What is a Medium Voltage Drive?

From zero to 600 V is called low voltage; medium voltage is 600 V and above; there is no high-voltage designation.

Don't feel bad about being confused by "medium voltage drives" terminology. It’s a confusing because it sounds like a technical term, but really it’s marketing driven. Actually, there are two terms that can be confusing to anyone new to the field of automated control of electric motors.

Let's start with the word "drive." Also sometimes called an “inverter,” a drive is essentially a power amplifier that puts out the electric power needed by an electric motor based on the torque and/or speed called for by the controller, which is a digital computer tasked with overall system control. As the industry has shifted in favor of variable-speed drives, the role of the drive has become more important.

Variable-speed motor/drive combinations use 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 drive’s power efficiently out to the load as mechanical power 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 an impedance for the drive/motor combination.

Copper resistance in the motor coils and power cables, as well as contact resistance in all the connections rob the system of power, however. The higher the circuit impedance compared to these parasitic impedances, the more efficient the overall system will be.

The graph below compares motor/drive impedance to a typical parasitic impedance level of one Ohm. Assuming we want the ratio to be one or two orders of magnitude, the chart shows how the supply voltage needed correlates with output power. Small, fractional horsepower motors (red line) work well from a few tens to a few hundreds of Volts. Larger motors require hundreds of Volts to run efficiently. As horsepower requirements climb, so do the drive voltage requirements.

What really drives people crazy is the term "medium voltage drive." It is a purely marketing term. From zero to 600 V is called "low voltage." "Medium voltage" is 600 V and above. There is no "high-voltage" designation.

Comparing this 600 V cutoff to the chart shows that low-voltage drives provide good efficiency up to several horsepower (10,000 W). Above 10,000 W, however, medium voltage drives are needed. Voltages above a few thousand Volts, however, are needed only for the relatively few electric motor applications requiring hundreds of horsepower. Most applications requring that much mechanical output are currently served by internal combustion engines.