124a on each line, for a total of 124a.A 100HP motor (480V, 3ph.)...draws 124 full load amps.
Will each phase see 124A or 41A (124/3)?
Well, given that 100 hp is about 75kW, there is no way a 50kVA transformer would be adequate.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).
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).
Then you have have 150 kva of transformer. If I were taking bets, I'd say you would be fine, .......
This is a vfd application.
That would have been useful information in your original post.This is a vfd application.
What is the highest bet you're willing to take?:grin:
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.
If it's a VFD, the supply shouldn't see locked rotor current.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.
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.