480 VFD feeding a 480\4160 stepup transformer

[GT6Steve]

I've just seen a novel installation that has me disturbed. I'd like to hear opinions on this. It was installed as an emergency short term repair and the owner now wants to make it permanent.

It is at a remote well site. 350 HP 4160 Volt motor. They are using a generator to feed a VFD which then feeds into the 480 volt side of a transformer with the pump motor being fed from the 4160 volt side. I don't recall any disconnects other than the one to isolate the drive from the generator. They run this at a steady 48 Hz. Owner now has the opportunity to be fed from utility power and has opted for 480 volt over a 4160 volt feeder.

I question using the transformer outside of it's design parameters at first glance and then question the entire concept upon reflection. Has anyone seen such a thing and care to comment? Thanks in advance

 

[iceworm]

... They are using a generator to feed a VFD which then feeds into the 480 volt side of a transformer with the pump motor being fed from the 4160 volt side. I don't recall any disconnects other than the one to isolate the drive from the generator. ...

I question using the transformer outside of it's design parameters at first glance and then question the entire concept upon reflection. Has anyone seen such a thing and care to comment? Thanks in advance

Q1: What design parameter is the transformer outside of? (That phrase would likely have gotten me beat by my high school English teacher.) I'm not seeing any.

 

[iceworm]

...They are using a generator to feed a VFD which then feeds into the 480 voltside of a transformer with the pump motor being fed from the 4160 volt side. I don't recall any disconnects other than the one to isolate the drive from the generator. ...

I question using the transformer outside of it's design parameters at first glance and then question the entire concept upon reflection. Has any one seen such a thing and care to comment? Thanks in advance

Around 1995 I was looking for a 3000hp, 4160V VFD. AB proposed a 4160/480V xfm, feeding paralleled 480V drives, feeding a 480V/4160V xfm, that would then feed our 3000hp motor. We did not buy it, so I didn't see it operational - but the proposal looked good. I don't see any issues with inrush, there isn't any.

By "disconnects", are you meaning transformer overcurrentprotection? If so:

The drive will have input OCP and a disconnect.
The drive output will limit current​

Not seeing any issue here.

What am I missing?

 

[Besoeker]

I've just seen a novel installation that has me disturbed. I'd like to hear opinions on this. It was installed as an emergency short term repair and the owner now wants to make it permanent.

It is at a remote well site. 350 HP 4160 Volt motor. They are using a generator to feed a VFD which then feeds into the 480 volt side of a transformer with the pump motor being fed from the 4160 volt side. I don't recall any disconnects other than the one to isolate the drive from the generator. They run this at a steady 48 Hz. Owner now has the opportunity to be fed from utility power and has opted for 480 volt over a 4160 volt feeder.

I question using the transformer outside of it's design parameters at first glance and then question the entire concept upon reflection. Has anyone seen such a thing and care to comment? Thanks in advance

Not good in my opinion if you are running a transformer rated for 480V 60 Hz at 48Hz.

 

[iceworm]

Not good in my opinion if you are running a transformer rated for 480V 60 Hz at 48Hz.

Volts to Hertz looks good. What is the issue?

 

[Besoeker]

Volts to Hertz looks good. What is the issue?

I see only 480V mentioned. Did I miss something?

 

[GT6Steve]

Q1: What design parameter is the transformer outside of? (That phrase would likely have gotten me beat by my high school English teacher.) I'm not seeing any.

I'm questioning using the transformer rated at 60 Hz when the frequencies drop and the reactances change in it. I suspect current is offset by the reduced vector voltages at that point but am not the engineer to calculate what goes on within the transformer. Granted a motor is just a rotary transformer so the curve may be entirely suitable?

It's an application I've never seen and am trying to understand the effects or various frequencies upon a standard transformer.

 

[Besoeker]

I'm questioning using the transformer rated at 60 Hz when the frequencies drop and the reactances change in it. I suspect current is offset by the reduced vector voltages at that point but am not the engineer to calculate what goes on within the transformer. Granted a motor is just a rotary transformer so the curve may be entirely suitable?

It's an application I've never seen and am trying to understand the effects or various frequencies upon a standard transformer.

