Three main methods of speed regulation for three-phase induction motors used on tankers are rotor resistance, cascade system, and pole-changing.

Each method operates on a different principle and is suited to particular shipboard applications depending on the load, torque, and speed control requirements.

(a) Rotor Resistance Method

Principle:

• This method is applicable only to slip-ring (wound-rotor) induction motors.
• Additional resistance is inserted into the rotor circuit through the slip rings.
• By increasing the rotor resistance, the slip increases, resulting in a reduction in motor speed.

Speed can be controlled smoothly while maintaining high starting torque.

Application & Advantage:

• Suitable for applications requiring high starting torque and variable speed under load.
• Provides fine speed control and is simple and cost-effective, though it suffers from power loss in the external resistors and reduced efficiency.

Examples:

• Cargo winches
• Crane motors
• Grain elevators
• Cargo and ballast pumps (where gradual speed control is required)

(b) Cascade System (Concatenation)

Principle:

• Two slip-ring induction motors are mechanically coupled.
• The rotor circuit of the first motor is electrically connected to the stator circuit of the second motor.
• Depending on the polarity and connection, this system provides up to four discrete speeds.
• The combined system allows the supply frequency to be divided between the two motors, producing multiple synchronous speeds.

Application & Advantage:

• Useful where two or more fixed speeds are required without complex circuitry.
• Offers higher torque at lower speeds and smooth transition between speed stages.
• Though more complex mechanically, it allows efficient control in heavy-duty machinery requiring multiple fixed speeds.

Examples:

• Multi-stage centrifugal pumps
• Compressors
• Large ventilation fans and machinery requiring distinct speed stages on tankers

(c) Pole-Changing Method

Principle:

• In this method, the number of poles in the stator winding is altered by reconfiguring the connections.
• As synchronous speed depends on the number of poles, changing the pole number changes the speed.

$$N_{s}=\frac{120f}{P}$$

• This method is used mainly with squirrel-cage induction motors.

Application & Advantage:

• Provides two or more discrete fixed speeds (commonly a two-speed arrangement).
• Mechanically simple, reliable, and requires no external resistors or complex controls.
• Efficient and well-suited where two-speed operation (high/low) is sufficient for operational flexibility.

Examples:

• Ballast pumps (high speed for filling, low speed for stripping)
• Cargo oil pumps
• Engine room and cargo ventilation fans

Summary: