Impedance Induction motor transient

The damper bars or winding act in a manner very similar to an induction motor and provide a breaking torque against the transient disturbances in shaft speed. To be effective the damper needs to have a steep torque versus shp characteristic in the region near synchronous speed. The equivalent impedance of the damper requires a low resistance and a high reactance. High conductivity copper bars are embedded into the pole face to provide a low reluctance path for the leakage flux. 

The sub-transient impedance determines the initial decay, i.e. in the first cycle or so. Therefore the emfs E" and E, together with the reactances Xj and X j, need to be used for calculating the fault currents. In a similar way to induction motors, the synchronous motors will contribute to fault-making dnty reqnirements. However, they will also contribute towards the fault-breaking duty because of the transient effects. 

When a running induction motor has a short-circuit applied to its terminals the air-gap flux creates an emf that drives a current into the fault. The motor is then driven by the inertia of its load. The speed may be assumed to be unchanged for the duration of the fault current, which in practice for small motors is only a few cycles at the supply frequency i.e. less than 60 milliseconds. For large motors the duration may as long as 250 milliseconds, see Reference 23. This is due to the higher X-to-R ratio in the short circuit than is the case with small motors. The impedance to the fault current consists of the transient reactance (equal to the sub-transient reactance) and the stator resistance. This will be shown below. 

Figure G.4 Volt-drop when starting an induction motor. Volt-drop in per-unit versus the ratio of the motor kVA rating to the generation kVA capacity, for different values of generator per transient impedance Zg and standing load.

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. 

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