Transcript 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
 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