AC Drives and Soft Starter (5)

Motor Characteristics Using VFD’s

During acceleration, the inverter applies different frequencies to the motor. It also changes the voltage but in direct proportion to the frequency. This is know as constant volts per hertz and provides constant torque while the motor accelerates.
A series of speed torque curves is shown in figure 9. These relate to speed torque curves at various frequencies. The “constant torque” line represents the full load or rated torque of the motor.

This “constant torque” line is actually the full load point on a locus of curves representing the speed torque curves of the motor from 0 to full speed. The inverter produces rated motor torque from 0 to rated speed. It will produce full load torque while drawing much less than full load current from the power line during starting. This is due to the fact that the motor is effectively always running at speed for the applied frequency.
When full voltage starting, the slip of the motor at 0 speed is 100 percent and the motor is highly inductive. This results is the very high inrush current, 600–800 percent, and relatively low starting torque, 150–180 percent of full load torque, compared to the current draw. Almost all of the motor current here is reactive. Reactive current, by nature, does not produce torque.
When a motor runs at speed the slip is typically in the area of one to three percent. Under this condition the reactive current is much less and the motor produces rated torque at rated current. With a VFD the motor runs virtually at speed during acceleration. Since the voltage is reduced at low speeds, the input current can be 10 percent or less with more than 150 percent torque.
Since the motor always runs at speed, or within rated slip, the acceleration time is dependent on the ramp time setting. This assumes that the drive has been properly selected for the load.


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