If you are easily confused by the term “medium voltage AC drives“, do not feel bad, seeing as you are probably not the only one.  It is usually confusing terminology because it sounds like a technical term; however, it is really only marketing driven.  From zero to 600 V is called “low voltage (LV).”  “Medium voltage (MV)” is 600 V and above.  There is no “high-voltage (HV)” designation.  This is just one of many terms that can be confusing to anyone new to the field of automated control of electric motors.

The 600 V cut off is quite significant, seeing as low-voltage drives provide good efficiency up to several horsepower (10,000 W).  However, above 10 kW, medium voltage drives are necessary.  Voltages above a few thousand Volts, however, are needed only for the relatively few electric motor applications requiring hundreds of horsepower.  Most applications requiring that much mechanical output are currently served by internal combustion engines.

AC drive combinations utilize a synchronous AC motor with an encoder built in.  The encoder signals the motor’s speed and phase angle to the drive, which must then match its output frequency to the motor speed and its output phase to produce the required torque.  To get the AC drive’s power efficiently out to the load as mechanical power, it requires having enough voltage to drive enough current through the motor.  The output power is, after all, the product of voltage times current.  The ratio of the voltage to the current, on the other hand, defines impedance for the AC drive/AC motor combination.