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Electrical Sensing Devices

Electrical Sensing Devices. What are the basic functions of electrical control?. safety convenience comfort. Non-Automatic Controls. plug at end of cord snap switch magnetic switch. Parts of Automatic Devices. Sensing Element Switch. Types of Switches.

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Electrical Sensing Devices

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  1. Electrical Sensing Devices

  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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