Q4Electric Drives and Their Control

Slip Recovery Scheme with Static Kramer Drive

The static Kramer drive is a slip-power-recovery scheme for wound-rotor induction motors that, instead of dissipating the rotor circuit's slip-frequency power as wasted heat in external rotor resistance (as in simple rotor resistance speed control), recovers this slip power and returns it to the AC supply, substantially improving overall drive efficiency for a motor operated over a wide sub-synchronous speed range.

In the static Kramer drive, the rotor's slip-frequency AC output is first rectified to DC using a diode bridge rectifier connected to the rotor slip rings, and this rectified DC slip power is then fed to a line-commutated inverter (synchronized to and commutated by the fixed-frequency AC supply), which converts it back to AC at the supply frequency and feeds it into the grid, recovering energy that would otherwise be dissipated in rotor resistance. Speed control is achieved by varying the firing angle of the line-commutated inverter, controlling how much slip power is drawn from the rotor circuit and hence controlling the motor's operating slip for a given load torque.

A key limitation of the basic static Kramer drive is that, since the diode rectifier only allows power flow from rotor toward the DC link (one direction), this configuration can only operate the motor at sub-synchronous speeds, and cannot achieve super-synchronous speed operation, which instead requires the static Scherbius drive configuration (discussed in relation to another question in this examination), replacing the diode rectifier with a bidirectional, controllable converter.

Speed Control of Induction Motor Drive Using Current Source Inverter

A Current Source Inverter (CSI) fed induction motor drive controls speed by regulating a substantially constant DC link current (established by a large series DC link inductor fed from a controlled rectifier), which the CSI then commutates into the three motor stator phases at the desired frequency, producing a stepped (quasi-square-wave) current waveform rather than the PWM voltage waveform of a VSI. Speed is controlled by simultaneously adjusting the DC link current magnitude and the CSI's output frequency, maintaining an appropriate current-to-frequency ratio to hold the motor's air-gap flux approximately constant, analogous to the V/f control principle used in VSI-fed drives but expressed in terms of controlled current rather than controlled voltage. CSI drives are particularly well suited to large motor drives and offer inherent short-circuit protection (since the DC link inductor limits fault current rise), though they generally produce more torque pulsation (due to the stepped current waveform) than a well-designed PWM VSI drive.

The static Kramer drive and the CSI-fed induction motor drive discussed together in this question represent two distinct but complementary power-electronic approaches to induction motor speed control, one exploiting slip-power recovery specifically for wound-rotor motors operated below synchronous speed, and the other exploiting current-source inverter technology applicable to squirrel-cage motors across a considerably wider speed range spanning both below and potentially above the motor's nameplate synchronous speed rating.

It is useful to note that the static Kramer drive belongs to the broader family of slip-power-recovery schemes that exploit the fact that, in a wound-rotor induction motor operating below synchronous speed, the rotor circuit carries real power at slip frequency proportional to the slip itself, and rather than wastefully dissipating this slip power in external rotor resistors (as in simple rotor resistance speed control), the Kramer scheme rectifies this slip-frequency power via a diode bridge and returns it to the supply via a line-commutated inverter and a matching transformer, so that the net effect is a substantial improvement in overall drive efficiency compared to resistive slip-power dissipation, at the cost of only allowing sub-synchronous speed operation, since the classical static Kramer scheme cannot recirculate power in the reverse direction and hence cannot achieve super-synchronous speeds.

The complete slip power recovery arrangement in a static Kramer drive is realized using four main components connected in sequence in the rotor circuit: first, the wound-rotor slip rings bring out the three-phase rotor EMF at slip frequency; second, a three-phase diode bridge rectifier converts this AC slip-frequency EMF into DC; third, a smoothing inductor (choke) in the DC link limits ripple and current fluctuation; and fourth, a naturally line-commutated thyristor inverter, synchronized to the AC mains supply frequency, converts the DC back into AC at supply frequency and feeds it back to the mains through a step-down matching transformer whose turns ratio is selected to match the rotor open-circuit voltage to the mains voltage across the intended speed range. Speed control is achieved by varying the firing angle of the line-commutated inverter, which controls the effective DC voltage it presents to the rotor circuit and hence controls the equivalent resistance the rotor sees, in turn setting the operating slip and hence the motor speed - increasing the inverter firing angle towards 180 degrees increases the effective rotor circuit resistance (raising slip and lowering speed), while reducing it towards 90 degrees reduces effective resistance (lowering slip and raising speed towards, but never reaching, synchronous speed).

It is also useful to note the practical speed range limitation of the static Kramer scheme: because the diode bridge in the rotor circuit can only conduct current in one direction, and the line-commutated inverter can only operate in its inverting mode over a firing angle range that keeps its output DC voltage negative (delivering power to the mains rather than absorbing it), the classical static Kramer drive is restricted to a sub-synchronous speed range only, typically down to perhaps 50 percent of synchronous speed for a practical, cost-effective converter and transformer rating, since achieving a wider speed range would require the converter and transformer to be rated for correspondingly larger slip power and voltage, increasing cost substantially; this restricted but useful sub-synchronous range is nonetheless valuable in applications such as large pump and fan drives, where the required torque at reduced speed is often much lower than rated torque, allowing overall system cost savings compared to a full-rating variable-voltage-variable-frequency drive covering the same speed range.

In summary, the static Kramer drive exemplifies the broader principle of slip-power recovery as an efficiency-improving alternative to simple resistive rotor speed control, at the cost of restricting operation to sub-synchronous speeds only and requiring a rotor slip-ring machine rather than the simpler and more common cage induction motor.