Efficiency measurements on electric motors

Increasingly, electric motors are controlled with variable speed drives. This is to achieve higher efficiency in control. But how efficient is this motor control actually? For accurate measurements of the entire drive, we need to measure the entire drive chain. In this example, we assume a three-phase AC supply which is rectified to a DC supply for the inverter. We then measure the electrical output of the frequency inverter and the mechanical energy of the electric motor to get an overall view of the various losses or the efficiency of the entire system.

In EV traction systems, the AC input and rectifier are omitted and we have as the input of the three phase inverter the battery of the car.

Synchronization

In these measurements, synchronization of the measurements is vital so that the instantaneous efficiency is calculated correctly. It is common for a power analyzer to use the nominal update rate set by the user (e.g., 500 ms) as the size of the internal data acquisition window, regardless of the frequency of the motor being evaluated. However, for optimal measurement, the measurement window must be synchronized with the fundamental time period or “motor frequency” (relative to time). If the measurement window is correctly sized to a single or even multiple fundamental time periods, the RMS calculations of voltage, current and power will be calculated correctly. If the acquisition window does not contain an exact integer number of cycles, this will cause significant measurement error as the update frequencies approach the motor frequency time period, Figure 1 illustrates the problem.

Harmonics

Besides synchronization, the specifications of the power analyzer must be carefully considered given the higher harmonics created by the use of the AC drive. This needs some explanation. The speed of the motor is controlled by the variable speed drive. This is done using IGBTs and pulse width modulation. The square wave nature of pulse width modulation (switching the maximum voltage on and off by a certain width) creates higher harmonics in the control signal to the motor.

As we know, the “in phase” components of voltage and current will result in real power. Therefore, the true power of a PWM system can only be determined with measuring equipment that accurately measures power signals over a frequency range that includes all frequency components.

Thus, we must ask whether the bandwidth and sample rate of the power meter is sufficient to include the contribution of the higher harmonic to the total power and system efficiency.

Accuracy

With efficiency improvements on the overall system, we often talk in hundredths of percent improvements. This is because efficiency within electronics is pretty high anyway. The efficiency of rectifiers is around 98 to 99%. For inverters, we now see efficiencies of 94% to 95%. If you want to improve this, then the accuracy of the power meter is crucial.

We’ll work this out with an example. If you have a power meter with a basic accuracy of 0.1% for voltage and 0.1% for current, the effective measurement uncertainty for the power measurement is 0.14%. For the second power measurement, we have the same measurement uncertainty where we effectively end up with 0.2% for the efficiency measurement. With an accuracy of 0.03% for voltage and current, we arrive at 0.06%. For critical efficiency determinations, we would prefer an even smaller percentage here.

Then back to our example for a moment.

In this setup, we use a Newtons4th 6 channel power analyzer with a dedicated input for a torque and speed sensor. For the input power, we assume a three phase two wattmeter technique to calculate the total power of the AC input(PH1 + PH2). In addition, we measure the DC power (with PH3), the output power of the frequency controller (PH4+PH5+PH6) and the mechanical power through torque and speed sensors. In this way, we have enough of a 6 channel power analyzer for the complete test setup.

The phase 7 measurement as mentioned above is the calculated power of the three phase of the input. Thus, the total power of the AC input is 1590.91W (532.77 + 521.64 + 536.50). The DC power is 1569.0W, making the efficiency of the rectifier 98.62%. The electrical output power of the frequency regulator is 1437.8W which equates to an efficiency of 91.64% for the frequency regulator and calculated 90.37% for the total electrical power (90.43% measured). The mechanical power is unfortunately not shown in this example. As you can see, selecting the right power meter for your application involves quite a bit in terms of different specifications. Let one of our consultants properly inform you so that you can make the right choice in terms of the number of channels, bandwidth and accuracy of both the power meter and the necessary accessories for your application.

Back to the overview