HP and amperage

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jaykool

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A 100HP motor (480V, 3ph.)...draws 124 full load amps.

Will each phase see 124A or 41A (124/3)?

The reason I ask, I need to specify a 7200-480V transformer, so either I install a 50KVA (60A secondary) or 167KVA (200A secondary).
 
jaykool said:
A 100HP motor (480V, 3ph.)...draws 124 full load amps.

Will each phase see 124A or 41A (124/3)?

The reason I ask, I need to specify a 7200-480V transformer, so either I install a 50KVA (60A secondary) or 167KVA (200A secondary).
Click to expand...
Well, given that 100 hp is about 75kW, there is no way a 50kVA transformer would be adequate.

Whether the 167kVA unit would be adequate is debatable. If starting is direct-on-line (across the line) the motor will take maybe six to eight times rated full load current. Taking six times FLC gives around 618kVA and could result in a voltage drop of around 20% or more depending on motor characteristics and transformer impedance. That would significantly reduce the motor starting torque capability with the possibility of the motor failing to reach rated speed.

That said, the circumstances you have given are, in my experience, quite uncommon.
We have used unit transformers for individual drives but the smallest I can recall was about 1000 hp and that was for a variable frequency inverter so starting current was quite low.

I'm sorry it isn't a simple answer to what you may have perceived as a simple question.
 
Then you have have 150 kva of transformer. If I were taking bets, I'd say you would be fine, but as Besoeker notes, there are many variables to consider. The type of starting (across the line or other ?), the Code Letter of the motor (determination of LRA), etc.
An engineer associated with the manufacturer of the transformer (or POCO engineer in POCO's case) should be able to give you the proper guidance.
 
jaykool said:
A 100HP motor (480V, 3ph.)...draws 124 full load amps.

Will each phase see 124A or 41A (124/3)?
The reason I ask, I need to specify a 7200-480V transformer, so either I install a 50KVA (60A secondary) or 167KVA (200A secondary).
Click to expand...

A bit of a scary question, being as it touches on a very basic bit of knowledge an electrician just has to know.

I am not so sure even the 200A transformer is large enough to supply voltage at starting current levels (could be in the 1000A range). It depends on some factors like motor design that are not specified. I would be wary if you are going to try and start it across the line.
 
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Check with the VFD mfg website and get some data sheets or call a rep. If motor is existing see nameplate if not see data sheet and relay the information to the rep or cross-referrence with data sheet. Is this the first time you're doing this? You should really consider consulting an engineer or at least an experienced contractor.
 
Hah!!!

Hah!!!

petersonra said:
A bit of a scary question, being as it touches on a very basic bit of knowledge an electrician just has to know.

I am not so sure even the 200A transformer is large enough to supply voltage at starting current levels (could be in the 1000A range). It depends on some factors like motor design that are not specified. I would be wary if you are going to try and start it across the line.
Click to expand...

Bob,

My sentiments exactly!!
OP.. you're given a HP. You need to be able to relate that to KVA.
If it were a perfect world, with unity PF, you'd need at least a 74.6 kVA transformer.
Forest for the trees.

JM
 
Transformer sizing

Transformer sizing

Know your load first. What is the motor's locked-rotor amps? (or should I ask "What code letter is marked on the nameplate saying Locked-rotor kVA?). Another way of asking is: How much starting current are going to limit when starting? You need this data to calculate the size of the transformer taking into account starting of the motor. Else, you will end up with too small a transformer and the secondary voltage might be too low for your VFD to function properly.
 
topgone said:
Know your load first. What is the motor's locked-rotor amps? (or should I ask "What code letter is marked on the nameplate saying Locked-rotor kVA?). Another way of asking is: How much starting current are going to limit when starting? You need this data to calculate the size of the transformer taking into account starting of the motor. Else, you will end up with too small a transformer and the secondary voltage might be too low for your VFD to function properly.
Click to expand...
If it's a VFD, the supply shouldn't see locked rotor current.
 
Besoeker said:
Well, given that 100 hp is about 75kW, there is no way a 50kVA transformer would be adequate.

Whether the 167kVA unit would be adequate is debatable. If starting is direct-on-line (across the line) the motor will take maybe six to eight times rated full load current. Taking six times FLC gives around 618kVA and could result in a voltage drop of around 20% or more depending on motor characteristics and transformer impedance. That would significantly reduce the motor starting torque capability with the possibility of the motor failing to reach rated speed.

That said, the circumstances you have given are, in my experience, quite uncommon.
We have used unit transformers for individual drives but the smallest I can recall was about 1000 hp and that was for a variable frequency inverter so starting current was quite low.

I'm sorry it isn't a simple answer to what you may have perceived as a simple question.
Click to expand...

We have many field irrigation pumps in this area 100hp is a common size all of them are across the line start - for 100 hp utility usually provides 3-37.5 kva transformers for total of 112.5

usually no problem starting motor sometimes if conductors are too small for distance the problem that is most noticeable is controls dropping out from too much voltage drop during starting
 
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