Sizing Variable Frequency Drives
One of the most common misunderstood aspects of applying variable frequency drives (VFD) happens when you retrofit a motor with a drive for the first time. Often the VFD purchased is too small. This happens when a person says, “I have a 10 hp motor so I need a 10 hp drive.”
The problem is that when you start a motor across the line, it has unlimited current from the starter powering it—this current creates the high torque needed to start the motor with its associated load.
A VFD can only supply a limited amount of current. Current equals torque, hence less current less torque. While you may be able to run your load with that 10 hp drive you cannot start it up with that same drive. Many drives can only supply 150% of what they are listed for.
This means you may have to purchase a 25 hp (or larger) VFD to start your 10 HP motor.
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