- More About AC Motors
- Types of AC Motors
- Different types of motors and their use
- Types of AC Motor - Different Types of Motors - Electric Motor Types
- AC Motors vs DC Motors
- More About DC Motors
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- Types of AC motors – Classification and uses of alternating current motors
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More About AC Motors
Therefore, the motor has four poles per phase. If the number of poles per phase is given on the nameplate, the synchronous speed can be determined by dividing times the frequency by the number of poles per phase.
In the example used above, the synchronous speed is equal to 7, divided by 4, or 1, rpm. The rotor of an induction motor consists of an iron core having longitudinal slots around its circumference in which heavy copper or aluminum bars are embedded.
These bars are welded to a heavy ring of high conductivity on either end. The composite structure is sometimes called a squirrel cage, and motors containing such a rotor are called squirrel cage induction motors.
Types of AC Motors
See figure The induced voltage causes a current to flow through the bars. This current, in turn, produces its own magnetic field which combines with the revolving field so that the rotor assumes a position in which the induced voltage is minimized.
As a result, the rotor revolves at very nearly the synchronous speed of the stator field, the difference in speed being just sufficient enough to induce the proper amount of current in the rotor to overcome the mechanical and electrical losses in the rotor. If the rotor were to turn at the same speed as the rotating field, the rotor conductors would not be cut by any magnetic lines of force, no e.
Different types of motors and their use
The rotor would then slow down. For this reason, there must always be a difference in speed between the rotor and the rotating field. This difference in speed is called slip and is expressed as a percentage of the synchronous speed. For example, if the rotor turns at 1, rpm and the synchronous speed is 1, rpm, the difference in speed is 50 rpm.
The first effort in the development of a self-starting, single phase motor was the shaded pole induction motor figure This motor has salient poles, a portion of each pole being encircled by a heavy copper ring. The presence of the ring causes the magnetic field through the ringed portion of the pole face to lag appreciably behind that through the other part of the pole face.
Types of AC Motor - Different Types of Motors - Electric Motor Types
The net effect is the production of a slight component of rotation of the field, sufficient to cause the rotor to revolve. As the rotor accelerates, the torque increases until the rated speed is obtained.
Such motors have low starting torque and find their greatest application in small fan motors where the initial torque required is low. In figure , a diagram of a pole and the rotor is shown.
The poles of the shaded pole motor resemble those of a dc motor. The synchronous motor is one of the principal types of ac motors.
Like the induction motor, the synchronous motor makes use of a rotating magnetic field. Unlike the induction motor, however, the torque developed does not depend on the induction of currents in the rotor. Briefly, the principle of operation of the synchronous motor is as follows: A multiphase source of ac is applied to the stator windings, and a rotating magnetic field is produced.
AC Motors vs DC Motors
A direct current is applied to the rotor winding, and another magnetic field is produced. The synchronous motor is so designed and constructed that these two fields react to each other in such a manner that the rotor is dragged along and rotates at the same speed as the rotating magnetic field produced by the stator windings.
An understanding of the operation of the synchronous motor can be obtained by considering the simple motor of figure Assume that poles A and B are being rotated clockwise by some mechanical means in order to produce a rotating magnetic field, they induce poles of opposite polarity in the soft iron rotor, and forces of attraction exist between corresponding north and south poles.
To design a series motor for satisfactory operation on ac, the following changes are made:. The eddy current losses are reduced by laminating the field poles, frame and armature. Sparking at the commutator is reduced by the use of preventive leads P1, P2, P3, and so forth, as shown in figure , where a ring armature is shown for simplicity.
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When coils at A and B are shorted by the brushes, the induced current is limited by the relatively high resistance of the leads.
Sparking at the brushes is also reduced by using armature coils having only a single turn and multipolar fields. High torque is obtained by having a large number of armature conductors and a large diameter armature. Thus, the commutator has a large number of very thin commutator bars and the armature voltage is limited to about volts.
Induction Motor Slip When the rotor of an induction motor is subjected to the revolving magnetic field produced by the stator windings,. A low resistance, short circuited coil or copper band is placed across one tip of each small pole, from which, the motor gets the name of shaded pole.
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The rotor of this motor is the squirrel cage type. As the current increases in the stator winding, the flux increases. A portion of this flux cuts the low resistance shading coil. This induces a current in the shading coil, and by Lenz's law, the current sets up a flux which opposes the flux inducing the current. Hence, most of the flux passes through the unshaded portion of the poles, as shown in figure Consequently, as poles A and B rotate, the rotor is dragged along at the same speed.
However, if a load is applied to the rotor shaft, the rotor axis will momentarily fall behind that of the rotating field but, thereafter, will continue to rotate with the field at the same speed, as long as the load remains constant. If the load is too large, the rotor will pull out of synchronism with the rotating field and, as a result, will no longer rotate with the field at the same speed.
The motor is then said to be overloaded. Such a simple motor as that shown in figure is never used.
The idea of using some mechanical means of rotating the poles is impractical because another motor would be required to perform this work. Also, such an arrangement is unnecessary because a rotating magnetic field can be produced electrically by using phased ac voltages.
Types of AC motors – Classification and uses of alternating current motors
In this respect, the synchronous motor is similar to the induction motor. Hysteresis losses are minimized by using high permeability, transformer-type, silicon steel laminations. The reactance of the field windings is kept satisfactorily low by using shallow pole pieces, few turns of wire, low frequency usually 25 cycles for large motors , low flux density, and low reluctance a short airgap.
The reactance of the armature is reduced by using a compensating winding embedded in the pole pieces.
If the compensating winding is connected in series with the armature, as shown in figure , the armature is conductively compensated. If the compensating winding is designed as shown in figure , the armature is inductively compensated.
If the motor is designed for operation on both dc and ac circuits, the compensating winding is connected in series with the armature.
This arrangement is similar to the compensating winding used in some dc motors and generators to overcome armature reaction. The compensating winding establishes a counter magnetomotive force, neutralizing the effect of the armature magnetomotive force, preventing distortion of the main field flux, and reducing the armature reactance.
The inductively compensated armature acts like the primary of a transformer, the secondary of which is the shorted compensating winding. The shorted secondary receives an induced voltage by the action of the alternating armature flux, and the resulting current flowing through the turns of the compensating winding establishes the opposing magnetomotive force, neutralizing the armature reactance.
Different types of ac and dc motors pdf reader
Fractional horsepower ac series motors are called universal motors. They do not have compensating windings or preventive leads. They are used extensively to operate fans and portable tools, such as drills, grinders, and saws.