Methods used to control the speed of a 3-phase induction motor:

• Pole changing: by reconnecting the stator winding (Dahlander or consequent-pole connection) the number of poles is changed, giving discrete synchronous speeds (N = 120 f/P). Simple and efficient but only a few fixed speeds.
• Rotor resistance (slip-ring motors): inserting resistance in the rotor circuit increases the slip and reduces the speed. Simple but inefficient (slip power is lost as heat) and gives poor speed regulation.
• Variable voltage: reducing the stator voltage increases the slip and reduces the speed, but the torque is also reduced and the method is inefficient and gives poor regulation.
• Variable frequency (VFD): varying the supply frequency changes the synchronous speed. This is the most efficient and gives smooth, continuous speed control over a wide range. The voltage is varied in proportion to the frequency (V/f constant) to maintain constant flux and torque.
• Cascade and Scherbius/Kramer systems: used for large motors to recover slip power and give a few efficient speeds.

Variable Frequency Drive (VFD) for energy efficiency of auxiliary machinery:

• A VFD consists of three main stages:
• Rectifier: converts the a.c. supply to d.c. (a diode or thyristor bridge).
• D.C. link: a capacitor (and inductor) smooths the d.c. voltage.
• Inverter: converts the d.c. back to a.c. at a variable frequency and voltage using IGBTs switched by pulse-width modulation (PWM).
• The control unit varies the output frequency and voltage (maintaining a constant V/f ratio) to control the motor speed.
• Operation: the VFD supplies the motor with a variable-frequency, variable-voltage supply. By controlling the frequency, the synchronous speed and hence the motor speed are controlled. The V/f ratio is kept constant so the air-gap flux and torque capability are maintained. The motor runs at low slip at each speed, so the efficiency is high.
• Energy efficiency: for auxiliary machinery such as pumps, fans and compressors, the load power varies with the cube of the speed (for fans and pumps). By reducing the speed with a VFD instead of throttling or using a fixed speed, the power consumption is greatly reduced. The VFD matches the motor speed to the actual demand, saving energy, reducing wear, and giving soft starting (reduced starting current and mechanical shock). This is why VFDs are widely used to optimise the energy efficiency of auxiliary machinery on board vessels.