Speed control for industrial ac motors is one of the most important applications in today's process/industrial control environment. It involves designing an inverter of appropriate size and capacity to control the power for a given motor. The motor speed responds in proportion to the voltages and frequencies coming from the inverter. A host of vendors provide off-the-shelf and custom inverters, known as adjustable-speed motor drives, for applications ranging from irrigation pumps to wind tunnels.
However, just providing the raw power isn't enough these days. Motor drives present a nonlinear load to the power grid feeding them. Characteristically the diodes in an inverter's input stage create harmonic distortion as they switch on and off. Reflected back into the line, these harmonics tend to sap usable power and cause overload in transformer neutrals, circuit breakers, and motors. They can even damage other sensitive equipment connected to the same source. Some industry estimates predict that harmonics will eventually consume up to 50% of the energy on the nationwide power grid-clearly anunacceptable situation.
The solution, of course, is to suppress harmonics. The common approach is to install filters intervening between the power line and the inverter (Fig. 1). In effect, a filter gives the motor controller a source of lower impedance from which to draw energy. The filter itself then recovers without sending harmonics back into the line. This is largely a function of the filter's inductance. Similarly, the filter's capacitance is tailored to maximize the system's total power factor. Like motor speed controllers, harmonic filters are widely available today in standard and custom configurations.
Computer-based load modeling programs let installers forecast the harmonics that will be produced by a motor controller. Thus the site can be prepared to accept the needed filters and power cabling before the first switch is thrown. Modeling results are usually rather accurate if the initial system specifications are correct. Even so, many customers demand proven compliance with the IEEE 519-1992 specification, which defines maximum harmonic levels and prescribes measurement methods for determining "actuals." As proof of compliance, it's common for the motor drive vendor to provide paper copies of test results showing before-and-after harmonic readings.
Harmonic measurements traditionally have been the province of dedicated, single-purpose harmonic analyzers. Although these provide satisfactory results, many of the instruments are expensive, not particularly portable, and they must be operated by skilled technicians. Consequently, harmonic measurements are often subcontracted to specialists. Of course, this service adds cost to the installation and consumes time as well. Motor drive vendors have long sought a solution that allows them to bring the harmonic measurement function "in-house" and offer a complete installation service package to their customers.
Recently, a new generation of multipurpose solutions has appeared-instruments that combine voltage, current, power, waveform, and harmonic measurement capabilities in one compact hand-held battery-powered tool. This new class of tools eliminates the need to carry separate instruments to perform each of the many measurements required at installation time. As a result, hand-held "do-everything" instruments are finding their way into many installation tool kits.
Using modern equipment, harmonic measurements are not difficult. The procedure involves connecting an appropriate probe (high-voltage passive probe for voltage harmonics, current probe for current harmonics, or both for power measurements) to one phase of the incoming power line and invoking the harmonics mode. Normally the three phases are balanced and symmetrical, meaning that it is only necessary to take readings on one phase. The result, captured with a Tektronix model THS720P hand-held oscilloscope set to its "Harmonics" mode, is shown in Fig. 2.
The display shows one phase of a typical three-phase motor drive distortion profile, minus filtering. It contains large amounts of odd-order harmonic distortion, especially on the 5th, 7th, and 11th harmonics. The bar graph is an accurate and easily understood way to present the information. Note that the third harmonic is conspicuously absent-a characteristic of many types of three-phase loads.
The IEEE 519-1992 specification calls for less than 5% total harmonic voltage distortion returned to the power grid (and lower yet if the installation is near a hospital). As Fig. 2 reveals, the distortion here is much higher. Because almost all motor controllers are installed with at least some filtering to begin with, this is an exaggerated situation. But it takes only a small design miscalculation to produce harmonics exceeding 5%. Line length-the distance between the controller and the motor itself-can also affect the harmonic content.
With proper filtering, the harmonics are drastically attenuated, as shown in Fig. 3. Now the total harmonic contribution is far below 5% and well within the tolerances outlined in IEEE 519-1992. Again, the bar graph makes it easy to interpret the measurement results at a glance.In addition to the ha rmonic bar graph, some of today's power measurement instruments offer tabular displays of power factor, phase shift, and statistical power information. Power factor is important because it affects the motor user's cost of power. Most utilities charge higher rates for lower, less efficient power factors.
Commercial PC software packages such as Tektronix WaveStar are available to aid scope users in capturing screen images (for example, the harmonic display) for publication and further analysis. This is a valuable feature when documenting proof-of-performance for motor drive customers or power providers.
Harmonic distortion in ac industrial motor drives is a common, but correctable, problem. Given the right measurement tools and a few simple readings, it is easy to ensure compliance with industry standards such as IEEE 519-1992. Today it is possible with just one instrument to fully validate a motor drive installation in terms of its harmonic distortion compliance, dc functions, waveform content, and power factor rating.