3644 (RP-667) -- Measurements of Electrical Power Inputs to Variable-Speed Motors and Their Solid-State Power Converters PDF

An increasing number of variable-speed, variable-output heating, ventilating, and air-conditioning (HVAC) equipment units continue being installed due to their high energy savings. However, because of the nonlinearity inherent in the semiconductor devices used in these power converters, the voltage and current waveforms supplied to the motors are not purely sinusoidal and thus contain an extremely high harmonic content. Many available instruments do not accurately measure the power or energy of this distorted current waveform due to limitations in the meter's frequency response, range, internal impedance, etc. Moreover, some of the few meters that can measure distorted waveforms and provide a total harmonic distortion (THD) of voltage and current take into account the first 50 harmonics. However, preliminary results show that in the case of a pulse-width-modulated (PWM) drive, if the THD of the input currents of the drive-fed motor is calculated taking into account the first 50, 100, and 200 harmonics, the values 19.18%, 20.28%, and 71%, respectively, are obtained - showing that a THD of the first 50 harmonics is not enough. Furthermore, currently available standards do not take into account the existence of harmonics for the proper determination of efficiency or separation of losses.

KEYWORDS: measuring, variable speed motors, electronic, controls, fluctuating voltage, instruments, measuring equipment, accuracy, meters.