AC Machine

CHAPTER 4

EKT 103

By: Dr Rosemizi Abd Rahim

AC Machine Alternating current (ac) is the primary source of electrical

energy. It is less expensive to produce and transmit alternating

current (ac) than direct current (dc). For this reason, and because ac voltage is induced into the

armature of all generators, ac machines are generally more practical.

It is very easy to step up or step down the voltage for AC electricity through the use of transformers. It is not very straight forward to step up or step down the voltage for DC electricity- for power loads, it is typically done through inversion to AC- stepping up/down using a transformer- and rectification back to DC.

May function as a generator (mechanical to electrical) or a motor (electrical to mechanical)

DC Machine & AC machine

• DC motor - ends of the coil connect to a split ring to 'rectify' the emf produced

• AC motor - no need rectification, so don't need split rings.

AC Motor

As in the DC motor case,• a current is passed through the coil, generating a torque on the coil.• Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave.

AC Generator • This process can be described in terms of

Faraday's law when you see that the rotation of the coil continually changes the magnetic flux through the coil and therefore generates a voltage

Generator and Motor

How Does an Electric Generator Work?

i) Synchronous Machines: • Synchronous Generators: A primary source of

electrical energy.• Synchronous Motors: Used as motors as well as

power factor compensators (synchronous condensers).

ii) Asynchronous (Induction) Machines:• Induction Motors: Most widely used electrical motors in

both domestic and industrial applications.• Induction Generators: Due to lack of a separate field

excitation, these machines are rarely used as generators.

Classification of AC Machines

Two major classes of machines;

Synchronous Machine

Origin of name: syn = equal, chronos = time Synchronous machines are called ‘synchronous’ because

their mechanical shaft speed is directly related to the power system’s line frequency.

the rotating air gap field and the rotor rotate at the same speed, called the synchronous speed.

Synchronous machines are ac machine that have a field circuit supplied by an external dc source.– DC field winding on the rotor,– AC armature winding on the stator

Synchronous Machine

Synchronous machines are used primarily as generators of electrical power, called synchronous generators or alternators.

They are usually large machines generating electrical power at hydro, nuclear, or thermal power stations.

Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives

Application as a motor: pumps in generating stations, electric clocks, timers, and so forth where constant speed is desired.

Synchronous Machine

Synchronous Machine

where P is the number of magnetic poles

fe is the power line frequency. Typical machines have two-poles, four-poles, and six-poles

The frequency of the induced voltage is related to the rotor speed by:

Construction

• Energy is stored in the inductance• As the rotor moves, there is a change in

the energy stored• Either energy is extracted from the

magnetic field (and becomes mechanical energy – motor)

• Or energy is stored in the magnetic field and eventually flows into the electrical circuit that powers the stator – generator

Synchronous Machine

Construction

• DC field windings are mounted on the (rotating) rotor - which is thus a rotating electromagnet

• AC windings are mounted on the (stationary) stator resulting in three-phase AC stator voltages and currents

The main part in the synchronous machines arei) Rotorii) Stator

Synchronous Machine

Synchronous MachineRotor There are two types of rotors used in synchronous

machines:

i) cylindrical (or round) rotors

ii) salient pole rotors Machines with cylindrical rotors are typically found in

higher speed higher power applications such as turbogenerators. Using 2 or 4 poles, these machines rotate at 3600 or 1800 rpm (with 60hz systems). 

Salient pole machines are typically found in large (many MW), low mechanical speed applications, including hydrogenerators, or smaller higher speed machines (up to 1-2 MW).

Salient pole rotors are less expensive than round rotors.

L » 10 m

D » 1 mTurbine

Steam

Stator

Uniform air-gap

Stator winding

Rotor

Rotor winding

N

S

    High speed

3600 r/min Þ 2-pole

1800 r/min Þ 4-pole

î    Direct-conductor cooling (using hydrogen or water as coolant)

î   Rating up to 2000 MVA

Turbogenerator

d-axis

q-axis

Synchronous Machine – Cylindrical rotor

Stator

Cylindrical rotor

Synchronous Machine – Cylindrical rotor

1. Most hydraulic turbines have to turn at low speeds (between 50 and 300 r/min)

2. A large number of poles are required on the rotor

Hydrogenerator

Turbine

Hydro (water)

D » 10 m

Non-uniform air-gap

N

S S

N

d-axis

q-axis

Synchronous Machine – Salient Pole

Stator

Salient-pole rotor

Synchronous Machine – Salient Pole

Synchronous machine rotors are simply rotating electromagnets built to have as many poles as are produced by the stator windings.

