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Synchronous Motors
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Harmonics In Synchronous Machines

What Are ? Harmonics In Synchronous Machines

Harmonics: When the uniformly sinusoidally distributed air gap flux is cut by either the stationary or rotating armature sinusoidal emf is induced in the alternator. Hence the nature of the waveform of induced emf and current is sinusoidal. But when the alternator is loaded waveform will not continue to be sinusoidal or becomes non-sinusoidal. Such non-sinusoidal wave form is called complex wave form.

By using Fourier series representation it is possible to represent complex non-sinusoidal waveform in terms of series of sinusoidal components called harmonics, whose frequencies are integral multiples of fundamental wave. The fundamental wave form is one which is having the frequency same as that of complex wave.The waveform, which is of the frequency twice that of the fundamental is called second harmonic. The one which is having the frequency three times that of the fundamental is called third harmonic and soon. These harmonic components can be represented as follows.

Fundamental: e1 = Em1 Sin ( t ± θ1)
2nd Hermonic e2 = Em2 Sin (2 t ± θ2)
3rd Harmonic e3 = Em3 Sin (3 t ± θ3)
5th Harmonic e5 = Em5 Sin (5 t ± θ5) etc.
In case of alternators as the field system and the stator coils are symmetrical the induced emf will also be symmetrical and hence the generated emf in an alternator will not contain any even harmonics.

Slot Harmonics: As the armature or stator of an alternator is slotted, some harmonics are induced into the emf which is called slot harmonics. The presence of slot in the stator makes the air gap reluctance at the surface of the stator non uniform. Since in case of alternators the poles are moving or there is a relative motion between the stator and rotor, the slots and the teeth alternately occupy any point in the air gap. Due to this the reluctance or the air gap will be continuously varying. Due to this variation of reluctance ripples will be formed in the air gap between the rotor and stator slots and teeth. This ripple formed in the air gap will induce ripple emf called slot harmonics.

Minimization Techniques of Harmonics: To minimize the harmonics in the induced waveforms following methods are employed:
1. Distribution of stator winding.
2. Short Chording
3. Fractional slot winding
4. Skewing
5. Larger air gap length.

Effect of Harmonics on induced emf:

The harmonics will affect both pitch factor and distribution factor and hence the induced emf. In a well designed alternator the air gap flux density distribution will be symmetrical and hence can be represented in Fourier series as follows.

The RMS value of the resultant voltage induced can be given as

Eph2 =  [(E1)2+ ....+ …………… (En)2]

**(A)2 Means A Square 

Effect of Harmonics of pitch and distribution Factor:

The pitch factor is given by Kp = cos /2, where is the chording angle.
For any harmonic say nth harmonic the pitch factor is given by Kpn = cos n α/2
The distribution factor is given by Kd = (sin mβ /2) / (m sin β/2)

For any harmonic say nth harmonic the distribution factor is given by 
Kdn = (sin mn β/2)/(m sin nβ/2)

This is the detailed info about Harmonics In Synchronous Machines,Minimization Methods of Harmonics.Effect of Harmonics on induced emf.

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Differences Between Synchronous and Induction Motor

Differences Between Synchronous and Induction Motor

We have two major types of AC motors.They are synchronous and induction motor,synchronous motor runs at synchronous speed where induction motor runs at less than synchronous speed.In this article we are going to discuss on comparisons between synchronous and induction motor. There are so many differences between synchronous motor and induction motor like excitation,speed control.etc.Most important points are listed below.

Synchronous Motor Induction Motor
Construction is complicated. Construction is simpler , particularly in
case of cage rotor.
Not self starting.(why read here) Self starting.
Separate DC source is required for rotor
Rotor gets excited by the induced e.m.f
so separate source is not necessary.
The speed is always synchronous irrespective
of the load.
The speed is always less than synchronous
but never synchronous.
Speed control is not possible. Speed control is possible though difficult.
As load increases, load angle increases,keeping
speed constant at synchronous.
As load increases , the speed keeps on
By changing excitation , the motor p.f can be
changed from lagging and leading.
It always operates at lagging p.f and p.f
control is not possible.
It can be used as synchronous condenser for p.f
It can not be used as synchronous condenser.
Motor is sensitive to sudden load changes and
hunting results.
Phenomenon of hunting is absent.
Motor is costly and requires frequent maintenance. Motor is cheap, especially cage rotors and
maintenance free.

