Alternating Current Machines-Synchronous Machines

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Classroom Rules:2. I talk, you listen

3. I don’t allow:A. Heads on deskB. Feet on deskC. Going to sleep in classD. MP3 players

4. Mobile phones are to be off, or on “silent” unless OK’d with me, and in you bag (not on desk)

5. Should they be OK’d, and you receive a call, you will answer it outside the classroom

6. No SMS-ing in class

7. Keith Butler’s number is: 0417 637 909

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Topics (from Learning Outcomes)

10. Three-phase synchronous machines • operating principles • construction feature • application

11. Three-phase synchronous machines • effects of load changes • effects of excitation change • load/current characteristics

12. Single phase synchronous machines • alternators • motors • applications

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Assessment

(a)All prac’s must be completed• A final written exam will be given on the

learning outcomes covered. Should a student fail, they will be allowed one further attempt during block. Should they fail this they will be allowed one further attempt within six weeks of completing the block. Should they fail this they will be in a “Show Cause” situation.

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6500 KW Genset / Generator Set, powered with a Cummins VTA28G1 Engine

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2 x 14MW Synchronous Motors…apparently, they use permanent magnets!

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3 phase Dunlite machine

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Synchronous machines are not just the big units, but they can

be small also.

http://my.ecplaza.net/yiden/1.asp

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V

AC Supply R

VI

I

Current is in phasewith voltage.

Time->

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LAC supply

V

I V

I

Current lags theVoltage by 90o

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CAC Supply

But if an ammeter were placed in series it would most definitely read a current.

Current appears to pass through the capacitor.In reality, it is charging in one direction, and thendischarging and recharging in the other direction.

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V I

CAC Supply

V

I

Current leads theVoltage by 90o

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This opposition to current flow is called:Inductive reactance, in inductors. (XL)Capacitive reactance in capacitors. (XC)

Both Inductors and Capacitors oppose, or “resist” current flow when connected to AC supplies.

While it opposes current flow, it is NOT called resistance.

Current flow through resistance produces HEAT. Current flow in inductors and capacitors doesn’t!

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Generator / Transformer / Motor

S F

S

F

S

F S F

S F

S F

B

A

C

AC

B

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AC

B

Motor

Why isn’t a neutral run to a balanced three phase Star connected load?

A

BC

N?????

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AC

B

Motor

Because the Star point is at 0V

A

BC

0V0VN

And the neutral is at zero volts also.So if they were joined no current would flow.

0A

So why join it?

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AC

B

MotorA

BC

The neutral is not connected to a balanced three phase star connected load.

Only connected to unbalanced loads!!!

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Transformer

S FB

S FC


AB C

F

S F

S

FS

S FA

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Transformer

SFB

SFC


AB C

S

F S

F

SF

SFA

Swapped

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3-phase Transformer Secondary

S FB CS F

A S F

A B C

Note that all windings are connected in series, with the twoends joined together.

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If we did that with three batteries, there would be majorproblems!

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The voltmeter should read the sum of the three voltages?Right?

VA

VBVC

AC B

F

S F

S

FS

Transformer

V

The voltmeter reads the phasor sum of the voltages.

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The voltmeter reads, in effect, the distance between thebeginning of VA and the end of VC. ie. 0V

VA

VBVC

AC B

F

S F

S

FS

Transformer

V

We can connect the two ends together because the phasor sum adds up to zero!

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Transformer

VA

VBVC

AC B

F

S F

S

FS

The voltmeter reads, in effect, the distance between thebeginning of VA and the end of VC. ie. 0V

We can connect the two ends together because the phasor sum adds up to zero!

No Arc!

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Generator Load

STARVL = 3 VPH

IL = IPH

DELTAIL = 3 IPH

VL = VPH

P = 3 x VPH x IPH x Cos = 3 x VL/3 x IL x Cos = 3/3 x VL x IL x Cos = 3 VL x IL x Cos

P = 3 x VPH x IPH x Cos = 3 x VLx IL/3 x Cos = 3/3 x VL x IL x Cos = 3 VL x IL x Cos

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P = 3 VL x IL x Cos

NOT:P = 415 x I x pf.

