The Power to Measure

The renowned Charles Steinmetz managed to develop AC power theory and applications while associated with General Electric, even though he didn’t have a modern power analyzer. In fact, he didn’t need one a century ago because AC power devices were driven by continuous sinusoidal voltage and current waveforms.

More recently, the trend has been toward discontinuous, variable-rate motor drives, and smooth sine waves are seldom encountered. Measurement capabilities beyond those of a voltmeter or a CRT that only displays Lissajous figures are required to cope with today’s complicated signals.

AC power analyzers are specialized data acquisition and analysis instruments used in both single- and three-phase applications. They have greater resolution and accuracy than digital oscilloscopes, multiple voltage and current channels for three-phase measurements, faster sampling than digital voltmeters, and, most importantly, built-in power-analysis functions. Power factor, efficiency, and harmonic content can be determined as easily as peak or rms voltage and current.

A number of considerations affect the suitability of a particular power analyzer for an application. For example, resolution and accuracy must correspond to the levels required by the tests being done. The type and number of analysis functions provided may be important. There must be enough input channels. But signal speed is the major factor that determines the necessary sample rate and bandwidth and much of the cost of the instrument.

For equipment such as motors and heaters that are driven directly from AC power and don’t rectify or chop the input voltage, the current and voltage waveforms are sinusoidal. In these cases, the power factor is given by

pf = cosq (1)

where q represents the phase angle between the voltage and current. Waveform distortion is relatively low, and instrument bandwidth is only a few kilohertz.

The power factor also can be expressed as the ratio of instantaneous to effective power or

pf = instantaneous W/VA (2)

where VA is commonly used to refer to Vrms * Irms. Instantaneous power is determined by multiplying simultaneous values of voltage and current. This definition proves to be more useful for applications involving discontinuous voltages and currents caused by switch-mode techniques.

Many products rely on fast semiconductors with high voltage and current ratings to provide efficient and flexible power control. Power dissipation is minimal when the switching device is fully on or off, and a fast transition minimizes losses between states.

Two examples of such products are variable-speed AC motor drives and electronic ballasts for fluorescent lamps. In these types of applications, the fundamental switching frequency may be kilohertz to tens of kilohertz. Consequently, instrument sampling rates typically range from 100 kS/s to a few megasamples per second to capture the high-frequency waveforms.

Variable-Frequency AC Motor Drives

AC motor drives now account for a large percentage of variable-speed motor applications and offer advantages over DC drives that include lower initial motor price, reduced operating and maintenance costs, and very high power ratings. In addition, the power-factor-corrected input characteristic of AC motor drives reduces power corruption and simplifies plant wiring where several large machines are involved.

Motor drives typically store energy in a DC link inductor or a capacitor between the motor load and the AC input. A pulse-width modulated (PWM) inverter produces variable-frequency AC from the raw DC. See Figure 1. Torque is related to the amplitude of the DC voltage, and speed is proportional to the output frequency of the inverter.

Unfortunately, fast switching creates harmonics and possibly EMI. In addition, audible noise and overheating can be caused in motors driven with harmonically rich waveforms, and high dV/dt waveforms stress winding insulation.

Audible noise results from motor lamination vibration and fan windage. A higher fundamental inverter switching frequency generally corresponds to a higher-fidelity sine waveform and less vibration.

A so-called full spectrum inverter control algorithm is one way that drive manufacturers reduce noise levels. The algorithm spreads the frequency content of the PWM output to produce a lower average noise level while still satisfying the motor speed and torque requirements.

“Proper phase, amplitude, and frequency characteristics of motor drive signals under start-up and steady-state conditions can have a significant impact on motor performance and life span,” said Herman van Eijkelenburg, product marketing manager at California Instruments. “Growing EMC regulations also are forcing drive designers to pay close attention to design choices for drive circuits. The need to reduce audible noise often requires higher PWM frequencies. The availability of new insulated gate bipolar transistors (IGBT) devices with lower losses and higher switching rates makes these design criteria easier to meet.”

For motors with internal cooling fans, winding temperature can rise to high levels at slow speeds because the airflow slows down. Overheating also can occur at full torque and maximum speed. In this case, the PWM harmonics are responsible for the increased losses compared with an AC-driven motor.

Bill Gatheridge, product manager for power measuring instruments at Yokogawa Corp. of America, said, “The 5^th harmonic current, nominally 300 Hz, is commonly referred to as a negative sequencing current. This actually can cause a negative or reverse torque in the motor. In turn, overheating can result due to an increase in the fundamental current required to overcome this opposing torque effect.”

