Transformer questions

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nickelec

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Need some help understanding this

We have a step down 480-120 3kva transformer for some controls, x1-x2 I get 0 ohms on Megger but I still get 120 when energized. Also get 0hms from x1- ground. To me transformer is no good but if there was a bad coil is it still possible to get 120vm?

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Is the secondary completely disconnected and you're reading only on the transformer terminals?
 
Secondary was disconnected and reading only on transformer leads x1 x2 I get 120

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I'm getting 0m ohms across x1-x2 and h1-h4 with h2-h3 spliced together. I'm also getting continuity from x1- to ground that blowing supply fuses

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If you are meggering the transformer windings I'd expect you to get 0hms as the wire is continuous through the winding.

What kind of transformer is this? Autotransformer or isolated?

I gather the 120V side is supposed to be ungrounded? (no x0) Then the continuity between x1 and ground is a ground fault. If it's an autotransformer then that's probably what's blowing the fuses. If it's an isolated transformer I don't know what's blowing the fuses. Yes, it is still possible to get 120V on the secondary even if the winding is faulted to ground. It's just like a grounded system, just unintentional.
 
This is the transformer I'm working with
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Also to mention I was only getting a short to ground when the x2 was hooked up to neutral

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Ok, that's an isolation transformer.

Again, the fact that you have 120V on the secondary is totally possible with one output conductor grounded, whether intentionally or not. That's how things are normally wired, though maybe your secondary is not supposed to be grounded in this case.

What are the x1 and x2 voltages to ground?
 
nickelec said:
Also to mention I was only getting a short to ground when the x2 was hooked up to neutral

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Presumably your 'neutral' is grounded somewhere, regardless of what system it belongs to. (I don't know why you'd hook up x2 to a grounded conductor of another system, but anyway...)

So if x2 was connected to a grounded conductor, and x1 is faulted to ground inside the transformer case, then of course you got a short, since both sides of the circuit were connected to ground.
 
nickelec said:
Need some help understanding this

We have a step down 480-120 3kva transformer for some controls, x1-x2 I get 0 ohms on Megger but I still get 120 when energized. Also get 0hms from x1- ground. To me transformer is no good but if there was a bad coil is it still possible to get 120vm?

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Click to expand...
These are the correct readings. The megger is reading through the coil and it should see a dead short. If x2 is connected to ground you should see a dead short from X1 to ground, again reading through the coil.

What makes you think there's something wrong with the transformer?
 
This is what Im dealing with.i disconnected the ground and all is clear. Should this transform be grounded?

Long story short the just transformer feeds some receptacles and lighting on a movable platform.

Once the platform is pulled away from its docking station the power that supplies the platform is being used once it goes back into its doocking station it hooks up to 120 volt 24-hour power that switches a contactor from platform power hard to explain

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That diagram appears to show that X2 is supposed to be grounded outside the transformer. It's a bit vauge, as it doesn't label the terminals or denote the transformer case. But it confirms the secondary is supposed to be a grounded system.

To clarify...The problem is blowing fuses?
Which fuses were blowing? One or both to the right of the transformer in the drawing?
 
I'll try to explain as best I can..

We're on a movable platform that usually sits in a dock connected to a 120v power supply for constant power for lights receptacles. This platform is fed via 480v rail system. Once the platform moves there is a contactor that changes position to feed these receptacles and outlets via this transformer
66dcbb33680e6af315313614a85a7677.jpg


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