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AC Machines

Alternate Current Machines

AC Machines

10.12.2012

Contents

Electric Motors

AC Motors

Synchronous AC Motors


Asynchronous AC Motors

Losses in Motors

225.11.2011

Power Calculations for Motors

Electric Motors

If an electric current flows through a conductor in a magnetic

field, a magnetic force effects the conductor. A simple electric

motor can be formed if this conductor has a point to rotate

around. Faraday electric motor and Barlow Wheel are the first

experimental representations of the electric motor.

Alternate Current Machines3

Electric Motors Faraday Electric Motor

There is a free rotating wire

which is inserted in a glass

full of mercury (or salt

water) in Faraday Electric

Motor. The glass full f

mercury has a permanent

magnet on center.

If a current flow through


If a current flow through

the wire, it starts to rotate.

This motion is the

representation of the

magnetic field produced

because of the current

flows on a wire.

Electric Motors

An electric current passes through the

hub of the wheel to a mercury contact

on the rim; this is contained in a small

trough through which the rim passes.

Due to health and safety

considerations brine (salt water) is

sometimes used today in place of


sometimes used today in place of

mercury. The interaction of the current

with the magnetic field of a U-magnet

causes the wheel to rotate. The

presence of serrations on the wheel is

unnecessary and the apparatus will

work with a round metal disk, usually

made of copper.

What is an Electric Motor?

An electric motor is a machine that converts

electrical energy to mechanical energy.

• Used is compressors, pumps, air condition

fans, electric vehicles, robot mechanisms,

cranes, etc.


cranes, etc.

• More than the two thirds of the load in

industry are the load of electric motors.

6

Electric Motors

It is the ‘Lorentz Force’

that effects the charge

of ‘q’ which has the

velocity of ‘V’ in

magnetic field ‘B’. The

directions of this force,


directions of this force,

the current and the

magnetic field can be

seen in the figure.

|FL|=q V B Sin α

Electric Motors

The rotating part of an electric motor is called as rotor whereas

the fixed part as stator. If the rotor is consist of windings,

brushes are used to transfer the current. The brush is a carbon

part which has a contact with the terminals of the coils on the

axle of the rotor. In some DC motors, permanent magnets are

used in rotors and these types are called as Brushless DC Motors


used in rotors and these types are called as Brushless DC Motors

(BLDCM). The problem for these types is to sense the position of

the rotor. Information about the position of the rotor is needed

to be sent to the driver of the motor.

Electric Motors


In some AC motors, Aluminum bars are used as the rotor of the

motor. These type of motors are called as Squirrel Cage type

electric motors. When the current is changing periodically, the

rotor follows the current.

Electric Motors


Electric Motors

Electric Motors

Alternate Current Motors

(ACM)

Direct Current Motors

(DCM)

Synchronous Induction Self-ExitedExternally


Synchronous Induction

(Asynchronous

Three PhaseMono Phase

Self-ExitedExternally

Excited

Series SchuntCompound

Alternatif Akım Motorları


(ACM)

Synchronous ACM Induction (Asynchronous) ACM


Synchronous ACM Induction (Asynchronous) ACM

Squirrel Cage ACMSlip Ring ACM


The ACM’s are simplier in structure

and more economic than DCM’s.

An ACM generates more power

comparing with a DC motor that has

the same weight. Maintenance of

ACM’s is easier. However, their speed

control is harder. They can be


control is harder. They can be

connected to the AC source directly.

If accuracy in velocity or position

control is needed, DCM’s are used.

But, ACM’s are used more than

DCM’s in industry.

ACM’s

Free Running Current (I0): It is the current consumed from the grid with

nominal voltae and frequency, but without any load on motor.

Maximum Starting Current (Ik): It is the maximum current on nominal

voltage and frequency when starting a motor.

Starting Torque (MA): It is the torque generated by the motor during

starting under nominal voltage and frequency.

