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IT 435 Digital Instrumentation and Control

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Series wound - high torque at low speeds. Shunt wound - constant torque over speed range ... Soft steel and like gear teeth. Electromagnets arranged around the ... – PowerPoint PPT presentation

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Title: IT 435 Digital Instrumentation and Control


1
IT 435 Digital Instrumentation and Control
  • Week 5
  • Chapters 9, 10 and 12

2
Describe the magnetics motor principle
  • Unlike fields attract, Like fields repel
  • Electrical current flow creates magnetic field
  • Attraction and repulsion creates rotation

3
List the parts of an industrial motor
  • Rotor - Armature
  • Stator - Field Coil and Field Poles
  • Laminations - Retard eddy currents
  • Cooling fan
  • Brushes
  • Minimum gap between rotor and stator

4
Explain the operation of a DC motor
  • Magnetic fields create repulsion
  • Rotary motion causes fields to shift polarity
  • Brushes transfer current from stationary to the
    rotating armature
  • CEMF - Counter Electromagnetic Force
  • Resists build up of magnetic field
  • Although DC is required, will run on AC
  • In order to reverse direction of rotation,
    reverse commutator connections

5
DC Motor Components
6
DC Motor
  • Types and advantages
  • Series wound - high torque at low speeds
  • Shunt wound - constant torque over speed range
  • Compound - combines series and shunt
    characteristic
  • With electronic control - flat torque versus speed

7
DC Motor speed control
  • Problems related to CEMF
  • Rheostat to vary supply voltage
  • Staged starting to get to speed
  • SCR - Silicon Controlled Relay
  • Effects pulse width
  • Pulse width modulation
  • 555 and appropriate Rs and Cs

8
DC Motor Control
  • Rheostat
  • AutoTransformer
  • Pulse Width Modulation
  • Slicon Controlled Rectifier

9
Pulse Width Modulation
10
Describe the function of a silicon controlled
rectifier
11
Describe the function of a silicon controlled
rectifier
  • SCR - Useful device for rectifying AC
  • Converting it to DC
  • Blocks lower half
  • Can invert signal
  • Can be controlled to pass a limited amount of
    electricity
  • Modern capable of high power
  • Extremely useful for controlling modern motors

12
Motor Directional ControlH Drive
13
Choice of motor for different applications
  • DC
  • Torque over wide speed range
  • More costly
  • More complicated
  • Require variable DC current
  • Have brushes
  • AC
  • Induction (no wires on armature)
  • Constant speed
  • Three phase
  • Smallest size per HP

14
Explain the operation of an AC motor
  • Magnetic field induced into the rotor
  • Squirrel Cage Rotor
  • Field rotates with AC signal
  • Magnetic attraction drags the rotor around
  • Speed is determined by the speed (frequency) of
    the AC signal
  • Synchronous speed
  • Asynchronous speed infers slip

15
AC Motor Components
16
AC Motor - Single Phase
  • Usually less than 1 HP
  • Generally use capacitor to start
  • Lags the driving voltage
  • Creates a electrical slip
  • Centrifugal switch to add in starter winding

17
AC Motor speed control
  • Capacitor start
  • Varying frequency supply
  • Controller to create desired frequency
  • Use high speed switching
  • Uses IGBT - Integrated Gate Bipolar Transistor
  • Problems with power spiking
  • Becoming approach of choice

18
AC Motor Variable Frequency Speed Control
19
AC Motor Variable Frequency Speed Control
20
AC Motor Variable Frequency Speed Control
21
AC Motor - Three Phase
  • Three separate coils
  • Three separate signals

22
3 Phase Variable Frequency Drive
23
3 Phase Variable Frequency Drive
24
Stepper Motors
  • Simple electromagnet device
  • Rotor made of several magnets
  • Variable reluctance
  • Soft steel and like gear teeth
  • Electromagnets arranged around the circumference
  • Separately powered
  • Electrical magnetic attraction causes rotor
    magnets to align with the electromagnets

25
Stepper Motor
26
Stepper Motor
  • Accurate angular location of the rotor
  • Error only associated with position of magnet and
    electromagnet
  • Specific number of steps per revolution
  • Holding power limited by electromagnet and magnet
    strength
  • Usually driven by square wave
  • Rotation speed function of signal and switching
    frequency

27
Stepper Motor Parameters
  • Step response rate (milli-seconds)
  • Stepping rate - 300 normal, 800 max
  • Step angle
  • Step angle accuracy
  • Overshoot
  • Slew rate - max rotational speed

28
Stepper Motor
  • Picture of thin stepper rotor

29
Synchros and Resolvers
  • Three fixed stators (Like a motor field)
  • Single rotor coil
  • Location of the coil indicates angular location
    of the rotor
  • Used to determine angular location of a shaft
  • Resolvers have two coils on the rotor and stator

30
Servo Motors (torque)
  • High torque small size motors
  • Effectors on robots
  • Machine table movements
  • No direct feedback of angular location
  • Used with
  • Resolver
  • Optical encoder

31
Linear Motor
  • Unrolled stator - stationary
  • Moving rotor - on moving device

32
Motor Control Terms
  • Jogging - Momentary on and off with one button -
    inching
  • Sealing - two button operation
  • Start/stop with hold-in rung
  • Plugging - forward/reverse motor operation
  • To remove jam

33
AC Motor speed control
  • Capacitor start
  • Varying frequency supply
  • Controller to create desired frequency
  • Use high speed switching
  • Uses IGBT - Integrated Gate Bipolar Transistor
  • Problems with power spiking
  • Becoming approach of choice

34
AC Motor Variable Frequency Speed Control
35
AC Motor Variable Frequency Speed Control
36
AC Motor Variable Frequency Speed Control
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