Slip power recovery for slip ring induction motor

In a slip-ring induction motor, part of the air-gap power, the slip power $sP_g$, appears in the rotor circuit. With rotor-resistance speed control this power is wasted as heat in external resistors, making low-speed operation inefficient. In a slip power recovery system, the slip power is taken out of the rotor through power converters and fed back to the supply (or to the shaft), so speed can be controlled efficiently.

Static Kramer drive (sub-synchronous)

 3-ph supply ---------+-----------------------+
                      |                       |
                 [Stator]              [Transformer]
                 Slip-ring IM                 |
                 [Rotor]                      |
                    | slip rings              |
               [Diode bridge]--[L]--[Line-commutated
                 (rectifier)  DC    thyristor inverter]
                              link

Working

1. Rotor voltage $sE_2$ at slip frequency is rectified by an uncontrolled diode bridge:

$$V_{d} = 1.35\,sE_2 \quad (E_2 = \text{standstill rotor line voltage})$$

2. A line-commutated thyristor inverter, operating with firing angle $90^\circ < \alpha < 180^\circ$, converts the DC power back to AC at supply frequency and returns it to the mains through a transformer (turns ratio adapts the voltage). Its DC voltage is

$$V_{I} = -1.35\,V_L'\cos\alpha$$

where $V_L'$ is the inverter-side AC line voltage.

3. Neglecting the drop in the DC-link inductor, $V_d = V_I$:

$$1.35\,sE_2 = -1.35\,V_L'\cos\alpha \;\Rightarrow\; s = -\frac{V_L'}{E_2}\cos\alpha$$

4. Changing $\alpha$ changes the counter-voltage seen by the rotor, so the slip, and hence speed, changes: at $\alpha = 90^\circ$, $s \approx 0$ (near synchronous speed); as $\alpha \to 180^\circ$, slip increases (lower speed).

5. Torque is proportional to DC-link current: $T \propto I_d$; the drive gives a shunt-like speed characteristic.

Features

• Speed control only below synchronous speed (power flows only out of the rotor through the diode bridge).
• High efficiency, since slip power is recovered; converter rating is proportional to the speed range (for pumps and fans with limited range, a small converter suffices).
• Drawbacks: poor power factor (inverter draws reactive power), harmonics; starting needs rotor resistors because the converter is rated for limited slip.

Static Scherbius drive

• The diode bridge and inverter are replaced by a bidirectional converter (cycloconverter, or back-to-back PWM converters, as in doubly-fed induction generators).
• Slip power can flow both ways, so speed control is possible below and above synchronous speed, with regenerative braking and power-factor control.

Applications

Large pumps, fans, compressors, cement-mill drives, and wind turbines (doubly-fed induction generator), where a limited speed range is needed with high efficiency.