Q8Electric Drives

Several factors influence the overall efficiency of an induction motor drive system: motor-internal losses, including stator and rotor copper (I²R) losses (which increase with load current/torque), iron/core losses (hysteresis and eddy-current losses in the stator/rotor magnetic cores, which depend on flux level and frequency), and mechanical friction/windage losses; power-electronic converter losses, including semiconductor conduction losses (proportional to current and device on-state voltage drop) and switching losses (proportional to switching frequency and the voltage/current being switched, as discussed in the hard-switching/soft-switching comparison elsewhere in this paper); harmonic losses, since a PWM inverter's output voltage waveform is not a perfect sinusoid but contains switching-frequency harmonic content, which induces additional eddy-current and stray losses in the motor beyond what an ideal sinusoidal supply would cause; and the motor's operating point relative to its design rating, since induction motors (like most electrical machines) are typically designed to achieve their peak efficiency at or near their rated load and speed, with efficiency generally decreasing somewhat at very light load (where fixed losses become a larger fraction of the reduced output power) or when operated significantly below rated speed/flux in certain control modes.