Synchronous and Special Machines

UNIT – III

Synchronous and Special Machines

Compiled By

M Saad Bin Arif Course Incharge Mohd Anas Anees

Department of Electrical Engineering Aligarh Muslim University, Aligarh

Synchronous and Special Machines

Content

1. Synchronous Machine

▪ Introduction : Construction and Types

▪ EMF equation

▪ Circuit Model

▪ Power developed in Cylindrical rotor Synchronous Machine.

▪ Introduction to Synchronous Motor

2. Stepper Motor

▪ Construction and Working

3. Servo Motor and Permanent Magnet Motors

Introduction

▪ Synchronous machines are principally used as alternating current generators. They supply the electric power used by all sectors of modern society.

▪ Synchronous machine is an important electromechanical energy converter.

▪ Synchronous generators usually operate in parallel forming a large power system supplying electrical power to consumers or loads. For these applications the synchronous generators are built in large units, their rating ranging form tens to hundreds of Megawatts.

▪ These synchronous machines can also be run as synchronous motors.

▪ Synchronous machines are AC machines that have a field circuit supplied by an

external DC source.

▪ Synchronous machines are having two major parts namely stationary part stator and a rotating field system called rotor.

▪ In a synchronous generator , a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then driven by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding.

▪ Field windings are the windings producing the main magnetic field (rotor windings

for synchronous machines); armature windings are the windings where the main

voltage is induced (stator windings for synchronous machines).

Types

of synchronous machines

According to the arrangement of armature and field winding, the synchronous machines are classified as

▪ Rotating armature type.

▪ Rotating field type.

Rotating armature type

▪ In rotating armature type the armature winding is on the rotor and the field winding is on the stator. The generated emf or current is brought to the load via the slip rings. These type of generators are built only in small units.

Rotating field type

▪ In case of rotating field type generators field windings are on the rotor and the armature windings are on the stator. Here the field current is supplied through a pair of slip rings and the induced emf or current is supplied to the load via the stationary terminals.

According to the type of rotor used synchronous machines are classified as

▪ Salient pole Machines

▪ Non-salient pole or Cylindrical rotor or Round rotor Machines

Salient pole Machines

▪ These type of machines have salient pole or projecting poles with concentrated field windings. This type of construction is for the machines which are driven by hydraulic turbines or Diesel engines.

Non-salient pole or Cylindrical rotor or Round rotor Machines:

▪ These machines are having cylindrical smooth rotor construction with distributed field winding in slots. This type of rotor construction is employed for the machine driven by steam turbines.

Types

of synchronous machines

Construction

of synchronous machines

The armature winding of a conventional synchronous machine is almost invariably on the stator and is usually a three phase winding. The field winding is usually on the rotor and excited by dc current, or permanent magnets. The dc power supply required for excitation usually is supplied through a dc generator known as exciter, which is often mounted on the same shaft as the synchronous machine. Various excitation systems using ac exciter and solid state rectifiers are used with large turbine generators.

Stator

▪ The stator is the outer stationary part of the machine, which consists of

✓ The outer cylindrical frame called yoke, which is made either of welded sheet steel, cast iron.

✓ The magnetic path, which comprises a set of slotted steel laminations called stator core pressed into the cylindrical space inside the outer frame.

✓ The magnetic path is laminated to reduce eddy currents, reducing losses and heating.

CRGO laminations of 0.5 mm thickness are used to reduce the iron losses.

▪ A set of insulated electrical windings are placed inside the slots of the laminated stator. The cross sectional area of these windings must be large enough for the power rating of the machine.

▪ For a 3- phase generator, 3 sets of windings are required, one for each phase connected in star. Fig. 1 shows one stator lamination of a synchronous generator.

▪ In case of generators where the diameter is too large stator lamination can not be punched in on circular piece. In such cases the laminations are punched in segments.

▪ A number of segments are assembled together to form one circular laminations.

All the laminations are insulated from each other by a thin layer of varnish.

