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SYNCHRONOUS MOTOR DRIVES

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In a synchronous motor, a 3-phase set of statorcurrents produces a rotating magnetic field causingthe rotor magnetic field to align with it. The rotormagnetic field is produced by a DC current applied tothe rotor winding.

Field windings are the windings producing the main

magnetic field (rotor windings for synchronousmachines); armature windings are the windingswhere the main voltage is induced (stator windingsfor synchronous machines).

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Characteristics

High operating efficiency

Smooth constant starting & accelerating torque

Versatile power factor control

Constant speed

Considerably more expensive than induction motors

Zero starting torque

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Types

Wound field

Permanent magnet

Synchronous Reluctance Hysteresis Motors

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Wound Field Syn. Motor

Cylindrical and salient pole rotor

Cylindrical Rotor

1.Higher Mechanical strength high power and high

speed applns.2.In syn.machine, there is no stator induced

induction in the rotor and therefore, rotor mmf issupplied exclusively by the field winding.Whereas in

I.M., the stator supplies the rotor excitation thatmakes the machine pf always lagging.

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Construction of synchronous

machines

The rotor of a synchronous machine is a large electromagnet. The magnetic poles can be

either salient (sticking out of rotor surface) or non-salient construction.

Non-salient-pole rotor: usually two- and four-pole rotors. Salient-pole rotor: four and

more poles.

Rotors are made laminated to reduce eddy current losses.

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machines

Salient pole with field

windings

Salient pole without

field windings

observe laminations

A synchronous rotor with 8 salient poles

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machines

Two common approaches are used to supply a DC current to the field circuits on the

rotating rotor:

1. Supply the DC power from an external DC

source to the rotor by means of slip rings

and brushes;

2. Supply the DC power from a special DC

power source mounted directly on the

shaft of the machine.

Slip rings are metal rings completely encircling the shaft of a machine but insulated from it.

One end of a DC rotor winding is connected to each of the two slip rings on the machines

shaft. Graphite-like carbon brushes connected to DC terminals ride on each slip ring

supplying DC voltage to field windings regardless the position or speed of the rotor.

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machines

Slip rings

Brush

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machines

Slip rings and brushes have certain disadvantages: increased friction and wear (therefore,

needed maintenance), brush voltage drop can introduce significant power losses. Still this

approach is used in most smallsynchronous machines.

On large generators and motors, brushless exciters are used.

A brushless exciter is a small AC generator whose field circuits are mounted on

the stator and armature circuits are mounted on the rotor shaft. The exciter

generators 3-phase output is rectified to DC by a 3-phase rectifier (mounted on

the shaft) and fed into the main DC field circuit. It is possible to adjust the field

current on the main machine by controlling the small DC field current of the

exciter generator (located on the stator).

Since no mechanical contact occurs between the rotor and the stator, exciters of this type

require much less maintenance.

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machines

A brushless exciter: a low

3-phase current is

rectified and used to

supply the field circuit of

the exciter (located on

the stator). The output of

the exciters armature

circuit (on the rotor) is

rectified and used as the

field current of the main

machine.

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Starting synchronous motors

Three basic approaches can be used to safely start a synchronous motor:

1. Reduce the speed of the stator magnetic field to a low enough value that the

rotor can accelerate and two magnetic fields lock in during one half-cycle of

field rotation. This can be achieved by reducing the frequency of the applied

electric power (which used to be difficult but can be done now).

2. Use an external prime mover to accelerate the synchronous motor up to

synchronous speed, go through the paralleling procedure, and bring the

machine on the line as a generator. Next, turning off the prime mover will

make the synchronous machine a motor.

3. Use damper windings or amortisseur windings the most popular.

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Motor starting by amortisseur or

damper windings

Amortisseur (damper) windings are special bars laid

into notches carved in the rotor face and then shorted

out on each end by a large shorting ring.

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damper windings

A diagram of a salient 2-pole rotor with an amortisseurwinding, with the shorting bars on the ends of the two

rotor pole faces connected by wires (not quite the

design of actual machines).

We assume initially that the

rotor windings aredisconnected and only a 3-

phase set of voltages are

applied to the stator.