Nominal Moment (MN): It is the toque generated by the motor under

nominal power and speed.

Basic Definitions


nominal power and speed.

Stall Torque (Mk): It is the maximum torque generated by the motor with

nominal voltage and frequency.

Pull-up Torque (Ms): It is the minimum torque delivered by the motor

with nominal voltage and frequency, between zero velocity and the

velocity with the stall torque.

1 kgm = 9,81 Nm ~ 10 Nm,

MN

= 9550 x Nominal Power [kW] /Nominal Velocity of Rotor [RPM]

Torque

[Nm]

ACM’sBasic Definitions


MA

Revolution [RPM]

MS

MK

MN

http://www.ebmpapst.se/sv/dat/media/informati

on/definitions_for_ec_motors.pdf

Synchronous Machines

“ Synchronous Machine is a machine that runs at a constant speed which

is proportional to frequency and number of poles. It can be run as a

generator or a motor. However, because of the constant running speed

these machines are generally used as generators. They are the most

common machines used in power plants. They can be manufactured to

generate electricity up to 2000 [MVA]. Cost effectivity due to unit power

generated, higher efficiency in greater power generation, less

maintenance and control processes made them to be manufactured in


maintenance and control processes made them to be manufactured in

greater powers.

(*see references)


Stators of Synchronous Machines are

manufactured using laminated cores

which have slots to place the coils on

them.

Synchronous Machines are divided into

two groups according to the structure

of the rotor that has exiting coil on it.


of the rotor that has exiting coil on it.

If the airgap between the stator and

rotor is constant every where, then it is

a round rotor (turbo) machine. Unless,

it is a salient pole synchronous

machine.

22 [MW], 13.8 [kV], 3,600 [RPM]* http://www.ips.us/industries/fossil-fuel-

power/


Round rotor synchronous generators are manufactured in small

number poles and high synchronous revolution per minute. They are

used in high velocity steam turbines. The length of the rotor is long

and radius of the rotor is small in this type of turbines.

The salient pole synchronous machines are generally have more

poles and are designed for lower synchronous rotational velocity.


poles and are designed for lower synchronous rotational velocity.

Length of the rotors are short and the radius of the rotors are long.

Salient pole synchronous machines are used in hydro elecric power

plants and for compensating the power factor of the grid.

ns = 120 f / p

ns : velocity of the synchronous

machine

f : frequency of the source

p : number of poles

Asynchronous Machine

• Mono Phase Induction Machine

• Has only one stator coil.

• Uses only one phase.

• Rotor of an asynchronous machine can be a squirrel

cage.


cage.

• Needs a unit to start to motor.

• Are used in applications needs 3 ~ 4 HP (Fans, washing

machines, household devices… etc.)

Asynchronous Machines

• Three Phase Induction Machine

• Magnetic field is generated by three phases

• Rotor can be either squirrel cage or composed of coils

• Can be started easily

• Has great power capacities


• Has great power capacities

• There are applications from 1/3 HP to hundreds of

HPs: Pumps, compressors, conveyor drums, grinding

machines and etc.

• More than 70 % of the motors in industry are three

phase induction machines.

Three Phase Asynchronous Machine

Industrial loads or high

power loads are

needed to be

connected to three

phase grid whose

phases follow each


phases follow each

other in 120 degrees

instead of mono phase

grid. Result of this

usage is smaller

currents.



t1


In a three phase AC motor, a rotating field might be achived using

the coils which are located geometrically around stator (see Figure

below).