▪ Details of construction of stator are shown in Figs 1 – 5.

Figure. 5: Stator lamination and Stator of a salient pole alternator

Rotor

▪ There are two types of rotor structures:

▪ Round or cylindrical rotor or Non- Salient pole rotor

▪ Salient pole rotor

Round or Cylindrical Rotor Type

▪ Generally, round rotor structure is used for high speed synchronous machines, (1000 to 3000 RPM), such as steam turbine generators.

▪ This type machine has Small diameter and has large axial length.

▪ This type of machines are used in Thermal power plant and Gas turbine

power plant Where speed required is high.

▪ It is constructed from a solid steel forging so as to withstand the large centrifugal stresses inherent in high speed operation.

▪ Cylindrical rotors cannot accelerate high inertia loads. They are limited in

application to pumps, fans, blowers, and other loads with similar low starting

torque requirements.

Salient pole rotor

▪ This type of rotor is built for use with high-inertia, low-speed loads.

▪ It has many poles projecting radially outward from a steel spider. These salient poles are bolted or keyed to the spider, and the spider is keyed to the shaft.

▪ Salient pole rotor does have poles that are projecting out from surface.

▪ The air gap between the stator and rotor is non-uniform.

▪ The salient pole rotor has large no of poles. Salient Pole rotors are mechanically weak. They have large diameters and small axial length.

▪ They are preferred for low speed alternators which range from 100 RPM to

500 RPM.

Rotating magnetic field

In the case of a synchronous generator , three balanced emf's of frequency f=Pn/120 Hz are induced in the three phase windings when the rotor is driven by a prime mover rotating at a speed n. If the three phase stator circuit is closed by a balanced three phase electrical load, balanced three phase currents of frequency f will flow in the stator circuit, and these currents will generate a rotating magnetic field of a speed nf = 120f/P = n.

When the stator winding of a three phase synchronous motor is supplied by a balanced three phase power supply of frequency f, the balanced three phase currents in the winding will generate a rotating magnetic field of speed n

= 120f/P.

This rotating magnetic field will drag the magnetized rotor, which is essential a

magnet, to rotate at the same speed n= n

. On the other hand, this rotating rotor

will also generate balanced three phase emf's of frequency f in the stator winding,

which would balance with the applied terminal voltage.

EMF Equation of an alternator

Consider the following

Φ = flux per pole in wb, P = Number of poles, Ns = Synchronous speed in rpm f = frequency of induced emf in Hz, Z = total number of stator conductors

Zph = conductors per phase connected in series, Tph = Number of turns per phase Assuming concentrated winding, considering one conductor placed in a slot.

According to Faradays Law electromagnetic induction, The average value of emf induced per conductor in one revolution

e

= dФ/dt

e

=

Time taken for one revolution = 60/Ns seconds

Hence

e

We know f = PNs /120 hence PNs /60 = 2f

If there are T^ph, number of turns per phase connected in series, then average emf induced in T^phturns is

E

= T

x e

= 4 f Ф T

volts

Hence RMS value of emf induced

E = 1.11 x E

E = 1.11 x 4 f Ф T

volts E = 4.44 f Ф T

volts

▪ This is the general emf equation for the machine having concentrated and full pitched winding.

Note

▪ In practical machines the windings will be generally short pitched and distributed over the periphery of the machine. Hence in deducing the emf equation both pitch factorand distribution factor has to be considered.

▪ Hence the general emf equation including pitch factor and distribution factor can be given as EMF induced per phase , E^ph = 4.44 f Ф T^phx K^pK^dvolts

E^ph = 4.44 K^pK^df Ф T^phvolts

▪ Hence the line Voltage E^L = √3 x phase voltage = √3 E^ph

Pitch Factor

The factor by which the emf induced in a short pitched coil gets reduced is called pitch factor and defined as the ratio of emf induced in a short pitched coil to emf induced in a full pitched coil.

