Hi, Tom
The way I look at it is this. The percent line drop is a function of Motor starting MVA and available Short Circuit MVA available at the terminals.
Or
% VD = (Motor Start MVA)/(available SC MVA) X 100 this will give you a percent line drop for starting. however, in this case KVA would be appropriate.
For example a 10 horsepower motor at 240 volts code letter G would have a starting KVA of 63KVA
A 25 KVA transformer with a 5-7 % impedance would have a SC kva or 500kva to 357kva between the two the line drop would be between 13 to 17 % during starting.
This is just the beginning of the problem. With the extent of the voltage dip and allowing for further voltage drop due to the size of the feeder to the motor,the voltage at the motor terminals is now seriously depressed. since motor torque is proportional to the square of the motor terminal voltage the starting torque will be well below normal starting torque. This in turn will cause longer starting times for the motor/compressor. That is acceleration time will be longer and the motor will be under duress (severe stress) for longer than necessary.
As Scott pointed out, 'if this is the case then the power company should be involved and maybe some type of soft start should be used to help lessen these effects.'
woc