One way to keep a motor cool is to use a separate, constant-speed fan. Another approach to handle increased winding temperature incorporates an insulation system with a higher temperature rating. For example, Underwriters’ Laboratories rates the varnish insulation system used on MagneTek aluminum wire at 20°C higher than the equivalent copper magnet wire:

“The temperature ratings are based on the Arrhenius curve, which predicts the rate of degradation of a given insulation system based on its survival under specific test conditions. UL has established that for the MagneTek insulation system, every increase of 8.2°C reduces the life of the insulation by one half….

“One of the inherent properties of aluminum is that it quickly forms a very tight oxide skin that is also a good insulator. This adds to the wire varnish’s ability to provide good insulation over a long time. Copper, on the other hand, inherently catalyzes the thermal degradation reaction of all organic polymers. In other words, the chemical nature of copper accelerates the thermal breakdown of the insulating varnish over time.”¹

Variable-speed motor drives produce the best results when they are matched to the motor being driven. Some of the differences between inverter- and AC-power-rated motors include separate fans, higher-temperature insulation, attention to the rotor-bar design to minimize high-frequency skin-effect losses, and increased rotor-bar insulation even in high torque applications. With regard to this last point, variable-speed drives provide a soft-start capability that avoids excessive mechanical stress between the bars and the rotor in which they are mounted. Increased insulation can be introduced, and although it lowers the stiffness of the rotor assembly, it also stops inter-bar currents which reduces losses.

Electronic Ballasts

Electronic ballasts for fluorescent lighting reduce costs because they are about 30% more efficient than electromagnetic ballasts, and they ensure a proper starting sequence that prolongs the life of the lamp. Ballasts deliberately run the fluorescent tubes with a high-frequency sinusoidal waveform to improve efficiency.

The importance of lamp-starting details depends upon how the lamp is used. In many factories, for example, lamps are seldom turned off. Here, the simplest approach is to apply a voltage high enough to cause the lamp electrodes to arc without preheating the filaments.

This is called instant starting and a method that ANSI defines, although it does shorten the life of a tube if repeated often. Using the instant-start approach reduces the number of wires that must be run because only one side of each filament is connected. Blackened areas near the ends of a fluorescent tube are caused by sputtered cathode material and eventually will result from instant starting.

Rapid starting, also defined by ANSI, applies a lower voltage and simultaneously preheats the filaments. It is a more controlled approach than instant starting, but tests by MagneTek and others have found little difference in lamp life between the two starting systems in frequently switched applications.²

In contrast, Figure 2 shows a typical sequence of start-up events for a lamp driven by an electronic ballast. Here a preset time is allowed for filament preheating. By the end of the preheat period, the tube voltage has risen sufficiently to strike an arc. This approach is much less violent than either instant or rapid starting and prolongs tube life.

A recent innovation that has emphasized the importance of lamp starting is occupancy sensing. Instead of using a manually operated switch, a person controls the light simply by walking into a room or leaving it. Consequently, a starting method is required that reduces lamp burnout to achieve the full economy of such a system.

Power-Analyzer Specifications

Both PWM motor drives and electronic ballasts involve high-speed switching signals, and efficiency is an important characteristic in actual applications. This means that you need high-accuracy measurements of the AC voltage and current as well as ballast output or motor torque and rpm measurements.

According to Tom Mahr, vice president and general manager at Voltech Instruments, “Typically, measurement accuracy should be on the order of 0.05% for the detection of losses and efficiency within a motor. This is vital because even a minor performance improvement that may not be visible on a DSO, for example, can make a significant difference to enhancing a motor’s efficiency.

“Precision analysis requires input stages with a very high common-mode rejection ratio, excellent basic accuracy of the analog amplifiers and analog-to-digital converters, and precise simultaneous sampling of both the voltage and current channels,” he continued. “If the samples of voltage and current are not simultaneous, then phase errors will severely affect the watts measurement.”

In fact, for the case of sine-wave power, if any delay difference between the voltage and current measurements is expressed as a phase delay d, then equation 1 becomes

pf = cosq * (1-dtanq) (3)

The delay error (dtanq) increases with a decreasing power factor.

For the ballast, Figure 3 shows how six measurements are required to derive the lamp current and the ballast efficiency, even though this is a single-phase lighting example. A three-phase motor drive would require eight inputs if the power in each phase as well as the output power were to be measured.

Figure 3. Fluorescent Lamp and Ballast Measurements