As BS

sweeps along in s counter-clockwise direction, it induces a

voltage in bars of the amortisseur winding:

At t= 0, assume that BS

(stator

field) is vertical.

inde v B l (7.88.1)

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damper windings

Here v the velocity of the bar relative to the magneticfield;

B magnetic flux density vector;

l length of conductor in the magnetic field.

The bars at the top of the rotor are moving to the right

relative to the magnetic field: a voltage, with direction out

of page, will be induced. Similarly, the induced voltage is

into the page in the bottom bars. These voltages produce a

current flow out of the top bars and into the bottom bars

generating a winding magnetic field Bw

to the right. Two

magnetic fields will create a torque

ind W SkB B (7.89.1)

The resulting induced torque will be counter-clockwise.

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damper windings

At t= 1/240 s, BS has rotated 900 while the rotor has barely moved.Since v is parallel to B

S, the voltage induced in the amortisseur

windings is zero, therefore, no current in wires create a zero-torque.

At t= 1/120 s, BS

has rotated

another 900 and the rotor is still.The voltages induced in the bars

create a current inducing a

magnetic field pointing to the left.

The torque is counter-clockwise.

Finally, at t= 3/240 s, no voltage is induced

in the amortisseur windings and, therefore,

the torque will be zero.

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damper windings

We observe that the torque is either counter-clockwise or zero, but it is alwaysunidirectional. Since the net torque is nonzero, the motor will speed up.

However, the rotor will never reach the synchronous speed! If a rotor was running at the

synchronous speed, the speed of stator magnetic field BS

would be the same as the speed

of the rotor and, therefore, no relative motion between the rotor and the stator magnetic

field. If there is no relative motion, no voltage is induced and, therefore, the torque will bezero.

Instead, when the rotors speed is close to synchronous, the regular field current can be

turned on and the motor will operate normally. In real machines, field circuit are shorted

during starting. Therefore, if a machine has damper winding:

1. Disconnect the field windings from their DC power source and short them out;2. Apply a 3-phase voltage to the stator and let the rotor to accelerate up to near-

synchronous speed. The motor should have no load on its shaft to enable motor speed

to approach the synchronous speed as closely as possible;

3. Connect the DC field circuit to its power source: the motor will lock at synchronous

speed and loads may be added to the shaft.

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Synchronous machine ratings

The speed and power that can be obtained from a synchronous motor or generator arelimited. These limited values are called ratings of the machine. The purpose of ratings is to

protect the machine from damage. Typical ratings of synchronous machines are voltage,

speed, apparent power (kVA), power factor, field current and service factor.

1. Voltage, Speed, and Frequency

The rated frequency of a synchronous machine depends on the power system to which it

is connected. The commonly used frequencies are 50 Hz (Europe, Asia), 60 Hz (Americas),

and 400 Hz (special applications: aircraft, spacecraft, etc.). Once the operation frequency is

determined, only one rotational speed in possible for the given number of poles:

120e

m

fn

P (7.93.1)

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Load angle

The rotor poles are engaged with stator polesand both run synchronously in the samedirection. As the load on the motor increases,

the rotor tends to fall back in phase by someangle load angle. This value depends on theload.

For load angle 0 to 180 degrees lagging ,torque is +ve and load angle 0 to 180degrees leading Torque isve.

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Synchronous machine with a cylindrical

round-rotor

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Introduction

Permanent magnet synchronous motors have

the rotor winding replaced by permanent

magnets. These motors have several advantages

over synchronous motors with rotor fieldwindings, including:

Elimination of copper loss

Higher power density and efficiency

Lower rotor inertia

Larger airgaps possible because of larger

coercive force densities.

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Introduction (contd)

Some disadvantages of the permanent magnet

synchronous motor are:

Loss of flexibility of field flux control Cost of high flux density permanent magnets is

high

Magnetic characteristics change with time

Loss of magnetization above Curie temperature

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Permanent Magnets

Advances in permanent magnetic materials over

the last several years have had a dramatic impact

on electric machines. Permanent magnet

materials have special characteristics which mustbe taken into account in machine design. For

example, the highest performance permanent

magnets are brittle ceramics, some have chemical

sensitivities, all have temperature sensitivity, and

most have sensitivity to demagnetizing fields.

