Step down transformer being used as a step up device.

[knoppdude]

I have a transformer that is being installed for a 5 hp pump motor, and the plant electrician ordered a 120/240-480 step down transformer that is being used as a step up transformer, to boost 240 to 480. The problem with this, is that using the transformer in this fashion allows no place to ground the transformer on the "new" secondary, with the exception of corner grounding it. This is the only way I can see to allow the O/C device to trip on a fault. However, the grounded leg would not cause the breaker to trip, except through the motor single phasing. This would probably cause the O/L device to trip. I have searched through the 2008 NEC for clarification, but the fact is, I think the transformer is simply being used in a way not in compliance with the manufacturers specifications. I am not happy with this situation, but the plant electrician seems ok with it. Does anyone know if there is a code section dealing with this? There is no place to center tap a winding, it is a delta-delta transformer.

 

[WIMaster]

The first thing I would do is contact the manufacturer and see if the approve of it being used this way.
If they do you will need to apply the NEC rules that apply to ungrounded systems. I very rarely deal with this, but I believe that ground detectors are required.

 

[iwire]

The problem with this, is that using the transformer in this fashion allows no place to ground the transformer on the "new" secondary, with the exception of corner grounding it.

Why is corner grounding it a problem?

If there were no specifications requiring a wye supply he could have used a delta delta transformer.

This is the only way I can see to allow the O/C device to trip on a fault. However, the grounded leg would not cause the breaker to trip, except through the motor single phasing.

No the grounded leg would not trip the breaker, there is no reason it has to or should. It is an NEC violation to place an over current device on the grounded conductor unless that device opens all circuit conductors at the same time.

I think the transformer is simply being used in a way not in compliance with the manufacturers specifications.

Square D for one allows their transformers to be used in either direction.

I am not happy with this situation, but the plant electrician seems ok with it.

OK, you are unhappy about it, but is it unsafe?

There is no place to center tap a winding, it is a delta-delta transformer.

Delta deltas are commonly corner grounded or ungrounded with ground fault indicators.

250.21 Alternating-Current Systems of 50 Volts to 1000
Volts Not Required to Be Grounded.

(B) Ground Detectors. Ungrounded alternating current
systems as permitted in 250.21(A)(1) through (A)(4) operating
at not less than 120 volts and not exceeding 1000
volts shall have ground detectors installed on the system.

So in my opinion the choices are either corner ground what you have or install ground fault indicators.

 

[hillbilly1]

Why is corner grounding it a problem?

If there were no specifications requiring a wye supply he could have used a delta delta transformer.

iwire, he may already have a delta-delta since he states its a 480 to 120/240 volt stepdown transformer, unless he meant 120/208. Of course this dose not change anything though unless his service is a 240 volt delta, in which using a 120/208 wye transformer will produce close to a 575-600 volt output

 

[iwire]

iwire, he may already have a delta-delta since he states its a 480 to 120/240 volt stepdown transformer, unless he meant 120/208. Of course this dose not change anything though unless his service is a 240 volt delta, in which using a 120/208 wye transformer will produce close to a 575-600 volt output

I have no idea at all what you are trying to say. :?:happysad:

 

[barclayd]

Seems like a lot of trouble for a 5hp motor.
db

 

[hillbilly1]

I have no idea at all what you are trying to say. :?:happysad:

480 to 120/240 three phase: secondary has to be delta, cannot be 120/208 unless secondary is wye. If 120/208 secondary is backfed with 120/240, output voltage will be much higher than 480, i did this for some 575 volt Canadian equipment years ago. Actually had 600 volt output, backed the taps down to get 575.

 

[knoppdude]

Thanks for the replies. The motor is actually 10 HP, and is running fine. Since one phase is already grounded, it has 0 volts when referenced to ground. This phase cannot have a ground fault as it is already grounded. If one of the other two go to ground, they now have a current path through the transformer. This is still probably not what the mfg. had in mind, but it is the situation for now.

 

[ActionDave]

The motor does not care what the voltage to ground is. I doubt the manufacturer does either.

 

[hillbilly1]

Thanks for the replies. The motor is actually 10 HP, and is running fine. Since one phase is already grounded, it has 0 volts when referenced to ground. This phase cannot have a ground fault as it is already grounded. If one of the other two go to ground, they now have a current path through the transformer. This is still probably not what the mfg. had in mind, but it is the situation for now.

This type of installation is quite common, the transformer does not care where the ground is, or if it has one at all, as long as it is at one point only. As long as the insulation is rated for the voltage, it works fine.

 

[iwire]

Since one phase is already grounded, it has 0 volts when referenced to ground. This phase cannot have a ground fault as it is already grounded. If one of the other two go to ground, they now have a current path through the transformer.

Again, that is not a problem, it is not unsafe and in fact it is very common.

Both single and three phase motors are often supplied from corner grounded sources.

 

[kwired]

Seems like a lot of trouble for a 5hp motor.
db

My thoughts also. Unless this motor is single voltage with special frame, shaft or other characteristic a new motor will cost less than the transformer and other needed items.