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Three Single Phase Power Equations:

True Power = Watts = V x I x Cos

Apparent Power = VA = V x I

Reactive Power = VAR’s = V x I x Sin

Power Factor = Cos where Cos = Cosine of the angle

between Voltage and Current

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VA

Watts

Var’s

Phase angle between current and volts

This can be put as a triangle:

VA2 = Watts2 + Var’s2

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V=240V

Alternators, where the windings are limited by the current through them, are rated in VA.

To rate them in watts, (ie. watts delivered to the load) would give no idea of the current through them.

Load 3 = 14.14A at 45º

Load 1 = 10A

Load 2 = 20A at 60º

P = V x I x Cos 45º = 240 x 14.14 x 0.707 = 2.4kW

P = V x I x Cos 0º = 240 x 10 x 1 = 2.4kW

P = V x I x Cos 60º = 240 x 20 x 0.5 = 2.4kW

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Q What dictates the phase angle of the current supplied by a single alternator supplying a single load?

V=240V

Load 3 = 14.14A at 45º

Load 1 = 10A

Load 2 = 20A at 60º

P = V x I x Cos 45º = 240 x 14.14 x 0.707 = 2.4kW

P = V x I x Cos 0º = 240 x 10 x 1 = 2.4kW

P = V x I x Cos 60º = 240 x 20 x 0.5 = 2.4kW

A The load

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V

Al currents here take the same power

Constant power line

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V


Higher power

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V


Constant power line

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V


Lower power

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Alternator

Mechanical Energy Electrical Energy

Losses

AlternatorPrime Mover

- Diesel Engine- Steam Turbine- Small petrol engine

Alternator: PoutEff% = x 100 Pin

Alternator:Pin = Pout + Losses

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Losses

Synchronous Motor

Mechanical Energy MSBElectrical Energy

Motor: PoutEff% = x 100 Pin

MotorPin = Pout + Losses

Motor Load

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Synchronous Machine

Stator- Identically wound to an induction motor.- Connected to supply.

Rotor- Constant DC field- Connected to supply via sliprings.

ElectricalPower

DCSupply

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• The stator produces a rotating magnetic field exactly the same as an induction motor.

• The rotor is a magnet and locks in to the RMF• Rotor travels at SYNCHRONOUS SPEED.

SYNCHRONOUSMOTOR

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Synchronous Machine

If a synchronous motor is OVER driven by the load (eg electric train going down a hill), then it will generate power, still at synchronous speed.

If an alternator coupled to the grid is UNDER driven by the prime mover (eg steam stops), then it will motor, and drive the turbine at synchronous speed.

ElectricalPower

DCSupply

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Synchronous Machine

In other words, the two machines are identical in construction.

ElectricalPower

DCSupply

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3000RPM 1500RPM 1000RPM 750RPM

185kw 310A, .88pf 315A, .86pf 343A, .80pf 348A, .78pf

220kw 362A, .89pf 375A, .86pf 408A, .78pf 412A, .78pf

150kw 242A, .90pf 265A, .87pf 279A, .80pf 278A, .77pf

Lower the RPM, • Larger value IS

• More lagging IS

Characteristic of WEG® Induction Motors.

110kw 182A, .90pf 200A, .84pf 205A, .80pf 203A, .81pf

22kw 39A, .87pf 41A, .83pf 42A, .80pf 47A, .74pf

4kw 7.8A, .87pf 8.2A, .82pf 9A, .74pf 11A, .63pf

What is thetendency as RPM gets lower?

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So why use a Synchronous Motor?Uses:– Low Speed Drives. Low speed induction motors

draw very large currents at poor power factors. This cannot be altered or corrected. In synchronous motors, the p.f. can be altered to cause the motor to draw minimum current. (The alternative is to use a high speed induction motor through a gearbox.)

