Tuesday, October 4, 2011

Types Of Electric Motors (4)

Induction motor

Induction motors are the most common motors used for various equipments in industry. Their popularity is due to their simple design, they are inexpensive and easy to maintain, and can be directly connected to an AC power source.

a. Components

An induction motor has two main electrical components :

􀂃 Rotor. Induction motors use two types of rotors:

- A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings.

- A wound rotor has a three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The  three phases are wired internally and the other ends are connected to slip-rings mounted on a shaft with brushes resting on them.

􀂃 Stator. The stator is made up of a number of stampings with slots to carry three-phase windings. It is wound for a definite number of poles. The windings are geometrically spaced 120 degrees apart.

An Induction Motor (Automated Buildings)

c. Classification of induction motors:
Induction motors can be classified into two main groups :
􀂃 Single-phase induction motors. These only have one stator winding, operate with a single-phase power supply, have a squirrel cage rotor, and require a device to get the
motor started. This is by far the most common type of motor used in household appliances, such as fans, washing machines and clothes dryers, and for applications for 

up to 3 to 4 horsepower.

􀂃 Three-phase induction motors. The rotating magnetic field is produced by the balanced three-phase supply. These motors have high power capabilities, can have squirrel cage or wound rotors (although 90% have a squirrel cage rotor), and are self-starting. It is estimated that about 70% of motors in industry are of this type, are used in, for example,pumps, compressors, conveyor belts, heavy-duty electrical networks, and grinders. They are available in 1/3 to hundreds of horsepower ratings.

d. Speed of induction motor

Induction motors work as follows. Electricity is supplied to the stator, which generates a magnetic field. This magnetic field moves at synchronous speed around the rotor, which in
turn induces a current in the rotor. The rotor current produces a second magnetic field, which tries to oppose the stator magnetic field, and this causes the rotor to rotate.
In practice however, the motor never runs at synchronous speed but at a lower “base speed”.
The difference between these two speeds is the “slip”, which increases with higher loads. Slip only occurs in all induction motors. To avoid slip, a slip ring can be installed, and these
motors are called “slip ring motors”. The following equation can be used to calculate the percentage slip :

% Slip =(Ns – Nb x 100)/(  Ns)
Ns = synchronous speed in RPM
Nb = base speed in RPM

e. Relationship between load, speed and torque:

When the motor :
􀂃 Starts there is a high starting current and low torque (“pull-up torque”).
􀂃 Reaches 80% of the full speed, the torque is at its highest level (“pull-out torque”) and the current begins to drop.
􀂃 Is at full speed, or synchronous speed, the torque and stator current drop to zero.
Typical Torque-Speed Curve of 3-Phase AC
Induction Motors


jabroon piece said...

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Induction motors are the very useable motor.Their reputation is due to their simple style, they are affordable and simple to sustain, and can be immediately linked with an AC automobiles.

engineer said...

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