Read Here: Differences B/W Induction Motor & Transformer
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V-Curves and Inverted V-Curves

 V-Curves and Inverted V-Curves of Synchronous Motor

A synchronous motor is a double-excited machine, its armature winding is energized from an a.c source and its field winding from d.c source.Synchronous motor operates at unity power factor when field current is enough to set up the air-gap flux, as demanded by constant applied voltage.This field current, which causes unity power factor operation of the synchronous motor, is called normal excitation or normal field current.

If the the motor is under excited i.e, current is less than normal excitation,there will be some deficiency in flux to compensate that winding draws a magnetizing current from the a.c source and as a result of it, the motor operates at a lagging power factor.In other case, in over excited i.e, field current more than normal excitation machine operates at leading power factor.In the below figure you can observe leading and lagging variations.
In synchronous motors  performance characteristics obtained by v-curves and inverted v-curves. Synchronous machines have parabolic type characteristics.If excitation is varied from very low (under excitation) to very high (over excitation) value, then current Ia decreases, becomes minimum at unity p.f. and then again increases.But initial lagging current becomes unity and then becomes leading in nature.V-curves and inverted V-curves are used to analyze motor efficiency with no-load and with load.

What are V and inverted V curves ?

  • V-curve is a graph between Ia Vs If.
  • Inverted V curve are drawn between cosΦ Vs If.
In the above two cases If (field current) taken on x-axis.We can control the field excitation of synchronous motor by increasing/decreasing the field current.

Experiment to Obtain V-Curves and Inverted V-Curves 

If graph of armature current drawn by the motor (Ia) against field current (If) is plotted, then its shape looks like alphabet V. If the power factor (cos Φ) is plotted against field current (If), then the shape of the graph looks like an inverted V.
v and inverted v curves of synchronous motor circuit diagram
  • Connect stator of motor to 3 phase supply as shown in the above figure through watt meters and ammeter.
  • In this experiment two watt meter method is used to measure input power of motor.Ammeter reads the line current which is same as armature (stator) current. Voltmeter is reads line voltage.
  • A rheostat in a potential divider arrangement is used in the field circuit to control the excitation.By changing its value note down the voltage,armature current,watt meter readings(W1&W2) in a table like below.

 Now IL = Ia, per phase value can be determined, from the stator winding connections. 
       IL  = Iaph for stator connection
       IL/√3 = Iaph for delta connection
       The power factor can be obtained as
From above values make a final table like this,

The graph can be plotted from this result table.
1) Ia Vs If  → V-curve
2) cosΦ Vs If  → Inverted V-curve
This is the entire procedure can be repeated for various load conditions to obtain of V-curves and Inverted V-curves of any synchronous motor.

Tags: v and inverted v curves of synchronous motor wikipedia,v and inverted v curves of 3 phase synchronous motor

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Starting Methods of Synchronous Motor

In the previous article we came to know that synchronous motors are not self starting so to make them start automatically we should add some tweaks.We should rotate rotor near to the speed of synchronous speed,so it attains synchronous speed.We have some fine methods to start the synchronous motor.They are,
1. Using pony motors.
2. Using small d.c. motor.
3. As a slip ring induction motor.
4. Using damper winding.

1. Pony Motor Method / Induction Motor

Pony motors are small motors mostly induction motors,which are used to run the rotor near to the synchronous speed. Once the rotor attains the synchronous speed, the d.c. excitation to the rotor is switched on.Pony motors are connected to the shaft of synchronous motor only up to it attains synchronous speed.Once the synchronism is established pony motor is decoupled. The motor then continues to rotate as synchronous motor.

In the below figure we tried to show you pony motor connection to synchronous motor.This is not exact arrangement what we use in industries.
Methods of Starting Synchronous Motor

2. Using Small D.C. Machine

This is also similar to pony motor method with some advantages.Mostly this method employed for large synchronous motors.For synchronous motor a dc machine will be connected.This dc machine is used as a d.c. motor to rotate the synchronous motor at a synchronous speed.. Once motor starts running as a synchronous motor, the same d.c. machine acts as a d.c. generator to supply dc to the rotor of syn. motor.This have some advantages like low power usage and easy starting.

3.Using Damper Winding

This is the most used method of starting synchronous motor.In addition to the normal field winding an additional winding consisting of copper bars placed in the slots in the pole faces.Ends of the copper bars are short circuited with the help of end rings.This winding is called damper winding.This damper winding acts as a squirrel cage rotor of an induction motor.
Methods of Starting Synchronous Motor
First synchronous motor runs as induction motor due to the damper winding at sub synchronous speed.Then d.c. supply is given to the field winding. At some instant motor gets pulled into synchronism and starts rotating at a synchronous speed.As rotor rotates at synchronous speed, the relative motion between damper winding and the rotating magnetic field is zero. Hence when motor is running as synchronous motor, there can not be any induced e.m.f. in the damper winding. So damper winding is active only at start, to run the motor as an induction motor at start. 