– Power Factor Correction– Constant Speed drives

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Salient Pole Rotor Cylindrical Rotor

2 basic types:Cylindrical rotorSalient Pole

-Low speed-Diesel Prime Mover-Hydro systems

-High speed-Steam Turbine

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Small salient pole synchronous machine rotor

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Losses

Synchronous Motor

Mechanical Energy MSBElectrical Energy

Motor: PoutEff% = x 100 Pin

MotorPin = Pout + Losses

Motor Load

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Same as an induction motor.

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A1

A2

2-Pole Machineie. 3000RPM

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A1

A2

B1B2

C1

C2

2-Pole Machineie. 3000RPM

In reality, the coilsspan more slots in a 2-pole motor.

N

S

Notice that for a two pole stator we have a 2-pole rotor

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N N

S

S

A

A

AA

B

B

B

B

C

C

C

C

4-pole machine

A four pole stator must have a four

pole rotor

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Flux+

-Time->

1

Resultant flux = 1.5 x flux of one phase

N

S

N

S

S

N

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Flux+

-Time->

2

Resultant flux = 1.5 x flux of one phase

N

S

N

S

N

S

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Flux+

-Time->

3 4 5 6

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Flux+

-Time->

3 4 5 61 2

So the flux rotates one full rev in one cycle, for our two pole machine.

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Flux+

-Time->

3 4 5 61 2

Because the flux is a constant value, it gives: 1. Very quiet operation 2. Constant torque as the rotor rotates.

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Flux+

-Time->

3 4 5 61 2

This rotating magnetic field rotates at:3000RPM for a 2-pole motor1500RPM for a 4-pole motor

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Flux+

-Time->

3 4 5 61 2

To reverse the direction of rotation:reverse any two phases to the motor.

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where N = RPM f = frequency P = Number of poles (per phase).

N = 120f/P

So the speed of the rotating magnetic field isaffected by:

Frequency, andNumber of poles.

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As the rotating magnetic field rotates, the rotor is locked in synchronism with it and is dragged along for the ride.

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N

S

As the rotating magnetic field rotates, the rotor is locked in synchronism with it and is dragged along for the ride.

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What will happen as a load is put on the shaft?

N

S

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N

S

What will happen as a load is put on the shaft?

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The load tries to slow it down.But it must do synchronous speed!So it stretches the lines of flux.

N

S

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N

S

C/L of RMF

C/L of Rotor Field

Torque Angle

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If the lines stretch to breaking point (ie too much load), then the rotor stalls

This is referred to as “Pull Out Torque”.

N

S

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What would the Torque Curve look like?

RPMNs0

Torque Curve for an induction motor

Torque

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What would the Torque Curve look like?

Torque

RPMNs0

Torque “Curve” for a Synchronous Motor

Pull outTorque

Zero Torque below synchronous speed

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1. Amortisseur winding

Starting a Synchronous Motor?

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Rotor Construction

Squirrel Cage

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1. Amortisseur windingThis gets the motor up to speed as an induction motor. When it is close to synchronous speed it will lock in.

2. Shorting the rotor DC winding and starting it as a wound rotor motor. When it is close to synchronous speed, the short is removed and DC is applied to the rotor. It will (hopefully) lock in.

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3. Using a pony motor to get the synchronous motor up to speed, then applying AC to the stator and DC to the rotor.(Not applicable if there is a high starting torque load connected)

Note that these starting methods will only work if the load on the motor at start can be reduced or eliminated.


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• Amortisseur windings also reduce hunting.

• Hunting is rhythmic fluctuations of the RPM around an average value.

• If not subdued, hunting can cause the rotor to swing out of synchronism.

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Revs

Time

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N

S

N

S

N

S

N

S

N

S

And all this while it is whizzing around at synchronous speed!

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And all this while it is whizzing around at synchronous speed!

