Induction motor time constants

In an industry there may be many drives that may not be required to operate at their optimum capacity at all times. The process requirement may require a varying utilization of the capticity of the drive at different times. In an induction motor, which is a constant speed prime-mover, such a variation is conventionally achieved by throttling the How valves or by employing dampers. 

PrImary-Voltage-Control-AC Motor Driver. Induction motor torque at any slip s is proportional to primary V. Rotor-power dissipation is equal to s times the air-gap power. These two relationships define the boundary of operation of an induction motor with primary voltage control of speed. As the speed is reduced (s increased) at constant torque, the air-gap power remains fixed, but the power divides between rotor circuit dissipation and mechanical shaft power. 

Generators and motors are often connected to their associated switchboards or networks by an impedance. This impedance can be a cable, an overhead line, a unit transformer or a combination of these components. The intermediate circuit introduced in the stator circuit will contain resistance and inductive reactance, the effect of which is to modify the time constants in the generator and motor equations, and the performance of these machines under most transiently disturbed conditions. This aspect has been mentioned in the literature e.g. References 24, 25 and 26 but is easily overlooked when developing computer programs. 

Derived Reactances and Time Constants for an Induction Motor... 

Figure 20.4 Short-circuit current decrement for a 2500 kW and a 37 kW induction motor. These motors have a relatively high armature time constant Ta that causes the initial offset of the waveform. The deep bar effect in the rotor has been taken into account.

Derived reactances and time constants for an induction motor... 

The AC drive has a shorter thermal time constant than the induction motor, and a separate RMS duty cycle calculation is required for both the drive and the motor using the applicable thermal time constant. 

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