This example shows a PWM VSI induction motor drive with a braking chopper for a 3HP AC motor.
H.Blanchette, L.-A. Dessaint (Ecole de technologie superieure, Montreal)
The induction motor is fed by a PWM inverter, which is built using a Universal Bridge Block. The speed controller consists in a PI regulator that produces a slip compensation, which is added to the rotor speed in order to derive the commanded stator voltage frequency. A constant volts per hertz ratio is also applied to the motor. The motor drives a mechanical load characterized by inertia J, friction coefficient B, and load torque TL.
Motor current, speed, and torque signals are available at the output of the block.
Start the simulation. You can observe the motor stator current, the rotor speed, the electromagnetic torque and the DC bus voltage on the scope. The speed set point and the torque set point are also shown.
At time t = 0 s, the speed set point is 1000 rpm. As shown in Figure 0-10, the speed follows precisely the acceleration ramp.
At t = 0.5 s, the full load torque is applied to the motor shaft while the motor speed is still ramping to its final value. This forces the electromagnetic torque to increase to a high value and then to stabilize at 11 N.m once the speed ramping is completed and the motor has reached 1000 rpm.
At t = 1 s, the speed set point is changed to 1500 rpm and the electromagnetic torque reaches again a high value so that the speed ramps precisely at 1800 rpm/s up to 1500 rpm under full load.
At t = 1.5 s, the mechanical load passed from 11 N.m to -11 N.m , which causes the electromagnetic torque to stabilize at approximately at -11 N.m shortly after. Note that the DC bus voltage increases since the motor is in the braking mode. This increase is limited by the action of the braking chopper.
1) The power system has been discretised with a 2 us time step. The speed controller uses a 100 us sample time and the space vector modulator uses a 20 us sample time in order to simulate a microcontroller control device.
2) In order to reduce the number of points stored in the scope memory, a decimation factor of 25 is used.
3) A simplified version of the model using average-value inverter can be used by selecting 'Average' in the 'Model detail level' menu of the graphical user-interface. The time step can then be increased up to the control system sample time value.This can be done by typing 'Ts = 20e-6' in the workspace in the case of this example. See also ac2_example_simplified model.