Transformer feeding motor

[pedorrero79]

Hi all,

I have a question regarding sizing a transformer. The situation is this: a new building is being built and the owner is going from a 240V/1-phase service to 480V/3-phase service. However, they have an existing air compressor that they're going to move to the new facility but the air compressor is rated for 240V only. What we're planning to do is have a branch circuit from a 480V panel to a 480:240 transformer and feed the air compressor from there. (I know it's simpler for them to buy a new compressor but they're adamant about keeping this one because it's relatively new.)

I've determined the load that the compressor will put on the system (4,320VA) and think I need a 5kVA transformer. However, my question is: should the transformer be upsized to take into account the inrush current of the compressor?

Any and all help is appreciated.

Thanks,
- P

 

[Jraef]

Hi all,

I have a question regarding sizing a transformer. The situation is this: a new building is being built and the owner is going from a 240V/1-phase service to 480V/3-phase service. However, they have an existing air compressor that they're going to move to the new facility but the air compressor is rated for 240V only. What we're planning to do is have a branch circuit from a 480V panel to a 480:240 transformer and feed the air compressor from there. (I know it's simpler for them to buy a new compressor but they're adamant about keeping this one because it's relatively new.)

Well, it would be far less to just buy a new motor, not a new compressor!

I've determined the load that the compressor will put on the system (4,320VA) and think I need a 5kVA transformer. However, my question is: should the transformer be upsized to take into account the inrush current of the compressor?

Yes, you must account for starting current (inrush is something else, but don't sweat it, common mistake). The quick-and-dirty rule of thumb is that for Across-the-Line stating, the transformer kVA should be 3X the motor HP. You didn't say the HP of the motor, but I'd guess it's 3HP, So 10kVA would be the right size. You may be able to use a 7-1/2kVA though, because what would happen is that the 240V side would drop voltage, but since (if) the motor will be the only load, it becomes in effect a reduced voltage starter of sorts. But it's hard on the transformer and for something like a compressor that may come on and off quite a bit, I wouldn't do it.

 

[kingpb]

Well, it would be far less to just buy a new motor, not a new compressor!


But it's hard on the transformer and for something like a compressor that may come on and off quite a bit, I wouldn't do it.

Hard on the motor as well if it's not rated for RVS, but then you can buy a new motor and ditch the transformer:happyyes:

 

[DavidA]

It may be worth a closer look at the compressor or a call to the manufacture. In a similar situation that I worked on recently all it took to switch the compressor to 480v was changing the taps on the control transformer and changing the make up at the motor.

 

[dkarst]

The original poster stated the current service was 240V 1Φ so it seems unlikely the motor could be wired for 480V 1Φ operation.

 

[Cow]

They need to change the motor. It makes absolutely no sense to install a transformer(and possibly a fused disconnect or two) adding more failure points to the circuit instead of just installing the right motor. Do they realize they'll have transformer losses 24/7 that I'm sure will cost far more in the long run than just a motor? Don't forget to take a look at the contactor as well and make sure it'll work at 480v.

I might catch hell for this but sometimes you have to tell the customer what they need because they don't see the big picture.

 

[mbeatty]

It may be worth a closer look at the compressor or a call to the manufacture. In a similar situation that I worked on recently all it took to switch the compressor to 480v was changing the taps on the control transformer and changing the make up at the motor.

I agree that many compressor control systems are made so they can be configured to operate at multiple voltage levels. If so, care must be taken to ensure that proper labeling and UL/NEC requirements are followed. Different forums, I know, but they should be considered anytime changes are made to an operating system.

 

[Jraef]

I agree that many compressor control systems are made so they can be configured to operate at multiple voltage levels. If so, care must be taken to ensure that proper labeling and UL/NEC requirements are followed. Different forums, I know, but they should be considered anytime changes are made to an operating system.

Again, take a look at the original post. It is a SINGLE phase 240V motor. There are no changes to control circuits or motor hookups that will allow that to be run from 480V without changing the motor or adding a transformer.

I still think it would be cheaper to change the motor to a 480V 3 phase version of the same HP, speed and frame. The point about adding an additional disconnect and fuses for the transformer is valid, albeit the entire starter arrangement will likely have to change for 480V 3ph too. still, it would be much simpler.

 

[DavidA]

Oops, that's what I get for not paying attention. In my situation I had instructions from the manufacture who then also provided stickers to indicate the line voltage change.

 

[nishchal]

want to confirm is it 3 phase 4 wire or 3 phase 3 wire system

want to confirm is it 3 phase 4 wire or 3 phase 3 wire system

Just want to confirm , is it a 3 phase 4 wire system you are shifting to , if yes then you have a phase (220V) and a neutral ,, so you can connect motor without transformer ,

If its a 3 phase 3 wire system then change the motor to from 1 phase 220 V to 3 phase 480 V and use your compressor .

The Transformer can be choosen as 10KVA in this case .
reagrds
Attri

 

[T.M.Haja Sahib]

Well, it would be far less to just buy a new motor, not a new compressor!


Yes, you must account for starting current (inrush is something else, but don't sweat it, common mistake). The quick-and-dirty rule of thumb is that for Across-the-Line stating, the transformer kVA should be 3X the motor HP. You didn't say the HP of the motor, but I'd guess it's 3HP, So 10kVA would be the right size. You may be able to use a 7-1/2kVA though, because what would happen is that the 240V side would drop voltage, but since (if) the motor will be the only load, it becomes in effect a reduced voltage starter of sorts. But it's hard on the transformer and for something like a compressor that may come on and off quite a bit, I wouldn't do it.

Where did you get that rule?Because in 'American Electrician Handbook',it is stated that motor starting transformer capacity in KVA is equal to the h.p of the motor.

 

[mbeatty]

Again, take a look at the original post. It is a SINGLE phase 240V motor. There are no changes to control circuits or motor hookups that will allow that to be run from 480V without changing the motor or adding a transformer.

I still think it would be cheaper to change the motor to a 480V 3 phase version of the same HP, speed and frame. The point about adding an additional disconnect and fuses for the transformer is valid, albeit the entire starter arrangement will likely have to change for 480V 3ph too. still, it would be much simpler.

Jraef,
I agree with your statement. My response was more directed to DavidA's example of changing transformer taps and motor connections. I probably should have clarified that. :slaphead: