10-04-2017, 08:31 PM
For industrial and mining applications, 3-phase AC induction motors are the prime movers for the vast majority of machines. These motors can be operated either directly from the mains or from adjustable variable frequency drives. In modern industrialized countries, more than half the total electrical energy used in those countries is converted to mechanical energy through AC induction motors. The applications for these motors cover almost every stage of manufacturing and processing.
Applications also extend to commercial buildings and the domestic environment. They are used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools, cranes, etc, etc.
It is not surprising to find that this type of electric motor is so popular, when one considers its simplicity, reliability and low cost. In the last decade, it has become increasingly common practice to use 3-phase squirrel cage AC induction motors with variable voltage variable frequency (VVF) converters for variable speed drive (VSD) applications.
To clearly understand how the VSD system works, it is necessary to understand the principles of operation of this type of motor.
Although the basic design of induction motors has not changed very much in the last 50 years, modern insulation materials, computer based design optimization techniques and automated manufacturing methods have resulted in motors of smaller physical size and lower cost per kW. International standardization of physical dimensions and frame sizes means that motors from most manufacturers are physically interchangeable and they have similar performance characteristics.
The reliability of squirrel cage AC induction motors, compared to DC motors, is high. The only parts of the squirrel cage motor that can wear are the bearings. Sliprings and brushes are not required for this type of construction. Improvements in modern prelubricated bearing design have extended the life of these motors.
Although single-phase AC induction motors are quite popular and common for low power applications up to approx 2.2 kW, these are seldom used in industrial and mining applications. Single-phase motors are more often used for domestic applications.
The information in this article applies mainly to 3-phase squirrel cage AC induction motors, which is the type most commonly used with VVF converters.
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Basic construction
The AC induction motor comprises 2 electromagnetic parts:
Stationary part called the stator
Rotating part called the rotor, supported at each end on bearings
The stator and the rotor are each made up of:
An electric circuit, usually made of insulated copper or aluminum, to carry current
A magnetic circuit, usually made from laminated steel, to carry magnetic flux