Electric motors convert electrical energy into rotational mechanical energy. Motors may drive pumps, fans, compressors, or any other number of loads that may be found in a typical building. The concept seems simple, but a review of motor basics is necessary to understand how VFDs operate and also how they can destroy a motor.

Variable Frequency Drive (VFD) can be used to control the speed of three-phase induction motor. A variable frequency drive is a system equipped for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.

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A VFD drive supplied from a 3 phase system will have a bank of rectifiers at the input feeding a capacitor bank. The capacitor bank serves two purposes --- to filter some of the input ripple from the rectifiers, and to absorb some energy during deceleration. Rectifier-capacitor input filters produce non-sinusoidal pulses of current in the input lines. This means the RMS input current is higher than if the same energy was transferred from a DC source. If you take a VFD drive designed for a 3 phase input and use it on a single phase system, then the diodes and the filter bank may be under sized. This is why you may need a 3 times larger VFD. It is not because of the DC to AC conversion electronics, but because of the input diodes and capacitors.

All VFD applications require programming. You have to make it do what you want. Parameters like minimum and maximum frequencies, number of motor poles, maximum current, overload response, braking behavior, acceleration profiles, etc. must be set by the user to match your installation's needs. The voltage/frequency curve is user configurable too, which is very useful for your kind of application where you are operating outside the normal frequency range of the motor. Good VFDs come with a display that can be set to show the motor speed (or any number of other parameters). If I were doing this, I would use a 1HP 208V two-pole motor and a 1HP VFD with a 240V input.