High amps on transformer primary

[dave121]

We have a 1 ph. Dry type trans. 100 kva. Using it to step up from the power co.supplied 240v to 480 v. ( 240v to x taps, 480 out of h taps), Feed a VFD which runs a submersible 75hp. pump
480v 3 ph. load. The secondary amps are ok. The primary amps are too high. If we
Run the motor fast enough to pull say 100 amps. on the secondary of the trans. The primary
Amps are 300a. We install a lot of these, wire them the same and the amps are always, primary double the secondary. We have replaced the trans. and moved it further from the po. co's pad
mount trans. Neither change has helped. Other than wiring, any suggestions?

 

[ptonsparky]

Primary side straight 240, no XO connection?

How is the secondary, 480 v grounded? Mid point?

Just curious, cause if you have done a lot of them the same way and no problems, nothing should have changed.

 

[ptonsparky]

How would moving the your transformer further away from the POCO pad change anything?

I know, this is a lot of help:roll:

 

[AdrianWint]

We have a 1 ph. Dry type trans. 100 kva. Using it to step up from the power co.supplied 240v to 480 v. ( 240v to x taps, 480 out of h taps), Feed a VFD which runs a submersible 75hp. pump
480v 3 ph. load. The secondary amps are ok. The primary amps are too high. If we
Run the motor fast enough to pull say 100 amps. on the secondary of the trans. The primary
Amps are 300a. We install a lot of these, wire them the same and the amps are always, primary double the secondary. We have replaced the trans. and moved it further from the po. co's pad
mount trans. Neither change has helped. Other than wiring, any suggestions?

I think maybe its time to brush up on your theory.......

If you double the voltage across the transformer, then the primary current must be twice the secondary current. The thing that is constant is the power MUST be the same on both sides (ignoring transformer losses), so if you have 480V @ 100A on one side, you will have 240V @ 200A on the other (480V x 100A = 240V x 200A ie. the VA is the same).

Also, do you really mean that the transformer has a single phase input but has a three phase output? I think you have a typo here, a transformer alone cannot create the missing two phases......

 

[iwire]

Also, do you really mean that the transformer has a single phase input but has a three phase output? I think you have a typo here, a transformer alone cannot create the missing two phases......

I would bet it is a standard single phase transformer supplying a three phase VFD that is producing a three phase output.

 

[mivey]

We have replaced the trans. and moved it further from the po. co's padmount trans.

As Iwire showed, you have a math problem not an install problem.

I think maybe its time to brush up on your theory...

If you double the voltage across the transformer, then the primary current must be twice the secondary current.

The thing that is constant is the power MUST be the same on both sides

You almost had it

I would bet it is a standard single phase transformer supplying a three phase VFD that is producing a three phase output.

Bingo: Conservation of power rule.

100 A * 480 V * sqrt(3) = 83138 VA

83138 kVA / 240 V = 346 A

Replaced transformer and moved it? Oops.

 

[Besoeker]

Bingo: Conservation of power rule.

100 A * 480 V * sqrt(3) = 83138 VA

83138 kVA / 240 V = 346 A

The transformer is single phase and the 100A, according to the OP, was measured on the transformer secondary thus would also be single phase so sqrt(3) isn't applicable.
If, on the other hand, it was measured at the VFD output............but that's not what was stated.

 

[mivey]

The transformer is single phase and the 100A, according to the OP, was measured on the transformer secondary thus would also be single phase so sqrt(3) isn't applicable.
If, on the other hand, it was measured at the VFD output............but that's not what was stated.

I see he did say 100 amps at the transformer secondary. But using Occam's Razor would say he meant 100 motor amps since he had a 75 HP motor.

If he gets 300 amps on the primary he is evidently running the motor at full load. What do you think the chances are that he has 24 kW of loss that is not appearing as smoke?

 

[templdl]

I see he did say 100 amps at the transformer secondary. But using Occam's Razor would say he meant 100 motor amps since he had a 75 HP motor.

If he gets 300 amps on the primary he is evidently running the motor at full load. What do you think the chances are that he has 24 kW of loss that is not appearing as smoke?

That is what I was thinking. That's a lot of power that just doesn't disappear. If the OP were to just consider the heating watts where is the smoke?

 

[junkhound]

Feed a VFD which runs a submersible

Power factor; possible that the power factor correction circuit in the VFD is fried but that the inverter bridge sections of the VFD are still functioning. Thus, the input is going into a full wave bridge rectifier directly into the DC link capacitor without the benefit of the power factor boost conversion circuit. Single phase full wave bridge circuits have a very poor PF under high load. OP did not say what type meter is being used for the current measurements, so the 100A may be something other than a sinusoidal 100 A rms.

Has the OP tried replacing the VFD?

 

[Jraef]

The transformer is single phase and the 100A, according to the OP, was measured on the transformer secondary thus would also be single phase so sqrt(3) isn't applicable.
If, on the other hand, it was measured at the VFD output............but that's not what was stated.

Dave121,
There is a mistake somewhere in this equation, and my bet is that it is in your statement about secondary amps. Most likely you looked at the VFD display and saw that it was drawing 100A, then jumped to the conclusion that the secondary amps at the transformer were the same. Evidence to this fact is that you have already has demonstrated that someone in this sceario was unfamiliar with the proper way to size a step-up transformer for a VFD being fed with a single phase source.

