AC machines ebook

Electric MotorsIn some AC motors, Aluminum bars are used as the rotor of the motor.. Salient pole synchronous machines are used in hydro elecric power plants and for compensating the po

AC Machines

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.

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

the wire, it starts to rotate

This motion is the representation of the

magnetic field produced

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

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

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

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,

the current and the

magnetic field can be

seen in the figure

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

Electric Motors

Self-Exited Externally

Excited

Series Compound Schunt

Alternatif Akım Motorları

Alternate Current Motors

(ACM)

Synchronous ACM Induction (Asynchronous) ACM

Squirrel Cage ACM Slip Ring ACM

Alternate Current Motors

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

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

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

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 greater powers

Synchronous Machines

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

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-Synchronous Machines

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

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

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

• 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

Three Phase Asynchronous Machine

Industrial loads or high

power loads are needed to be connected to three

phase grid whose

phases follow each

other in 120 degrees

instead of mono phase

grid Result of this

usage is smaller

Three Phase Asynchronous Machine

Three Phase Asynchronous Machine

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:

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 Machine

Rotating Magnetic Field in a Three Phase Machine

t1 t2 t3 t4 t5 t6

Three Phase Asynchronous Machine

Rotating Magnetic Field in a Three Phase Machine

-Three Phase Asynchronous Machine

WYE Connection

Three Phase Asynchronous Machine

Delta Connection

Three Phase Asynchronous Machine

Three 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.

N = 60 f / p f [Hz]: Frequency of the source

Ns = 60 f / p

p [ ]: 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,

Three Phase Asynchronous Machine

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)

Speed and Slip

then the electric machine is running as a generator

s = [(Ns – Nr)/ Ns ] 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 Machine

Efficiency – Speed – Torque Curves

Asynchronous Machine

Slip Ring Type

Losses in Electric Motors

Ohmic power loss of

Losses in Electric Motors

Losses in Electric Motors

Losses in Asynchronous Motors

P fw

P e

P m

P fe P

Friction and Air Flow They are constant losses during motor run,

independent from load and occur in bearings and

cooling fan propellers

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

Losses in Electric Motors

Losses in Asynchronous Motors

Pfw

P e

P m

PfeP

Conductor Loss (Stator)

It is heat loss The current flow through the stator

coils creats this loss (I 2 RS ).

Conductor Loss (Rotor)

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

cage bars creats this loss (I 2

RR ).

P S

P R

P XL

Additional 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

Power Calculations in Electric Motors

Power Calculations in Electric Motors

The nominal power of a DC motor might be expressed as theequation 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] isthe mechanical poweror the nominal power, ωm [RPM] is therotational speed of the axle of the motor, Tm [Nm] is the torquegenerated by the motor Ploss [W] is the power loss, η [%] is theefficiency of the motor

Pinput =UDC IDC

Poutput =ωm Tm

Ploss = Pinput − Poutput

Power Calculations in Electric Motors

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

Power Calculations in Electric Motors

Reactive (blind) Power

Power Calculations in Electric Motors

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]

φ

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 Electric Motors

Power Calculations in Three Phase Electric Motors

In a balanced three phase circuit:

P = √ 3 x U x I x cos φ

Q = √ 3 x U x I x sin φ

S = √ 3 x U x I

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 Electric Motors

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