The Driving Force

The Driving Force

In “the good old days” we simply connected a motor to the power lines and that created motion — but those days are disappearing fast. Almost all motion today is controlled in some form, either by a high-end servo or at least a simple V/F speed control.

Initially, the need for accurate positioning was driving the development of hydraulic and electrical servo systems, and the emergence of suitable power electronics and microprocessors in the 1970s and 1980s dramatically changed the motion industry. But these systems were expensive and in many applications servos were considered ‘overkill’ when only variable speed was required. This led to the development of lower-cost speed controls for AC induction motors where users could add these devices and upgrade their applications cost-effectively to reap the benefits of variable speed. The most common solution today is the V/F drive that allows a standard AC induction motor to run at variable speed.

In this posting I want to focus on V/F drives and why they have become so popular and important. The original prime motivation for using these devices was to convert applications from fixed speed (dictated by the line frequency and pole count of the motor) to variable speed, where processes could be adjusted for synchronization and work flow; and that is how most users still see these drives. However, the addition of the V/F drive had other significant benefits that are becoming even more important today.

First, they reduce and or eliminate entirely the in-rush currents that are typically associated with starting a line-driven AC motor. The V/F drive coordinates both the frequency (speed) that the motor sees and also the effective voltage that is applied to the motor. Thus when the motor is at a standstill, a very small speed is applied (controlled slip) to maximize the AC motor’s torque output and efficiency. This would typically result in very large currents in the motor at full line voltage, but the V/F drive reduces the voltage that the motor sees, thereby controlling the current not to exceed rated motor current. As the motor speeds up the V/F drive will increase the field frequency and also the voltage that is applied to the motor for maximum torque output.

Since this controller varies both the voltage (V) and frequency (F), it is called a V/F drive. A typical AC motor has a low starting torque when it is directly connected to the power lines, along with a very high in-rush current. To achieve a specific starting torque it is often necessary to oversize the motor to insure that it will deliver the required starting torque. This has two negative side effects: first it increases the cost of the motor and secondly the motor may not run as efficiently at rated speed compared to a motor that is rated at the required running power consumption because it is oversized.

By adding the V/F we may now be able to reduce the size — and thus the cost — of the motor, which will partially offset the cost of the drive. But we also will be saving energy costs while running a motor that is now much more closely sized to the application and that will therefore operate more efficiently; this will most likely offset the higher acquisition cost of the motor/controller combination.

Regulatory pressure is building — especially in Europe — to eliminate the spikes on the power distribution system that are caused by the spikes resulting from the inrush currents. These spikes may interfere with sensitive computing equipment that is now common on the plant floor. New efficiency and power quality regulations are expected to make the addition of drives all but mandatory in the U.S. in the next decade.

Lastly, if power factor correction (PFC) is incorporated into these V/F controllers they will become even more efficient and improve power quality. With the advent of new wide bandgap power switches, PFC can now be implemented very economically and we can expect to see a new generation of V/F drives enter the market that will further improve the power quality and system efficiency while saving the user money in the long run.

About Author

George Holling

With over 70 publications and 9 U.S. patents on sensorless and efficient motor controls and low-cost power circuits to his credit, George Holling (PI) is an in-demand consultant to many major U.S. and International corporations for motors and drives. At present he holds significant influence in two companies — as technical director of Electric Drivetrain Technologies (2011– present) Moab, UT and as CTO of Rocky Mountain Technologies (2001– present), Basin, MT. Holling is a graduate of the University of Aachen, earning his B.S. (1974), M.S. (1978) and Ph.D. degrees there, while picking up his MBA here at the University of Wisconsin. His career has spanned both the commercial and academic arenas, the latter including stops at (Dean, Computer Science & Engineering) Utah Valley University, 2001 – 2003; and (Adjunct Professor), Western Michigan University, 1997 – 2002. From the commercial side, apart from his current positions, Holling has served as Project Engineer and Product Line Manager, UNICO; Franksville, WI (1978-1981); Project Engineer, General Electric, Medical Products Division, Milwaukee, WI (1981-1983; Manager R&D, Pacific Scientific/Honeywell Motor Products, Rockford, IL (1983-1985); Vice President of Engineering, Regdon Solenoid, Brookfield, IL (1988-1990); President, Advanced Motor Controls, Sun Prairie, WI (1990 – 1999); and Vice President of Engineering, Cordin Company, Salt Lake City, UT (1999 – 2000). Holling has also spearheaded projects for the development of high-efficiency motors and drives up to 400 kW, and has successfully negotiated licensing agreements with U.S., Chinese, Japanese and Indian customers for the licensing of motor and drive technology.

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