# Transformers for Motors



## Journeymanjoe13 (Oct 2, 2015)

Afternoon,

Looking for some help for properly sizing a transformer for a motor load. The motor load is 460Volt, 3Phase, 29.5FLA, 40Amp MOCP. The building supply is 120/208Volt, 3phase, 200Amp. Also when installing wires and OC for the new TX would the motor load be used or the TX FL rating

Thanks


----------



## Journeymanjoe13 (Oct 2, 2015)

Afternoon,

Looking for some help for properly sizing a transformer for a motor load. The motor load is 460Volt, 3Phase, 29.5FLA, 40Amp MOCP. The building supply is 120/208Volt, 3phase, 200Amp. Also when installing wires and OC for the new TX would the motor load be used or the TX FL rating

Thanks


----------



## telsa (May 22, 2015)

Journeymanjoe13 said:


> Afternoon,
> 
> Looking for some help for properly sizing a transformer for a motor load. The motor load is 460Volt, 3Phase, 29.5FLA, 40Amp MOCP. The building supply is 120/208Volt, 3phase, 200Amp. Also when installing wires and OC for the new TX would the motor load be used or the TX FL rating
> 
> Thanks


The circuit equations for a 3-phase motor and a 3-phase transformer are identical.

The ratios in a transformer work either way: stepping down or stepping up.

So this situation is as if you had a 480 V delta to 208Y120 transform. The math is identical.

Due to the NEMA standards, the only workable fit is a 30 kVA 3-phase transformer. Anything smaller is much too small, the next bump up is 45 kVA -- true overkill.

Size the conductors for the 30 kVA transformer. You want to be conservative -- and voltage drop during spool up is best held to a minimum. 

The delta load connection is effectively an ungrounded electrical power system. You can corner ground one of its phases to eliminate objections. If so, don't fuse that leg, insert a copper bar, instead.

I'd place the transformer as close to the motor as is practical. That way the OCPD (mounted on the transformer) could do double duty as the disconnecting means for the motor. ( ie Disconnect )

These puppies are small enough to be suspended or wall mounted. Just avoid foul atmospheres.


----------



## bkmichael65 (Mar 25, 2013)

Without knowing any of the specifics, I would say 3 times the motor's HP would give you a ballpark kVA rating of the transformer you need


----------



## JRaef (Mar 23, 2009)

telsa said:


> ...
> 
> Due to the NEMA standards, the only workable fit is a 30 kVA 3-phase transformer. Anything smaller is much too small, the next bump up is 45 kVA -- true overkill.


FLA of a 40kVA at 480V is 36A, the motor FLA is 29.5, that's cutting it a little too close for my comfort. The next standard size up is 37.5kVA, with an FLA of 45A, personally I would go that route as a minimum size, given the caveat I state below.



bkmichael65 said:


> Without knowing any of the specifics, I would say 3 times the motor's HP would give you a ballpark kVA rating of the transformer you need


That rule-of-thumb would apply if you wanted to avoid an appreciable voltage drop on the load side of the transformer, for instance if other things were being supplied by that same transformer. But if the transformer is ONLY feeding that one motor, it's OK to saturate it and let the motor voltage drop temporarily; its like having a Reduced Voltage Starter of sorts. As long as the load doesn't need the full Locked Rotor Torque of the motor to accelerate, it should work fine. But if the load DOES need the full LRT from the motor for some reason, of you are not sure, then you WOULD want to over size the transformer, and 3X is about right for an Across-the-Line start.


----------



## telsa (May 22, 2015)

JRaef said:


> FLA of a 40kVA at 480V is 36A, the motor FLA is 29.5, that's cutting it a little too close for my comfort. The next standard size up is 37.5kVA, with an FLA of 45A, personally I would go that route as a minimum size, given the caveat I state below.


In all my days I've NEVER seen 37.5 kVA as a standard NEMA size in 3-phase.

I've noted that it's a popular size in SINGLE phase.

The NEMA crowd is conservative. 

A 30 kVA transformer can go into mild overload for hours on end -- and not protest.

And it's easy to specify a deluxe heat rating for such a transformer -- available right off the shelf. Such transformers can handle even more abuse.

I must say that I NEVER see motors -- properly engineered -- maxxed out. The chance of the motor in question pulling 30 FLA for hours on end -- that is a stretch.

But, again, I'd change my attitude if I knew more about the nature of the load.

Over sizing a transformer costs money up front -- and bleeds electricity ( parasitic losses ) forever afterward. 

36A/ 30A ==> 120% of FLA.... good enough for me.

And that's before noting how robust the NEMA designs are.

In sustained over load, the heaters within the dedicated contactor should pop LONG before the transformer even breaks a sweat.


----------



## varmit (Apr 19, 2009)

Is this motor a single voltage unit? 480 volt only? If not why not run the motor on 208?


----------



## micromind (Aug 11, 2007)

varmit said:


> Is this motor a single voltage unit? 480 volt only? If not why not run the motor on 208?


Agree, and if it's part of a machine, it's usually easy to change the taps on the control transformer. 

The only 460 only motors this small (I'm guessing 25 HP) that I've seen are either Automotive Duty, IEEE 841 or 2 speed.


----------

