Look at a motor torque speed curve.
Current and torque is mostly dependent on the slip speed or frequency.
That is the difference between the speed associated with the applied frequency and the motor actual speed.
Example. 60 Hz applied to a 4 pole motor, synchronous speed = 1800 RPM.
Motor full load speed, 1760 RPM.
Slip speed, 1800 - 1760 = 40 RPM.
So depending on the load, a motor may be started with less than full load current and torque.
Consider a fan or centrifugal pump:
With a slowly rising frequency, the motor may accelerate at less than full load torque and current.
Full load torque and current may only be reached as the load closely approaches rated speed.
The actual results will depend on the load torque/speed demands and the speed at which the frequency is ramped up.
A VFD may accelerate a motor faster than DOL starting with less current.
Consider:
DOL; Starting torque about 150%, Starting current about 600%.
VFD, Best case starting; Breakdown torque about 200%, breakdown current about 200%.
Reading the torque/speed curve:
The part of the curve of interest when using a VFD is the part between 100% and the breakdown torque.
First relabel the curve:
For a 1760 RPM motor, 1800 RPM will become zero RPM.
Full load torque will become 1800 - 1760 = 40 RPM.
This is the slip frequency/speed.
At breakdown torque the slip speed will be about 80 RPM to 100 RPM depending on the individual motor.
Now read the curve backwards.
At zero slip, the torque will be zero.
Think of a 1760 RPM motor turning at 1800 RPM with 60 Hz applied.
As the applied frequency/speed is increased, the slip is increased and as the slip is increased, the torque and current are increased.
Bill
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"Why not the best?"
Jimmy Carter
