- Perhaps we need a short electric motor tutorial. All electric motors need a rotating field current to power the stator or a fixed field (can be ferrite, rare earth or electrical) with a rotating stator current. The typical DC motor uses a commutator and brushes to feed the stator and create this rotating field. Its advantage is the speed of the motor is easily controlled (think car header blower motor), but somewhat inefficient (think heavy and expensive). DC motors can also use a square wave form (current switched on and off many times a second) to control the motors speed (think Dremel tool). This more efficient than varying the voltage but some additional electronic switching circuity is required. AC motors use the fact that the current reverses direction 60 times a second to rotate the field and the stator is self-energized. AC motors are efficient and cheap but inherently single speed without major electronics.
Along came a new type of DC motor but with an inverter to change the wave form to mimic an AC motor. Since the frequency of an inverter can be easily changed, the motor's speed changed economically. The most efficient motors use rare earth magnets in the stator (think Tesla). Interestingly the same system can be used to create a variable speed AC electric motor, but it requires a rectifier to power the inverter which powers the motor.
Actually everything is more complicated, as the reluctance, inductance, back EMF, etc to be considered in choosing the ideal motor type for a particular application, that is why there are electrical engineers. By the way I am not one, so there will be a few things I missed.
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