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Calculating Starting Currents
- Thread starter jm264
- Start date
If you're referring to the normal running kVA and power factor, then you can only guess at the starting current.
If you know the **locked rotor** kVA and the locked rotor power factor, you can calculate a good approximation of the starting current.
If you know the **locked rotor** kVA and the locked rotor power factor, you can calculate a good approximation of the starting current.
The starting current (and torque) also depends on the rotor design, in particular the shape and electrical resistivity of the rotor bars. In the US the induction motor starting torque (relative to full load torque) is classified by NEMA, in the rest of the world I don't think there is a specific IEC classification (but I could be wrong).
The best way to find out is to look at manufacturers data, e.g.
- the ratio of starting current to full load current is given for each machine.
The best way to find out is to look at manufacturers data, e.g.
- the ratio of starting current to full load current is given for each machine.
Hello jm264
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
Best regards,
Mark Empson
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
Best regards,
Mark Empson
On U.S. made motors there should be a starting code letter from A to V that indicates how much starting KVA per horsepower of the motor. The table for decoding the code letter is in National Electrical Code article 430. The lowest code letter motor that I have installed is a code B (low starting torque) and the highest was code S for a 1/2 HP submersible well pump.
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
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
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