In a common AC induction motor, a magnetic field is induced in the rotor and the power is supplied directly to the stator. The power used to induce the magnetic field in the rotor is commonly known as reactive power since it does not produce work. The power supplied to the stator is best known as active or real power for it produces the torque of the motor. Together, the real and reactive power form the total or apparent power in the system. This is often depicted on a power triangle.

In the early days of variable speed drives, Nema rated motor contactors were used. The more economical IEC rated contactor had not be introduced. As variable speed drive technology improved and sequencing the opening of the contactor to coordinate with the termination of current flow to the motor became reliable, the need for a contactor to break an arc took second place to the use of a contactor for isolation purposes. This meant that the more robust and more expensive Nema contactor was not required.

I recently had a situation occur where a customer had a VFD fail and asked me to set up the spare in its place.