Sohail Ansari

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Why Synchronous Motor is not a self starting?????


Whenever a three phase supply is given to the stator, a rotating magnetic field is produces which is rotating at very high speed depending up on the no. of poles and frequency of supply. Due to change in flux w.r.t time an emf is induced in the rotor and torque in developed in anticlockwise direction (direction is taken acc. to fleming right hand rule)
 Now stator fields are rotating very fast i.e. at a speed Ns  r.p.m. Due to inertia, before rotor hardly rotates in the direction of  anticlockwise torque, to which it is subjected, the stator poles change their positions. Consider an instant half a period latter where stator poles are exactly reversed but due to inertia rotor is unable to rotate from its initial position. This is shown in the Fig.
Fig.  1
       At this instant, due to the unlike poles trying to attract each other, the rotor will be subjected to a torque in clockwise direction. This will tend to rotate rotor in the direction of rotating magnetic field.
       But before this happen, stator poles again change their position reversing the direction of the torque exerted on the rotor.
As a result, the average torque exerted on the rotor is zero. And hence the synchronous motor is not self starting. So under any case, whatever may be the starting position of the rotor, synchronous motor is not self starting.

How synchronous motor starts


suppose the rotor is rotated by some external means at a speed almost equal to synchronous speed. And then the rotor is excited to produce its poles. At a certain instant now, the stator and rotor unlike poles will face each other such that their magnetic axes are near each other. Then the force of attraction between the two, pulls both of them into the magnetic locking condition.
 
       Once magnetic locking is established, the rotor and stator poles continue to occupy the same relative positions. Due to this, rotor continuously experiences a unidirectional torque in the direction of the rotating magnetic field. Hence rotor rotates at synchronous speed and said to be in synchronism with rotating magnetic field. The external device used to rotate rotor near synchronous speed can be removed once synchronism is established. The rotor then continues its rotation at Nsdue to magnetic locking. This is the reason why synchronous motor runs only at synchronous speed and does not rotate at any speed other than the synchronous. This operation is shown in the Fig 1(a) and (b).

Fig. 1 Unidirectional torque experienced by rotor
       It is necessary to keep field winding i.e. rotor excited from d.c. supply to maintain the magnetic locking, as long as motor is operating.
       So a general procedure to start a synchronous motor can be stated as :
1. Give a three a.c. supply to a three phase winding. This will produce rotating magnetic field rotating at synchronous speed Ns  r.p.m.
2. Then drive the rotor by some external means like diesel engine in the direction of rotating magnetic field, at a speed very near or equal to synchronous speed.
3. Switch on the d.c. supply given to the rotor which will produce rotor poles. now there are twp fields one is rotating magnetic field produced by stator while the other is produced by rotor which is physically rotated almost at the same speed as that of rotating magnetic field.
4. At a particular instant, both the fields get magnetically locked. The stator field pulls rotor field into synchronism. Then the external device used to rotate rotor can be removed. But rotor will continue to rotate at the same speed as that of rotating magnetic field i.e. Ns  due to magnetic locking. 
Key Point : So the essence of the discussion is that to start the synchronous motor, it needs some device to rotate the rotor at a speed very near or equal to the synchronous speed.

Synchronous Impedance Method or E.M.F Method (for finding Voltage Regulation)


“THIS IS JUST LIKE FINDING THEVENIN’S IMPEDANCE WHEN BOTH DEPENDENT AND INDEPENDENT SOURCE ARE PRESENT IN THE NETWORK “. So plz dont think this is different topic. This is one of the application of Thevenin Theorem….. 🙂

The method is also called E.M.F. method of determining the voltage regulation. The method requires following data to calculate the regulation.

