Transformer problem .

Join Date
Feb 2007
Location
Oklahoma
Posts
275
Hi to all,
Last week we had a frequency drive that runs a conveyor belt keep tripping out with a loss phase fault. The frequency drive is 3 phase for a 480 v motor with a gear box. The problem would occur when the line ran for a while then would trip the frequency drive with a phase loss error. The frequency drive, motor and wires were changed by the other crew in which it did not solve the problem. I changed the transformer used to drop the 480v to 120v for the control circuit. The 3 phase comes off the bus bars in the MCC panel and goes to the main breaker then from the bottom of the main breaker; it goes to the top of the frequency drive in which 2 of the leads goes to the transformer for the control circuit. Because the problem would occur after the belt was running for a while made me think there was a problem with the transformer shorting across the windings when it got heated up causing the frequency drive to think it had lost a phase in which it would shut down with the phase loss error. My question is ,if the way I am thinking about this it correct about the transformer causing the phase loss or was there something else going on with the transformer. All I know is that the problem went away when I had put in the new transformer. Any feedback would be greatly appreciated. Thank you very much for you help.
Sincerely
Maintenance Man
 
Not much data here to base an opinion. I would expect, however, that an intermittent short in the transformer would result in either a fuse or breaker release or bits and pieces of the transformer splattered about the inside of the cabinet.

Since you didn't mention either of those, I really don't have an opinion.
 
Thank you all for answering to my post. That would be true that the fuses would have blown (transformer is fused on both of the input legs and fused on the output 120 v leg) if the windings had shorted together; but what if some damage to the insulating coating was causing a short to a single winding, causing the resistance to be decreased in itself, in which the path of least resistance was to that shorted winding making the VFD trip out with a phase loss error when the transformer warmed up. I just like having good ideas or theories on problems when they occur. Thank you.o_O(y)đź“š
 
From what I read, it looks like you only changed out the control transformer (the 480V - 120V). This was probably fused at 1 amp or less on the 120V side. Even if there was some problems and the fuses didn't operate (which is unlikely), I don't see how a problem on the control transformer would affect the current in the main transformer.

What is on the control transformer? Is is just the pushbuttons and start signal to the VFD?

Is the 480 V feed an ungrounded system?
 
A short in a transformer winding creates another tap. This additional tap is shorted. It starts to burn up, causing the rest of the transformer to burn up. I doubt that was happening.
The phase detector is a simple circuit, looking at the peaks of each sine wave. Large noise spikes can fool it. If the transformer had an intermittent, the back EMF from the primary would cause a large spike on the line. Because the transformer is close by, the RF component still has plenty of energy by the time it reaches the VFD, and can easily bypass the internal spike filters. The phase loss detector can be fooled with extra pulses, or be clamped if the spike is opposite polarity.
 
is there a choke between the VFD and the line or the phase detector read the VFD noise ?

I don't uderstand how a small control transformer would give a phase failure fault ?
as stated, a short transformer will smoke or blow the fuse quickly...

sometimes the power factor of each line is much different and when a single phase good inductive load goes on or off it make voltage move enought to unbalance phases...A capacitor bank would help but VFD noise should be filtered first.
 
I hate to bring up an old sore spot but, is this possibly a floating delta or high-resistance grounded wye power supply?

If so, any number of phase to ground events could have caused false faulting. You won't be able to determine with any certainty just what is going on until you spring for a drive isolation transformer and connect the wye secondary center point solidly to ground.

Then you will have a system where faults mean something and the drive is not so vulnerable to damage.
 
What is on the control transformer? Is is just the pushbuttons and start signal to the VFD? Is the 480 V feed an ungrounded system?[/quote]

Yes it is just the pushbuttons and start signal to the VFD that is on the control transformer. I am beginning to think that changing the transformer was not the reason to having the problem go away. The bottom line is that the problem went away and everyone is happy that the belt is functioning properly. We do not have a phase detector unless it is on the VFD. Well thank you all for your help.

 

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