Q4Electric Drives and Their Control

Slip Power Recovery Scheme with Stator Scherbius Drive

The static Scherbius drive extends the static Kramer drive concept (discussed in relation to another question in this examination) by replacing the Kramer drive's uni-directional diode rectifier on the rotor side with a fully controllable, bidirectional power converter, allowing slip power to flow in either direction between the rotor circuit and the AC supply, rather than only from rotor to supply as in the Kramer drive.

This bidirectional capability allows the Scherbius drive to operate at both sub-synchronous speeds (extracting slip power from the rotor and feeding it back to the supply, as in the Kramer drive) and super-synchronous speeds (instead injecting power into the rotor circuit, effectively adding to the mechanical power delivered by the stator alone, allowing the motor to run above synchronous speed while still operating efficiently) - this wider speed range spanning both below and above synchronous speed is the key advantage of the Scherbius drive over the sub-synchronous-only Kramer drive, making it well suited to applications such as large pumped-storage generator-motor sets and doubly-fed induction generator wind turbines, both requiring efficient operation across a speed range spanning both below and above synchronous speed.

Static Rotor Resistance Control of Induction Motor Drive

Static rotor resistance control achieves the same speed-control effect as classical, mechanically-switched rotor resistance control (varying rotor circuit resistance to control slip and hence speed) but replaces the mechanical resistance-switching contactors with a static (power-electronic) implementation: the rotor's AC output is rectified to DC via a diode bridge, and a chopper (DC-DC converter) connected across a fixed resistor on the DC side varies the effective resistance seen by the rotor circuit by rapidly switching the resistor in and out of circuit at a high duty-cycle-controlled rate, presenting an electronically continuously variable effective resistance to the rectified rotor current rather than the discrete, stepped resistance values that mechanical rotor resistance controllers provide. This static approach offers smoother, more precisely continuous speed control, faster response, and elimination of the mechanical wear and maintenance associated with contactor-based rotor resistance switching, though like classical rotor resistance control, it remains fundamentally a lossy, sub-synchronous-only speed control method (since the slip power is still ultimately dissipated as heat in the resistor rather than being recovered, unlike the Kramer and Scherbius drives discussed elsewhere in this examination).

The static Scherbius drive and static rotor resistance control discussed together in this question both address wound-rotor induction motor speed control, but occupy quite different positions along the efficiency-versus-simplicity spectrum, with the Scherbius drive offering superior energy efficiency through slip-power recovery at the cost of a more complex bidirectional power-electronic converter, while static rotor resistance control offers a simpler, lower-cost implementation at the cost of the ongoing energy loss inherent to any resistance-based, non-recovering speed control method.

For contrast, it is instructive to note that the static Scherbius drive discussed here overcomes the fundamental sub-synchronous-only limitation of the simpler static Kramer scheme by replacing the Kramer scheme's uncontrolled diode-bridge-plus-line-commutated-inverter rotor interface with a fully controlled, bidirectional power-electronic converter (historically a cycloconverter, or in modern practice a back-to-back PWM converter), allowing slip power to flow in either direction between the rotor circuit and the supply - this bidirectionality is what enables the Scherbius scheme to operate the induction motor at both sub-synchronous speeds (extracting slip power from the rotor, as in the Kramer scheme) and super-synchronous speeds (injecting additional power into the rotor circuit from the supply), giving it a substantially wider useful speed range than the Kramer scheme at the expense of a more complex and costly power-electronic converter.

Static rotor resistance control is one of the simplest methods of speed control for a wound-rotor (slip-ring) induction motor, achieved by inserting variable external resistance in series with the rotor circuit through the slip rings; increasing this external resistance increases the total effective rotor circuit resistance, which shifts the induction motor's torque-speed characteristic so that the same load torque is now developed at a higher slip (lower speed), thereby providing a means of speed reduction below the natural (rotor-short-circuited) characteristic speed. In the static (power-electronic) version of this scheme, the external mechanical resistance bank (traditionally varied by a contactor-operated tapped resistor or a liquid rheostat) is replaced by a fixed resistor placed after a rotor-circuit diode bridge rectifier, with a chopper (a controllable power semiconductor switch, typically an IGBT or thyristor with forced commutation) connected across the resistor and switched at high frequency with a controllable duty cycle to vary the effective (average) resistance seen by the rectified rotor circuit - increasing the chopper's on-time (duty cycle) short-circuits the resistor for a larger fraction of each switching cycle, reducing the effective resistance, while decreasing the duty cycle increases the effective resistance, giving smooth, stepless, electronically controlled speed variation without the need for mechanically switched resistor taps, though this method inherently wastes all of the slip power as heat in the resistor rather than recovering it, making it less efficient than the Kramer or Scherbius slip-power-recovery schemes at low operating speeds.

A further point worth noting for static rotor resistance control is that, because it operates by intentionally dissipating slip power as heat (whether in a fixed external resistor with mechanical tap-changing, or in the chopper-controlled fixed resistor arrangement described above), its energy efficiency deteriorates progressively as the required speed reduction increases, since the fraction of total air-gap power dissipated as rotor copper loss is directly proportional to the operating slip; at very low speeds (high slip), a large fraction of the total power delivered to the motor is wasted as heat in the rotor circuit resistance rather than being converted to useful mechanical output, making this method economically unattractive for continuous, wide-range speed control applications, but perfectly acceptable for applications requiring only occasional or short-duration reduced-speed operation, such as starting torque control or short-term speed trimming, where the energy efficiency penalty is incurred only briefly rather than continuously.

In summary, static rotor resistance control offers a straightforward, moderate-cost means of achieving stepless speed control in a wound-rotor induction motor, trading away the higher efficiency of slip-power-recovery schemes such as the Kramer and Scherbius drives in exchange for a substantially simpler power-electronic converter arrangement, making it well suited to applications with only occasional need for reduced-speed operation.