Rotating Magnetic Field:

Terminals for a three phase asynchronous

machine:


t1

machine:

Phase R� input terminal: U, output terminal X

Phase S� input terminal: V, output terminal Y

Phase T � input terminal: W, output terminal Z

Three Phase Asynchronous MachineRotating Magnetic Field in a Three Phase Machine

i,u

R S T

1200

12001200


t1

1

23

t1

t2

t3

t4

t6

t5

Three Phase Asynchronous MachineRotating Magnetic Field in a Three Phase Machine

t1

t2 t

3 t4

t6

t5


t1

Time Interval IR

IS

IT

t1 + - 0

t2 + 0 -

t3 0 + -

t4 - + 0

t5 - 0 +

t6 0 - +

Three Phase Asynchronous MachineWYE Connection


t1

Three Phase Asynchronous MachineDelta Connection


t1

Three Phase Asynchronous MachineThree Phase Asynchronous Motor

If the frequency of the flowing current is f ,the number of

rotation (or synchronous number of rotation or nember of

rotation of rotating field) is n. Equation of the number of

rotation of magnetic field is given below in unit of RPM.

Ns = 60 f / pf [Hz]: Frequency of the source

p [ ]: Number of pole pairs


t1

Ns = 60 f / pp [ ]: Number of pole pairs

Three phase asynchronous machines do not form sparks. Their

number of rotation do not change so much with changing

loads. Thus they are said to be constant speed motors. Thus,

they are called as constant speed machines. Their efficiencies

are high. If the three phase grid is not present then the

monophase motors are used.


In an asynchronous motor, the speed of the magnetic field

generated by the stator coils and the rotation speed of the rotor is

not the same. The value of the rotational speed of the rotor is

always smaller than the speed of the stator’s magnetic field. The

reason of the word ‘asynchronous ’ is this. The difference of these

speed is called as the slip. If ‘s’ is negative (rotor’s speed is greater)

then the electric machine is running as a generator.

Speed and Slip


t1

then the electric machine is running as a generator.

s = [(Ns

– Nr)/ N

s] x 100

s [ %]: Slip

Ns

[RPM]: Speed of the magnetic field.

Nr

[RPM]: Rotational speed of the rotor

The slip s is defined as 'the

difference between synchronous

speed and operating speed, at

the same frequency, expressed

in rpm or in percent or ratio of

synchronous speed'.

Asynchronous MachineEfficiency – Speed – Torque Curves


t1

Asynchronous Machine

Slip Ring Type


Losses in Electric Motors

Losses Notation

Losses of mechanical

frictionsPks

Iron loss(hysteresis and

eddy current losses)PkFe


eddy current losses)

Ohmic power loss of

armaturePka

Losses in Electric Motors

Losses Notation

Friction and air flow

lossesPfw

Iron loss Pfe

Loss of conductor

(stator - copper)PS

Losses in Asynchronous Motors

Pe

P1

13

4

2


(stator - copper)PS

Loss of conductor

(rotor - alluminium)PR

Additinal load loss PXL

Pfw

Pm

PfePSPR

PXL

1 4

5

Losses in Electric MotorsLosses in Asynchronous Motors

Pfw

Pe

Pm

PfePS

Friction and Air Flow

They are constant losses during motor run,

independent from load and occur in bearings and

cooling fan propellers.

Iron Loss

Total affects of losses in cores of coils (hysteresis and eddy


PS

PRPXL

Total affects of losses in cores of coils (hysteresis and eddy

current losses). It can be neglected even the rotor composed of

coils since the frequency of the induced voltage is low. It might

be observed as heat in laminated cores when the motor is

running. It is dependent to the material, thickness and

dimensions of the laminated core, the frequency applied to the

motor and the square of the voltage applied to the motor. It is

constant if the frequency and the voltage that the motor is

connected do not change.

Losses in Electric MotorsLosses in Asynchronous Motors

Pfw

Pe

Pm

PfePS

Conductor Loss (Stator)

It is heat loss. The current flow through the stator

coils creats this loss (I2RS ).

Conductor Loss (Rotor)

It is heat loss. The current flow through the stator coils or

cage bars creats this loss (I2RR

).


PS

PRPXLAdditional Load Loss

It is the loss occurs in metal parts of the motor except

the laminated cores in rotor and stator because of the

leakage because of the load.

