Question

What are the three basic types of ac motors? State their advantages, Disadvantages

The majority of motors on ships with alternating current as the main electrical power are squirrel-cage induction motors with direct on-line starting.

With the changeover from the use of direct to alternating current, these motors were a simple and robust replacement for d.c. motors.

Compared with d.c. motors (which needed constant maintenance of starting contacts, brushes and commutators; replacement of starting resistances and cleaning) the routine work on an ac. motor is negligible.

Additionally, they are much safer, being non-sparking and having no resistances liable to overheat.

Ac motors will be divided into

(1) Series,

(2) Synchronous,

And (3) induction motors.

AC SERIES MOTORS are nearly identical to the dc series motors.

Special construction techniques allow ac series motors to be used as UNIVERSAL MOTORS, operating on either ac or dc power.

SYNCHRONOUS MOTORS are specifically designed to maintain constant speed, with the rotor synchronous to the rotating field.

Synchronous motors require modification (such as squirrel-cage windings) to be self-starting.

INDUCTION MOTORS are the most commonly used of all electric motors due to their simplicity and low cost. Induction motors may be single-phase or multiphase.

They do not require electrical rotor connection.

Split-phase motors with special starting windings, and shaded-pole motors, are types of single-phase induction motors.

Comparisons of cage and slip ring rotors

Squirrel cage

Advantages

Cheaper and more robust

Slightly higher efficiency and power factor

Explosion proof, since absence of slip rings and brushes eliminates risk of sparking

Virtually constant speed machine

Disadvantages

High starting current ( 5 to 8 times F.L.)

Low starting torque

Wound rotor with slip rings

Advantages

High starting torque

Lower starting current

Speed can be varied if required

Disadvantages

cost

Danger from sparking

Synchronous motors

Advantages

The ease with which the power factor can be controlled. An overexcited synchronous motor with a leading power factor can be operated in parallel with induction motors having a lagging power factor to improve the overall power factor of the supply system

The speed is constant and independent of the load. This characteristic is mainly of use when the motor is required to drive another alternator to generate a supply at a frequency, as in frequency changers

Uses-A.C. electric propulsion schemes but generally not for auxiliary purposes.

Disadvantages

Cost per h.p. is greater than induction motors