Simplified Block Diagram for Variable Frequency Induction Motors

A simplified block diagram representation valid for small slips is given for frequency controlled induction motors. The perturbation equations around an operating point in the synchronously rotating reference frame are decoupled and a block diagram representation with changes in supply voltage, frequency and load torque as inputs and rotor speed as output is obtained. The transfer functions relating changes in electromagnetic torque with incremental changes in supply voltage, frequency and rotor speed are simplified neglecting stator resistance and stator transients. The approximate transfer functions are used to develop the simplified block diagram. The time response of rotor speed for step changes in supply voltage, frequency, and load torque is calculated using the simplified transfer functions and compared with those of the linearised exact and reduced order models.