Mine Equipment Design Selection and Maintenance

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Mine Equipment Design Selection and Maintenance

• Electrical Motors

Lecture 4
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What are electrical motors?
Motors are electro-mechanical devices that
convert electrical energy into mechanical.
Magnetic field
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Types of Electrical Motors

• DC
• Require DC power supply
• AC
• Require AC power supply
• Single phase
• 3-phase
• Universal Motor
• DC series motor connected to a single phase AC
  supply
• Behaves like a DC series motor

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Construction Details

• Stator
• Stationary part
• Rotor
• Rotating part
• Armature (winding)
• Winding which carries the main current
• Armature is placed on
• Rotor (DC motors)
• Stator (AC motors)

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Magnetic field in a DC machine
Q What is the role of the stator ?
A To create and provide path for magnetic
flux.
http//helium.ee.tut.fi/Computational_Electromagne
tics_files/animation/rot_motor.gif
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DC motor
Stator
Rotor
6 Pole wound
commutator
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AC motor examples
Squirrel-cage rotor
3-phase wound rotor
Slip-rings
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(No Transcript)
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DC motors vs AC motors

• Complex construction (commutator)
• more maintenance
• More Expensive
• requires DC source
• good speed control, but not so important any more
  due to electronic drives
• still so many already employed and produced

• 3-phase induction motors most commonly used in
  industry
• simple construction
• almost no maintenance
• cheep
• Uses 3-phase AC source
• smooth power flow
• poor speed control, but improving with new
  electronic drives

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DC motors

• Types
• self-excited (one DC source)
• shunt
• series
• compound
• separately excited
• permanent magnet (only small motors)
• second voltage source for field
• expensive
• in mines for speed control (Ward-Leonard System)

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Motor Speed

• Speed rpm
• directly proportional to supply voltage
• if voltage reverses polarity, the direction of
  rotation reverses
• inversely proportional to flux per pole
• Speed Control
• by varying armature voltage -armature speed
  control
• by varying field - field speed control

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Speed Control

• Armature speed control
• controls armature voltage
• Ward-Leonard System
• efficient (low losses),
• good speed regulation, can increase the speed
  above nominal
• for large motors
• expensive (additional motor-generator set)
• Armature rheostat
• only for small motors due to losses in rheostat
• this method can only reduce the speed below
  rated
• poor speed regulation i.e. speed drops with load

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Speed Control..

• Field speed control
• can increase speed above rated (up to 3 times)
• disadvantage is runaway condition
• due to accidental interruption in field current,
• field becomes very small (only remanent
  magnetism)
• can cause dangerously high speeds

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Starting a Motor

• Armature current decreases with the increase of
  the speed
• Current is the highest at standstill
• Can burn the armature winding if not protected
• Motor starters reduce the armature current while
  motor accelerates
• Manual starters -old!
• Automatic starters - for user just the matter of
  pressing the switch

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Stopping a Motor

• When switched off motor will continue to run
• Rapid stopping using breaking
• dynamic breaking
• external resistor connected in place of source
• causes armature current in the opposite direction
  producing torque in the opposite direction
• torque is proportional to current
• current depends on resistance
• plugging
• very rapid breaking
• reversing the polarity of the supply voltage
• current must be limited by an external resistor
  otherwise arcing may damage commutator
• less popular than dynamic breaking due to
  complexity

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Motor Characteristics

• Most important
• Torque - Speed Characteristic
• Determines the application
• Also important
• Torque - Armature Current

Cumulative compound
series
n
Differential comp.
shunt
T
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Efficiency

• Efficiency is defined as
• Pin EsIin (electrical input power)
• Pout Tw (mechanical power out)
• Losses
• copper losses (RI2) in armature and in field
• mechanical losses
• Losses increase machine temperature
• need for cooling
• resistance increases with temperature

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Efficiency Curve
efficiency
losses
Mechanical power
Rated power
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AC Motors

• Large industrial motors are 3-phase induction
  motors
• Two types
• squirrel cage
• robust, simple construction
• low cost and low maintenance
• wound rotor
• has a 3-phase winding both on rotor and stator
• rotor has brushes and slip-rings, but not
  commutator.

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Induction Motor

• Runs at speeds (n) slightly lower than
  synchronous speed
• synchronous speed
• depends on supply frequency (f 50Hz)
• and number of poles (P)
• Slip is defined as

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Torque-Speed Characteristic
Speed decreases with the load, therefore it is
not suitable for constant speed and variable
speed applications, but electronic speed
controllers available that can fix this problems,
except when wide range of speeds required (61).
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Rotor Resistance

• Rotor resistance can change the shape of T-n
  curve
• Increasing resistance can increase starting
  torque, but after it reaches pull-out torque
  value further resistance increase will decrease
  the starting torque

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Wound -Rotor Induction Motor

• More Expensive
• Advantages
• reduction of starting current (motor starter)
• speed can be varied by external resistors
• fast acceleration of high inertia loads

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Selection and Application of 3-phase Induction
motor

• More than 90 of all industrial motors are
  3-phase induction motors
• Uncommon applications
• Linear motors - magnetic levitation for fast
  trains
• Frequency changer

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Industrial Motor Control

• Control Devices
• Switches,Cam switches, push-buttons
• circuit breakers
• control relays, time delay relays, magnetic
  contactors
• thermal relays and fuses
• motor starters
• manual
• magnetic for remote starting and for large motors
• reduced-voltage starting (current reduced but
  torque also reduced)
• adding resistance
• plugging
• reversing polarity for rapid stopping, but
  running in reverse direction is prevented

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Industrial Motor Control..

• Reversing direction of rotation
• by reversing phase sequence
• Electric Drives - control motor operation
• Electronic Drives
• power electronic devices that adjust motor
  characteristic.