Dc currents flowing in the field coils surrounding each pole magnetize the rotor poles.

The magnetic field produced by the rotor poles locks in with a rotating stator field, so that the shaft and the stator field rotate in synchronism.

Salient poles are too weak mechanically and develop too much wind resistance and noise to be used in large, high-speed generators driven by steam or gas turbines. For these big machines, the rotor must be a solid, cylindrical steel forging to provide the necessary strength.

Axial slots are cut in the surface of the cylinder to accommodate the field windings.

Since the rotor poles have constant polarity they must be supplied with direct current.

This current may be provided by an external dc generator or by a rectifier. In this case the leads from the field winding are connected to insulated

rings mounted concentrically on the shaft.

Stationary contacts called brushes ride on these slip rings to carry current to the rotating field windings from the dc supply.

The brushes are made of a carbon compound to provide a good contact with low mechanical friction.

An external dc generator used to provide current is called a “ brushless exciter “.

Synchronous Machine

Stator The stator of a synchronous machine carries the armature or load

winding which is a three-phase winding.

The armature winding is formed by interconnecting various conductors in slots spread over the periphery of the machine’s stator. Often, more than one independent three phase winding is on the stator. An arrangement of a three-phase stator winding is shown in Figure below. Notice that the windings of the three-phases are displaced from each other in space.

Construction Stator

Synchronous Machine

Magnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings

Synchronous Machine

Flux produced by a stator winding

Magnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings

Synchronous Machine

Magnetomotive Forces (MMF’s) and Fluxes Due to Armature and Field Windings

Synchronous Machine

Two Cycles of mmf around the Stator

Synchronous Generator

Equivalent circuit model – synchronous generator

If the generator operates at a terminal voltage VT while supplying a load corresponding to an armature current Ia, then;

In an actual synchronous machine, the reactance is much greater than the armature resistance, in which case;

Among the steady-state characteristics of a synchronous generator, its voltage regulation and power-angle characteristics are the most important ones. As for transformers, the voltage regulation of a synchronous generator is defined at a given load as;

Synchronous Generator

Phasor diagram of a synchronous generator

The phasor diagram is to shows the relationship among the voltages within a phase (Eφ,Vφ, jXSIA and RAIA) and the current IA in the phase.

Unity P.F (1.0)

Synchronous Generator

Leading P.F.

Lagging P.F

Synchronous Generator

Power and Torque

In generators, not all the mechanical power going into a synchronous generator becomes electric power out of the machine

The power losses in generator are represented by difference between output power and input power shown in power flow diagram below

Synchronous Generator

LossesRotor - resistance; iron parts moving in a magnetic field causing currents to be generated in the rotor body - resistance of connections to the rotor (slip rings)Stator - resistance; magnetic losses (e.g., hysteresis)Mechanical - friction at bearings, friction at slip ringsStray load losses - due to non-uniform current distribution

Synchronous Generator

The input mechanical power is the shaft power in the generator given by equation:

The power converted from mechanical to electrical form internally is given by

The real electric output power of the synchronous generator can be expressed in line and phase quantities as

and reactive output power

Synchronous Generator

In real synchronous machines of any size, the armature resistance RA is more than 10 times smaller than the synchronous reactance XS (Xs >> RA). Therefore, RA can be ignored

Synchronous Motor

Synchronous Motor

Power Flow

Example : Synchronous Generator.

A three-phase, wye-connected 2500 kVA and 6.6 kV generator operates at full-load. The per-phase armature resistance Ra and the synchronous reactance, Xd, are (0.07+j10.4).

Calculate the percent voltage regulation at

(a) 0.8 power-factor lagging, and

(b) 0.8 power-factor leading.

Solution.

