· Measurement Bandwidth Needed for Inverter Measurement
The most common use of inverters is for motors, and motors constitute loads with resistance and inductance connected in series. As an example, if a PWM voltage of fundamental frequency 30 Hz and carrier frequency 10 kHz is applied to an R-L load of R = 1 Ω and L = 1 mH, figure 9 shows the R-L load frequency characteristics and the content ratio spectrum of PWM voltage signals and active power.
Even if a PWM voltage that has high-frequency components is applied to the R-L load, the load characteristics almost totally prevent harmonic current from flowing. As stated in section 2.2, it is enough to use a measuring instrument having low frequency bandwidth characteristics in terms of either voltage or current for measurement of active power. Therefore even if extremely high frequency components are included in the voltage PWM signal, high measurement bandwidth is not necessarily required as these components are not included in the current signal. Considering the example in figure 9, in the case of motor drive inverters, it is sufficient to have a measurement bandwidth of up to several times the carrier frequency to maintain a certain degree of high measurement accuracy.

· Notes on Voltage Measurement with Recent Inverter Drive Motors
When testing inverter motors, the motor’s drive characteristics are thought to be influenced by the RMS fundamental wave of the inverter’s output voltage. Also, as the fundamental wave RMS value of sinewave modulated PWM is almost identical to the measured value obtained with the rectified mean value calibrated to the rms value (voltage MEAN), measurement by rectified mean value calibrated to the rms value seems to be sometimes used for inverter voltage measurements. However, with variable PWM control in recent years and other non-sinewave PWM modulation signals, there are cases in which the rectified mean value calibrated to the rms value yields measurements that are far removed from the fundamental wave. In such cases, as in section 3.4, the powermeter can accurately measure the fundamental wave if it employs a harmonic measurement function. Conventional powermeters required a changing of modes when measuring active power or RMS current with high-speed sampling, but more recent powermeters can measure voltage, current, power, and harmonics simultaneously. This not only makes measurement easier, but also allows simultaneous measurement of power and fundamental wave values. With inverter drive circuits, heat emissions that change with the passage of time have a substantial effect on instrument characteristics, so it can be very important to capture data at the same time.