For all machines

1
For all machines

• Basic concepts of construction and operation
• How to start machine?
• How to calculate the circuit parameters?
• How to achieve voltage (generator) or speed
  (motor) control?
• The power flow diagram and the power losses

2
Review of Motor Concepts

• A motor converts ?
• Electrical Energy ? Mechanical Energy
• Basic Construction of a motor?

ROTOR
AIR GAP
STATOR
3
Review of Motor Concepts

• Basic Operation of a motor?
• Produce Stator Magnetic Field, BS
• Produce Rotor Magnetic Field, BR
• ?ind kBR x BS
• In other words, as BS rotates, BR will chase BS
  and this induces a torque that causes the rotor
  to rotate at speed, ?

Torque Induced
4
Chapter 7 Induction Motor

• EEEB 283
• Semester 2 Year 2006 / 2007
• Aishah Mohd Isa

5
Induction Motor -concept

• How do we get BS in induction motor?
• Apply 3 phase sinusoidal current to stator
  windings
• How to get BR in induction motor?
• Current is induced on rotor winding by stator
  magnetic field
• ? Torque is induced

6
Induction Motor- CoNcept

• On stator 3 ? IS ? BS
• On rotor IR ? BR
• ? By definition, an induction motor is an AC
  machine where no direct field current is supplied
  to its field windings, instead field current is
  induced by the stator magnetic field.

?ind
7
Induction motor construction

• Stator is the hollow cylindrical core with slots
  for windings
• Rotor
• revolves inside the stator
• has slots for windings
• has two types of construction
• Squirrel Cage Rotor
• Wound Rotor

8
Cage Rotor construction
Casing
Conductor shorting rings
Stator windings
Fins to cool the rotor
Embedded rotor conductors
Iron core
Shaft

• Squirrel Cage Rotor has conducting bars shorted
  at the end by shorting (or end) rings

9
Wound Rotor Construction
Stator windings
Rotor windings
Slip rings
Shaft
Stator winding connections to power source
Brushes

• Wound rotor has a set of 3-phase windings,
  usually Y-connected with the ends tied to slip
  rings. Rotor windings are shorted by brushes
  riding on the slip rings.
• Wound rotor is more expensive and requires more
  maintenance

10
Principle of operation

• 3 phase winding on stator produces BS and speed
  of rotation for BS is
• where ?sync synchronous speed of BS
• fe electrical frequency
• Pnumber of poles
• BS induces current and voltage (eind v x B . ?)
  on the rotor bars which will then produce BR
• Motor action comes from ?ind kBR x BS

11
PRINCIPLE OF OPERATION

• BS BR
• ?ind kBR x BS
• Note that if rotor speed is the same as stator
  magnetic field speed,
• eind 0, IR 0, BR 0 and ?ind 0
• ? Induction motor is operated NEAR synchronous
  speed but not at synchronous speed

?
12
Rotor Slip

• Stator magnetic field speed is known as
  synchronous speed, ?sync
• Rotor speed is known as motor speed, ?M
• Rotor bar induced voltage is dependent upon the
  relative speed between ?sync and ?M.

13
Rotor Slip

• Slip speed, ?slip ?sync - ?M.
• Slip,
• If rotor is stationary, ?M 0, s 1. This
  condition is also known as locked rotor
  condition.
• If rotor is at the same speed as BS,?M ?sync ,
  s0.
• Note that 0ltslt1 and ?M (1-s) ?sync

14
Electrical frequency of rotor

• An induction motor is like a rotating
  transformer, i.e. Stator (primary) induces
  voltage in the rotor (secondary)
• Stator winding electrical frequency comes from
  its 3 phase windings, thus stator frequency, fS
  fe
• For locked rotor condition, ?M 0 and s1,
• When rotor at synchronous speed, ?M ?sync and
  s0,
• For 0lt ?M lt ?sync ? fR sfe

fR fe
fR0
15
Summary

• 3 ? Is ? BS?IR?BR
• kBR x BS? ?ind
• ?slip ?sync - ?M.
• ?M (1-s) ?sync
• fR sfe

Vaa -
16
Example

• A 220 V 15 hp, 8, 50 Hz pole induction motor has
  full load slip of 10. (1hp746W)
• Find
• ?sync
• ?M.
• fR.
• ?ind