The machines are called induction machines because of the rotor voltage which produces the rotor current and

the rotor magnetic field is induced in the rotor windings. Induction generator has many disadvantages and low

efficiency. Therefore induction machines are usually referred to as induction motors.

Induction Machine

• Three-phase induction motors are the most common and frequently encountered machines in industry– simple design, rugged, low-price, easy maintenance– wide range of power ratings: fractional horsepower to 10 MW – run essentially as constant speed from no-load to full load– Its speed depends on the frequency of the power source

• not easy to have variable speed control • requires a variable-frequency power-electronic drive for optimal

speed control

Induction Machine

Construction

Cutaway in a typical wound-rotor IM. Notice the brushes and the slip rings

Brushes

Slip rings

Construction• An induction motor has two main parts

i) a stationary stator • consisting of a steel frame that supports a hollow,

cylindrical core• core, constructed from stacked laminations (why?),

– having a number of evenly spaced slots, providing the space for the stator winding

Stator of IM

Constructionii) a revolving rotor

• composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor winding

• conventional 3-phase windings made of insulated wire (wound-rotor) » similar to the winding on the stator

• aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit (squirrel-cage)

Construction• Two basic design types depending on the rotor design

– squirrel-cage: conducting bars laid into slots and shorted at both ends by shorting rings.

– wound-rotor: complete set of three-phase windings exactly as the stator. Usually Y-connected, the ends of the three rotor wires are connected to 3 slip rings on the rotor shaft. In this way, the rotor circuit is accessible.

1. Squirrel cage – the conductors would look like one of the exercise wheels that squirrel or hamsters run on.

Construction

2. Wound rotor – have a brushes and slip ring at the end of rotor

Notice the slip rings

Construction

Rotating Magnetic Field• Balanced three phase windings, i.e.

mechanically displaced 120 degrees form each other, fed by balanced three phase source

• A rotating magnetic field with constant magnitude is produced, rotating with a speed

Where fe is the supply frequency and

P is the no. of poles and nsync is called

the synchronous speed in rpm

(revolutions per minute)

)(120

rpmP

fn e

sync

Synchronous speed

P 50 Hz 60 Hz

2 3000 3600

4 1500 1800

6 1000 1200

8 750 900

10 600 720

12 500 600

Principle of operation• This rotating magnetic field cuts the rotor windings and produces an

induced voltage in the rotor windings• Due to the fact that the rotor windings are short circuited, for both

squirrel cage and wound-rotor, and induced current flows in the rotor windings

• The rotor current produces another magnetic field• A torque is produced as a result of the interaction of those two magnetic

fields

Where ind is the induced torque and BR and BS are the

magnetic flux densities of the rotor and the stator respectively

ind R skB B

Induction motor speed• At what speed will the IM run?

– Can the IM run at the synchronous speed, why?– If rotor runs at the synchronous speed, which is the same speed

of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed

– When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced

Induction motor speed

• So, the IM will always run at a speed lower than the synchronous speed

• The difference between the motor speed and the synchronous speed is called the Slip (s)

Where nslip= slip speed

nsync= speed of the magnetic field

nm = mechanical shaft speed of the motor

msyncslip nnn

The Slip

Where s is the slip

Notice that : if the rotor runs at synchronous speed

s = 0

if the rotor is stationary

s = 1

Slip may be expressed as a percentage by multiplying the above eq. by 100, notice that the slip is a ratio and doesn’t have units

sync

msync

n

nns

Induction Motors and Transformers

• Both IM and transformer works on the principle of induced voltage– Transformer: voltage applied to the primary windings produce an

induced voltage in the secondary windings– Induction motor: voltage applied to the stator windings produce

an induced voltage in the rotor windings– The difference is that, in the case of the induction motor, the

secondary windings can move– Due to the rotation of the rotor (the secondary winding of the IM),

the induced voltage in it does not have the same frequency of the stator (the primary) voltage

Frequency• The frequency of the voltage induced in the rotor

is given by

Where fr = the rotor frequency (Hz)

P = number of stator poles

n = slip speed (rpm)

120r

P nf

( )

120

120

s mr

se

P n nf

P snsf

Frequency• What would be the frequency of the rotor’s

induced voltage at any speed nm?

