Comparison of methods of speed regulation of three-phase induction motors used in tankers:

• Method: additional resistance is inserted in the rotor circuit of a slip-ring (wound rotor) induction motor. Increasing the rotor resistance increases the slip for a given torque, so the speed falls. The speed is varied by varying the rotor resistance.
• Characteristics: gives speed control only below synchronous speed (from full speed down to standstill). The speed regulation is poor (speed varies with load). The efficiency is low because the slip power is dissipated as heat in the rotor resistance. The starting torque can be increased.
• Advantages: simple, cheap, gives high starting torque and smooth acceleration.
• Disadvantages: wasteful (heat loss), poor speed regulation, only stepwise control unless a liquid rheostat is used.
• Example: cargo pump motors, winches, windlasses, and other deck machinery where high starting torque and some speed reduction are needed.

• Method: two induction motors are mechanically coupled, and the rotor of the first (main) motor is connected electrically to the stator of the second (auxiliary) motor. The slip power of the main motor is fed to the auxiliary motor, which adds to the mechanical output. By changing the number of poles of the auxiliary motor (or by using a Scherbius or Kramer arrangement), the speed of the combined set can be varied.
• Characteristics: gives a limited number of discrete speeds (usually two or three), all below synchronous speed. The efficiency is better than rotor resistance because the slip power is usefully employed.
• Advantages: better efficiency than rotor resistance, useful for large motors.
• Disadvantages: complex, expensive, requires two machines, only a few fixed speeds.
• Example: large cargo pump drives and other large constant-speed applications where a few discrete speeds are acceptable.

• Method: the stator winding is reconnected to change the number of poles, giving two (or more) discrete synchronous speeds. The Dahlander connection gives a 2:1 speed ratio (e.g. 4-pole/8-pole). Speed = 120 f / P.
• Characteristics: gives discrete speeds only (e.g. half and full speed), not continuous control. The efficiency is high at each speed because the motor runs at its rated slip. The torque can be maintained constant or the power constant depending on the connection.
• Advantages: simple, robust, cheap, high efficiency at each speed, no extra losses.
• Disadvantages: only a few fixed speeds, no continuous speed variation, the changeover requires a special starter.
• Example: engine room fans, ventilation fans, ballast and bilge pumps, and other auxiliaries where two or three fixed speeds are sufficient.

Summary: rotor resistance gives smooth but inefficient low-speed control; cascade gives a few efficient speeds for large drives; pole-changing gives simple, efficient discrete speeds for fans and pumps.