Proper machine design requires understanding

the materials well.

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B-H Loop

A typical B-H loop for a permanent magnet is

shown below. The portion of the curve in which

permanent magnets are designed to operate in

motors is the top left quadrant. This segment isreferred to as the demagnetizing curve and is

shown on the next slide.

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Demagnetizing Curve

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Demagnetizing Curve (contd)

The remnant flux density Br will be available if

the magnet is short-circuited. However, with an

air gap there will be some demagnetization

resulting in the no-load operating point, B.Slope of no-load line is smaller with a larger air

gap. With current flowing in the stator, there is

further demagnetization of the permanentmagnet causing the operating point to shift to

C at full load.

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Demagnetizing Curve (contd)

Transients or machine faults can lead to a worst-

case demagnetization as shown which results in

permanent demagnetization of the permanent

magnet. The recoil line following the transient isshown and shows a reduced flux density

compared to the original line. It is clearly

important to control the operation of themagnets to keep the operating point away from

this worst-case demagnetization condition.

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Permanent Magnetic Materials

Alnico - good properties but too low a coercive

force and too square a B-H loop => permanent

demagnetization occurs easily

Ferrites (Barium and Strontium) - low cost,

moderately high service temperature (400C),

and straight line demagnetization curve.

However, Br is low => machine volume and sizeneeds to be large.

P M M i l

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Permanent Magnet Materials

(contd)

Samarium-Cobalt (Sm-Co) - very good

properties but very expensive (because

Samarium is rare)

Neodymium-Iron-Boron (Nd-Fe-B) - very

good properties except the Curie

temperature is only 150C

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Permanent Magnet Materials

(contd)

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PM Motor Construction

There are two types of permanent magnet motor

structures:

1) Surface PM machines

- sinusoidal and trapezoidal

2) Interior PM machines

- regular and transverse

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Circuit Model of PM Motor (contd)

Based on the recoil line, we can write:

where Prc, the permeance, is the slope of

the line. From this equation we can write:

0

0

( ) rcPF F

0r rcP F

E i l t Ci it M d l f PM

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Equivalent Circuit Model of PM

Motor

Rearranging the slope equation, we get:

This equation suggests the following equivalent

circuit for a permanent magnet:

0

rc

F FP

i l i i d l f

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Motor (contd)

It can be shown that the mmf, flux and

permeance are the mathematical duals of

current, voltage, and inductance,

respectively. Therefore, the followingelectrical equivalent circuits can be used to

represent the magnetic circuit:


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Motor (contd)

We can now use this equivalent circuit of the

permanent magnets on the rotor and the

previous equivalent equivalent circuits of the

synchronous motor to develop a set of qd0equivalent circuits for the permanent

magnet synchronous motor. Assuming the

PM synchronous motor has damper cagewindings but no g winding, the qd0

equivalent circuits are as shown on the next

slide.


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Motor (contd)


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Motor (contd)

Here the PM magnet inductance Lrc can be

lumped with the common d-axis mutual

inductance of the stator and damper windings,

and the combined d-axis mutual inductanceindicated by Lmd. Also, the current im is the

equivalent magnetizing current for the

permanent magnet referred to the stator side.

d0 E ti f P t

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qd0 Equations for Permanent

Magnet Synchronous Motor

The qd0 equations for a permanent magnet

motor are given in the table below:


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qd0 Equations for PermanentMagnet Synchronous Motor

(contd)


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(contd)The developed electromagnetic torque expressionhas three components:

1) A reluctance component (which is negative for

Ld

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(contd)The mutual flux linkages in the q- and d-axesmay be expressed by:

The winding currents can be expressed (as before)

as:

'( )mq mq q kqL i i

' '( )md md d m kd

L i i i

q mq

q

ls

iL

d md

d

ls

i

L

'

'

'

kq mq

kq

lkq

iL

'

'

'

kd md

kd

lkd

i

L


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(contd)Combining these equations gives:

where .

Similar expressions for mq and LMQcan be

written for the q-axis.