Pitch factor K^p= emf induced in a short pitched coil/ emf induced in a full pitched coil

Distribution Factor

▪ The factor by which the emf induced in a distributed winding gets reduced is called distribution factor and defined as the ratio of emf induced in a distributed winding to emf induced in a concentrated winding.

Distribution factor K^d= emf induced in a distributed winding/ emf induced in a concentrated winding

Distribution Factor Deriviation

Let

E = emf induced per coil side

m = number of slots per pole per phase, n = number of slots per pole

β = slot angle = 180/n

The emf induced in concentrated winding with m slots per pole per phase = mE volts

.

Fig on next slide shows the method of calculating the vector sum of the voltages in a distributed winding having a mutual phase difference of β.

When m is large curve ACEN will form the arc of a circle of radius r. From the figure AC = 2 x r x sin β/2

Hence arithmetic sum = m x 2r sin β/2

Now the vector sum of the emfs is AN as shown in figure = 2 x r x sin mβ/2

Hence

The distribution factor

K^d= vector sum of the emf / arithmetic sum of the emf K^d= (2r sin mβ/2) / (m x 2r sin β/2)

K^d= ( sin mβ/2) / (m sin β/2)

Pitch Factor:

Pitch factor Kp= emf induced in a short pitched coil/ emf induced in a full pitched coil Kp = (2E cos α/2 )/ 2E

Kp = cosα/2

where α is called chording angle.

Example Numerical

1. A 3Φ, 50 Hz, star connected salient pole alternator has 216 slots with 5 conductors per slot. All the conductors of each phase are connected in series;

the winding is distributed and full pitched. The flux per pole is 30 mwb and the

alternator runs at 250 rpm. Determine the phase and line voltages of emf

induced. (Assuming Kp =1 and Kd = 0.9597).

Equivalent Circuit Model

▪ The voltage E

is the internal generated voltage produced in one phase of a synchronous generator. However, this voltage is not usually the voltage that appears at the terminals of tire generator.

➢ Why is the output voltage from a phase not equal to E

?

➢ and what is the relationship between the two voltages?

✓ The answer yields the equivalentcircuit model of a synchronous machine.

▪ There are a number of factors that cause the difference between E

and V

:

1. The distortion of the air-gap magnetic field by the current flowing in the stator, called armature reaction.

2. The self-inductance of the armature coils.

3. The resistance of the armature coils.

4. The effect of salient-pole rotorshapes.

▪ We will explore the effects of the first three factors and derive a machine model from them.

1. Effect ofArmature reaction

▪ The first effect mentioned, and normally the largest one, is armature reaction.

▪ When a synchronous generator’s rotor is spun, a voltage EA is induced in the generator’s stator windings. If a load is attached to the terminals of the generator, a current flows.

▪ But a three-phase stator current flow will produce a magnetic field of its own in the machine. This stator magnetic field distorts the original rotor magnetic field, changing the resulting phase voltage.

▪ This effect is called armature reaction because the armature (stator) current affects the magnetic field which produced it in the first place.

▪ To understand armature reaction, refer to Figure below, shows a two-pole rotor spinning inside a three-phase stator.

▪ There is no load connected to the stator. The rotor magnetic field produces an internal generated voltage EA whose peak value coincides with the direction of Bs.

▪ With no load on the generator, there is no armature current flow, and E^A will be equal to the phase voltageV^Φ.

▪ Now suppose that the generator is connected to a lagging load - The stator magnetic field Bs produces a voltage of its own in the stator -calledE^stat.

▪ With two voltages present (E^A and E^stat) in the stator windings, the total voltage in a phase is

V

= E

+ E

▪ How can the effects of armature reaction on the phase voltage be modeled?

▪ First, note that the voltage Estatlies at an angle of 90° behind the plane of maximum current I^A.

