Factoring In the Power Factor

Factoring In the Power Factor

Recently our customers have indicated a great concern over power factors when starting new development projects for higher-efficiency motors.

While in Europe there has been an emphasis on power factor and power quality for many years, while the U.S. has largely ignored any emphasis on these concerns — but that is changing.

First, let us quickly define what is the “power factor,” or PF, as it is often referred to.  The power factor originates in the world of AC motors and power transmission, and it is a measure of the relationship between the AC voltage and the AC current.  In any power distribution system, we have transmission lines (inductor) that have coupling to the earth plane (capacitor) and AC motors that have parasitic inductances. All of these stray components offset the AC current from the AC voltage and hence the PF, which is always equal to or less than 1.

Why are we concerned? Useful power at the consumer load (motor, plant) is a function of both the voltage and current that are in phase with the AC voltage (real power). If there is a phase shift, then the total current to the consumer must be larger than the “real” component that performs the work, i.e. — Real Current = Total Current/PF.

The power company is primarily concerned since the generators that provide the current must be rated for the total current, and the generating plant will typically have a PF = 1. Thus the power company will have to generate power that equals the total current but they will only get paid for the “real” power consumed by the load.

The customer will also pay for losses due to power factor since the power losses in the plant’s wiring are based on total current rather than “real” current, and all the wiring and circuit breakers must be sized to handle the total current which can easily add 20% or more to the size of internal wiring and breaker capacity, which can be a sizeable cost. In addition the customer will be billed for the I2R losses in the internal wiring as that is real power which is consumed even though it never provides any useful work and worse yet may require additional air conditioning to get rid of this parasitic heat.

Inverters, especially smaller ones can significantly distort this picture since the input current is no longer a sinusoidal waveform and they often can have a very low power factor. Unlike the PF in large machines that still results in sinusoidal waveforms these inverters can generate a large amount of harmonics that can interfere with computing equipment and sensors.

Hence, power companies and regulators are pushing for better quality and improved PF which is reflected by the demands from our customer base. While improved motor design can help improve PF most motors today are driven by inverters and the solution to better PF and power quality lies primarily with the inverter manufacturer.

Electronic power factor correction is a well-known way to achieve an ideal power factor of virtually PF = 1, but until recently it added much cost to the design of the inverter.  This is about to change as new, fast switching technology enters the marketplace at increasingly competitive prices: wide bandgap switches such as gallium nitride (GaN) and silicon carbide (SiC).

These devices allow us to incorporate active PF correction in an inverter as virtually no added cost, save for the cost differential for the power switches, since the cost for the added circuitry can be offset with savings in other components such as the DC link capacitors. These designs will also reduce other power quality issues and they can add to increased overall motor efficiency especially when operating at reduced loads. The prices for these new wide bandgap switches are rapidly decreasing and they are proliferating rapidly.

We can look forward to a new generation of inverters with built-in PF correction and excellent power quality characteristics which will save plant owners significant amounts of energy costs while offering the added benefit that they are a “green technology” that pays. We can reasonably expect that within the next decade the power factor should no longer be a factor.

 

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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