How do I reduce the required starting current of an AC Motor?

When an AC motor is started “across the line” it takes as much as seven to eight times the motor full-load current to start the motor and load. This current flexes the motor windings and generates heat, which will, over time, reduce the longevity of the motor. An adjustable speed AC Drive starts a motor at zero frequency and voltage. As the frequency and voltage “build,” it “magnetizes” the motor windings, which typically takes 50 to 70 % of the motor full-load current. Additional current above this level is dependent upon the connected load, the acceleration rate and the speed being accelerated, too. The substantially reduced starting current extends the life of the AC motor, when compared to starting across the line. The customer payback is less wear and tear on the motor (motor rewinds), and extended motor life.

Starting an AC motor across the line, and the subsequent demand for seven to eight times the motor full-load current, places an enormous drain on the power distribution system connected to the motor. Typically, the supply voltage sags, with the amplitude of the sag being dependent on the size of the motor and the capacity of the distribution system. These voltage sags can cause sensitive equipment connected on the same distribution system to trip offline due to the low voltage. Items such as computers, sensors, proximity switches and contactors are voltage sensitive and, when subjected to a large AC motor line started nearby, can drop out. Using an adjustable speed AC Drive eliminates this voltage sag, since the motor is started at zero voltage and ramped up.

Lower power demand on start: If power is proportional to current times voltage, then power needed to start an AC motor across the line is significantly higher than with an adjustable speed AC drive. This is true only at start, since the power to run the motor at load would be equal regardless if it were fixed speed or variable speed. The issue is that some distribution systems are at their limit, and demand factors are placed on industrial customers, which charges them for surges in power that could rob other customers or tax the distribution system during peak periods. These demand factors would not be an issue with an adjustable speed AC drive.

This means that using a variable speed drives, given normal start conditions, then only full load current rating of AC Motor will be used.

Quantum Controls offer Variable Speed Drives for hire from 5.5Kw – 1.5mWatt, so if you are having a problem with an application pulling too much current on start-up why not hire a Variable Speed Drive from Quantum to prove that it will solve the problem before committing to a purchase. Send your enquiry to

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