DIGITAL PHASE CONVERTERS THREE-PHASE MOTORS ROTARY PHASE CONVERTERS STATIC PHASE CONVERTERS BUCK/BOOST TRANSFORMERS

 


 

Phase Converters & Power Factor
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     • Harmonic Distortion
Three-Phase Motors
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Motor Starting Currents

Phase Converters and Three-Phase Motors

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Motors that convert electrical energy to mechanical energy comprise two-thirds of the industrial demand for electricity. 

Most of these motors are three-phase squirrel-cage induction motors which consist of an arrangement of coils wound in slots in a stack of iron laminations shown in cross section in figure 1 below. This part of the motor is stationary and is called the stator. The coils in the stator are connected in a manner to produce at least three separate windings which are at angles of 120 degrees with respect to each other. This is shown schematically in figure 1.

If a set of three-phase voltages is applied to the windings shown, a magnetic field will be produced in the center portion of the stator.  This magnetic field is constant in magnitude, and rotates at the frequency of the applied voltages (either 50 or 60 Hz depending on what country you're in).

The second part of the motor (the rotor) is a set of round iron laminations that have been attached to a shaft with bearings. There are slots in this set of laminations as well.  In this instance the slots are filled with very low resistance bars of aluminum that are shorted together at the outer edges of the laminated stack of iron.  If the rotor is inserted into the center part of the stator, the magnetic field generated by the stator will cross through the shorting bars of the rotor causing a large current in the rotor. These rotor currents react with the magnetic field generated by the stator and cause the rotor to spin.  The rotor will continue to accelerate until the shaft rotation speed is nearly equal to the velocity at which the magnetic field of the stator is spinning.

The important point here is that if the stator had only a single coil driven by a single-phase voltage, then the magnetic field generated by the stator would not rotate it could for example point either up or down, but not left or right. The motor could never start because there would be no rotational component of the magnetic field.  Thus a three-phase system allows the mechanical energy being fed into the generator to be transferred to three-phase induction motors very efficiently. Three-phase motors also have the advantage of being very simple and reliable--there are no electrical switches contained in them.  If they are not overheated, the only thing that wears out is the bearings, which are replaceable.

       
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