Brushless DC Motor Fed by Six-Step Inverter
This example shows the use of a Six-Step Switch-on mode for a trapezoidal PMSM motor rated 1kW, 3000 rpm and speed regulated.
Olivier Tremblay, Louis-A. Dessaint (Ecole de technologie superieure, Montreal)
A three-phase motor rated 1 kW, 500 Vdc, 3000 rpm is fed by a six step voltage inverter. The inverter is a MOSFET bridge of the Specialized Power Systems library. A speed regulator is used to control the DC bus voltage. The inverter gates signals are produced by decoding the Hall effect signals of the motor. The three-phase output of the inverter are applied to the PMSM block's stator windings. The load torque applied to the machine's shaft is first set to 0 and steps to its nominal value (3 N.m) at t = 0.1 s.
Two control loops are used. The inner loop synchronises the inverter gates signals with the electromotive forces. The outer loop controls the motor's speed by varying the DC bus voltage.
Observe the sawtooth shape of the motor currents. That's caused by the DC bus which applies a constant voltage during 120 electrical degrees to the motor inductances. The initial current is high and decreases during the acceleration to the nominal speed. When the nominal torque is applied, the stator current increases to maintain the nominal speed. The sawtooth waveform is also observed in the electromagnetic torque signal Te. However, the motor's inertia prevents this noise from appearing in the motor's speed waveform.
Change the "Back EMF flat area" of the motor from 120 to 0 and observe the waveform of the electromotive force e_a.