Electrical power measurement on 3-phase motors
Testing drive-and-motor systems is a three-step process.
Complete testing of a pulse width modulation (PWM)-based drive and motor system is a three-step process. Step 1 is accurate measurement of PWM VFD input and output power to identify drive efficiency and power losses. Step 2 is accurate measurement of motor input power. Step 3 is accurate measurement of motor mechanical power. The optimum method is to integrate all three steps using a single power analyzer to eliminate time skew. This provides excellent efficiency calculations as well, in one software and hardware solution.
In the first part of this three-part series, we examined basic electric motor power measurements and analysis. In the second part, we examined a three-step process for making precision electrical and mechanical power measurements on motors and variable frequency drive (VFD) systems with complex and distorted waveforms, and how these measurements are used to calculate motor and drive system efficiencies. In this third and final article on electric motor power measurement and analysis, we will cover power measurements for 3-phase ac motors and drive systems.
Some power analyzers have a motor option in which the speed and torque signals can be integrated in this manner. These power analyzers can measure electrical power and mechanical power, and send the data to a PC running software from the original analyzer manufacturer, or custom software from a system integrator (see Lead Photo).
PWM drive measurements for ac motors
When using a PWM VFD to operate a motor, it is often necessary to measure both the input and output of the VFD using a 6-phase power analyzer. Not only can this setup measure the 3-phase power, it can also measure dc or single-phase power (see Figure 1).
Depending on the analyzer, the setup mode will be performed in the normal or RMS mode. The wiring configuration should be set to match the application, such as 3-phase input and 3-phase output.
Any line filter or low-pass filter should be off because the filtering will obscure the measurements. However, the zero-cross filter or frequency filter should be on because it will filter the high-frequency noise so the fundamental frequency can be measured. This measurement is necessary when tracking the frequency of a drive.
Figure 2 shows a PWM output voltage waveform with a highly distorted voltage, chopped high frequencies, and a lot of noise on the current side, making for a difficult measurement. High-frequency switching on the voltage signal creates a much distorted waveform and with high harmonic content. The frequency varies from 0 Hz up to the operating speed.
For such a noisy signal, special current sensors are needed for measurement. Accurate PWM power measurements also require wide bandwidth power analyzers capable of measuring these complex signals.
Figure 3 is an example of the voltage harmonic content from a PWM output. Beat frequencies are present, and voltage harmonic content exceeds 500 orders (approximately 30 kHz). Most of the harmonic content is in the lower frequencies on the current side.
PWM motor, drive measurement issues
Inverter voltage is typically measured in one of two ways. A true RMS measurement that includes total harmonic content can be used. However, because the fundamental waveform is primarily what contributes to the torque of the motor, a simpler measurement can be made and used. Most applications only require measurement of the fundamental waveform.
There are two main methods for measuring the fundamental amplitude of the voltage wave. The first and simplest is to use a low-pass filter to remove high frequencies. If the power analyzer has this filter, simply turn it on. Proper filtering will give an RMS voltage of the inverter fundamental frequency. However, this type of filtering does not offer a true total power measurement, so filtering isn't the most exacting method.
The second method is the rectified mean measurement method, which delivers an RMS voltage of the fundamental wave without filtering by using mean-value voltage detection scaled to the RMS voltage. The algorithm of the rectified mean of a cycle average will provide the equivalent of the fundamental voltage that will be very close to the RMS value of the fundamental wave.
Using this method, the total power, total current, and fundamental voltage can be measured.