Q11Electrical Machines and Drives

Static Rotor Resistance Control

In a wound-rotor (slip-ring) induction motor, the rotor circuit's effective resistance can be varied without mechanical rheostats by replacing the conventional resistance bank with a static (thyristor-controlled) equivalent: the three-phase rotor slip-ring output is first rectified using a diode bridge into a DC circuit, and a fixed external resistance R is connected across this DC link through a chopper (a single thyristor/IGBT switch operated in a duty-cycle-controlled chopping mode).

By periodically switching the chopper on and off with duty cycle d, the resistance R appears to the rotor circuit (through the diode bridge, which cannot pass reverse current) as an effective, continuously variable resistance Reff = R*(1-d): when the chopper is ON continuously (d=1), the resistance R is short-circuited (Reff=0, minimum rotor resistance); when the chopper is OFF continuously (d=0), the full resistance R is in circuit (Reff=R, maximum resistance). Varying d smoothly and steplessly between 0 and 1 therefore provides continuously adjustable effective rotor resistance without any moving contacts or mechanical rheostat tap-changing.

In a closed-loop speed control scheme, the actual motor speed is sensed by a tachogenerator or encoder and compared with a reference (desired) speed to generate a speed error signal. This error, processed through a controller (typically PI), generates the duty-cycle command for the chopper: if actual speed is below reference (positive error, e.g., due to an increase in load torque causing the motor to slow down), the controller increases the effective rotor resistance (or decreases it, depending on convention) to reposition the operating point on the torque-speed characteristic to restore speed - since increasing rotor resistance shifts the motor's torque-speed curve so that the same load torque is developed at a lower speed and vice versa, the specific correction direction is chosen to counteract the sensed deviation and drive the error toward zero.

This closed-loop arrangement gives the motor an approximately constant, speed-independent torque-speed characteristic over the controllable range (similar in shape to a separately excited DC motor with armature resistance control), useful for crane hoist drives, fan drives, and other applications needing smooth, continuously adjustable speed with automatic load-torque compensation. The technique avoids the poor efficiency of large continuous rotor resistance dissipation for extended periods at very low speeds (since the chopper's duty cycle can be optimized), and it eliminates the contact wear, sparking, and maintenance associated with mechanical rotor rheostats and their moving contacts, while retaining a fully static, solid-state, and hence more reliable power-electronic implementation suitable for automatic feedback control.

A key limitation of this method, inherited from any rotor-resistance-based speed control technique, is that it necessarily operates by increasing slip, and hence increasing rotor copper loss (which is dissipated as heat in the external resistance R via the chopper circuit rather than recovered), meaning the overall drive efficiency falls significantly at reduced speeds - this trade-off is acceptable for drives that only occasionally need reduced-speed operation (such as during starting, low-speed positioning, or occasional load adjustment) but is not energy-efficient for continuous low-speed operation, where more sophisticated slip-power-recovery schemes (static Kramer or Scherbius drives, which recover the slip power back to the supply rather than dissipating it) would be preferred.