▪ Second, the voltage Estat is directly proportional to the current IA. If X is a constant of proportionality, then the armature reaction voltage can be expressed as

E

= - jX I

The voltage on a phase is thus

V

= E

- jX I

▪ Look at the circuit shown below. The Kirchhoffs voltage law equation for this circuit is

V

= E

- jX I

▪ This is exactly the same equation as the one describing the armature reaction voltage.

▪ Therefore, the armature reaction voltage can be modeled as an inductor in series with the internal generated voltage.

2. Effect of self-inductance and resistance of the armature coils

▪ If the stator self-inductance is called L^A (and its corresponding reactance is called X^A) while the stator resistance is called R^A, then the total difference between E^A and V^Φ is given by

V = E - jX I - jX I - R I

▪ The armature reaction effects and the self-inductance in the machine are both represented by reactances, and it is customary to combine them into a single reactance, called thesynchronous reactanceof the machine.

Xs = X + X

Therefore, the final equation describingV^Φis

V

= E

- jX

I

- R

I

▪ From above equation, it is now possible to sketch the equivalent circuit of a three-phase synchronous generator. The full equivalent circuit of such a generator is shown in Figure.

▪ This figure shows a dc power source supplying the rotor field circuit, which is modeled by the coil’s inductance and resistance in series.

Full Equivalent Circuit

Figure : The full equivalent circuit of three phase synchronous machine (generator)

Per Phase Equivalent circuit

▪ The fact that the three phases of a synchronous generator are identical in all respects except for phase angle normally leads to the use of a per-phase equivalent circuit. The per-phase equivalent circuit of this machine is shown in Figure.

▪ One important fact must be kept in mind when the per-phase equivalent circuit is used: The three phases have the same voltages and currents onlywhen the loads attached to them are balanced.

Figure : Per Phase equivalent circuit of synchronous machine (generator)

Phasor Diagram of a Synchronous Machine (Generator)

At Unity Power Factor

At Lagging Power Factor

At Leading Power Factor

Power developed in Cylindrical rotor Synchronous Machine (Generator)

▪ Not all the mechanical power going into a synchronous generator becomes electrical power out of the machine.

▪ The difference between input power and output power represents the losses of the machine.

A power-flow diagram for a synchronous generator is shown on next slide.

▪ The input mechanical power is the shaft power in the generator Pⁱⁿ = ζ^appω^m, while the power converted from mechanical to electrical form internally is given by

Pconv = ζinωm

P^conv = 3 E^A I^Acosϒ where ϒis the angle between E^A and l^A.

▪ The difference between the input power to the generator and the power converted in the generator represents the mechanical, core, andstray losses of the machine.

▪ The real electrical output power of the synchronous generator can be expressed in line quantities as

Pout = √3 V^T I^L cosθ and in phase quantities as

Pout = 3 V^ΦI^Acosθ

▪ The reactive power output can be expressed in line quantities as

Qout = √3 V^T I^Lsinθ and in phase quantities as

Qout = 3 V^ΦI^A sinθ

Synchronous Motor

▪ Synchronous motors are synchronous machines used to convert electric power to mechanical power.

▪ 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.

▪ As the name implies, a synchronous motor runs at synchronous speed (Ns = 120f/P) i.e., in synchronism with the revolving field produced by the 3-phase supply.

▪ The speed of rotation is therefore tied to the frequency of the source. Because the frequency is fixed, the motor speed stays constant irrespective of the load or voltage of the three phase line.

▪ A synchronous motor runs either at synchronous speed or not at all. Its speed is constant (synchronous speed) at all loads.

▪ The only way to change its speed is to alter the supply frequency (Ns = 120 f/P).

▪ The outstanding characteristic of a synchronous motor is that it can be made to operate over a wide range of power factors (lagging, unity or leading) by adjustment of its field excitation.

▪ Therefore, a synchronous motor can be made to carry the mechanical load at constant speed and at the same time improve the power factor of the system.

▪ Synchronous motors are generally of the salient pole type.

▪ A synchronous motor is not self-starting and an auxiliary means has to be used for starting it - we use either induction motor principle or a separate starting motor for this purpose.

