6._Induction_Motors_2011

6._Induction_Motors_2011 - Induction Motors Revised July...

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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 1 Induction Motors Revised July 12, 2011
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 2 Induction Motors: We just learned how damper or amortisseur windings on a synchronous motor could develop a starting torque without the necessity of supplying an external current to the field winding. This idea works so well that a motor can be made without the synchronous motors main field windings at all. A machine with only amortisseur windings is called an induction machine . These machines are called induction machines because the rotor voltage, and hence the rotor current and rotor magnetic field is induced in the rotor windings instead of being physicaly connected by wires.
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 3 Induction Motors: The distinguishing feature of an induction machine is that no DC field current is required to run the machine. Although an induction machine can be used as either a motor or a generator, it has many problems when used as a generator and so these are rare. For this reason, when studying induction machines are really studying induction motors . An induction motor is called a singly excited machine (as opposed to a doubly excited machine ) because power is applied only to the stator.
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 4 Induction Motors: An induction motor has the same physical stator as a synchronous machine but with a different rotor construction. There are two different types of induction motor rotors. 1. A squirrel cage induction motor rotor consists of conducting bars laid into slots carved in the face of the rotor and shorted at either end by end rings or shorting rings (see picture next page). 2. A wound rotor that has a complete set of three phase windings that are mirror images of the windings on the stator.
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 5 Squirrel Cage Rotor Al or Cu bars End Rings
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 6
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 7 Squirrel Cage Rotor Embedded Al or Cu bars End Rings Laminated Fe Laminated Stator Pole Shaft
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 8
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 9 Induction Motors: A wound rotor that has a complete set of three phase windings that are mirror images of the windings on the stator. The three phases of the rotor windings are usually Y-Connected, and the ends of the three-phase wires are tied to slip rings on the motor’s shaft. The rotor windings are accessed through brushed riding on the slip rings. Such a rotor arrangement is more expensive, requires maintenance, and hence is rarely used. We will not consider wound rotors any further in this course.
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EEL 3211 (© 2011, H. Zmuda) 6. Induction Motors 10 Induction Motors – Basic Operation: Operation is basically the same as that of synchronous motors with amortisseur (damper) windings.
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6._Induction_Motors_2011 - Induction Motors Revised July...

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