Speed control systems for a.c. motors?

What different methods of speed control are there for a.c. motors??





thanxSpeed control systems for a.c. motors?
Modern solid state electronics increase the options for speed control. By changing the 50 or 60 Hz line frequency to higher or lower values, the synchronous speed of the motor may be changed. However, decreasing the frequency of the current fed to the motor also decreases reactance XL which increases the stator current. This may cause the stator magnetic circuit to saturate with disastrous results. In practice, the voltage to the motor needs to be decreased when frequency is decreased. Conversely, the drive frequency may be increased to increase the synchronous speed of the motor. However, the voltage needs to be increased to overcome increasing reactance to keep current up to a normal value and maintain torque. The inverter (Figure ) approximates sinewaves to the motor with pulse width modulation outputs. This is a chopped waveform which is either on or off, high or low, the percentage of “on” time corresponds to the instantaneous sine wave voltage.





Once electronics is applied to induction motor control, many control methods are available, varying from the simple to complex:





# Scaler Control Low cost method described above to control only voltage and frequency, without feedback.


# Vector Control Also known as vector phase control. The flux and torque producing components of stator current are measured or estimated on a real-time basis to enhance the motor torque-speed curve. This is computation intensive.


# Direct Torque Control An elaborate adaptive motor model allows more direct control of flux and torque without feedback. This method quickly responds to load changes