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Three Phase Motors

Posted by IER Services on Oct 9, 2017 3:47:00 PM

3_phase_motor.pngA three-phase induction motor derives its name from the rotor current that is induced by the magnetic field, instead of electrical connections. A three-phase induction motor has a simple design, inherently high starting torque and high efficiency. 

They are widely used in industrial applications such as large air handlers, cranes and water pumps. They are rugged, reliable and it is easy to control the speed of a three-phase induction motor by using a Variable Frequency Drive (VFD). 

The Stator and the Rotor are the two major parts of the three-phase motor: 

The stator consists of numerous slots that are designed for a 3 phase winding circuit. The winding circuit is arranged to produce a rotating magnetic field. This is has an over-lapping winding offset electrical angle of 120°. This circuit is connected to a three-phase AC source.

 The motor’s rotation direction depends on the phase sequence of supply lines, and the order in which these lines are connected to the stator. By interchanging the connection of any two primary terminals to the supply, you can reverse the direction of rotation.

There are two types of rotors. They are the squirrel cage rotor and phase wound rotor (or simply - wound rotor). 

A Squirrel cage type rotor has very simple and sturdy construction. This rotor has a cylindrical laminated core with parallel slots on it. These slots carry rotor conductors. Heavy bars of copper, aluminum or alloys are used as rotor conductors instead of wires. 

The rotor bars are brazed or electrically welded to end rings that short circuit at both ends. Thus this rotor construction resembles a squirrel cage. 

A Phase wound rotor is wound with a 3 phase, double layer, distributed winding. The number of rotor poles are usually equal to the number of poles of the stator. (The rotor is always wound three-phase even if the stator is wound two-phase.)

The three-phase rotor winding is internally star connected. The other three terminals of the winding are taken out via three insulated sleep rings mounted on the shaft and the brushes resting on them. These three brushes are connected to an external star connected rheostat. This introduces an external resistance in the rotor circuit for starting purposes and for changing the speed and torque characteristics.

When the motor is running at its rated speed, slip rings are automatically short circuited by means of a metal collar and the brushes are lifted above the slip rings to minimize the frictional losses. 

Advantages of Three-Phase Motors: 

  • Simple and sturdy construction
  • Very reliable and low cost
  • High efficiency and good power factor
  • Minimal maintenance requirements
  • Self starting 
  • Less armature reaction and brush sparking

We are an authorized distributor for a wide range of electric motors from manufacturers like WEG, Baldor, GE, Siemens, A.O. Smith, Emerson and Westinghouse. Our trained professionals will install your motor, and perform scheduled maintenance to be sure that it runs in top shape.

IER Services offers additional services such as diagnostic services, thermal-imaging, laser alignment, and advanced motor winding analysis. These tools help us to determine quickly where the problem is, and often we can fix it onsite! Call us for all your commercial electric motor repair projects.  614.298.1600

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Topics: three phase induction motor, WEG, 3 phase motors

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