AC3 - Sensorless Field-Oriented Control Induction Motor Drive

This example shows the Sensorless Field-Oriented Control Induction Motor Drive during speed regulation.

Souleman Njoya M., Louis-A. Dessaint (Ecole de technologie superieure, Montreal))

Description

This circuit uses a modified version of the AC3 block of Specialized Power Systems library. It models a sensorless field-oriented control (FOC) induction motor drive with a braking chopper for a 200HP AC motor. The motor speed is estimated from terminal voltages and currents based on the MRAS (Model Referencing Adaptive System) technique [1]. Consequently, the speed sensor (necessary in AC3) is no more required.

The induction motor is fed by a PWM voltage source inverter, which is built using a Universal Bridge Block. The speed control loop uses a PI controller to produce the flux and torque references for the FOC controller. The FOC controller computes the three reference motor line currents corresponding to the flux and torque references and then feeds the motor with these currents using a three-phase current regulator.

Motor current, speed (reference, real and estimated), and torque signals are available at the output of the block.

Simulation

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 500 rpm. Observe that 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 the user-defined maximum value (1200 N.m) and then to stabilize at 820 N.m once the speed ramping is completed and the motor has reached 500 rpm.

At t = 1 s, the speed set point is changed to 0 rpm. The speed decreases down to 0 rpm by following precisely the deceleration ramp even though the mechanical load is inverted abruptly, passing from 792 N.m to - 792 N.m, at t = 1.5 s. Shortly after, the motor speed stabilizes at 0 rpm.

Finally, note how well the DC bus voltage is regulated during the whole simulation period.

Notes

1) The power system has been discretised with a 2 us time step. The speed controller uses a 100 us sampling time and the vector controller uses a 20us sampling time in order to simulate a microcontroller control device.

2) 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 40 us. This can be done by typing 'Ts = 40e-6' in the workspace and by changing the speed controller sampling time to 120e-6 and the vector controller sampling time to 40e-6. See also ac3_sensorless_simplified model.

References

1. Bose, Bimal K., "Modern Power Electronics And AC Drives", Prentice-Hall, Inc., Upper Saddle River, NJ 07458, 2002.