MATLAB Examples

AC1 - Six-Step VSI Induction 3HP Motor Drive

This example shows the AC1 Six-Step VSI Induction Motor Drive during speed regulation.

H.Blanchette, L.-A. Dessaint (Ecole de technologie superieure, Montreal)

Contents

Description

The induction motor is fed by a voltage source inverter, which is built using the Universal Bridge Block. The DC bus voltage is produced by a thyristor rectifier and regulated using a PI controller in order to maintain a constant volts per hertz ratio. A braking chopper limits the DC bus voltage increase during motor deceleration or when the load torque tends to accelerate 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.

Simulation

A speed reference step from 0 to 1800 rpm is applied at t = 0. The speed set point doesn't go instantaneously at 1800 rpm but follows the acceleration ramp. The motor reaches steady state at t = 1.3 s.

At t = 2 s, an accelerating torque is applied on the motor's shaft. You can observe a speed increase. Since the rotor speed is higher than the synchronous speed, the motor is working in the generator mode. The braking energy is transferred to the DC link and the bus voltage tends to increase. However the over voltage activates the braking chopper which causes the voltage to reduce. In this example, the braking resistance is not big enough to avoid a voltage increase but the bus is maintained within tolerable limits.

At t = 3 s, the torque applied to the motor's shaft steps from -11 N.m to +11 N.m .You can observe a DC bus voltage and speed drop. At this point, the DC bus controller switches from braking to motoring mode.

At t = 4 s, the load torque is removed completely and the electromagnetic stabilizes around zero shortly after.

Notes

1) The power system has been discretised with a 20us time step