'

'

'

d kdmd MD m

ls lkd

L iL L

'

1 1 1 1

MD ls lkd mdL L L L


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(contd)Under steady state conditions where =e as in thecase of Efin the wound field synchronous motor,

we can express em or xmdim by Em, the

permanent magnets excitation voltage on thestator side. If the stator resistance is neglected and

the Efterm in the earlier torque expression

replaced by Em, the torque of a permanent magnet

synchronous motor in terms of the rms phasevoltage Va at its terminal can be written as:

2 1 13 sin sin 2

2

a m

e a

e d q d

V EPT V

X X X

Trapezoidal Surface Magnet

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Trapezoidal Surface Magnet

Motor

A trapezoidal surface permanent magnet motor is

the same as a sinusoidal PM motor except the

3 winding has a concentrated full-pitch

distribution instead of a sinusoidal distribution.

Trapezoidal Surface Magnet Motor

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(contd)

This 2-pole motor has a gap in the rotor

magnets to reduce flux fringing effects and the

stator has 4 slots per phase winding per pole.

As the machine rotates the flux linkage willvary linearly except when the magnet gap

passes through the phase axis. If the machine

is driven by a prime mover, the stator phasevoltages will have a trapezoidal wave shape as

shown on the next slide.


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(contd)


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(contd)

An electronic inverter is required to establish a

six-step current wave to generate torque. With

the help of an inverter and an absolute-

position sensor mounted on the shaft, bothsinusoidal and trapezoidal SPM motors can

serve as brushless dc motors (although the

trapezoidal SPM motor gives closer dcmachine-like performance).

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Synchronous Reluctance Motor

A synchronous reluctance motor has the same

structure as that of a salient pole synchronous

motor except that it does not have a field

winding on the rotor.


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(contd)

The stator has a 3, symmetrical winding which

creates a sinusoidal rotating field in the air gap.

This causes a reluctance torque to be created on

the rotor because the magnetic field induced inthe rotor causes it to align with the stator field in

a minimum reluctance position. The torque

developed in this type of motor can be expressed

as:

2( )

3 sin 22 2

ds qs

e s

ds qs

L LPT

L L


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(contd)

The reluctance torque stability limit can be seen to

occur at (see figure below)./ 4


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(contd)

Iron laminations separated by non-magnetic

materials increases reluctance flux in the qe-axis.

With proper design, the reluctance motor

performance can approach that of an induction

motor, although it is slightly heavier and has a

lower power factor. Their low cost and robustness

has seen them increasingly used for low powerapplications, such as in fiber-spinning mills.

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Variable Reluctance Motors

A variable reluctance motor has double saliency,i.e. both the rotor and stator have saliency. There

are two groups of variable reluctance motors:

stepper motors and switched reluctance motors.

Stepper motors are not suitable for variable speed

drives.

Ref: A. Hughes, Electric Motors and Drives, 2nd. Edn. Newnes

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Switched Reluctance Motors

The structure of a switched reluctance motor isshown below. This is a 4-phase machine with 4

stator-pole pairs and 3 rotor-pole pairs (8/6

motor). The rotor has neither windings norpermanent magnets.


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(contd)

The stator poles have concentrated winding

rather than sinusoidal winding. Each stator-

pole pair winding is excited by a converter

phase, until the corresponding rotor pole-pair

is aligned and is then de-energized. The stator-

pole pairs are sequentially excited using a rotor

position encoder for timing.


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(contd)The inductance of a stator-pole pair andcorresponding phase currents as a function of

angular position is shown below.


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(contd)

Applying the stator pulse when the inductance

profile has positive slope induces forward

motoring torque.

Applying the stator pulse during the time that

the inductance profile has negative slope

induces regenerative braking torque.

A single phase is excited every 60 with four

consecutive phases excited at 15 intervals.


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(contd)

The torque is given by:

where m=inductance slope and

i=instantaneous current.

21

2eT mi


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(contd)

Switched reluctance motors are growing in

popularity because of their simple design and

robustness of construction. They also offer the

advantages of only having to provide positivecurrents, simplifying the inverter design. Also,

shoot-through faults are not an issue because each

of the main switching devices is connected in series

with a motor winding. However, the drawbacks ofthis type of motor are the pulsating nature of their

torque and they can be acoustically noisy (although

improved mechanical design has mitigated this

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ADVANTAGES AND DISADVANTAGES OF

BLDC MOTOR

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