CONSTRUCTION

✓ It consists of a stator which houses 3-phase armature winding in the slots of the stator core and receives power from a 3-phase supply - Develop the rotating magneticfield.

✓ A rotor has a set of salient poles excited by direct current to form alternate N and S poles.

✓ The exciting coils are connected in series to two slip rings and direct current is fed into the winding from an external exciter mounted on the rotor shaft

PRINCIPLES OF MOTOR OPERATION

▪ A three-phase set of balance voltages is applied to the stator of the machine, which produces a three-phase current flow in the windings.

▪ The field current I^F of the motor produces a steady-state magnetic field B^R.

▪ The three-phase set of currents in the armature winding produces a uniform rotating magnetic field BS.

▪ Therefore, there are two magnetic fields present in the machine.

▪ The rotor field will tend to line up with the stator field, just as two bar magnets will tend to line up if placed near to each other.

▪

To understand the basic concept of synchronous motor, Consider the Figure below which shows a two-pole synchronous motor.

▪ The basic principle of synchronous motor operation is that the rotor “chases” the rotating stator magnetic field around in a circle never quite catching up with it.

▪ The disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase ac power to the stator.

▪ A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed.

✓ Since a synchronous motor is the same physical machine as a synchronous generator, all of the basic speed, power, and torque equations .

Starting of synchronous motor – Basic Principle

As we already know that synchronous Motor is not self-starting and we already discussed why the synchronous motor is not self-starting. So here is a general procedure to start synchronous motor:

➢ Three phase winding is given a three phase a.c. supply. Now a rotating magnetic field is produced which is rotating at synchronous speed Ns r.p.m.

➢ Now make the rotor to rotate in the direction of the rotating magnetic field at a speed very near to that of synchronous speed using some external equipment like a diesel engine.

➢ Switch on the d.c. supply given to the rotor so that rotor poles are produced.Now there are two fields one is rotating magnetic field produced by stator while the other is produced by the rotor which is physically rotated almost at the same speed as that of rotating magnetic field.

➢ At a particular instant, both the fields are magnetically locked. The stator field pulls rotor field into synchronism.

➢ Now we can remove 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

➢ This magnetic interlocking between stator and rotor poles causes the synchronous motor to run at the speed of revolving flux i.e., synchronous speed.

➢ So the main point of this discussion is to start the synchronous motor, it needs some device to rotate the rotor at a speed very near or equal to the synchronous speed.

Methods of starting Synchronous Motor:

We have to think about an alternative to rotate the rotor at a speed almost equal that of synchronous speed. This can be possible by employing various methods to start the

synchronous motor.

The following are the different methods to start a synchronous motor.

▪ Using Pony Motors

▪ Using Damper Winding (squirrel cage induction motor)

▪ As a slip ring Induction motor

▪ Using Small d.c machine coupled to it.

Making Synchronous Motor Self-Starting

▪ In order to make the motor self-starting, a squirrel cage winding (also called damper winding) is provided on the rotor - damper winding serves to start the motor.

▪ To start with, 3-phase supply is given to the stator winding while the rotor field winding is left un- energized.

▪ The rotating stator field induces currents in the damper or squirrel cage winding and the motor starts as an induction motor.

▪ As the motor approaches the synchronous speed, the rotor is excited with direct current.

▪ Because the bars of squirrel cage portion of the rotor now rotate at the same speed as the rotating stator field, these bars do not cut any flux and, therefore, have no induced currents in them.

▪ Hence squirrel cage portion of the rotor is, in effect, removed from the operation of the motor.

➢ It is important to excite the rotor with direct current at the right moment.

Stepper Motor

Introduction

▪ A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements.

▪ The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.

Main Features

▪ The sequence of the applied pulses is directly related to the direction of motor shafts rotation.

▪ The speed of the motor shafts rotation is directly related to the frequency of the input pulses.