17
answer

• a)
• b) ?M (1-s) ?sync
• (1-0.10) 750
• 675 rpm c) fR s
  fe
• 0.10 x 50 5 Hz

• d) ?load Pout / ?M

18
Induction motor Equivalent Circuit

• Induction motor operation relies on rotor
  voltages and currents induced by the stator
  circuit
• This is similar to transformer action.
• ? Induction motor equivalent circuit is similar
  to transformer equivalent circuit

19
Induction Motor Equivalent Circuit
Stator
Excitation Branch
Rotor
20
Induction Motor Equivalent Circuit

• For Primary Side
• Essentially the same as the transformer
• R1 Stator winding resistance
• jX1 Stator winding leakage reactance

21
Induction Motor Equivalent Circuit

• The flux in the induction motor depends on the
  integral of the applied voltage, E1 .
• The magnetization curve for the induction motor
  is as shown.
• Notice that the slope for induction motor is
  shallower than the one for transformer.
• This is due to the air gap reluctance which
  requires more magnetomotive force to obtain a
  certain flux level

22
Induction Motor Equivalent Circuit

• The excitation current is modeled by RC and XM

23
Induction Motor Equivalent Circuit

• The primary stator voltage E1 is coupled to
  secondary rotor voltage ER by effective turns
  ration, aeff
• Voltage ER produces current flow in the rotor
  circuit

24
Induction Motor Equivalent Circuit

• The primary impedances and magnetising current in
  the induction motor is very similar to
  corresponding components in a transformer
  equivalent circuit.
• Difference comes from the effect varying rotor
  frequency on rotor voltage and rotor impedances.

25
Induction Motor Equivalent Circuit

• The Rotor Circuit Model
• When voltage is applied to the stator windings,
  voltage will be induced in the rotor circuit.
• The amount of induced rotor voltage is dependent
  upon slip.
• In general, as the relative motion between the
  rotor and the stator magnetic fields increases,
  the resulting rotor voltage and rotor frequency
  increases. (s?, ER?, FR?)

26
Induction Motor Equivalent Circuit

• Components in the rotor circuit model that
  depends on slip are
• Rotor induced voltage, ER sER0
• Rotor frequency, frsfe
• Rotor windings leakage reactance, XRsXR0 where
  XR 2?fLR
• Rotor windings resistance, RR is the only
  constant component

27
Induction Motor Equivalent Circuit

• At locked rotor conditions (?M 0, s1)
• ER0 sER ER ? maximum voltage induced
• fr sfe fe
• XRsXR0 XR0 2?fLR 2?feLR
• At synchronous conditions (?M ?Sync, s0)
• ER 0? no voltage induced
• fr sfe 0
• XRsXR0 2?fLR 0

28
Induction Motor Equivalent Circuit

• As before
• ER sER0
• frsfe
• XRsXR0

29
Induction Motor Equivalent Circuit

• Referring to primary side
• E1 aeff ER0
• R2 aeff2 RR
• X2 aeff2 XR0

30
Final Equivalent circuit model
I? I1
jX2
j

V?
R2/S
-
31
Power and Torque

• What is the input and output of an ideal
  induction motor?

Input Power (electrical)
Output Power (mechanical)
32
Power and Torque

• Power losses on the stator side
• Stator Copper Loss, PSCL 3 I12R1 at the stator
  windings
• Core Losses, PCORE, consists of Hysterisis and
  eddy current losses.
• As power is transferred through the air gap to
  the rotor, power loss as air gap power PAG 3
  I22R2/s ?ind ?sync

33
Power and Torque

• Power losses on rotor side
• Rotor copper loss PRCL I22R2 at the rotor
  windings
• As power converts from electrical to mechanical
  power, power loss as
• For electrical
• For mechanical

34
Power and Torque

• Miscellaneous losses
• Friction and windage losses, PFW
• Stray losses, Pstray

35
Power Flow Diagram
PSTRAY
PFW
36
Power and torque

• Please note that at higher speeds (?M?sync),
  core losses are lumped with the miscellaneous
  losses as rotational losses,
• ProtationalPcorePstrayPFW

37
Power Flow Diagram
P rotational
38
Losses at the Circuit
PSCL
PAG
PCORE
39
Losses at The Circuit

• But PAGPRCLPCONV

40
Losses at the Circuit
PSCL
PRCL
PCORE
PCONV
41
Induction Motor TorQue Speed Characteristics

• How does speed vary when shaft load increases?
• Case I No Load
• ?M?sync ? s 0
• Since s is very small, relative motion small
• fr?, Er ?, iR ? , BR ?
• - fr small, X2 2? fr L small, R2 /s big, current
  in phase with ER
• since IR induces torque, torque induced is very
  small and just enough to overcome rotational
  losses

