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
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AC Motor Variable Frequency Speed Control
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AC Motor Variable Frequency Speed Control
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AC Motor - Three Phase

• Three separate coils
• Three separate signals

22
3 Phase Variable Frequency Drive
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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
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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