▪ The length of rotation is directly related to the number of input pulses applied.

Stepper Motor Characteristics

Open loop

▪ The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.

Brushless

▪ Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing.

Incremental steps/changes

▪ The rotation angle of the motor is proportional to the input pulse.

➢ As speed increases, torque decreases

Torque vs. Speed

▪ Torque varies inversely with speed.

▪ Current is proportional to torque.

▪ Torque → ∞ means Current →

∞, which leads to motor damage.

▪ Torque thus needs to be limited

to rated value of motor.

Working principle

▪ Stepper motors consist of a permanent magnet rotating shaft, called the rotor and electromagnets on the stationary portion that surrounds the rotor, called the stator.

▪ When a phase winding of a stepper motor is energized with current, a magnetic

flux is developed in the stator. The direction of this flux is determined by the “Right

Hand Rule” .

At position 1

▪ the upper electromagnet active, (voltage applied to it).

At position 2

▪ To move the rotor clockwise (CW), the upper electromagnet is deactivated and the right electromagnet is activated, causing the rotor to move 90 degrees CW, aligning itself with the active magnet.

At position 3-5

▪ This process is repeated in the same manner at the south and west electromagnets until we once again reach the starting position.

Working principle

Working principle

Resolution

Resolution is the number of degrees rotated per step.

Step angle = 360/(N

* Ph) = 360/N

Where, N

= Number of equivalent poles per phase = number of rotor poles.

Ph = Number of phases.

N = Total number of poles for all phases together.

Types of Stepper Motors There are three main types of stepper motors:

▪ Variable Reluctance Stepper Motor

▪ Permanent Magnet Stepper Motor

▪ Hybrid Synchronous Stepper Motor

▪ Servomotors are special electromechanical devices that produce precise degrees of rotation.

▪ A servo motor is a DC or AC or brushless DC motor combined with a position sensing device.

▪ Servomotors are also called control motors as they are involved in controlling a mechanical system.

▪ The servomotors are used in a closed-loop servo system as shown in Figure.

▪ A reference input is sent to the servo amplifier, which controls the speed of the servomotor.

▪ A feedback device is mounted on the machine, which is either an encoder or resolver. This device changes mechanical motion into electrical signals and is used as a feedback.

▪ This feedback is sent to the error detector , which compares the actual operation with that of the reference input.

▪ If there is an error, that error is fed directly to the amplifier, which will be used to make necessary corrections in control action.

▪ In many servo systems, both velocity and position are monitored. Servomotors

provide accurate speed, torque, and have ability of direction control.

DC servomotors

▪ DC operated servomotors are usually respond to error signal abruptly and accelerate the load quickly.

▪ A DC servo motor is actually an assembly of four separate components, namely:

▪ DC motor

▪ gear assembly

▪ position-sensing device

▪ control circuit AC servo motor

▪ In this type of motor, t he magnetic force is generated by a permanent magnet and current which further produce the torque.

▪ It has no brushes so there is little noise/vibration. This motor provides high precision control with the help of high resolution encoder.

▪ The stator is composed of a core and a winding. The rotor part comprises of shaft,

rotor core and a permanent magnet.

Advantages of servo motors

▪ Provides high intermittent torque, high torque to inertia ratio, and high speeds

▪ Work well for velocity control

▪ Available in all sizes

▪ Quiet in operation

▪ Smoother rotation at lower speeds

Disadvantages of servo motors

▪ More expensive than stepper motors

▪ Require tuning of control loop parameters

▪ Not suitable for hazardous environments or in vacuum

▪ Excessive current can result in partial demagnetization of DC type servo motor

Permanent Magnet motor

Assignment

1. Explain the working and construction of Permanent Magnet motor.

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# Please see material discussed on board in class.

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References

1. D. P. Kothari and I. Nagrath, Electric machines: Tata McGraw-Hill Education, 2004.

2. S. Chapman, Electric machinery fundamentals: Tata McGraw-Hill Education, 2005.