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Three Phase transformer and VFD

adrian

Cast Iron
Joined
Jan 11, 2005
Location
Kent, UK
I am trying to get my head around what's needed for a 415V three phase star configuration converter.

I am trying to steer away from an idler motor with capacitor setup and was wondering if any one has tried putting a three phase transformer with primary as delta and secondary as star on the output of a VFD.

Been looking on eBay for ages for a 5KVA 240-415 step up transformer at the right price I noticed a three phase 240V delta to 415V star transformer come up for sale and was curious as to would it work. May be someone on the forum with far more knowledge than me has the answer.

I can see there could be an issue with frequency changes to the transformer input, as the output will change.

Ciao Adrian.
 
The reason you don't find them is because in countries that use 415V, it is always 415Y240V, so if they need 240V, which for them is always single phase, they just use Line to Neutral; they don't need a transformer.

So can I assume you have a 240V single phase service and need 415V 3 phase for a machine, using a VFD to get you the three phases from your single phase source?
 
I am trying to get my head around what's needed for a 415V three phase star configuration converter.

I am trying to steer away from an idler motor with capacitor setup and was wondering if any one has tried putting a three phase transformer with primary as delta and secondary as star on the output of a VFD.

Been looking on eBay for ages for a 5KVA 240-415 step up transformer at the right price I noticed a three phase 240V delta to 415V star transformer come up for sale and was curious as to would it work. May be someone on the forum with far more knowledge than me has the answer.

I can see there could be an issue with frequency changes to the transformer input, as the output will change.

Ciao Adrian.

The only issue I see is possibly high transformer losses due to the chopped output of the VFD. A filter will fix that.

As far as frequency goes, transformers have a V/Hz ratio same as motors. So a 240V 50Hz transformer can only handle 120V at 25Hz or up to 288V at 60Hz. This is in terms of not saturating the transformer. At higher than the rated frequency you will get increased eddy currents in the core, so you should only do that intermittently, or derate the transformer and use a lower voltage or pass fewer amps. (You can even reduce the voltage for the same frequency and get a bit more amperage, but again, complex math)

The math for this is complex and specific to each transformer, but it is always safe to run a transformer at a lower voltage than the V/Hz ratio suggests. Going over can be done, but you must compromise.

The simple answer is if you aren't overloading your motor, you won't overload a transformer with the same rating. If you are using tricks to get more power/speed/torque out of your motor that doesn't result in overheating your motor, you *probably* will not overheat your transformer.

EDIT: another option is to use a typical 230V to 460V transformer with an output rated at the same FLA as the motor and use the VFD to reduce the voltage. Transformers often have multiple taps (e.g. 200, 208, 220, 230, 240 and 440, 450, 460, 480) so you may be able to get something much closer than you would expect.

Do you actually need a neutral? Someone correct me if I'm wrong, but if you don't need a neutral it doesn't matter if your power source is delta or star.

I'm cheap so I would just get a VFD, a filter, a 1:2 (230:460) transformer, and run the VFD around 216V at 60Hz. (That also gives you a little room for intermittent speed boosts without losing torque, if you are the kind of guy who likes to briefly overload equipment.)
 
My workshop is in a home environment so all my machines are three phase run through converters.

Most simple machines are not an issue, my lathe and mill had a major re-wire to update to VFD and personally I think it's a better setup with soft start braked stop and simple speed and direction changes.

It would be nice sometimes to be able to plug a machine in without the re-wire.

Ciao Adrian.
 
The only issue I see is possibly high transformer losses due to the chopped output of the VFD. A filter will fix that.

As far as frequency goes, transformers have a V/Hz ratio same as motors. So a 240V 50Hz transformer can only handle 120V at 25Hz or up to 288V at 60Hz. This is in terms of not saturating the transformer. At higher than the rated frequency you will get increased eddy currents in the core, so you should only do that intermittently, or derate the transformer and use a lower voltage or pass fewer amps. (You can even reduce the voltage for the same frequency and get a bit more amperage, but again, complex math)

The math for this is complex and specific to each transformer, but it is always safe to run a transformer at a lower voltage than the V/Hz ratio suggests. Going over can be done, but you must compromise.

The simple answer is if you aren't overloading your motor, you won't overload a transformer with the same rating. If you are using tricks to get more power/speed/torque out of your motor that doesn't result in overheating your motor, you *probably* will not overheat your transformer.

EDIT: another option is to use a typical 230V to 460V transformer with an output rated at the same FLA as the motor and use the VFD to reduce the voltage. Transformers often have multiple taps (e.g. 200, 208, 220, 230, 240 and 440, 450, 460, 480) so you may be able to get something much closer than you would expect.

Do you actually need a neutral? Someone correct me if I'm wrong, but if you don't need a neutral it doesn't matter if your power source is delta or star.

I'm cheap so I would just get a VFD, a filter, a 1:2 (230:460) transformer, and run the VFD around 216V at 60Hz. (That also gives you a little room for intermittent speed boosts without losing torque, if you are the kind of guy who likes to briefly overload equipment.)

I like the cheap bit, suits me fine.

Interestingly as previously mentioned I currently run both my lathe and mill through an inverter with a 4 pole motor. The original motors were two speed so needed the 415V supply. I run these motors up to 100Hz to give the 2800rpm of the original motor without a problem. There seems to be no issue with heat or vibration, however they are not run at that speed continuously.

It's all down to finding a suitable transformer. They would appear to be more common in the USA but few and far between in the UK.

Ciao Adrian.
 
I like the cheap bit, suits me fine.

Interestingly as previously mentioned I currently run both my lathe and mill through an inverter with a 4 pole motor. The original motors were two speed so needed the 415V supply. I run these motors up to 100Hz to give the 2800rpm of the original motor without a problem. There seems to be no issue with heat or vibration, however they are not run at that speed continuously.

It's all down to finding a suitable transformer. They would appear to be more common in the USA but few and far between in the UK.

Ciao Adrian.

What voltage do you run those motors at, and what are they rated for?
 
They are all standard dual voltage 240V delta 415V star motors only rated for 50/60Hz.
Ranging in size from .55Kw up to 2.2Kw and all 4 pole.

Ciao Adrian.
 








 
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