Final Exam Flashcards
How is Bimetallic strip made?
By bonding 2 dissimilar metals together
Why should electrical contacts never be permitted to open or close slowly
Produces poor contact pressure and causes contacts to burn
Two factors that determine the amount of voltage produced by a thermocouple?
1) the type of materials used
2) the temperature difference of the two junctions.
What do the letters RTD stand for?
Resistance temperature device
Type of wire used for RTD
Platinum
What material is a thermistor made of?
Semi conductor material
How can a silicon diode be made to measure temperature?
A constant current is passed through
What type of chemical is used to cause a pressure change in a bellows type thermostat?
Refrigerant
What two factors determine the polarity of the output voltage produced by a hall generator ?
1) the direction of current flow through semi conductor material
2) the polarity of magnetic fields
What two factors determine the amount of voltage produced by a hall generator?
1) the amount of current flow
2) the strength of magnetic field
What is a reluctor.
Is a ferrous metal disk used to shunt a magnetic field
Proximity detectors are basically ______?
Metal detectors
What type of electric circuit is used to increase the sensitivity of the proximity detector
Tank circuit
List 3 major categories of photodetectors
A) photovoltaic
B) photoemissive
C) photoconductive
Which category does the solar cell belong
Photovoltaic
Which category do phototransistors and photodiodes belong?
Photomissive
The term cad cell is a common name for what device ?
Cadmium solfide cell
What is the function of the transmitter in a photodetector unit ?
The light section
What is the advantage of a photodetector that uses a reflector to operate?
Needing electrical connection at only one piece of equipment
How much voltage is produced by a silicon solar cell ?
.5V
What is the operating voltage of most magnetic control systems?
120v
How many primary windings do control transformers have?
2
How are the primary windings connected when the control transformer is operated on 240V
Parallel
How are the primary windings connected when the transformer is 480V
Series
Why are two of the primary leads crossed on a control transformer
To make connection to primary easier
Two basic classifications of timers
On delay
Off delay
Explain the operation of an on delay relay?
When power is connected the contacts delay changing position
Explain the operation of a off delay relay
When voltage is applied the contacts immediately from open to closed
What are instantaneous contacts
Change their position immediately when energized, and immediately back when de energized
How are pneumatic timers adjusted ?
Needle valve
Name two methods used by electronic timers to obtain their time base
RC time constant
Crystal controlled clock
What is meant by the statement “solenoid valves are digital device ?
Completely ON or completely OFF
Why is it important the the inlet and outlet ports on a two way valve not be reversed?
Pressure of the inlet liquid and gas is used to maintain a seal
What type of valve is generally used to supply air pressure to a double acting cylinder ?
Four way valves
What two sections are generally used to operate motor operated valves ?
Local and remote
What type of switch is used to ensure motor operated valve is tightly seated
Special limit switch
Name three methods of braking a motor ?
Mechanical, dynamic, plugging
How is braking force of drum brakes controlled ?
Break shoes
Why are mechanical brakes used on cranes?
Because they can hold suspended loads
What is a advantage of dynamic over mechanical brakes
No brake shoes to wear out
What is a disadvantage of dynamic over mechanical
Can’t hold suspended loads
The amount of counter torque developed by a direct current generator is proportional to what?
To armature current
When using dynamic braking for a direct current motor, how is the braking time controlled ?
Adjusting resistance valve
Name three factors that determine the amount of induced voltage
Strength of magnetic field
Speed of cutting action
Length of conductor
How is a plugging stop accomplished ?
By disconnecting motor from power and momentarily reversing rotation
What device is generally used to accurately stop a motor when a plugging stop is used ?
Zero speed switch
What is the most common device used to disconnect the start winding of RS single phase motor
Centrifugal switch
What electronic component is used in the construction of a solid state starting relay ?
Thermistor
What percentage does a centrifugal switch disconnect
75%
Why is it detrimental to disconnect the auto transformer before motor reaches run stage
Conserve energy