• When the rotor is blocked (s=1) , the frequency of the induced voltage is equal to the supply frequency

• On the other hand, if the rotor runs at synchronous speed (s = 0), the frequency will be zero

er sff

Torque• While the input to the induction motor is electrical power,

its output is mechanical power and for that we should know some terms and quantities related to mechanical power

• Any mechanical load applied to the motor shaft will introduce a Torque on the motor shaft. This torque is related to the motor output power and the rotor speed

and ).( mNP

m

outload

)/(60

2srad

nmm

Horse power

• Another unit used to measure mechanical power is the horse power

• It is used to refer to the mechanical output power of the motor

• Since we, as an electrical engineers, deal with watts as a unit to measure electrical power, there is a relation between horse power and watts

wattshp 7461

Example

A 208-V, 10hp, four pole, 60 Hz, Y-connected induction motor has a full-load slip of 5 percent1. What is the synchronous speed of this motor?

2. What is the rotor speed of this motor at rated load?

3. What is the rotor frequency of this motor at rated load?

4. What is the shaft torque of this motor at rated load?

Solution1.

2.

3.

4.

120 120(60)1800

4e

sync

fn rpm

P

(1 )

(1 0.05) 1800 1710m sn s n

rpm

0.05 60 3r ef sf Hz

260

10 746 /41.7 .

1710 2 (1/ 60)

out outload

mm

P Pn

hp watt hpN m

Equivalent Circuit• The induction motor is similar to the transformer with the

exception that its secondary windings are free to rotate

As we noticed in the transformer, it is easier if we can combine these two circuits in one circuit but there are some difficulties

Equivalent Circuit• When the rotor is locked (or blocked), i.e. s =1, the

largest voltage and rotor frequency are induced in the rotor.

• On the other side, if the rotor rotates at synchronous speed, i.e. s = 0, the induced voltage and frequency in the rotor will be equal to zero.

Where ER0 is the largest value of the rotor’s induced voltage obtained at s = 1(locked rotor)

0RR sEE

Equivalent Circuit• The same is true for the frequency, i.e.

• It is known that

• So, as the frequency of the induced voltage in the rotor changes, the reactance of the rotor circuit also changes

Where Xr0 is the rotor reactance

at the supply frequency

(at blocked rotor)

er sff

fLLX 2

0

2

2

r

re

rrrrr

sX

Lsf

LfLX

Equivalent Circuit

• Then, we can draw the rotor equivalent circuit as follows

Where ER is the induced voltage in the rotor and RR is the rotor resistance

Equivalent Circuit• Now we can calculate the rotor current as

• Dividing both the numerator and denominator by s so nothing changes we get

Where ER0 is the induced voltage and XR0 is the rotor reactance at blocked rotor condition (s = 1)

0

0

( )

( )

RR

R R

R

R R

EI

R jX

sE

R jsX

0

0( )

RR

RR

EI

RjX

s

Equivalent Circuit• Now we can have the rotor equivalent

circuit

Equivalent Circuit

• Now as we managed to solve the induced voltage and different frequency problems, we can combine the stator and rotor circuits in one equivalent circuitWhere

22 0

22

2

1 0

eff R

eff R

R

eff

eff R

Seff

R

X a X

R a R

II

a

E a E

Na

N

Power losses in Induction machines

• Copper losses– Copper loss in the stator (PSCL) = I1

2R1

– Copper loss in the rotor (PRCL) = I22R2

• Core loss (Pcore)

• Mechanical power loss due to friction and windage• How this power flow in the motor?

