Speed control techniques in AC drives

Speed of an induction motor is $N = N_s(1-s) = \dfrac{120f}{P}(1-s)$. So speed can be changed by changing the slip $s$ (at fixed $N_s$), the supply frequency $f$, or the number of poles $P$. Slip is changed by stator voltage, rotor resistance or slip-power injection.

1. Stator voltage control

• AC voltage controller (anti-parallel thyristors) reduces the rms stator voltage at fixed frequency. Since $T \propto V^2$, the torque curve shrinks and the operating point moves to higher slip.
• Simple and cheap; used for fans and pumps ($T \propto N^2$).
• Narrow range, poor efficiency at low speed (rotor loss $= sP_g$), harmonics.

2. Pole changing

• Stator winding reconnected (e.g. Dahlander) or two separate windings, giving 2–4 fixed speeds. Only for squirrel cage motors; speed changes in steps.

3. Variable frequency (V/f) control

• An inverter supplies variable voltage and frequency, keeping $V/f$ constant so that air-gap flux stays constant.
• Wide, smooth speed range above and below base speed, high efficiency, good starting torque; most common modern method (VSI/PWM or CSI drives).

 3-ph ac -> [Rectifier] -> [DC link L/C] -> [Inverter] -> IM
                                 V/f controller ^

4. Rotor resistance control (slip ring motor)

• External resistance in rotor circuit raises slip at a given torque; static version uses a diode bridge, chopper and one resistor.
• Simple, high starting torque, but slip power is wasted as heat.

 rotor -> [diode bridge] -> L -> [chopper || R]

5. Slip power recovery (slip ring motor)

• Static Kramer drive: rotor power rectified, inverted and fed back to the supply; sub-synchronous speeds only.
• Static Scherbius drive: cycloconverter or back-to-back converter in rotor; power flows both ways, giving sub- and super-synchronous speeds.
• Efficient; used for large pumps, fans, compressors.

6. Vector (field oriented) control

• Stator current split into flux and torque components and controlled separately, so the induction motor behaves like a separately excited dc motor; fast response for high-performance drives.