synchronous motor - coursecontent

SYNCHRONOUS MOTOR

Introduction Synchronous motors is called so because

the speed of the rotor of this motor is same as the rotating magnetic field.

It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed is there or in other words it’s in synchronism with the supply frequency.

Synchronous speed is given by

Construction of Synchronous Motor

N o r m a l l y i t ’ s c o n s t r u c t i o n i s almost similar to that of a 3 phase induction motor, except the f ac t that the rotor is given dc supply.

The stator is given three phase supply and the rotor i s given dc supply.

Main Parts Stator – Armature Rotor – Field

Stator

Construction

The winding consists of copper bars insulated with mica and epoxy resin.

The conductors are secured by steel wedges.

The iron core is supported by a steel housing

Stator From an electrical standpoint, the stator

of a synchronous motor is identical to that of a 3-phase induct ion motor (cylindrical laminated core containing slots carrying a 3-phase winding).

T h e n o m i n a l l i n e v o l t a g e o f a synchronous generator depends upon its kVA rating — the greater the power, the higher the voltage

Stator Stationary armature Coils is placed in slots of stator

core. Stator is made of laminates. Whole stator is fixed in the

frame.

Rotor Field winding External D.C.supply

of 120-600 volt Supply by Brush &

Slip ring N pole & S pole

developed alternatively

Salient-pole rotors — Used fo r l ow speed appl icat ions

(<300rpm) which require large number of poles to achieve required frequencies

Cylindrical rotor — Used for high-speed applications — Minimum number of poles is 2, so for

50Hz the maximum speed is 3000rpm.

Synchronous Motor: Rotor

Cylindrical rotor Machines

Salient Rotor Machine

Main Features of Synchronous Motors

Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.

This motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement.

Principle of Operation Synchronous Motor

Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it.

The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux.

The rotor carrying DC supply also produces a constant flux.

At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force.

But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.

To overcome this inertia, rotor is initially fed some mechanical input which rotates it in same direction as magnetic field to a speed very close to synchronous speed.

After some time magnetic locking occurs and the synchronous motor rotates in synchronism with the frequency.


Application of Synchronous Motor

Synchronous motor having no load connected to its shaft is used for power factor improvement.

Owing to its characteristics to behave at any electrical power factor, it is used in power system in situations where static capacitors are expensive.

Synchronous motor f inds appl icat ion where operating speed is less (around 500 rpm) and high power is required.

For power requirement from 35 kW to 2500 KW, the size, weight and cost of the corresponding three phase induction motor is very high.

Hence these motors are preferably used . Ex- Reciprocating pump, compressor, rolling mills etc.

Number of Poles The number of poles on a synchronous

depends upon the speed of rotation and desired frequency

f = pn /120 Where f = frequency of the induced

voltage (Hz) p = number of poles on the rotor n =

speed of the rotor (rpm)

Methods of Starting of Synchronous Motor Synchronous motors are mechanically

coupled with another motor. It could be either 3 phase induction

motor or DC shunt motor. DC excitation is not fed initially. It is

rotated at speed very close to its synchronous speed and after that DC excitation is given.

After some time when magnetic locking takes place supply to the external motor is cut off.

Excitation The synchronous motor is doubly fed electrical motor

i.e it converts electrical energy to mechanical energy via magnetic circuit. Hence it comes under electromagnetic device.

It receives 3 phase ac electrical supply to its armature winding and DC supply is provided to rotor winding.

Synchronous motor excitation refers to the DC supply given to rotor which is used to produce the r e q u i r e d m a g n e t i c f l u x .

One of the major and unique characteristics of this motor is that it can be operated at any electrical power factor leading, lagging or unity and this feature is based on the excitation of the synchronous motor.

When the synchronous motor is working at constant applied voltage V,. the resultant air g a p f l u x a s d e m a n d e d b y V r e m a i n s substantially constant

This resultant air gap flux is established by the co operation of both AC supply of armature winding and DC supply of rotor winding

CASE 1: When the field current is sufficient enough to produce the a i r gap f lux , as demanded by the constant supply voltage V, then the magnetiz ing current or lagging reactive VA required from ac source is zero and the motor operate at unity power factor.

