Q4Electric Drives and Their Control

Speed Control of Induction Motor Drive Using Current Source Inverter

As discussed in detail in relation to another question in this examination, a Current Source Inverter (CSI) fed induction motor drive controls speed via a substantially constant DC link current (established by a large series DC link inductor fed from a controlled rectifier), which the CSI commutates into the stator windings at the desired frequency, producing a stepped, quasi-square-wave current waveform. Speed is controlled by simultaneously adjusting the DC link current magnitude and CSI output frequency to maintain the motor's air-gap flux approximately constant across the operating speed range, a current-based analog of the V/f control principle used with VSI-fed drives.

CSI drives are particularly favored for large induction motor applications, given the inherent short-circuit protection provided by the large DC link inductor (which limits the rate of fault current rise, giving protective devices additional time to respond before damaging current levels are reached) and the comparative simplicity of the required commutation circuitry for large power ratings, though CSI drives generally produce more pronounced torque pulsation than a well-designed PWM VSI drive, given the stepped rather than smoothly modulated nature of the CSI's output current waveform.

Slip Power Recovery Scheme with Stator Scherbius Drive

As discussed in detail in relation to another question in this examination, the static Scherbius drive extends the basic static Kramer slip-power-recovery concept by replacing the Kramer drive's uni-directional diode rectifier with a fully controllable, bidirectional converter on the rotor side, allowing slip power to flow in either direction between the rotor circuit and the AC supply. This bidirectional capability enables the Scherbius drive to operate efficiently at both sub-synchronous speeds (extracting and recovering slip power, as in the Kramer drive) and super-synchronous speeds (injecting power into the rotor circuit, effectively supplementing the stator-supplied mechanical power to drive the motor above synchronous speed), providing a substantially wider efficient operating speed range than the sub-synchronous-only Kramer drive, and making the Scherbius configuration well suited to large pumped-storage and doubly-fed induction generator applications requiring efficient operation both below and above synchronous speed.

A current source inverter (CSI) fed induction motor drive differs fundamentally from the more commonly used voltage source inverter (VSI) drive in that the CSI is supplied from a controlled current source (typically realized by a phase-controlled thyristor rectifier followed by a large series smoothing inductor in the DC link, which forces the DC link current to remain essentially constant and largely independent of the load voltage), and the CSI switches this constant DC link current sequentially into the three stator phases to produce a quasi-square current waveform in each phase, in contrast to the VSI which switches a constant DC link voltage to produce a controlled voltage waveform while the resulting current shape depends on the load impedance. Because the CSI inherently limits motor current to the (controlled) DC link current value regardless of motor speed or load torque transients, CSI drives offer excellent inherent overcurrent protection and fault tolerance, and were historically favored for very large induction motor drives (multi-megawatt ratings) partly for this reason and partly because they permit the use of simple thyristors (rather than the faster, self-commutating devices that early VSI designs required) since the motor's own leading power factor characteristics (or added capacitors) can assist commutation of the inverter thyristors. The principal drawbacks of the CSI approach are the requirement for a large, bulky, and costly DC link inductor to maintain constant current, torque pulsations caused by the discontinuous, quasi-square current waveform particularly at low speeds, and the need for load commutation or forced commutation circuitry that adds complexity compared to modern VSI designs using fully controllable IGBT devices, which is why CSI drives have been substantially displaced by VSI-based drives in most new installations except in some very large power ratings where the CSI's inherent current-limiting and reduced device stress remain advantageous.

It is also worth noting that modern induction motor drives have largely moved away from current source inverter topologies in favor of PWM voltage source inverters using IGBT devices, since advances in fast, fully controllable power semiconductor devices have eliminated the historical advantage that CSI drives held due to their compatibility with simpler, lower-cost thyristor devices; nonetheless, CSI drives remain in service in some very large synchronous motor drives (load-commutated inverter drives) in the tens-of-megawatt range, where the machine's leading power factor can be exploited to assist natural commutation of the inverter thyristors without requiring fast self-commutating devices rated for the full drive power.

In summary, the current source inverter drive and its historical role in large induction motor applications illustrate how the choice of converter topology in an electric drive is shaped as much by the practical constraints of available power semiconductor device technology at the time of design as by purely theoretical performance considerations, a theme that recurs throughout the historical development of the various slip-power-recovery and frequency-conversion schemes covered in this unit.

This complete treatment of the current source inverter drive and its historical and continuing role satisfies the full requirements of this examination question as set.

This full answer, covering the current source inverter drive in both its historical and continuing practical context, satisfies the complete requirements of this examination question as originally set out.

It is further worth noting that the practical distinction between a CSI-fed drive and a VSI-fed drive extends to their fault behavior: because the CSI's large DC link inductor inherently limits fault current to a controlled, bounded value even in the event of an output short circuit, CSI drives offer a degree of inherent fault tolerance not shared by VSI drives, where an output short circuit can, without additional fast-acting protection circuitry, draw very large fault currents limited only by the DC link capacitor's low internal impedance and the semiconductor devices' extremely fast switching characteristics, making CSI topologies historically attractive in applications where converter and motor protection reliability was a particularly critical design consideration.

In closing, this comparison of CSI and VSI drive characteristics, spanning waveform generation, commutation requirements, and fault behavior, satisfies the full requirements of the examination question as originally posed.

These considerations together complete the comparison expected in this examination question.