Three Phase Induction Motor V/F Control

A three phase source gets rectified by a diode bridge and then lowered using a simplified buck converter. The inverter uses mosfets

Some jotted notes on the system

3-phase rectifier:

Buck converter increased AC side source resistance to 0.01 Ω to dampen oscillations but not too much to induce a voltage drop.

Added a 1mF cap at exit of diode bridge to lower the transients and smoothen the DC output. Otherwise voltage was extremely unstable due to the fast switching of the mosfets

Added 1uF cap at exit of buck converter to create a DC link decoupling capacitor that absorbs voltage spikes caused by the rapidly changing currents.

Speed loop:

Slip limiter is very important. Too low and the error will not be enough to drive the motor. Especially when the error is very big. This is partially due to the maximum torque being applied when there is a good amount of slip. There is also an oscillatory effect that appears as the motor quickly accelerates, matches the current speed (no slip) and lowers its torque.

If too high, the controller will demand high frequency and high voltage but the slip will be too great and you won’t have maximum torque.

Frequency Limiter:

Helpful because beyond 60 hz, the motor enters flux weakening and the control loop, which is used to having the speed increase with increasing frequency, now experiences the opposite effect. Increasing the frequency actually decreases the speed since the field is weakening (V/F ratio decreases). Therefore used to stabilize the system.

V/F profile:

The output voltage has a minimal voltage used at lower than 10% speed. This ensures sufficient magnetic flux in the motor at low speeds.

Sine waves:

The amplitude of the sine waves remains 1. If the amplitude was modified at this stage, it would actually create PWM signals with varying voltage (which is the goal of V/F control). However, since the sine waves’ amplitude changes with respect to the triangular carrier wave (used to generate the PWM), the resulting PWM signal would have a varying modulation index. This would lead to a non-linear voltage change, increased harmonics, and decreased power efficiency.