The field current, which causes this unity power factor is called normal excitation or normal field current.

Excitation

CASE 2: If the field current is not sufficient enough to produce the required air gap flux as demanded by V, additional magnetizing current or lagging reactive VA is drawn from the AC source.

This magnetizing current produces the deficient flux (constant flux- flux set up by dc supply rotor winding).

Hence in this case the motor is said to operate under lagging power factor and the is said to be under excited

Excitation

CASE 3: If the field current is more than the normal field current, motor is said to be over excited.

This excess field current produces excess flux ( f lux set up by DC supply rotor winding – resultant air gap flux) must be neutralized by the armature winding.

Hence the armature winding draws leading reactive VA or demagnetizing current leading voltage by almost 90° from the AC source.

Hence in this case the motor operate under leading power factor.

Excitation

This whole concept of excitation and power factor of synchronous motor can be summed up in the following graph. This is called V curve of synchronous motor.

Excitation

V Curve of a Synchronous Motor

V curve is a plot of the stator current versus field current for different constant loads.

Since the shape of these curves is similar to the letter “V”, thus they are called V curve of synchronous motor.

The power factor of the synchronous motor can be controlled by varying the field current If.

The armature current Ia changes with the change in the field current If.

Let us assume that the motor is running at NO load.

If the field current is increased from this small value, the armature current Ia decreases until the armature current becomes minimum.

At this minimum point, the motor is operating at unity power factor.

The motor operates at lagging power factor until it reaches up to this point of operation.


If now , the f ield current is increased further, the armature current increases and the motor start operating as a leading power factor.

The graph drawn between armature current and field current is known as V curve.

If this procedure is repeated for various increased loads, a family of curves is o b t a i n e d .


Conclusion An overexc i ted synchronous motor

operate at leading power factor, under-excited synchronous motor operate at lagging power factor and normal excited synchronous motor operate at unity power factor.

Methods of starting synchronous motorBasically there are three methods that are

used to start a synchronous motor: By reducing the frequency of the applied

electric power By using an external prime mover to

accelerate the rotor of synchronous motor near to its synchronous speed and then supply the rotor as well as stator.

By using damper windings or amortisseur windings.

Motor Starting by Using damper (Amortisseur) Winding The large synchronous motors are

provided with damper windings. Damper windings are special bars laid

into slots cut in the pole face of a synchronous machine and then shorted out on each end by a large shorting ring, similar to the squirrel cage rotor bars.

Damper Winding

When the stator of such a synchronous machine is connected to the 3-Phase AC supply, the machine starts as a 3-Phase induction machine due to the presence of the damper bars, just like a squirrel cage induction motor.

Once the motor picks up to a speed near about its synchronous speed , the DC supply to its field winding is connected and the synchronous motor pulls into step i . e . i t c o n t i n u e s t o o p e r a t e a s a Synch ronous moto r runn ing a t i t s synchronous speed.

Damper Winding

Equivalent circuit model and phasor diagram of a synchronous motor

Equivalent-circuit model for one phase of a synchronous motor armature

Equivalent circuit

Combining reactances :

Applying Kirchhoff’s voltage law

where:Ra = armature resistance (/phase)Xl = armature leakage reactance (/phase)Xs = synchronous reactance (/phase)Zs = synchronous impedance (/phase)VT = applied voltage/phase (V)Ia = armature current/phase(A)

Phasor Diagram

EMF which is obtained from the phasor equation; Ef = VT − IaZsThe phase angle between the terminal voltage VT and the excitation voltage Ef is usually termed the torque angle. The torque angle is also called the load angle or power angle.

Synchronous-motor power equation Except for very small machines, the armature

resistance of a synchronous motor is relatively insignif icant compared to i ts synchronous reactance, so that Eqn. to be approximated to

VT = Ef + jIaXs

From this phasor diagram, we have, IaXs cos φi = −Ef sin δ Multiplying through by VT and rearranging terms we

have,

Since the left side of Eqn. is an expression for active power input and as the winding resistance is assumed to be negligible this power input will also represent the electromagnetic power developed, per phase, by the synchronous motor.