N

S

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VsupplyVinduced

Induced in the stator from the rotor

Phasor Diagram of Synchronous Motor

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Vsupply

Vinduced

Torque angle

Isupply

VR

Phasor Diagram of Synchronous Motor

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Phasor diagram for increased load:(Excitation current held constant)

Vsupply

Vinduced

Isupply

VR

Increased load = Increased Torque Angle

Vinduced

Increasing the load increases the power taken from supply

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Vsupply

Vinduced

Vinduced

Phasor diagram for increased excitation:(Constant Load)

Constant load = Constant Power line

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Vsupply

Vinduced


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Vsupply


Vinduced VRIsupply

So to force the supply current leading, we INCREASE excitation


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Vsupply

Vinduced

Isupply

VR

Phasor diagram for decreased excitation:(Constant Load)


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Vsupply

Isupply

VR

Phasor diagram for decreased excitation:(Constant Load)

Vinduced

So to force the supply current lagging, we DECREASE excitation


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Vsupply

Vinduced

IsupplyVR

Vsupply

Isupply

VRVinduced

Vinduced

Isupply

VR


Vsupply

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Vsupply

With a constant load, changing excitation changesthe phase angle and value of supply current.

Isupply

By increasing the DC excitation current to the rotor,the synchronous motor can act as a capacitorIt can be used for power factor correction.


120Excitation Current

Stat

or C

urre

nt

50% lo

ad

Unity pf

Lag Lead

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Deductions From Vee Curves

• At any particular load there is a certain value of rotor current which gives a minimum value of stator current and unity pf.

• If the rotor current is altered either way, the stator current will increase, and pf will decrease away from 1.

• For any given load there is a certain value of rotor current below which the rotor will fall out of synchronism.

• For any given load there are two values of rotor current that will give identical values of stator current. The lower value gives a lagging pf, and the higher value gives a leading pf.

122Excitation Current

Stat

or C

urre

nt

50% lo

ad

LagLead

75% lo

adStabilitylimit pf=1

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0.8 pf lag Unity 0.8 pf leadPer unitPower output

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Points:

• At a set load there is a value of excitation that will give minimum line current.

• Reducing OR increasing excitation from this value will only increase line current.

• At any other value of line current, there are two values of excitation current that can produce this.

• If a synchronous motor is heavily loaded, supply current may not be able to be driven highly leading.

• If a synchronous motor is lightly loaded, supply current can be driven highly leading.

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Single Phase Synchronous Motors• Used when constant speed is critical, with low

torque requirements. They have low efficiency, hence made in small sizes.

• Application:• clocks• record players• timers• recorders• communications• servo installations

• Two main types:• Reluctance motor• Hysteresis motor

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Reluctance Motor

Stator• Stator same as a single phase, split

phase motor.• Centrifugal Switch operates at

75% synchronous speed to open circuit the start winding.

Rotor• Assembled from laminated sheets

with defined teeth cut away. This forms salient poles.

• Windings are of the squirrel-cage type.

• Number of rotor poles equals the number of stator poles.

Two pole, 3000 RPM rotor

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Reluctance Motor• Operation

– Starts as an induction motor, with slip.– A single phase stator has a “Start” and “Run” winding. At

75% centrifugal the centrifugal switch operates.– As the load is light there is small slip– The salient poles become permanently magnetised by the

stator field– The salient poles will then lock to the stator field.– Once locked into synchronism the motor will continue to

operate at synchronous speed.– Not as much power output as a similar physical size 1-phase

motor.

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Hysteresis MotorRotor• Constructed from hardened steel rings, instead of thin,

magnetically soft, silicon steel laminations.• “Hysteresis” opposes any change once the flux is created, so

the rotor will lock into the RMF like a permanent magnet.Stator• Often a shaded pole stator principle is used.• If the shaded pole principle is used then the motor is self

starting.• Magnetic poles are established in the rotor.• These poles lock to the stator poles.• The rotor runs at synchronous speed determined by the

poles and frequency.

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Why generate AC?… Why not DC?

DC cant be “transformed” through a transformer. AC can go through a transformer.

Large brushless DC generators are not possibleLarge brushless AC alternators are!

Why do we want to transform it?It is easier to transmit to distant placesat higher voltages as the current will be lower. (P=V x I)

Induction motors are simpler and cheaper thanDC motors

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NS

V

Generating an AC Voltage

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N S


Volts

+

-

Time->

136

NS


Volts

+

-

Time->

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NS


Volts

+

-

Time->

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NS


Volts

+

-

Time->

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N S


Volts

+

-

Time->

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NS


Volts

+

-

Time->

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NS

V

Generating a AC Voltage3-Phase

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N S

Volts

+

-

Time->


Require:Three sets of coils physically displaced from each other by120º electrical.

