Digital measurements of the efficiency of inverter-induction machines

The authors describe a digital technique and the required instrumentation to measure the electric power output into inverter-induction machine drives, where the voltage and current signals are nonsinusoidal. The voltage and current signals representing the electrical power input into the motor are stored in a suitable storage oscilloscope with built-in computing capability. The power input is calculated by multiplying the appropriate voltage and current signals and integrating over one cycle. The mechanical power output is obtained from a standard strain-gage-type torque meter. The ratio of the output-to-input power yields the efficiency. The accuracy level attained is within ±1.0% and was verified by comparison with standard instruments using sinusoidal power and by checking the power flow into a resistive load for nonsinusoidal waveform. In both cases, the digital instrumentation gave results that were within ±0.5% of the standard instrumentation. Results of digital measurements obtained on a set of drives in the range of 2-50 hp that were operated over a speed range of 10-100% of rated speed and at rated load torque are presented