Power Factor Monitoring Improves Efficiency

Take a walk through any plant that processes chemicals or materials and you won’t get far before you spot an AC induction motor in action. These workhorse motors account for more than 90% of all installed industrial motors in use today, and they power many of the pumps, fans, compressors, mixers, processing machines, and material-handling systems that keep those plants humming. Many of those motors, however, simply do not run as efficiently as they could.

That is the case because induction motors are designed to run most efficiently at high loads, usually at least 70% of their full rated load. Yet in real-world applications, in which engineers must size motors for peak loads, induction motors often operate at less than 40% of full load for extended periods. In fact, according to an estimate from the U.S. Department of Energy (DOE), 44% of industrial motors routinely operate at less than 40% of full load.

The resulting efficiency penalty can be severe (Figure 1). Consider modern NEMA Premium efficiency motors. These best-of-class AC induction motors routinely attain full-load motor efficiencies that range from 80% to more than 93% in motors over 100 hp. Yet even in these highly efficient motors, efficiency drops well below 60% at low loads. The low-load efficiency penalty is even more pronounced in older legacy motors, which still make up most of the installed motor base.

All this lost efficiency wastes an enormous amount of energy and drives up energy costs, particularly in process industries, in which electric motors account for more than 70% of total electricity use. In a 2002 assessment of industrial motor use in the United States, the DOE estimated that motor efficiency upgrades would save nearly 20 billion kWh, or the equivalent of taking more than 2.6 million cars off the road. That figure includes only improvements in motor efficiency, not the even greater saving that could be achieved by redesigning motor-driven processes from top to bottom.

Fortunately, there are a number of well-known strategies for improving motor efficiency. One obvious strategy is to select the right motor for the load at hand, avoiding oversized motors that spend much of their time running inefficiently under low-load conditions. Another well-known and increasingly popular strategy employs variable-frequency drives (VFDs). Although VFDs often are implemented to enhance process control, they also can save significant amounts of energy by adjusting motor speed to actual load demands. Consider, for example, that a 50% reduction in speed typically reduces power consumption by a factor of 8. VFDs also can push the power factor of motor-driven systems above 0.9, shaving even more money off utility bills.

Both of these strategies can be effective in many applications, but they have their limits. Engineers do not always have the option of selecting new motors and must live with inefficient legacy motors. In addition, motors commonly are upsized for good reason:  to accommodate those peak loads. In these cases, running at less than an efficient condition is simply a fact of life.

VFDs can be pricey and are not best suited to applications in which a constant motor speed is a process requirement or the motor’s power requirements do not fall with speed, for example, in a mixing application that requires high torque at low motor speeds.

For these applications, the energy efficiency of motor-driven systems can be improved through a little-known motor control technique that is based on power factor monitoring. Many engineers know of power factor (Figure 2), or the ratio of real to apparent power, as an aspect of electrical systems that can drive up electric bills. Utilities have long imposed extra charges on systems with low power factor to make up for the fact that those systems require more current to perform a particular amount of work than do systems with high power factor.

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Source: Engineering Cases - Knovel