4. Slip Ring Induction Motor

In this method an external rheostat then can be introduced in series with the rotor circuit. So when stator is excited, the motor starts as a slip ring induction motor and due to resistance added in the rotor provides high starting torque.
Methods of Starting Synchronous Motor
The resistance is then gradually cut off, as motor gathers speed. When motor attains speed near synchronous. d.c. excitation is provided to the rotor, then motors gets pulled into synchronism ans starts rotating at synchronous speed. The damper winding is shorted by shorting the slip rings. The initial resistance added in the rotor not only provides high starting torque but also limits high inrush of starting current. Hence it acts as a motor resistance starter.
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Why Synchronous Motor Is Not Self Starting?

Everyone is saying synchronous motors are not self starting but why here we going to discuss if you don't have time to read this whole article to know why synchronous motor is not self starting ANSWER IS INERTIA OF ROTOR.Let us have a detailed information on this topic.
To get a clear idea about this question consider a rotating magnetic field as equivalent to physical rotation of two stator poles N1 and S1 as shown in the figure.Assume any instant like that two poles are in line with stator magnetic axis [A-B].At this instant, rotor poles are arbitrarily positioned as shown in the figure.
Read This : Synchronous Motor Working
When DC supply is given to stationary rotor unlike poles will try to attract each other.Because of this action rotor will be subjected to an instantaneous torque in anticlockwise direction.As we connected power supply to stator,stator poles will rotate at speed of  Ns  r.p.m.

Due to inertia of rotor it is unable to rotate in the direction of  anticlockwise torque, to which is driving force or stator rotating field.Just in that instant 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.Shown in figure (b).

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.

Key Point : Hence the average torque on the rotor is zero.So synchronous motor will not start it self .

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Synchronous Motor Working Principle

A motor is an electro-mechanical device which converts electrical energy to mechanical energy. and a 3 phase motor runs at synchronous speed is called synchronous motor.Synchronous motor mainly runs on principle of magnetic locking between rotor and RMF(Rotating magnetic field).When two unlike poles are brought near each other, if the magnets are strong, there exists a tremendous force of attraction between those two poles. In such condition the two magnets are said to be magnetically locked. 

Synchronous Motor Working Principle:-

When 3 phase voltage applied to electric conductors are placed in a certain geometrical positions which are  specially arranged (In certain angle from one another) rotating magnetic field produces.this RMF rotates with synchronous speed.The synchronous speed of a stator rotating magnetic field depends on the supply frequency and the number of poles for which stator winding is wound. if the frequency of the a.c. supply is f Hz and stator is wound for P number of poles, then the speed of the rotating magnetic field is synchronous given by,
Ns  = 120f/P  rpm.

Synchronous Motor Working Principle

How Rotor Rotates at Synchronous Speed?

Now to understand the concept of synchronous motor working,consider a two pole simple rotor(shown in fig).Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply.when DC supply given to rotor it acts like a we know rotating magnetic field rotates with synchronous speed when two opposite poles of rotor and RMF locked rotor also rotates with same speed of RMF in the direction of rotating magnetic field.Hence synchronous motor rotates at one and only one speed i.e. synchronous speed. But this all depends on existence of magnetic locking between stator and rotor poles. Practically it is not possible for stator poles to pull the rotor poles from their stationary position into magnetic locking condition. this is the reason why synchronous motors are not self starting.

Methods of Starting of Synchronous Motor:-

1.Pony motor method:

Synchronous motors are mechanically coupled with another 3 phase induction motor or DC shunt motor. DC excitation to the rotor is not fed initially. It is rotated at speed very close to its synchronous speed and after that DC excitation is given to the rotor. After some time when magnetic locking takes place supply to the external motor is cut off from coupling.

2.Damper winding Method:

In damper winding method synchronous motor is first run as three phase induction motor using  damper winding and finally it is synchronized with the RMF.

Application Of Synchronous Motors

1.It is used where high power at constant speed is required.EX: rolling mills, chippers, mixers, pumps, pumps, compressor etc.

2.As synchronous motor is capable of operating under either leading and lagging power factor, it can be used for power factor improvement.  A synchronous motor under no-load with leading power factor is connected in power system where static capacitors can not be used.

3.Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. 

Tags:Synchronous Motor Working Principle,Applications Of Synchronous Motor,Methods of Starting of Synchronous Motor,why synchronous motors are not self starting,How Rotor Rotates at Synchronous Speed?

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