Q. Slip-power controlled wound-rotor induction-motor drives?

Slip power is that portion of the air-gap power that is not converted into mechanical power. The methods involving rotor-resistance control and voltage injection into the rotor circuit belong to this

class of slip-power control. In order to implement these techniques using power semiconductor devices, some of the schemes are static rotor resistance control; the static Scherbius drive; and the static Kramer drive, of which only the ?rst is considered here.

The rotor circuit resistance can be smoothly varied statically (instead of mechanically) by using the principles of a chopper. Figure shows a scheme in which the slip-frequency ac rotor voltages are converted into direct current by a three-phase diode bridge and applied to an external resistance R. The self-commutated semiconductor switch S, operating periodically with a period T, remains on for an interval ton in each period, with a duty ratio δ de?ned by ton/T. The ?lter inductor helps to reduce the ripple in current Id and eliminate discontinuous conduction. Figure shows input phase voltage waveforms of the diode bridge and a six-step rotor phase-current waveform, assuming ripple-free Id. For a period T of the switch operation, the energy absorbed by resistance R is given by I²_d R(T -t_on), while the average power consumed by R is