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VA

VC

VB


N S

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A1

A2

N S

145


N S

A1

A2

B1B2

C1

C2

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N N

S

S

A

A

AA

B

B

B

B

C

C

C

C

4-pole machine

A four pole stator must have a four

pole rotor


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Alternator• Reasons for having the three phase winding on the

stator rather than the rotor:– More space on the stator for the three phase

winding.– Only one, low voltage winding on the rotor.

• Easier to insulate.• Less problems with centrifugal force.

– Only two sliprings required rather than four (3-ph + N)

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Alternator

Stator- Connected to load.

Rotor- Constant DC field- Connected to its own DC supply via sliprings.

ElectricalPower

MechanicalPower

Mag

neti

c Fi

eld

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Alternator

Q: What keeps an alternator producing 50Hz under all load conditions?

A: The governor on the prime mover. It detects any drop in speed, and tries to speed the unit up.

AlternatorPetrolEngine

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IFIELD

VOUT

Alternator Excitation Curve

(No Load)

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Alt

LoadR

XLInternal

Impedance

Alternator

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VOUTILOADVR

VZ VL

VGEN

VZ = Internal Impedance of the alternator

VR = Internal Resistance of the alternatorVL = Internal Reactance of the alternator

Resistive Load

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Resistive Load

VOUTILOADVR

VZ VL

VGEN

Notice that terminal volts DROP as load increases Load current and p.f. are dictated by the LOAD!

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Inductive Load

VOUT

ILOAD

VR

VZ

VL

VGEN

Parallel

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Inductive Load

VOUT

ILOAD

VR

VZ

VL

VGEN

Now there is a greater voltage drop under load

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Capacitive Load

VOUT

ILOAD

V R

V Z

V L

VGEN

Now there is a voltage RISE under load

Parallel

Because of the voltage rise under load, it is not desirable to run alternators at a

leading power factor.

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Leading pf

Unity pf

Lagging pf

Load Current

OutputVoltage

Effect of Power Factor on Output Voltage

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Voltage Regulation

(VNL – VFL)%Voltage Regulation = x 100 VFL

eg An alternator output falls from 240V to 200Vwith constant excitation. Calculate the % voltage regulation.

(Ans: 20%)

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Summary:When an alternator is standing by itself with a single load:

Output voltage is affected by excitation currentOutput frequency is affected by input power to the

alternator.

Alternators - stand alone

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When an alternator is tied to the grid, you cannot change:Grid voltageGrid frequency

So the output voltage of the alternator will not change, andthe output frequency of the alternator will not change.

Notice that, for a stand alone alternator with stand alone load, these are the two things that changed when:

(a) the excitation was altered, and(b) the power input to the alternator was increased

(ie. Put the foot down on the prime mover)

Alternators tied to the Grid

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Alternators tied to the Grid

VOUT VR

VZ VL

VGEN

If excitation is increased, and VOUT cannot alter, VGEN

will increase and push the triangle over.

ILOAD

1. Altering Excitation.

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VOUTILOAD

VR

VZ

VL

VGEN

If excitation is increased, and VOUT cannot alter, VGEN

will increase and push the triangle over.

Alternators tied to the Grid1. Altering Excitation.

Note that input power to the alternator is not changing,so output power does not change either.

Constant Power Line(Output power of the alternator has notChanged)

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VOUTILOAD

VR

VZ

VL

If excitation is reduced, and VOUT cannot alter, VGEN

will reduce and pull the triangle back.


VGEN

This drives the load current lagging

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VOUT

ILOAD

V R

V Z

V L

VGEN

If excitation is reduced, and VOUT cannot alter, VGEN

will reduce and pull the triangle back.


This will drive the load current leading

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If input power is reduced, and frequency and VOUT cannot alter, output power will reduce.

