Calculating Starting Currents
Calculating Starting Currents
(OP)
Good Afternoon.
Could anyone tell me how to calculate the starting current for a motor with only a KVA rating and PF value?
Obviously depends on starting method (VSD, Star/Delta and DOL)
Any Thoughts
Regards
JM264
Could anyone tell me how to calculate the starting current for a motor with only a KVA rating and PF value?
Obviously depends on starting method (VSD, Star/Delta and DOL)
Any Thoughts
Regards
JM264





RE: Calculating Starting Currents
If you know the **locked rotor** kVA and the locked rotor power factor, you can calculate a good approximation of the starting current.
RE: Calculating Starting Currents
The best way to find out is to look at manufacturers data, e.g.
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- the ratio of starting current to full load current is given for each machine.
RE: Calculating Starting Currents
The Full Voltage start current will initially be equal to the Locked Rotor Current and then gradually fall as the motor accelerates, only falling significantly one the motor reaches about 80% speed. If you do not have the rated LRC of the motor, then you have a difficult task, except the range of LRCs of motors is generally between 550% and 900%. You will find some, very few, that fall outside this range.
Low slip motors (High efficiency motors) tend to have a high LRC and low efficiency motors tend to have a low LRC. Larger machines are often higher on LRC also.
Provided there is sufficient start torque available on reduce voltage to accelerate the motor to full speed, the current will reduce directly with the voltage reduction. (Torque reduces with voltage reduction squared.)
There is more information on my web site at www.LMPhotonics.com
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Calculating Starting Currents
Motors for submersible well pumps have code letters that are all over the place. For a motor that fits into a 4 inch (100 mm) well the code letter tends to around R or S. This is because in a small diameter submersible motor the oil gap between rotor and stator is abnormally small and the designer cannot use a double squirrel cage or other rotor design that results in reasonable starting current. Motors for 6 inch wells are more like code F and G (normal range) because there is more design freedom.
Mike Cole mc5w at earthlink dot net