20-08-2012, 01:56 PM
SEMINAR REPORT ON SYNCHRONOUS MOTOR
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INTRODUCTION:-
A synchronous electric motor is an AC motor in which the rotation rate of the shaft is synchronized with the frequency of the AC supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain electromagnets on the stator of the motor that create amagnetic field which rotates in time with the oscillations of the line current. The rotor turns in step with this field, at the same rate.
Another way of saying this is that the motor does not rely on "slip" under usual operating conditions, and as a result produces torque at synchronous speed. Synchronous motors can be contrasted with induction motors, which must slip in order to produce torque. The speed of the synchronous motor is determined by the number of magnetic poles and the line frequency.
Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current excited industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting ac energy to work. Second, it can operate at leading or unitypower factor and thereby provide power-factor correction.
Type Of Synchronous Motor:-
There are two major types of synchronous motors: 'non-excited' and 'direct-current excited', which have no self-starting capability to reach synchronism without extra excitation means, such as electronic control or induction.
With recent advances in independent brushless excitation control of the rotor winding set that eliminates reliance on slip for operation, the 'brushless wound-rotor doubly fed electric machine' is the third type of synchronous motor with all the theoretical qualities of the synchronous motor and the wound-rotor doubly fed motor combined, such as power factor correction, highest power density, highest potential torque density, low cost electronic controller, highest efficiency, etc.
Non-excited motors
In non-excited motors, the rotor is made of solid steel. At synchronous speed it rotates in step with the rotating magnetic field of the stator, so it has an almost-constant magnetic field through it. The external stator field magnetizes the rotor, inducing the magnetic poles needed to turn it. The rotor is made of a high-retentivity steel such as cobalt steel, These are manufactured in permanent magnet, reluctance and hysteresis designs:
1 Reluctance motors: These have a rotor consisting of a solid steel casting with projecting (salient) toothed poles, the same number as the stator poles. The size of the air gap in the magnetic circuit and thus the reluctance is minimum when the poles are aligned with the magnetic field of the stator, and increases with the angle between them. This creates a torque pulling the rotor into alignment with the nearest pole of the stator field. Thus at synchronous speed the rotor is "locked" to the rotating stator field. This cannot start the motor, so the rotor poles usually have squirrel-cage windings embedded in them, to provide torque below synchronous speed. The machine starts as an induction motor until it approaches synchronous speed, when the rotor "pulls in" and locks to the rotating stator field.
DC-excited motors
Made in sizes larger than 1 hp, these motors require direct current supplied through slip rings for excitation. The direct current can be supplied from a separate source or from a dc generator directly connected to the motor shaft
Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed.
Synchronous motors fall under the category of synchronous machines which also includes the alternator (synchronous generator). These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.
Principle of operation
In order to understand the principle of operation of a synchronous motor, let us examine what happens if we connect the armature winding (laid out in the stator) of a 3-phase synchronous machine to a suitable balanced 3-phase source and the field winding to a D.C source of appropriate voltage. The current flowing through the field coils will set up sta- tionary magnetic poles of alternate North and South. ( for convenience let us assume a salient pole rotor, as shown in Fig.). On the other hand, the 3-phase currents flowing in the armature winding produce a rotating magnetic field rotating at synchronous speed. In other words there will be moving North and South poles established in the stator due to the 3-phase currents i.e at any location in the stator there will be a North pole at some instant of time and it will become a South pole after a time period corresponding to half a cycle. (after a time = 1 2f , where f = frequency of the supply). Let us assume that the stationary South pole in the rotor is aligned with the North pole in the stator moving in clockwise direction at a particular instant of time.
These two poles get attracted and try to maintain this alignment ( as per lenz’s law) and hence the rotor pole tries to follow the stator pole as the conditions are suitable for the production of torque in the clockwise direction. However the rotor cannot move instantaneously due to its mechanical inertia, and so it needs sometime to move. In the mean time, the stator pole would quickly (a time duration corresponding to half a cycle) change its polarity and becomes a South pole. So the force of attraction will no longer be present.
Motor Starting by Reducing the supply Frequency
If the rotating magnetic field of the stator in a synchronous motor rotates at a low enough speed, there will be no problem for the rotor to accelerate and to lock in with the stator’s magnetic field. The speed of the stator magnetic field can then be increased to its rated op- erating speed by gradually increasing the supply frequency f up to its normal 50- or 60-Hz value. This approach to starting of synchronous motors makes a lot of sense, but there is a big
problem: Where from can we get the variable frequency supply? The usual power supply systems generally regulate the frequency to be 50 or 60 Hz as the case may be. However, variable-frequency voltage source can be obtained from a dedicated generator only in the olden days and such a situation was obviously impractical except for very unusual or special
drive applications. But the present day solid state power converters offer an easy solution to this. We now have the rectifier- inverter and cycloconverters, which can be used to convert a constant fre- quency AC supply to a variable frequency AC supply. With the development of such modern solid-state variable-frequency drive packages, it is thus possible to continuously control the frequency of the supply connected to the synchronous motor all the way from a fraction of a hertz up to and even above the normal rated frequency.