Help with getting the right voltage output from a transformer.

synchro said:
Were those voltage readings taken with an averaging meter? If so, try measuring with a true RMS meter. If the phase-to-phase voltages get significantly closer to each other when using an RMS meter, that probably indicates that there is significant harmonic content (for example third harmonic). The problem is that the diode bridge rectifier in a VFD is responsive to peak voltages, not RMS or average voltages. However, if both RMS and average measurements show balanced voltages, then it's likely that there is low harmonic content and the peak voltages should also be balanced.

It should be possible to get the peak voltages more balanced in the presence of harmonics with, for example, buck boost transformers. But especially if we are dealing with an open delta service, harmonics could even make the peak voltage across the phases where 225V was measured even higher than where 250V was measured. So in this case you'd want a buck boost to reduce the voltage where 225 was measured, as counterintuitive as that sounds.
I believe this is why there was only current flowing on phases B and C on the feed to a VFD in this previous post, even though the measured B-C voltage was less than the A-C and A-B voltages:

Click to expand...
[QUOTE="synchro, post: 3044798, mem...-high-leg.2587440/page-4#post-3019099[/QUOTE]
It was not a true RMS. It was a Kline amp/combo meter. About $100 I think. I don’t know the model number. I do have a true RMS meter at my shop. If you think I should, I’ll test with it.
 
Thank you all. I appreciate it, I mean it.
My next steps are to see if the transformer i used on the first unit says wye primary or Delta primary. And to try to get a true RMS reading.
Is there anything else I should test?
If there is a different primary on the two transformers. I will call Larson. In any case do you think the delta to wye 208 unit would work, or do you see a problem with that?
Thank you very much
 
mike1061 said:
It was not a true RMS. It was a Kline amp/combo meter. About $100 I think. I don’t know the model number. I do have a true RMS meter at my shop. If you think I should, I’ll test with it.
Click to expand...

Yes, I think it would be a good idea to check it with a true RMS meter. It would be best if you also measured it again with that Kline meter again while your there, in order to get a good comparison between the two measurements. That's because there could be variability over time since your last measurement.
 
the nameplate shows delta primary, but the screen you pulled up shows Y primary (input). the wiring diagram also shows a H0 as well as Z0. any of you guys more familiar with transformers know if there is a particular reason you can't input on the secondary side and output the primary of this transformer, and simply tap the 208v feed, if that's what you're wanting?
 
mike1061 said:
Ive looked at that plate a dozen times. I didn’t see that. Wye on the input. That might be the problem, yea?
Click to expand...

I suspect that this may play a roll, but I admit I'm not sure. @jim dungar and I understand these transformers differently, and I need to review a bit.

My supposition is that a wye:wye transformer doesn't derive a low impedance neutral on the secondary, meaning that the L-N voltage is subject to large variation with changing L-N load. IMHO you _should_ have a delta:wye transformer, but I'm not willing to bet more than a beer on that :)

mike1061 said:
I did and experiment with the unit next to this one and it seems to have worked. This 2nd one won’t. They have replaced the VFD, the board and everything. So they say. Now they are kicking it back to me.
It is an open delta on the line. But the output is supposed to be balanced Y 240.
Click to expand...

Are you saying that you have two similar units, and used a transformer on the blower of the first unit and that solved the problem, but now the transformer isn't solving the problem on the blower for the second unit?

1) Did the two units have problems initially, but now one is working?

2) Did you use the _same_ transformer in both units?

3) Exactly how are you wiring the transformers that you install, particularly with respect to grounding.
 
mike1061 said:
My next steps are to see if the transformer i used on the first unit says wye primary or Delta primary. And to try to get a true RMS reading.
Is there anything else I should test?
If there is a different primary on the two transformers. I will call Larson. In any case do you think the delta to wye 208 unit would work, or do you see a problem with that?
Click to expand...

IMHO if a delta:wye 240:240/139V transformer works on one unit, then it should work on the other. Since it is in the voltage range, a delta:wye 240:208/120V transformer should also work.

-Jonathan
 
Master Nater said:
the nameplate shows delta primary, but the screen you pulled up shows Y primary (input). the wiring diagram also shows a H0 as well as Z0. any of you guys more familiar with transformers know if there is a particular reason you can't input on the secondary side and output the primary of this transformer, and simply tap the 208v feed, if that's what you're wanting?
Click to expand...
No, I was not asking if I could reverse feed it, if that’s what you meant. I looked into that for another job but this one is different.
 
winnie said:
I suspect that this may play a roll, but I admit I'm not sure. @jim dungar and I understand these transformers differently, and I need to review a bit.

My supposition is that a wye:wye transformer doesn't derive a low impedance neutral on the secondary, meaning that the L-N voltage is subject to large variation with changing L-N load. IMHO you _should_ have a delta:wye transformer, but I'm not willing to bet more than a beer on that :)



Are you saying that you have two similar units, and used a transformer on the blower of the first unit and that solved the problem, but now the transformer isn't solving the problem on the blower for the second unit?

1) Did the two units have problems initially, but now one is working?

2) Did you use the _same_ transformer in both units?

3) Exactly how are you wiring the transformers that you install, particularly with respect to grounding.
Click to expand...
Yes, 2 of the exact same model rtu’s. The transformers have the exact same path number but look different. One was pad mounted and this one is hung. Both units had the same problem, the VFD blower did not get up to speed. My solution worked for the first one but this one it is not.
 
mike1061 said:
...
The current readings are line side 125, 125, 215 phase to ground. 250, 250, 225, pause to phase. The Load side readings are, 140, 140, 140, phase to ground, 250, 250, 225, phase to ground.
Picture 1, is the transformer label.
Click to expand...

If it was a delta-wye transformer like the label says, the line to ground voltages on the output phases would not be the same as your measurements show they are. But the voltages measured are consistent with a wye-wye transformer having the H0 not connected.

I'm in agreement with @winnie that a wye-wye transformer with a floating H0 will not provide a low impedance to L-N loads or L-G faults (unless it has a tertiary delta winding).

I suggest that the L-L and L-G output voltages on the transformer with the working RTU be measured to see what you have there. Both an averaging meter and a true RMS meter should be used. That could indicate whether this transformer is a delta-wye. Also, measuring the line currents feeding the just the VFD and just the compressors would be helpful to see how balanced they are.
 
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