Electrical Machines Module SE3231

1
Electrical MachinesModule SE3231

• Lecturer
• Dr Vesna Brujic-Okretic
• Ext 9676 and 9681
• Email mes1vb_at_surrey.ac.uk

2
Energy Conversion

• due to the coupling action of magnetic and
  electric fields the conversion of mechanical to
  electrical energy is possible - and vice versa
• a simplified proof may be found if we calculate
  the electrical and mechanical power in the
  example from Lecture 2
• PEeiBlui W
• PM FextuBliu W
• in this ideal (lossless) case the electrical and
  mechanical energy are equal due to the energy
  conservation principle

3
Motor/Generator

• Lets use the same example, but assuming that in
  the loop there is a resistance, R, and the
  battery VB

-
4
Generator

• if the e.m.f. is greater than the battery
  voltage, VB, the current will be flowing in the
  direction shown on the sketch and the net
  voltage, V,
• V e - VB
• is being produced as a result of a mechanical
  motion
• eBlugt VB
• the system behaves like a generator

5
Motor

• for given values of B,l,R and Vb there exists a
  value of u for which the current will be positive
• but, if the velocity is lower than this value,
  then the current is negative and the conductor is
  forced to move to the RIGHT
• eBlult VB
• the battery acts as a source of energy and the
  system behaves as a motor
• in practice, we should consider the friction,
  inertia, elastic forces etc - certainly present
  on the mechanical side also, some inductance and
  capacitance on the electrical side

6
Rotating Machines

• rotational, rather than translational motion

7
Key Parts

• STATOR -
• provides a magnetic field either via electric
  current in the windings ferromagnetic core, or
  via permanent magnet
• static - does not move
• ROTOR -
• rotates inside the stator
• consists of a magnetic core and windings carrying
  the current (that produces its own magnetic
  field)
• mounted on a shaft - may be connected to
  mechanical load (or to a prime mover, in case it
  is a generator)

8
Basics

• Stator
• if the current serves the purpose of producing a
  magnetic field and is independent on the load it
  is called a magnetising current
• the related winding is termed field winding
• field currents - always DC and low power due to
  the presence of ferrous core - small current
  produces large magnetic field
• Rotor
• if the winding carries the load current it is
  called armature
• usually, separate windings carry field and
  armature currents but, there are types of motors
  where the same winding carries both (induction
  motor)

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Basics

• A machine acts as a GENERATOR if it converts
  mechanical energy from prime mover to electrical
  form
• Examples
• power-generating plants, automotive alternator
  etc.
• A machine is classified as a MOTOR if it converts
  electrical energy to mechanical form
• As shown in the figure it is the 2 magnetic
  fields
• the one from the rotor, BR
• and the one from the stator BS
• that are causing the machine to turn (rotate)
• magnetic attraction force permits the generation
  of torque

10
Electromagnetic Force - loop

• a conducting loop carrying current, I, in a
  constant magnetic flux density, B
• as a result - there are forces, F, acting upon it
  and forming a couple
• hence, torque T and a rotation

O
F
F
O
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Electromagnetic Force

• dFIdlxB force on the element dl of the segment
  L
• FBLI magnitude of total force on the segment
  L (if B I are constant)
• F, B dl are vectors, pointing as shown on the
  previous slide they form a couple
  hence, the torque
• TRxF T (torque) is a vector - a cross-product of
  the radius vector R and the force F (WRT a
  pre-defined reference point)
• T2RF is a magnitude of the total torque,
  where LAB and RAC/2 (previous slide)

12
Rotation
I

• Lorentz Force
• F on CD
• F on AB
• F on AB
• F on CD

I
I
I
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Commutator-Brushes System

• the torque is at its maximum when the angle
  between the rotor and the stator magnetic fields
  is 90º
• to keep this torque angle constant as the rotor
  spins, a mechanical switch, called COMMUTATOR is
  provided
• it ensures the current distribution in the rotor
  windings remains constant and the angle between
  BR and BS is constant, 90º (DC machines).
• in the following figure it is a split ring,
  rotating with a rotor
• BRUSHES are in contact with the commutator
  segments - they have definite polarities
  corresponding to the output voltage waveform

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Commutator-Brushes System
15

• 1 coil
• 2 coils
• 90 degrees
• apart
• resultant
• torque-
• minimum
• ripple

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Electric Motors - summary

• consist of stator and rotor
• stator - strong magnetic field
• rotor (also termed armature)
• cylindrical ferrous core
• rotates within the stator -
• carries a large number of windings (conductors)
• commutator rotates with the armature and consists
  of copper contacts attached to the ends of the
  windings
• brushes fixed to the motor casing - in contact
  with commutator - carry current to the coils
  resulting in the required motion

17
Armature

• traditional method stacked steel laminations
  slotted armature
• slots are not parallel to the motor shaft -
  skewing or skew winding

18
DC Motors

• The classification based on the generation of
  magnetic field
• Permanent magnet DC motors
• Separately excited
• Self-excited
• Series wound
• Shunt wound
• Compound-connected

19
Permanent Magnet DC Motor (PMDC)

• increasingly popular for applications requiring
  low torque and efficient use of space
• magnetic field of the stator produced by suitably
  located magnetic poles of magnetic materials
• no need for field excitation
• torque constant KPM depends on the geometry of
  the motor

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PMDC Features

• smaller and lighter
• high starting torque due to reduced armature
  reaction effect
• efficiency greater - no field losses
• essentially LINEAR torque-speed characteristics -
  easier to control
• reversal of rotation easy - just change polarity
  of the field
• Disadvantages
• can become demagnetised
• performance vary from motor to motor

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Permanent Magnet DC Motor

• Circuit model Steady-state characteristics

From K.V.L
a
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Permanent Magnet DC Motor

• Stall torque, T0, and no-load speed, w0 are

When Vs varies, torque-speed characteristics are
parallel straight lines (with the same
gradient) T0 and w0 change accordingly