Alternators tied to the Grid2. Altering input power to the alternator.

VOUT VR

VZ VL

VGEN

ILOAD

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VOUT VR

VZ VL

VGEN

ILOAD

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VOUT VR

VZ VL

VGEN

ILOAD

Size of trianglereduces

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VOUTV

RV

Z VL

VGEN

ILOAD

Size of trianglereduces

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If input power is increased, and frequency and VOUT cannot alter, output power will increase.


VOUT VR

VZ VL

VGEN

ILOAD

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VOUT VR

VZ VL

VGEN

ILOAD

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VOUT VR

VZ VL

VGEN

ILOAD

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VOUTVR

VZ

VL

VGEN

ILOAD

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Alternators - tied to the GridSummary:

Changing excitation changes the pf of output current.Changing input power changes output power

• Increasing excitation drives load current lagging• Reducing excitation drives load current leading• Increasing input power increases output power• Reducing input power reduces output power

• Output frequency and voltage do not change.

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Alternators – stand aloneSummary:• Changing excitation changes output voltage.• Changing input power changes RPM, which changes

output frequency.

• Here, output frequency and voltage do change

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Paralleling Alternators

To parallel alternators (or parallel one onto the grid), the following criteria must be met:

• Output voltage must be the same• Output frequency must be the same• Phase rotation must be the same • Supply voltage must be in phase

It is understood that they must both produce the same waveform – a sine wave!

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Alternator Rating

Alternators are rated according to:FrequencyVoltageCurrentkVA

The frequency dictates the RPM (3000, 1500, etc).Voltage and Current give the kVA rating.

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Efficiency

Losses:• By far the main loss in an alternator is HEAT loss.• If an alternator can be kept cool, more power can

be obtained from it. ie. Instead of a 300MW machine, it will become a 500MW machine.

• More power must be put into it to get this increased output power.

• Cooling large alternators is a big deal! They are often cooled using hydrogen.

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Efficiency

Losses:• Copper Losses:

I2R losses in the stator windingI2R losses in the rotor winding

• Iron Losses:Hysteresis loss in statorEddy current Loss in stator

• Friction and windage

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Single Phase Alternators

ElectricalPower

MechanicalPower

Mag

neti

c Fi

eld

Regulator

Stat

or

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Single Phase Alternators

• These are usually low rated units for portable use.• Prime mover is usually a small petrol or diesel engine.• The engine speed is kept constant by a governor.• This speed will usually be either 3000RPM or 1500RPM• Output voltage is kept constant using an automatic voltage

regulator. This senses the output voltage and adjusts the rotor excitation current automatically.

• They are usually self exciting, so if the load is left on at start, they may not build up output voltage.

• Many small alternators are brushless.• Usually, neither side is earthed. This is called a FLOATING

system.

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Rotor

Brushless Alternators

AC issampled

Regulator DC

Field P.S.

Note: Self Excited

3-phaseout

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Brushless AlternatorsRotor

Regulator

3-phaseout

Prime Mover3-phase

out

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Small Alternators-Factors when choosing:

•Voltage: 240V / 415V (1-phase or 3-phase)•kVA rating•RPM (3000RPM or 1500RPM)•Petrol or Diesel•Brushless or brushes•Ability to start loads such as motors•Extras: Soundproofing, starting, power outlets,

mounting holes, 12VDC / welding output

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http://images.google.com.au/imgres?imgurl=http://tractionmotor.com/Mvc-003f.jpg&imgrefurl=http://tractionmotor.com/9%2520PEMS%2520MOTOR%2520SHOP.htm&h=480&w=640&sz=85&tbnid=0epvLon39iAJ:&tbnh=101&tbnw=135&start=90&prev=/images%3Fq%3Dsynchronous%2Bmotor%26start%3D80%26hl%3Den%26lr%3D%26sa%3DN

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Alternating Current Machines-Synchronous Machines

Business

x vl x il x cos

x vl3 x il x cos

x vph x iph x cos

vl x il x cosp

vl x il x cosnot

x vlx il3 x cos

x cosapparent power

v x ireactive power