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Electrical Sensing Devices

Electrical Sensing Devices. Original Power Point Created by Casey Osksa Modified by Georgia Agricultural Education Curriculum Office June 2002. What are the basic functions of electrical control?. safety convenience comfort. Non-Automatic Controls. plug at end of cord snap switch

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Electrical Sensing Devices

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  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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