AC Machines
Mr. N. M. Malinga
Outline
• Types of AC machines
• Basic construction of induction motors
• Single phase induction motors
• Three phase induction motors
Types of AC machines
• Dfn: electromechanical rotating machine that converts mechanical
energy to AC electrical energy or convert AC electrical energy to
mechanical energy.
• Divided into two categories:
✓Synchronous machines
✓Induction machines
• Synchronous machines are such that rotor rotates at the same speed
as the rotating magnetic field produced by the stator , and
• Induction machines are such that the rotor rotates at a less speed
produced by the stator’s rotating magnetic field (asynchronous)
• Can either be single phase or three phase
Basic construction of induction motor
• There is no electrical connection between the stator and the rotor, the energy is
transferred entirely magnetically by means of the EMF induced in the rotor conductors
by the rotating field which is set up by the stator.
• Constitutes of the following main parts :
✓ Stator
✓ Rotor
✓ End plates
a) Stator
✓Stationary part of the motor‘s electromagnetic circuit.
✓Made of steel laminations are tend to reduce energy losses that would result if a
solid core were used.
✓ Constitute of stator frame, stator core and stator windings.
✓When connected directly to the power source, they become an electromagnet when
current is applied,
✓Generally choice of material is steel to keep down hysteresis losses
a) Stator frame
• provide mechanical protection for the field and the rotor system
• Provide mechanical support for internal parts
b) Stator core
• has series of thin plates that are electrically insulated from one another and are clamped
together to form a laminated structure
Q - Why laminated ?
c) Stator windings
• Fit into slots cur into the circumference of the core and inserted into the stator frame
• Produce main rotating magnetic field when connected to a three phase supply
• Can either be star connected or delta connected and these connections are done in the
terminal box.
Star connected windings
Delta connected windings
b) Rotor
✓rotating part of the motor‘s electromagnetic circuit.
✓Divided into TWO: squirrel cage rotor and wound rotor
1. Squirrel cage rotor
• Most common type of rotor used in a three-phase induction motor is a squirrel cage
rotor , Why ?
i.
ii.
iii.
iv.
Simple construction
Require no maintenance
Operation is smooth and silent
Prolonged lifespan
• Has slightly skewed slots to reduce magnetic noise and produce a smoother operation.
• Solid copper or aluminum are inserted into the slots and the rotor bars are then shortcircuited at the ends of the rings .
• Used in motors that drive lathes, fans, and blowers and also in single phase induction
motors
2. Wound rotor
• Also called the wound rotor (slip ring rotors) or phase wound rotors
• Rotor windings are placed inside the slots of the rotor and star connected or one end of
each winding is bridged out
• Other end of each winding is connected to three slip rings on the same rotor shaft.
• Allows for additional resistance to be introduced into the rotor ckt via brushes.
• Advantages:
✓ Has high starting torque
✓ High starting currents can be limited
✓ Speed can be controlled by simply varying the resistance added to the rotor ckt
• Applications: drive hoists , cranes and elevators
c) Bearings, end plates and end shields
✓protects the internal parts of the motor (such as bearings) from
water and other environmental elements.
Advantages and disadvantages of a three phase induction motor in
relation to a single phase induction motor
Basic operation of an induction motor
• Reversing direction of rotation of a three phase induction motor :
Interchange any two phases of the induction motor
• Applications
✓Pumps
✓Compressors
✓Elevators
✓Large blower fans
Reversing direction of rotation:
Testing of a three phase induction motor
Why ? Establish condition of the motor
• Tests performed on the stator and they include :
1. Winding resistance test (open and short circuit test) – test continuity of
windings (resistance must be within the 10% tolerance range)
2. Earth continuity test – check if there is a good connection btn the stator
frame and earth stud found inside the terminal box
3. Insulation test between windings – test effectiveness of insulation between
windings (Minimum acceptable reading should be 1 Mega Ohm)
4. Insulation tests between windings and earth - test effectiveness of
insulation between the three windings and the stator frame or earth stud
(Minimum acceptable reading should be 1 Mega Ohm)
Single Phase Induction Motors
• Used in gardens, household appliances and in workshops
• Types of Single phase induction motors:
1. Split phase motors:
✓ Resistance start induction run motors (lower torque)- fridges , blowers
✓ Capacitor start induction run motors (higher torque)- swimming pool pumps, electric lawn
mowers
✓ Capacitor start capacitor run motors (highest torque)- air conditioners
2. Shaded pole motors
3. Universal motors
Construction and operation split phase motors
• Non - self starting since single phase supply cannot produce a rotating
magnetic field.
• Converted into a two phase motor for staring purposes only.
Q – How?
✓Connect a starting or an auxiliary winding in parallel to the main or running
winding. These windings are displaced at 90˚ from each other.
✓To increase their starting torque, a capacitor is connected in series with the
auxiliary windings
Assignment :
Reversing the rotation of an induction motor