Electric Power Measurements on Three-Phase Motors

Electric Power Measurements on Three-Phase Motors

Three-phase topology has become the norm of all industrial motors due to the operational and economic benefits they possess. The most notable advantage of a three-phase motor is its ability to start automatically without needing additional windings. Furthermore, in a three-phase system the amount of copper connections required are only half of that required for single phase ones.

Testing a pulse width modulation (PWM) based drive and motor system can be broken down into three major steps:

  • Accurate measurement of PWM VFD input and output power in order to safely identify inefficiencies
  • Accurate measurement of motor input power
  • Accurate measurement of rotor mechanical power

Instead of carrying out these steps independently, they can be integrated with the help of a single tool such as a Power Analyzer. These nifty tools have both hardware and software capabilities to provide accurate and reliable measurements. Some power analyzers have options that allow the speed and torque signals of a motor to be integrated. They measure electrical as well as mechanical power, sending the collected data to a PC running client software, which may be provided either by the manufacturer or the System Integrator.

PWM Drive Measurements for AC Motors

Both input and output signals of a VFD hold importance when a PWM variant is being used to operate a motor. This is done using a six-phase power analyzer, which allows measurement of 3-phase power as well as dc power in the system. Depending on the analyzer, the setup mode will be performed in the normal or RMS mode.

All low-pass or line filters should be turned off as they may influence the results. However, zero-cross or frequency filters should remain on as it will filter the high-frequency noise while the fundamental frequency is being measured. This is necessary when the frequency of the drive is being tracked. High-frequency switching on the voltage signal can result in a distorted waveform that consists of high harmonics, with the frequency varying from 0Hz up to the operating speed. In order to carry out accurate measurements of such signals, special current sensors are needed. Furthermore, accurate PWM power measurements also need wide bandwidth power analyzers and special current sensors to measure complex signals.

PWM Motor, Drive Measurement Issues

Inverter voltage can be measured in two ways. First, a true RMS measurement can be taken that incorporates total harmonic content. But because the fundamental waveform contributes to the motor’s torque, a simpler measurement technique can be utilized. In most applications, the measurement of the fundamental waveform is sufficient.

Measuring the fundamental amplitude of the voltage wave can also be done in two ways. A low-pass filter can be used to remove the high frequencies, which is something usually provided in a power analyzer. Proper filtration would provide an accurate RMS voltage value of the fundamental frequency, but the true total power measurement is missed. Therefore, another method is employed that is known as rectified mean measurement method, delivering an RMS voltage of the fundamental wave without filtration through mean-value voltage detection scaled to the RMS voltage. The final value will be very close to the actual RMS value, while the true total power reading is also preserved. Through this method, the total power, total current and fundamental voltage are obtained.

Fundamental Wave Measurement through Harmonic Analysis

The harmonic analysis function can be utilized to determine the true fundamental voltage through Fast Fourier Transform, allowing the amplitude of each harmonic component to be computed. This gives a highly accurate RMS voltage measurement of the fundamental wave. Modern power analyzers have the capacity to make harmonic measurements along with true RMS measurements.

A true RMS signal is the only way to measure inverter current as all constituent harmonics result in rise in temperature of the motor.

A PWM drive should have a constant V/Hz ratio over the operating speed of the motor. The power analyzer can be used to compute this through the fundamental or RMS voltage value. An equation for this measurement is carried out by the analyzer’s user-defined math function.

Measuring DC Bus Voltage

Over- and under-voltage conditions can be checked in the PWM by measuring the DC bus voltage. The measurements can take place inside the drive across the terminals of the capacitor bank. But an easier method also exists, which is to use a power analyzer waveform along with cursor measurement.

When the waveform is displayed on the screen, the cursor’s position can be used to measure the DC bus voltage. It must be ensured that the cursor isn’t placed on top of the small spikes. Instead, it must lie across the waveform, so an accurate measurement can be made.

Mechanical Power Measurements

The mechanical power is computed as the product of motor speed and torque. There are various speed and torque sensors that are compatible with a wide variety of motors, However, before you rush in and purchase a generic model, be sure that it is compatible with your motors. The sensors can be integrated with the power analyzer, feeding it the necessary mechanical measurements for calculations.

Many sensors incorporate electronic components that carry out signal conditioning so that the readings can be used by the power analyzer. The conditioned signal may be analog or serial, acting as input to a PC and client software.

One notable option to carry out mechanical power measurements is to use sensors and a measurement instrument from a single manufacturer. This will ensure that there are no incompatibilities and the sensors’ outputs will seamlessly act as inputs for the analyzers. Measurements such as torque, speed and power will be readily available while options such as PC connectivity will always be present.

Another integrated approach is connecting the mechanical sensors to the power analyzer. The speed and torque outputs from the sensors act as inputs for the analyzers, allowing users to evaluate both electrical and mechanical parameters directly and continuously.

Motor, Drive and System Efficiency

Division between output and input powers provides us with the inverter efficiency. One method to calculate this is by measuring input and output powers through meters and then using the values to calculate efficiency by hand. On other hand, a more comprehensive approach exists, which is using multiple input power analyzers to measure both the input and output powers at the same time. This will result in highly accurate calculations while the real-time changes can also be tracked.

The drive loss and drive efficiencies can be computed by navigating to the internal math calculations options of the power analyzer.

Which method should be used?

IEEE 112: Standard Test Procedure for Polyphase Induction Motors and Generators is a well-established standard for motor testing in the US, outlining several methods of achieving accurate results. Most power analyzers are designed around this standard, allowing users to select the method as well.

  • Method A: as defined by IEEE 112, the efficiency is calculated as the ratio of the measured input and output powers, after any temperature and dynamometer corrections have been applied. Testing is carried out either through a dynamometer or a mechanical brake. The method is usually used for motors that have full load ratings of 1kW or less.
  • Method B: both input and output power measurements are made, however, losses are identified as well. These losses represent the heat produced by the motor’s assembly, representing energy that is otherwise not able to perform work. The method is typically used for motors that have full load ratings between 1 – 300 kW.

Both Methods, A and B, will give accurate results but will have differences as the latter requires a lot of instrumentation and is usually carried out be manufacturers. On the other hand, Method A is employed by users, and has a lower accuracy compared to B. Furthermore, there may also be differences between the manufacturer’s and user’s results as testing will be carried out at different speeds, loads and conditions.

Conclusion

When power is being measured in an electric motor there are several factors that need to be considered, notably, total and true power factor. These measurements are carried out after the data goes through complex equations, which is why a power analyzer is the best way to get reliable results.

Once the management is set on procuring a power analyzer, decisions on the range of frequencies and accuracy levels need to be made. Furthermore, compatibility of instruments with sensors must be ensured in the beginning to avoid problems later on, specifically with current transformers. If the sensors are compatible with the power analyzer, the mechanical measurements can also be carried out in an integrated manner. Therefore, selecting the right speed and torque sensors is the initial step towards determining mechanical power from the power analyzer.

A precise high-frequency power analyzer is a useful tool for measuring both electric and mechanical power. It has complex functionalities that allow analysis of the readings to be carried out, which can then be used to rectify any mistakes present in the motor’s operating conditions. This will ultimately lead to longevity in the motor’s lifespan, and thus, boost economic productivity.

 

 

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