42
Induction Motor TorQue Speed Characteristics

• Case II With Load
• ?Mgt?sync ? slip increases
• As slip inceases, ? fr?, Er ? iR ? BR ?
• fr large, X2 2? fr L large, R2 /s small, current
  lags ER
• since IR induces torque, torque increases too ?
  ir?, ?ind ?
• Pullout Torque
• - If load too high, there comes a point where iR
  lags Er too much that the torque starts to
  decrease. This is the pullout or maximum torque.
• - After that point, torque will decrease

43
Induction Motor Torque Speed Characteristics
Torque Induced
Synchronous Speed
44
Torque Speed Characteristic

• Low Slip Region
• As load increases, slip increases almost linearly
  (speed reduces)
• Power factor nearly unity (XR0)
• Torque increases as load increases
• Linear region is normal steady state operating
  range

45
Torque Speed Characteristic

• Moderate Slip Region
• Relative motion is higher, fR higher
• XR RR
• Pullout torque occurs in this region
• High Slip Region
• If load increased in this region, torque
  decreases and stops

46
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47
Induction Motor Torque Speed Curve

• At synchronous speed, ?ind 0.
• The curve is linear between no load and full
  load.
• The maximum torque is known as pullout torque or
  breakdown torque. It is approximately 2 to 3
  times the rated full-load torque of the motor.
• The starting torque is slightly larger than its
  full-load torque. So, IM will start carrying any
  load it can supply at full power
• Torque for a given slip varies as square of the
  applied voltage. This is useful as one form of IM
  speed control.
• If rotor is driven faster than synchronous speed,
  machine becomes a generator
• If motor is turning backward relative to the
  direction of magnetic fields (achieved by
  reversing the magnetic field rotation
  direction)?ind will stop the machine very rapidly
  (braking) and try to rotate in the other
  direction.

48
Induced Torque Equation

• How to find PAG?
• PAG 3 I22R2/s, so need to find I2
• To find I2 simplify the stator circuit to get the
  Thevenin Equivalent

49
Thevenin Values
50
Pullout torque

• Occurs when PAG maximum
• PAG incurred from R2/s
• For maximum power transfer

Magnitude of impedance
Magnitude of source impedance
51
Pullout Torque

• At maximum power transfer, slip max
• Substitute into induced torque equation

52
Variations in Induction Motor Torque Speed
Characteristic

• High R2
• High starting torque
• High slip
• But Pconv (1-s) PAG
• Efficiency low at normal operation
• Low R2
• Low starting torque (high starting current)
• Low slip
• Efficiency high at normal operation
• What do we need to do to get desired curve?
• High R2 at start but low R2 at normal operation

53
Rotor designs

• For wound rotor?
• For cage rotor?
• Design A - Large bars near surface
• Design D - Small bars near surface
• Design B - Variable rotor resistance
• Design C - Variable rotor resistance

54
Starting induction Motor

• -can connect straight to power line but this will
  cause voltage dip in power system due to high
  starting current required
• For wound rotor?
• Insert extra resistance during startup
• For cage rotor?
• Determine current required from code letter
  factor
• Either
• insert extra inductors or resistors into power
  line during startup
• Reduce VT using transformers

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56
Autotransformer
3 phase power system
1. Close 1 and 3
2. Open 1 and 3
3. Close 2
1
1
2
2
1
2
3
3
Induction Motor
57
Induction Motor Protective Circuits
58
Induction Motor Protective Circuits

• Start Protection Circuit
• When igtirated, fuses F1,F2 and F3 will melt
• Overload Protection
• OL heater elements and OL contacts will react
  when temperature is too high
• Undervoltage Protection
• When line voltage lt rated motor terminal voltage

59
Speed Control of Induction Motors

• Vary synchronous speed, ?sync
• Change electrical frequency,fe
• Change number of poles, P
• Vary slip,s
• Change terminal voltage, VT
• Change rotor resistance, R2

60
Determining Circuit Model Parameters

• How to find R1, R2, X1, X2 and XM ?
• Use three tests No Load Test, DC Test, Locked
  Rotor Test

61
No load Test
62
DC Test
63
Locked Rotor Test