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