At rated voltage and significantly lower than rated frequency there is a risk of the transformer saturating.

 

[iceworm]

I see only 480V mentioned. Did I miss something?

yes

... They are using a generator to feed a VFD ...

 

[Besoeker]

yes

What voltage is coming out of the VFD?

 

[iceworm]

I'm questioning using the transformer rated at 60 Hz when the frequencies drop and the reactances change in it. ....

As long as the volt to hertz ratio is constant the transformer will be fine - just like the motor. And VFDs are programmed to do that.

480V at 60 Hz ==> 8 volts/hertz

Although it more likely is:
460V at 60 HZ ==> 7.7 volts/hertz

So, at 48 hz, one would expect the VFD voltage to be 48 x 7.7 = 370V

Transformer will work just fine.

 

[Besoeker]

As long as the volt to hertz ratio is constant the transformer will be fine - just like the motor. And VFDs are programmed to do that.

480V at 60 Hz ==> 8 volts/hertz

Although it more likely is:
460V at 60 HZ ==> 7.7 volts/hertz

So, at 48 hz, one would expect the VFD voltage to be 48 x 7.7 = 370V

Transformer will work just fine.

I agree. But only 480V and 4169V were mentioned. Hence my concern.

 

[iceworm]

What voltage is coming out of the VFD?

Bes - You are well aware of this
I'm missing your point.

 

[Besoeker]

Bes - You are well aware of this
I'm missing your point.

Indeed I am well aware of what should be done. But none of that covered in the OP.

 

[iceworm]

I agree. But only 480V and 4169V were mentioned. Hence my concern.

Let me see if I got this right. You are concerned because Steve did not mention the voltage when the drive was running a 48 Hz? If so that is pretty funny.

Steve -
If Bes is honestly concerned - it is a misplaced concern. The VFD will bring the voltage down in ratio to the frequency

 

[iceworm]

Indeed I am well aware of what should be done. But none of that covered in the OP.

now you are just being mean

 

[Besoeker]

now you are just being mean

Shall we wait to see what the OP has to say rather than you engaging in personal insults?

 

[Adamjamma]

I'm questioning using the transformer rated at 60 Hz when the frequencies drop and the reactances change in it. I suspect current is offset by the reduced vector voltages at that point but am not the engineer to calculate what goes on within the transformer. Granted a motor is just a rotary transformer so the curve may be entirely suitable?

It's an application I've never seen and am trying to understand the effects or various frequencies upon a standard transformer.

But what if it is a 50 Hz system rather than. A60 Hz system? Is there a problem then? Items often designed for 60 Hz USA market get used on 50 Hz systems in Caribbean and other parts of the world without problems... in fact, most problems are minor, such as a pump turning slightly slower.. thus moving slightly less water... possibly two gallons a day less... but the water still gets moved... or a turntable runs slightly slower.. most other problems are not really noticed, unless the Hz is used for timing.

 

[Besoeker]

But what if it is a 50 Hz system rather than. A60 Hz system? Is there a problem then? Items often designed for 60 Hz USA market get used on 50 Hz systems in Caribbean and other parts of the world without problems... in fact, most problems are minor, such as a pump turning slightly slower.. thus moving slightly less water...

Quite a lot less if it's a centrifugal pump. But let's not wander off topic.

 

[GoldDigger]

As long as the volt to hertz ratio is constant the transformer will be fine - just like the motor. And VFDs are programmed to do that.

480V at 60 Hz ==> 8 volts/hertz

Although it more likely is:
460V at 60 HZ ==> 7.7 volts/hertz

So, at 48 hz, one would expect the VFD voltage to be 48 x 7.7 = 370V

Transformer will work just fine.

I would not make it quite that strong. They can be programmed to keep constant V/f, and that may be the default, but it is not the only VFD operating mode. Is the VFD set up to control the motor speed? Or is there a steady (adjustable?) speed signal being provided to it? Or is it programmed for a fixed output frequency?
I would also want to be sure that the pulse voltage waveform applied will not damage the transformer insulation, particularly if standing waves are present in the transformer feeder.