Here is what you SHOULD have as a minimum. A VFD will present a .95 power factor to the line, but the single phase load will increase the current requirement by the sq. rt. of 3. So at a full 75HP load, a 75kVA transformer was already on the ragged edge of too small; 75HP /.746 = 100kW, /.95 = 106kVA. But to supply a 3phase 75HP motor from a 1phase source, you now have to factor in the difference between 3phase and 1phase power. 106 x 1.732 = 183kVA, so you needed at LEAST that much of a transformer, and since they only come in certain sizes, you should have had a 225kVA transformer for this application.

Now the next question is, did you use a 75HP VFD? If so, expect an early demise! The VFD needs to be at double the current rating of the motor 3phase amps, so you need a drive capable of no less than 212A, which is most likely going to be a 150HP drive. I ask because if you had started from that point, the 100kVA transformer problem would have been more immediately obvious, so I suspect that mistake has been made as well.

The other possibility here is that you really have a 40HP motor being run by a 75HP drive with a single phase input. If that's the case, all is right with the component sizing, and you have a bad transformer.

Good luck with this, it sounds like a commercial mess...

 

[mivey]

Jraef,HP * 0.746 kW not divided by.

 

[Besoeker]

I see he did say 100 amps at the transformer secondary. But using Occam's Razor would say he meant 100 motor amps since he had a 75 HP motor.

He also did say "we wire them the same and the amps are always, primary double the secondary" which would infer measurements of the transformer primary and secondary currents.

 

[Besoeker]

Jraef,HP * 0.746 kW not divided by.

Yes. It's 56kW.

 

[mivey]

Ok. Time's up.

Evidence to this fact is that you have already has demonstrated that someone in this sceario was unfamiliar with the proper way to size a step-up transformer for a VFD being fed with a single phase source.

Speaking of which...

Here is what you SHOULD have as a minimum. A VFD will present a .95 power factor to the line, but the single phase load will increase the current requirement by the sq. rt. of 3. So at a full 75HP load, a 75kVA transformer was already on the ragged edge of too small; 75HP /.746 = 100kW, /.95 = 106kVA.

kVA = HP * kW/HP / efficiency / power factor. I'll use your 95% for power factor * efficiency:

So: kVA = 75 HP * 0.746 kW/HP / 95% = 58.9 kVA

But to supply a 3phase 75HP motor from a 1phase source, you now have to factor in the difference between 3phase and 1phase power. 106 x 1.732 = 183kVA

Not sure what that is supposed to mean. Conservation of power means power in = power out. You could account for conversion losses but I think you got mixed up with amp conversion or something. Asleep at the wheel?

 

[mivey]

He also did say "we wire them the same and the amps are always, primary double the secondary" which would infer measurements of the transformer primary and secondary currents.

It is a mystery but I am just trying to find a plausible explanation for the 300 amps. Any other ideas?

 

[mivey]

Yes. It's 56kW.

Yes. So we have 58.89 kVA at the three-phase end which gives us the 102 kVA {no, the amps change, not the power: see added note} at the single-phase end. The OP's 100 kVA unit was probably OK.

Add: Now I went and multiplied by sqrt(3). 58.89 kVA at 3-phase 480V gives 70 amps. 58.89 kVA at 1-phase 240 gives 245 amps. Lunch has made my brain turn off.

 

[Besoeker]

Yes. So we have 58.89 kVA at the three-phase end which gives us the 102 kVA {no, the amps change, not the power: see added note} at the single-phase end. The OP's 100 kVA unit was probably OK.

Add: Now I went and multiplied by sqrt(3). 58.89 kVA at 3-phase 480V gives 70 amps. 58.89 kVA at 1-phase 240 gives 245 amps. Lunch has made my brain turn off.

The input supply to the VFD is single phase rectified. If there is no dc link choke, the current will be very peaky resulting in a RMS value and high harmonic content
So simple linear circuit calculations are not really applicable in this case.

Maybe some parallel resonance on the supply side exacerbated by the high level of harmonics that is present on this site but has not been on others owing to differing supply characteristics.

 

[augie47]

I'm unsure how much it will effect the numbers, but historically I can testify that submersible pumps tend to pull more current than the equivalent conventional frame motors. For example, I have a data sheet on a 40HP GRUNDFOS pump that shows a 40 HP pump with a 64 FLA compared to 52 amps from 430.250
I am led to understand that the windings are much different do to the reduced diameter to fit the well.

 

[mivey]

The input supply to the VFD is single phase rectified. If there is no dc link choke, the current will be very peaky resulting in a RMS value and high harmonic content
So simple linear circuit calculations are not really applicable in this case.

Maybe some parallel resonance on the supply side exacerbated by the high level of harmonics that is present on this site but has not been on others owing to differing supply characteristics.

How does that give you 300 amps on the primary and 100 amps on the secondary of a 1:2 transformer? Is the flux not linking or something?

Or are you saying the measurements themselves are suspect? Why would he not have the same reading error on the secondary as he does on the primary?