Medium voltage drives

Back to Basics: Medium voltage drives are those putting out 600 V or more. Motor/drive sets operating at “medium” voltages up to 24,000 V have been built and operated. Generally, such drives are variable speed ac drives intended for high-power applications. They typically achieve power regulation by pulse-width modulation to match drive power with mechanical load requirements at the desired speed.

02/01/2008

Medium voltage drives are those putting out 600 V or more. Motor/drive sets operating at “medium” voltages up to 24,000 V have been built and operated. Generally, such drives are variable speed ac drives intended for high-power applications. They typically achieve power regulation by pulse-width modulation to match drive power with mechanical load requirements at the desired speed.

 The load impedance required for a given motor output depends on the drive's output voltage.

Many observers naively assume that the gains available by specifying medium voltage drives arise from the same phenomena that reduce losses in high-voltage power-transmission lines. That simply is not true. Power transmission lines have a fixed length—the distance between the generating station where the power originates to the load where it will be used. Electric motor windings, however, do not have fixed lengths.

In both cases, the power required depends on outside factors. In the motor case, however, the torque available from the motor is proportional to the product of current and the number of turns in the windings. To keep the same torque (and thus power) while reducing current, the motor designer must increase the number of turns proportionately. Assuming a constant current per unit cross-sectional wire area, it is easy to show that motor efficiency is independent of the supply voltage. At the same time, the increased number of turns negates the reduced wire diameter, wiping out any space savings.

While medium-voltage installations can be engineered to achieve both higher efficiency and smaller physical size than low-voltage counterparts, the primary phenomenon motivation for using a medium voltage drive is load-impedance control.

The general formula relating electrical power to supply voltage and load impedance is

P= V 2

____,

Z

where P is the delivered power, V is the supplied voltage, and Z is the complex load impedance. In the case of a motor drive, the impedance’s real part dissipates power directly as heat, while it’s the imaginary part that converts electrical power to mechanical power. Both components, however, combine to limit the current that the load sinks, and thus the power that can be delivered.

Rearranging the equation to solve for Z provides an expression for load impedance as a function of supply voltage at a given power requirement:

Z= V 2

_____

P

Graphing this equation on a log-log plot produces a family of parallel straight lines covering the relevant domain of 1 V to 100,000 V on the horizontal axis and the relevant impedance range of 0.01Ω to 1,000 Ω. The voltage domain comprises all of the low and medium drive voltages that are significant in industrial control applications. The impedance range goes from that seen in a 1,340 hp motor powered by a 30 kV drive to below the contact resistance achievable by mechanical connections.

That lower limit is what drives engineers to look at medium-voltage drives. It is difficult or impossible—certainly impractical—to reduce those parasitic resistances below the level of 0.1Ω and usually below 1 Ω. And, that’s not including the actual copper losses in the motor’s windings!

It’s no wonder that engineers planning to control motors developing in excess of 10,000 W of mechanical power increasingly opt for medium-voltage drives supplying 600 V or more.

 Author Information C.G. Masi is a senior editor. Reach him at charlie.masi@reedbusiness.com .