Power flow in induction motor

Power relations

3 cos 3 cosin L L ph phP V I V I 2

1 13SCLP I R

( )AG in SCL coreP P P P

22 23RCLP I R

conv AG RCLP P P

( )out conv f w strayP P P P convind

m

P

Equivalent Circuit

• We can rearrange the equivalent circuit as follows

Actual rotor resistance

Resistance equivalent to mechanical load

Power relations

3 cos 3 cosin L L ph phP V I V I 2

1 13SCLP I R

( )AG in SCL coreP P P P

22 23RCLP I R

conv AG RCLP P P

( )out conv f w strayP P P P

conv RCLP P 2 223

RI

s

2 22

(1 )3

R sI

s

RCLP

s

(1 )RCLP s

s

(1 )conv AGP s P conv

indm

P

(1 )

(1 )AG

s

s P

s

Power relations

AGP

RCLP

convP

1

s

1-s

: :

1 : : 1-AG RCL convP P P

s s

ExampleA 480-V, 60 Hz, 50-hp, three phase induction motor is

drawing 60A at 0.85 PF lagging. The stator copper losses

are 2 kW, and the rotor copper losses are 700 W. The

friction and windage losses are 600 W, the core losses are

1800 W, and the stray losses are negligible. Find the

following quantities:1. The air-gap power PAG.

2. The power converted Pconv.

3. The output power Pout.

4. The efficiency of the motor.

Solution1.

2.

3.

3 cos

3 480 60 0.85 42.4 kW

in L LP V I

42.4 2 1.8 38.6 kWAG in SCL coreP P P P

70038.6 37.9 kW

1000

conv AG RCLP P P

&

60037.9 37.3 kW

1000

out conv F WP P P

Solution

4.

37.350 hp

0.746outP

100%

37.3100 88%

42.4

out

in

P

P

ExampleA 460-V, 25-hp, 60 Hz, four-pole, Y-connected induction motor has the

following impedances in ohms per phase referred to the stator circuit:

R1= 0.641 R2= 0.332

X1= 1.106 X2= 0.464 XM= 26.3

The total rotational losses are 1100 W and are assumed to be constant.

The core loss is lumped in with the rotational losses. For a rotor slip of

2.2 percent at the rated voltage and rated frequency, find the motor’s1. Speed

2. Stator current

3. Power factor

4. Pconv and Pout

5. ind and load

6. Efficiency

Solution1.

2.

120 120 601800 rpm

4e

sync

fn

P

(1 ) (1 0.022) 1800 1760 rpmm syncn s n

22 2

0.3320.464

0.02215.09 0.464 15.1 1.76

RZ jX j

sj

2

1 1

1/ 1/ 0.038 0.0662 1.76

112.94 31.1

0.0773 31.1

fM

ZjX Z j

Solution

3.

4.

0.641 1.106 12.94 31.1

11.72 7.79 14.07 33.6

tot stat fZ Z Z

j

j

1

460 0

3 18.88 33.6 A14.07 33.6tot

VI

Z

cos33.6 0.833 laggingPF 3 cos 3 460 18.88 0.833 12530 Win L LP V I

2 21 13 3(18.88) 0.641 685 WSCLP I R

12530 685 11845 WAG in SCLP P P

Solution

5.

6.

(1 ) (1 0.022)(11845) 11585 Wconv AGP s P

& 11585 1100 10485 W

10485= 14.1 hp

746

out conv F WP P P

1184562.8 N.m

18002 60

AGind

sync

P

1048556.9 N.m

17602 60

outload

m

P

10485100% 100 83.7%

12530out

in

P

P

Induction Motor – Power and Torque

The output power can be found as

Pout = Pconv – PF&W – Pmisc

The induced torque or developed torque:

Example

A two-pole, 50-Hz induction motor supplies 15kW to a

load at a speed of 2950 rpm.1. What is the motor’s slip?

2. What is the induced torque in the motor in N.m under these conditions?

3. What will be the operating speed of the motor if its torque is doubled?

4. How much power will be supplied by the motor when the torque is doubled?

Solution1.

2.

120 120 503000 rpm

2

3000 29500.0167 or 1.67%

3000

esync

sync m

sync

fn

Pn n

sn

3

no given

assume and

15 1048.6 N.m

22950

60

f W

conv load ind load

convind

m

P

P P

P

Solution3. In the low-slip region, the torque-speed curve is

linear and the induced torque is direct proportional to slip. So, if the torque is doubled the new slip will be 3.33% and the motor speed will be

4.

(1 ) (1 0.0333) 3000 2900 rpmm syncn s n

2(2 48.6) (2900 ) 29.5 kW

60

conv ind mP