Thus, for a three-phase synchronous motor,

Above Eqn., called the synchronous-machine power equation, expresses the electro magnetic power developed per phase by a cylindrical-rotor motor, in terms of its excitation voltage and power angle.

Assuming a constant source voltage and constant supply frequency, Eqn. may be expressed as :

Effect of changes in load on armature current, power angle, and power factor of synchronous motor

Phasor diagram showing effect of changes in shaft load on armature current,power angle and power factor of a synchronous motor

During all load variations, the rotor assumes a new position in relation to the rotating magnetic field, the average speed of the machine does not change.

As the load is being increased, a final point is reached at which a further increase in fails to cause a corresponding increase in motor torque, and the rotor pulls out of synchronism.

the rotor poles at this point, will fall behind the stator poles such that they now come under the influence of like poles and the force of attraction no longer exists.

The max . value of Torque which motor can develop at rated voltage and frequency without losing synchronism is called Pull out Torque.


If the load on a synchronous motor is increased the following points are considered which are given below.

The motor continues to run at synchronous speed.

The torque angle δ increases. The excitation voltage Ef remains constant. The armature current Ia drawn from the supply

increases. The phase angle ϕ increases in the lagging

direction.


Effect of changes in field excitation on synchronous motor performance

Phasor diagram showing effect of changes in field excitation on armature current,power angle and power factor of a synchronous motor

When the shaft load is assumed to be constant, the steady-state value of Ef sinδ must also be constant.

An increase in Ef will cause a transient increase in Ef sin δ , and the rotor will accelerate.

As the rotor changes its angular position, decreases until Ef sin δ has the same steady-state value as before, at which t ime the rotor is again operating at synchronous speed, as it should run only at the synchronous speed.


Note that increasing the excitation from Ef1 to Ef3 caused the phase angle of the current phasor with respect to the terminal voltage VT (and hence the power factor) to go from lagging to leading.

The value of field excitation that results in unity power factor is called normal excitation.

Excitation greater than normal is called over excitation, and excitation less than normal is called under excitation.

when operating in the overexcited mode, |Ef | > |VT |.

In fact a synchronous motor operating under over excitation condition is sometimes called a synchronous condenser.


What is Hunting Unloaded synchronous machine has zero degree load

angle. On increasing the shaft load gradually load angle will increase.

Let us consider that load P1 is applied suddenly to unloaded machine shaft so machine will slow down momentarily.

Also load angle (δ) increases from zero degree and becomes δ1.

During the first swing electrical power developed is equal to mechanical load P1.

Equilibrium is not established so rotor swings further. Load angle exceeds δ1 and becomes δ2.

Hunting Now electrical power generated is greater

than the previous one. Rotor attains synchronous speed. But it does not stay in synchronous speed

and it will continue to increase beyond synchronous speed.

As a result of rotor acceleration above synchronous speed the load angle decreases.

So once again no equilibrium is attained. Thus rotor swings or oscillates about new equilibrium position. This phenomenon is known as hunting or phase swinging.

Causes of Hunting in Synchronous Motor Sudden change in load. Sudden change in field current. A load containing harmonic torque. Fault in supply system.

Effects of Hunting in Synchronous Motor It may lead to loss of synchronism. Produces mechanical stresses. Increases machine losses and cause

temperature rise. Cause greater surges in current and

power flow.

Reduction of Hunting in Synchronous MotorTwo techniques should be used to reduce hunting.

These are – Use of Damper Winding : It consists of low

electrical resistance copper / aluminium brush embedded in slots of pole faces in salient pole mach ine . Damper winding damps out hunting by producing torque opposite to slip of rotor. The magnitude of damping torque is proportional to the slip speed.

Use of Flywheels : The prime mover is provided with a large and heavy flywheel. This increases the i ne r t ia o f pr ime mover and he lps i n maintaining the rotor speed constant.

