Transformer Selection - 440 VAC to 415 VAC

what is the problem? there is no problem in connecting 440 V supply to the Pac Air Condiitoner since it is able to cope upto +10% voltage on it's rated voltage which is 415 +10% of 415V which is 456 V. So if you provide a 440 V supply the equipment is still being supplied within tolerance limits. You must then take care that your 440 V supply transformer should be kept at nominal tap or lower one if you can afford it for other loads.

Application is in a refinary. Power supply spec is 440V+/- 10%. I.E, the voltage can go up to 484V! This is the issue. They will not modify the power supply side as the entire refinary is designed for this voltage. Copeland compressors can work up to 462V. Same is the case with fan motors also. Customer also knows this. So insisting on using a transformer to step down the voltage to 415V. Is the sizing of transformer I did is correct?

You have a voltage reduction of about 6%. Therefore an autotransformer should be satisfactory. This means that the actual transformer size will be the equivalent of around3 kVA. (6% of the 50 kVA). I would go for anything rated at an equivalent of 3.5 to 4 kVA and leave the safety factor to the transformer manufacturer but make sure you give him/her all details such as ambient temperature, number of starts per hour/day etc. As a transformer manufacturer that would be enough for a good reliable design.

Best of luck

Thanks for the reply.

But, I have one doubt. Whether the 4 kVA transformer will handle the unit current of 70A? If you calculate back from 4kVA, the current per phase is around 6A. Please treat this as a Mechanical Engineers doubt.

Motor manufacturers generally design their motors to operate over a defined voltage range, generally +/- 7.5%. Check with the motor supplier what voltage range they are designed for. For a fixed load, an induction motor will take less current as the voltage increases until the voltage causes the magnetic field in the motor to move into magnetic saturation which then causes the motor current to start rising as the supply voltage increases and the heating of the magnetic steel in the motor increases.This usually happens when the motor voltage is greater than +10%, once again the motor supplier can give you these values.

As far as sizing the transformer is concerned, you will have to know the impedance of the transformer and what the voltage drop will be on starting which is effected by the supply systems voltage drop as well as the transformer voltage drop at starting to give you the voltage at the motor terminals which the motor supplier has designed it for. After that you can calculate the transformer size, % impedance and duty cycle. A "rule of thumb" is at least 1.5 times the motor KW as transformer KVA.

I hope this is of assistance.

PDSC