Losses

Friction and Air Flow Losses % 0,5 ~ 1,5

Iron loss % 1,5 ~ 2,5

Conductor loss (stator) % 2,5 ~ 4,0

Conductor Loss (rotor) % 1,5 ~ 2,5

Additional load losses % 0,5 ~ 2,5

Power Calculations in Electric Motors

i,uR S T

Colours of wires (TS 6429)


t1

t2

t3

t4

t6

t5

Blue- Brown – Black – Gray –

Yellow+Green


The nominal power of a DC motor might be expressed as the

equation below. UDC

[V] is the voltage applied to the motor, and IDC

[A] is the current flow. Pinput

[W] is the electrical power, Poutput

[W] is

the mechanical poweror the nominal power, ωm

[RPM] is the

rotational speed of the axle of the motor, Tm

[Nm] is the torque

generated by the motor. Ploss

[W] is the power loss, η [%] is the

efficiency of the motor.


Pinput

=UDC

IDC

Poutput

=ωm

Tm

Ploss

= Pinput

− Poutput

η=( Poutput

/ Pinput

)x100


In AC motors, because of the changing current characteristics, there is

an important point that , there are three powers called as apperant,

true and reactive. In AC motors, current is lagging voltage with angle

φ. This divides the power into two vector parts.

I [Amper]


[VAC]

[Hz]

I [Amper]

Lm

Rm


Reactive (blind)

Power

Q = U I sinφ[VAR]

Apparent (imaginary)

Power

S = I U[VA]


[VAR]

P = U I cos φ[Watt]

ϕ = P / S

True (real) Power


Example: A mono phase asynchronous motor draws 12.3[A] from

grid and its power factor is measured as 0.94. What are the powers

consumed?

Apparent Power= S = U I = 220 x 12,3 = 2706 [kVA]

Active Power = P = U I Cos φ =220 x 12,3 x 0,94 = 2,833 [kW]


Active Power = P = U I Cos φ =220 x 12,3 x 0,94 = 2,833 [kW]

Reactive Power = Q = U I sin φ or = 1,028 [kVAR]


Power Calculations in Three Phase Electric Motors

In a balanced three phase circuit:

P = √ 3 x U x I x cos φQ = √ 3 x U x I x sin φS = √ 3 x U x I

P : True Power [Watt] ;


P : True Power [Watt] ;

Q : Reactive Power [VAR];

S : App. Power [VA]

U : 380 [V] � phase to phase voltage: 380 [V].

I : Current drawn from one phase: [A]


Power Calculations in Three Phase Electric Motors

P : True Power [Watt] ; Q : Reactive Power [VAR]; S : App.Power [VA]

The current drawn from the three phase grid by an asynchronous

alternate current motor is 7 [A] and the power factor of the motor

is measured as 0.85. What are the powers consumed from the

grid?


Power in a balanced three phase circuit:

P = √ 3 x U x I x cos φ = √ 3 x 380 x 7 x 0,85 = 3916 [W]

Q = √ 3 x U x I x sin φ = √ 3 x 380 x 7 x 0,5268 = 2427 [VAR]

S = √ 3 x U x I = √ 3 x 380 x 7 = 4607 [VA]

P : True Power [Watt] ; Q : Reactive Power [VAR]; S : App.Power [VA]

U : 380 [V] � Voltage btw. Phases: 380 [V]

I : Current drawn from one phase: [A]

References:

1. http://ocw.mit.edu

2. http://en.wikipedia.org

3. http://www.energyefficiencyasia.org

4. www.amidesign.ch

5. Asenkron Elektrik Motorları, Ali Taner, 2011.


5. Asenkron Elektrik Motorları, Ali Taner, 2011.

6. http://avstop.com/ac/apgeneral/typesofacmotors.html

7. http://www.daviddarling.info/encyclopedia/E/electric_motor.html

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