Different Torques in Synchronous Motors STARTING TORQUE

It is the torque developed by the synchronous motor when full voltage at rated frequency is applied.

This torque is provided by the stator windings. This torque is also known as Breakaway

Torque or Locked rotor Torque.

RUNNING TORQUE It is the torque developed by the synchronous

motor under running conditions. It is determined by the output power and speed

of the driven machine. PULL-IN TORQUE

A synchronous motor started as induction motor till if runs near the synchronous speed.

Afterward, synchronous is excited with DC source and the rotor pull into step with the synchronously rotating stator field.

S o i t i s a b i l i t y o f m o t o r t o p u l l - i n t o synchronism when changing from induction to synchronous motor operation.

Different Torques in Synchronous Motors

PULL-OUT TORQUE It is the maximum value of torque which a

synchronous motor can develop at rated voltage to remain in synchronous under rated load condition.

It value varies from 1.25 to 3.5 times the full load torque.

Different Torques in Synchronous Motors

Synchronous Condenser Synchronous Condenser is also known as Synchronous Compensator or Synchronous Phase Modifier.

A synchronous condenser or a synchronous compensator is a synchronous motor running without a mechanical load.

It can generate or absorb reactive volt-ampere (VAr) by varying the excitation of its field winding.

It can be made to take a leading current with over-excitation of its field winding.

When the motor is operated at no load with over-excitation, it takes a current that leads the voltage by nearly 90 degrees.

Thus, it behaves like a capacitor and under such operating conditions, the synchronous motor is called a synchronous capacitor.

Since a synchronous condenser behaves like a variable inductor or a variable capacitor, it is used in power transmission systems to regulate line voltage.

Synchronous Condenser

Difference between Induction Motor and Synchronous Motor

DIFFERENCE SYNCHRONOUS MOTOR INDUCTION MOTOR

Type of ExcitationA synchronous motor is a doubly excited machine.

An induction motor is a single excited machine.

Supply System

Its armature winding is energized from an AC source and its field winding from a DC source.

Its stator winding is energized from an AC source.

SpeedIt always runs at synchronous speed. The speed is independent of load.

If the load increased the speed of the induction motor decreases. It is always less than the synchronous speed.

It is not self starting. It has to be run up to synchronous speed by any means before it can be synchronized to AC supply.

Induction motor has self starting torque

Operation

A synchronous motor can be operated with lagging and leading power by changing its excitation.

An induction motor operates only at a lagging power factor. At high loads the power factor becomes very poor.

Usage

It can be used for power factor correction in addition to supplying torque to drive mechanical loads.

An induction motor is used for driving mechanical loads only.

EfficiencyIt is more efficient than an induction motor of the same output and voltage rating.

Its efficiency is lesser than that of the synchronous motor of the same output and the voltage rating.

Cost

A synchronous motor is costlier than an induction motor of the same output and voltage rating

An induction motor is cheaper than the synchronous motor of the same output and voltage rating.

Difference between Induction Motor and Synchronous Motor

They are generally used in power stations to attain appropriate power factor. They operate in parallel to the bus bars and are often over-excited, externally, to reach the desired power factor.

They are also used in manufacturing industries where a large number of asynchronous motors and transformers are used to overcome the lagging p.f.

Used in power stations to generate electricity at a desired frequency.

Used to control voltage by changing its excitation in the transmission lines.

Synchronous Machine Applications

Synchronous-motor losses and efficiency The flow of power through a synchronous motor,

from stator to rotor and then to shaft output, is shown in Fig.

As indicated in the power-flow diagram, the total power loss for the motor is given by

where:Pscl= stator-copper lossPfcl = fie1d-copper.lossPcore = core lossPf,w = friction and windage lossPstray = stray load loss

Synchronous-motor losses and efficiency

Except for the transient conditions that occur when the field current is increased or decreased (magnetic energy stored or released), the total energy supplied to the field coils is constant and all of it is consumed as I2R losses in the field winding.

the overall efficiency of a synchronous motor is given by

Synchronous-motor losses and efficiency

synchronous motor - coursecontent

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