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Electrical Sensing Devices

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Title: Electrical Sensing Devices Subject: controls Author: Plentywood School Last modified by: Dr. Frank Flanders Created Date: 2/19/1998 7:49:26 AM – PowerPoint PPT presentation

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Title: Electrical Sensing Devices


1
Electrical Sensing Devices
  • Original Power Point Created by Casey Osksa
  • Modified by Georgia Agricultural Education
    Curriculum Office
  • June 2002

2
What are the basic functions of electrical
control?
  • safety
  • convenience
  • comfort

3
Non-Automatic Controls
  • plug at end of cord
  • snap switch
  • magnetic switch

4
Parts of Automatic Devices
  • Sensing Element
  • Switch

5
Types of Switches
  • Single Throw break one line of wire (120v)

6
Types of Switches
  • Double Pole break two lines of wire (240v)

7
Types of Switches
  • Three Pole 3-Phase

8
Types of Switches
  • Normally Open (NO)
  • door bell
  • Normally Closed (NC)
  • stop button

9
How are electric control switches classified?
  • Number of poles moveable contacts
  • Number of throws stationary contacts
  • NO or NC

10
How does a Mercury Switch operate?
  • switch is opened or closed by liquid mercury in a
    closed tube
  • as tube is tilted, mercury makes contact with
    wires, activates switch
  • as tube is tilted opposite, mercurey moves away
    from wires, breaks circuit
  • livestock water tanks

11
What is an overload protection device in a switch?
  • safety protection
  • snap switch is a thermal overload device
  • often have a small light on switch which
    indicates when a motor is running

12
What is a relay device?
  • control a circuit carrying a large current by
    means of a low-current circuit
  • photoelectric cell
  • Advantages
  • smaller control wires
  • less expensive
  • large distances between switch and load
  • low current in switch, lasts longer
  • can sequence certain loads

13
Parts of a relay
  • Solenoid or electromagnet
  • coil and a movable armature
  • set of contact points
  • Two electronic circuits
  • low voltage control circuits
  • load circuit

14
What is an Electro-Magnetic Switch?
  • similar to a relay, without the low voltage line
  • creates a fast make and break circuit
  • easy to apply remote control
  • easy to add overload protection to switch
  • car or tractor starting switch

15
Sensing Devices
  • Thermostats air temperature
  • Humidistats changes in moisture humidity
  • Time control clock
  • Photocell light intensity
  • Each reacts to a specific condition
  • High or low voltage circuits

16
Thermostats
  • Bimetallic
  • different metals expand at different rates
  • Liquid-gas
  • gas filled bellows expand/contract
  • Hydraulic
  • same as liquid-gas, uses liquid
  • Heating or Cooling
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