1. The armature resistance per phase (Ra).
2. Open circuit characteristics which is the graph of open circuit voltage against the field current. This is possible by conducting open circuit test on the alternator.
3. Short circuit characteristics which is the graph of short circuit current against field current. This is possible by conducting short circuit test on the alternator.
       Let us see, the circuit diagram to perform open circuit as well as short circuit test on the alternator. The alternator is coupled to a prime mover capable of driving the alternator at its synchronous speed. The armature is connected to the terminals of a switch. The other terminals of the switch are short circuited through an ammeter. The voltmeter is connected across the lines to measure the open circuit voltage of the alternator.
       The field winding is connected to a suitable d.c. supply with rheostat connected in series. The field excitation i.e. field current can be varied with the help of this rheostat. The circuit diagram is shown in the Fig.
Fig. 1  Circuit diagram for open circuit and short circuit test on alternator
1.O.C. Test:
 Procedure:
i) Start the prime mover and adjust the speed to the synchronous speed of the alternator.
ii) Keeping rheostat in the field circuit maximum, switch on the d.c. supply.
iii) The T.P.S.T switch in the armature circuit is kept open.
iv) With the help of rheostat, field current is varied from its minimum value to the rated value. Due to this, flux increasing the induced e.m.f. Hence voltmeter reading, which is measuring line value of open circuit voltage increases. For various values of field current, voltmeter readings are observed.
Observation table for open circuit test :

       From the above table, graph of (Voc)ph against If is plotted.
Note : This is called open circuit characteristics of the alternator, called O.C.C. This is shown in the Fig.

Fig. 2  O.C.C. and S.C.C. of an alternator
2. S.C.Test
       After completing the open circuit test observation, the field rheostat is brought to maximum position, reducing field current to a minimum value. The T.P.S.T switch is closed. As ammeter has negligible resistance, the armature gets short circuited. Then the field excitation is gradually increased till full load current is obtained through armature winding. This can be observed on the ammeter connected in the armature circuit. The graph of short circuit armature current against field current is plotted from the observation table of short circuit test. This graph is called short circuit characteristics, S.C.C. This is also shown in the Fig. 2.
Observation table for short circuit test :

The S.C.C. is a straight line graph passing through the origin while O.C.C. resembles B-H curve of a magnetic material.
Note : As S.C.C. is straight line graph, only one reading corresponding to full load armature current along with the origin is sufficient to draw the straight line.
3. Determination of Impedance from O.C.C. and S.C.C.
       The synchronous impedance of the alternator changes as load condition changes. O.C.C. and S.C.C. can be used to determine Zfor any load and load p.f. conditions.
       In short circuit test, external load impedance is zero. The short circuit armature current is circulated against the impedance of the armature winding which is Zs. The voltage responsible for driving this short circuit current is internally induced e.m.f. This can be shown in the equivalent circuit drawn in the Fig.
Fig. 3  Equivalent circuit on short circuit
 From the equivalent circuit we can write,

Z= Eph/ Iasc 

This is what we are interested in obtaining to calculate value of Zs. So expression for Zcan be modified as
                                                                             
               

So O.C.C. and S.C.C. can be effectively to calculate Zs.

4. Regulation Calculations:

From O.C.C. and S.C.C., Zcan be determined for any load condition.

The armature resistance per phase (Ra) can be measured by different methods. One of the method is applying d.c. known voltage across the two terminals and measuring current. So value of Rper phase is known.

So synchronous reactance per phase can be determined.

       No load induced e.m.f. per phase, Eph can be determined by the mathematical expression derived earlier.
                       

where     Vph = Phase value of rated voltage
I= Phase value of current depending on the load condition
cosΦ = p.f. of load

        Positive sign for lagging power factor while negative sign for leading power factor, Rand Xvalues are known from the various tests performed.

The regulation then can be determined by using formula,

                                            
5.Advantages and Limitations of Synchronous Impedance Method:
Advantage:
 synchronous impedance Zfor any load condition can be calculated. Hence regulation of the alternator at any load condition and load power factor can be determined.
Limitation:
The main limitation of this method is that the method gives large values of synchronous reactance. This leads to high values of percentage regulation than the actual results. Hence this method is called pessimistic method.
This is all about synchronous Impedance method for calculation voltage regulation of synchronous machine.
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