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3-phase Induction Motors

# 3-phase Induction Motors - Induction Motors Sattar Hussain...

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Induction Motors Sattar Hussain [email protected]

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1- Synchronous AC Machines The synchronous AC machines are built inside out : the armature winding is on the stator, and the field poles are on the rotor. This eliminates the commutator and reduces maintenance requirements. Since the armature circuit is the part where the energy conversion takes place, then by placing the armature winding on the stationary part, it can be secured better, both from the mechanical and electrical point view.
3-phase induction motor is a synchronous AC machines which is considered for many applications a constant speed motor ” since the variation from no-load to full load is very small. In 3-phase induction motor, a 3-phase supply is applied to the 3-phase windings in the stator. The rotor has also 3-phase windings . Each coil (or phase winding) is placed from the next coil by 120 degrees. As such it is called a three-phase winding . Phase A is represented by a-a’, phase B by b-b’ , and phase C by c-c’.

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The 3-phase AC supply has a phase-sequence gives the order in which the three voltages occur as time progresses. The sequence abc is called the positive phase sequence and will be considered for 3-phase induction motor analysis. The 3-phase windings in the stator may be connected as (Y) or as . Y-connection -connection
The induced voltage E in each coil is known as a phase voltage , V p , and the voltage between any two of the line conductors is known as the line voltage V L For Y-connection , the relations between each line voltage V L , line current I L and the phase voltage V p , phase current I P , respectively, are: For -connected connection: If the line quantities are used, the total 3-phase power can be expressed as: If the phase quantities are used, then P L p L I I V V = = , 3 p L P L I I V V 3 , = = θ cos 3 L L I V P = θ cos 3 p p I V P =

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2- Construction of Induction Motors There are two distinct types of rotor design, the so- called squirrel-cage rotor and the wound-rotor construction. Squirrel-cage rotor induction motor Wound-rotor induction motor
The squirrel-cage winding consists of aluminum or copper bars (conductor) embedded in the rotor slots and short-circuited on both sides of the rotor by end rings. There are no slip rings and brushes, making this construction maintenance free. The wound-rotor induction motor, on the other side, has a 3-phase distributed winding on the rotor as well, similar to that of the stator winding and wound for the same number of poles. The rotor winding is normally Y-connected and the three phase ends are connected to three slip rings, which allows external resistance to be connected in series with the rotor winding for speed control.

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3- Creation of rotating magnetic field The fundamental principle of synchronous AC machines operation is the generation of rotating magnetic field created by the 3-phase supply voltage in the stator armature windings.
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