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# Category Archives: Synchronous Machine

## 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)

_{s }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.

## How synchronous motor starts

_{s}due 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 |

**1**. Give a three a.c. supply to a three phase winding. This will produce rotating magnetic field rotating at synchronous speed N

_{s }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. N

_{s }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.

_{a}).

**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.

**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.

**1.O.C. Test:**

_{oc})

_{ph}against I

_{f}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**

**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.**

_{s }for any load and load p.f. conditions.

_{s}. 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 |

Z_{s }= E_{ph}/ I_{asc }

_{s}. So expression for Z

_{s }can be modified as

So O.C.C. and S.C.C. can be effectively to calculate Z_{s}.

**4. Regulation Calculations:**

From O.C.C. and S.C.C., Z_{s }can be determined for any load condition.

_{a}) 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 R

_{a }per phase is known.

So synchronous reactance per phase can be determined.

_{ph }can be determined by the mathematical expression derived earlier.

where V_{ph }= Phase value of rated voltage

I_{a }= Phase value of current depending on the load condition

cosΦ = p.f. of load

_{a }and X

_{s }values 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:**

_{s }for any load condition can be calculated. Hence regulation of the alternator at any load condition and load power factor can be determined.

**Limitation:**

**pessimistic method.**