Variable Speed Drives: Asking the Right Questions

In commercial and industrial facilities, the use of variable speed drives (VSDs) is growing, and for good reason. VSDs can reduce motor electricity usage by 25 to 85 percent, depending on the application. VSDs also minimize wear and tear on motors, reducing maintenance costs and increasing equipment life.

Despite these benefits, you may have concerns about cost and compatibility issues. This guide to frequently asked questions about VSDs can help you get the answers you need. Keep in mind we also offer rebates for VSDs that range from $35-$70/hp depending on the application.

What applications are best for VSDs? VSD controls produce the most savings on motors operating at full load a small amount of time (10 to 15 percent), but mostly at less than full load. Varying torque loads where equipment is easy to start and harder to operate at fast speeds are also good candidates. Examples include ventilation fans, water pumps and rotary screw-type air compressors.

Do VSDs increase motor efficiency? All motors have lower efficiency at part load, including those with VSDs. VSDs save energy mainly by reducing motor speed—and horsepower—to match demand reductions in the equipment that it’s driving—fans, pumps and so on.

Will there be a reduction in peak demand? Installing a VSD typically has little or no impact on peak demand. Demand is typically measured in 15-minute increments; the motor’s inrush current lasts only a few seconds during startup. However, a VSD can significantly reduce the inrush current, which is typically up to five times the full-load current. Reducing the inrush current helps minimize electrical and mechanical stresses at startup.

Can VSDs cause bearing or motor failure? VSDs produce high-frequency voltage pulses that can degrade the insulation on standard wound motors. Those pulses also produce stray high-frequency currents that can pass through motor bearings, causing arcing, sparking and pitting. This can result in bearing and motor failure. Adding grounding brushes to the motor shaft will help prevent this problem.

Will VSDs result in harmonic problems? In some cases, VSDs cause harmonic distortion in a motor’s electric currents, which can result in excessive heating. A number of solutions are available to reduce harmonic distortion, including active and passive harmonic filters, 18-pulse harmonic-reduction drives and active front-end drives. A harmonics analysis of the system can help in selecting the right solution.

Are VSDs necessary for all motors on a multiple pump or fan system? In the case where more than one motor supplies power to one fluid flow (gas or liquid) stream, it’s usually best to designate one motor as a trim unit and the remaining motors as baseload units. The trim unit will be controlled by a VSD and bring the flow level up to the baseload motor capacity. Then the non-VSD baseload motor will operate at full rated capacity while the trim unit drops down and smoothly adds capacity up to the simultaneous operation of a second baseload unit and so on.