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Transcript
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 magnetic switch Parts of Automatic Devices Sensing Switch Element Types of Switches Single (120v) Power Throw: break one line of wire Types of Switches Double (240v) Pole: break two lines of wire Types of Switches Three Pole: 3-Phase: Types of Switches Normally door bell Normally stop Open (NO) Closed (NC) button How are electric control switches classified? Number of poles: moveable contacts Number of throws: stationary contacts NO or NC 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 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 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 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 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 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 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