

I started by ohming the lugs on the main disconnect to ground

Click to expand...

Archie said:

The main supply at this plant is an ungrounded 480V delta system.

Click to expand...

Archie said:

The primary connections ohmed 0.3 to the secondary's neutral.

I made the same resistance measurements between primary and secondary and it also showed 0.3 ohms.

Click to expand...

Archie said:

attached is a page from a schematic showing one of these transformers.

Click to expand...

I only have a theoretical knowledge of ungrounded systems.

2 possibilities:
The wrong transformer was used, ie one that grounds both the primary and secondary star point. This wouldn't be a problem as it would be the only ground in the system. Until someone accidentally shorts a 480V line to ground, and then your little transformer becomes the return path.

OR because your ground is floating, the voltage between 480 and ground could be anything. Your European wound transformer probably has an insulation rating of 1000V, maybe even 690V. So every time it floats away from the average line voltage, the insulation breaks down and current flows through your transformer insulation to bring the ground voltage back down. Eventually this causes a short circuit between the two wye points.

Importantly, you don't get a fault with a single ground connection.

Ask them if anything changed elsewhere in the factory. Might have been a VFD replaced and they forgot to take out the filter screws.