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What are medium voltage drives?

　　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 “invertor,” 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.

　　The motor, drive, and encoder often form a tight little feedback loop within the control system.

　　Drive circuitry is, in general, based on power-supply technology, with simple drives being based on linear power supply circuits with voltage or current regulation. Modern drives are now based on switching power supply circuits, which are much more efficient.
　　The block diagram below shows one possible architecture for a single-phase motor drive. It is based on a switching power supply circuit with digital feedback control. Note that I present this for illustration only. It is not the only possible architecture, nor is it necessarily a particularly good architecture for any given application.
　　The process starts with a balanced rectifier and filter to produce balanced single-phase dc power. A pulse-width modulator (PWM) forms an approximation to the ac sine wave from a set of pulses of varying width. Since a PWM produces a single-ended output, to maintain efficiency, two PWMs are required: one to form the positive half cycle, and the other to form the negative half. These halves combine in a transformer, whose output is then filtered to remove the high frequency artifacts.

　　One possible drive architecture is based on pulse width modulation.

　　The microprocessor decides how to form the pulses based on input from the encoder and controller. The controller indicates what the system needs and the drive’s microcontroller figures out how to produce it. If, for example, the controller calls for increased speed, the microprocessor knows that this will require increased torque, which it can only get by increasing the phase angle between the motor’s magnetic field, and its rotor field (assumed to be a permanent magnet rotor for simplicity). The drive advances the phase by reducing the number of clock cycles between the encoder position pulse and when the delay counter initiates the next waveform.
　　As the rotor speeds up, the field frequency must increase to keep pace. The microprocessor accomplishes that by reducing the number of clock cycles between pulse leading edges.
　　The “instantaneous” voltage put out by the PWM depends on the width of each pulse. A third counter, the width counter, determines the pulse width by opening the PWM gate for a number of clock cycles depending on where in the waveform the drive is at that instant.
　　In short, the delay counter tells the repeat counter when to start the next cycle; the repeat counter tells the pulse former when to start the next pulse; and the width counter tells it when to end the pulse. The pulse former opens the PWM gate when triggered by the repeat counter, and closes it when triggered by the width counter.
　　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. I discussed this phenomenon at length in “Back to Basics: Medium voltage drives,” which appeared in the February 2008 issue of Control Engineering. 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 require requirements climb, so do the drive voltage requirements.

　For optimum operation, drive voltage correlates with motor output power.

　　That much is basic ac motor technology. 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.

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