Relays and Contactors Module 030204a Flashcards
What are the THREE main parts of a relay?
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Coil.
Core.
Armature.
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What is the purpose of laminations in relays and contactors?
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Reduce affects of eddy currents.
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What is the purpose of shading coils in relays and contactors?
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Creates a magnetic flux out of phase with the magnetic flux of the main coil.
Reduces chattering.
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What are the THREE most used metals for relay contacts?
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Silver.
Copper.
Cadmium.
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List the Use, Advantage and Disadvantage of Silver Contacts.
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Use: Lower voltage and current applications
Advantage: Considered Self-cleaning as silver oxide is conductive.
Disadvantage: Soft metal so it can easily weld together. Cannot be used in high current applications.
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Page 25
List the Use, Advantage and Disadvantage of Copper Contacts.
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Use: High voltage and current applications.
Advantage: Can be used with higher spring pressures.
Disadvantage: Prone to pitting and oxidization. Cannot be used for frequent switching applications.
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Page 25
List the Use, Advantage and Disadvantage of Cadmium Contacts.
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Use: High voltage and current applications as well as frequent switching under load conditions.
Advantage: Can withstand strong spring tension, has a longer contact life and maintains constant contact resistance.
Disadvantage: Does not conduct as well as silver.
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Page 25
What is Contact Bounce?
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When contacts slam against each other with such force that they bounce open again. This causes additional arcing and reduces the lifespan of the contacts.
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Page 27
What is the purpose of Contact Springs?
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Used to absorb the impact of closing contacts.
Maintain pressure when the contacts are closed.
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Page 28
What is the purpose of Armature Springs?
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Used to quickly return the armature to the original state when the coil is de-energized.
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Page 29
What is the purpose of a curved contact design?
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Helps reduce contact bounce.
Allows for self-cleaning.
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Page 30
What are the advantages and disadvantages of bridge contacts?
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Advantages: Good contact alignment, reduced arcing, self cleaning contacts.
Disadvantages: More complex and increase the size of the contactor enclosure.
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Page 31
What can happen if contact springs are too loose?
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Will not maintain proper contact pressure which increases resistance and heat.
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Page 30
What can happen if contact springs are too tight?
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Will not properly absorb the shock of contacts closing. Results in contact bounce and possibly contacts becoming welded together due to arcing.
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Page 30
What can happen if the armature spring is too loose?
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Will not open the armature fast enough to safely de-energize a power circuit.
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Page 30
What can happen if the armature spring is too tight?
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Magnetic field may not be strong enough to pull the armature closed which could result in the coil burning out.
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Page 30
What percentage of voltage across a coil is needed to seal-in the armature?
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85% of the rated voltage of the coil.
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Page 35
What percentage of voltage across a coil will cause the armature to drop-out?
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Approximately 50% of the rated voltage of the coil.
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Page 35
Why happens to current when the armature seals against a coil and why?
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Inductance increases due to no air gap which increases impedance.
Current decreases due to the increased impedance.
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Page 35
What can happen if a higher voltage is applied to a coil than what it is rated for?
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Armature can snap in too quickly and damage the armature poles faces and contacts.
Can cause the coil windings to break down and coil burnout.
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Page 36
What is the maximum percentage of voltage that should be applied to a coil?
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Never more than 110% of its rated voltage.
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Page 36
What can happen if a lower voltage is applied to a coil than needed?
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Not enough magnetic flux to pull the armature closed causing a higher current and coil burnout.
Can also cause contact chattering resulting in contact pitting, carbon buildup and high contact resistance.
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Page 36
What are some reasons an armature may fail to drop out when the coil is de-energized?
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Residual magnetism in the armature.
Contacts are welded together.
Jammed armature.
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Page 38
What is the main difference between a relay and a contactor?
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Contactors are for higher current applications.
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Page 3
What are the FOUR basic relay categories?
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Power relays.
Plug-in relays.
Industrial control relays.
Low-voltage lighting relays.
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Page 4
What are power relays designed for?
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Small motors and heating applications that do not require frequent cycling.
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Page 4
What is the maximum that contacts are rated for in power relays?
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30 amps and 2 horsepower.
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Page 4
What relay is an open-type with exposed contacts?
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Power relays.
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Page 4
What are plug-in relays also referred to as?
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General-purpose relays.
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Page 5
What type of relay can be replaced quickly without loosening wire terminals?
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Plug-in relays/General-purpose relays.
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Page 5
What are plug-in relays rated for?
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Rated for up to 15 amps and small motor loads under 1/2 horsepower.
Intended for control circuits.
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Page 5
What are the two types of pins on plug-in relays?
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Tubular - often arranged in a circle.
Spade - often arranged in rows and columns.
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Page 5
How many pins are typically available for plug-in relays?
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5, 8, 11, or 14.
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Page 5
How many pins are associated with each set of NO/NC contacts and coils on plug-in relays?
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Three pins for every set of NO/NC contacts
Two to Three pins for every operating coil.
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Page 5
What type of relay is a latching relay?
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Plug-in type.
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Page 5
What is the purpose of Industrial Control Relays?
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Heavy-duty applications.
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Page 6
What is the purpose of adder decks?
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Allows you to increase the amount of contacts for relays.
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Page 6
What is the operating voltage of low-voltage lighting relays?
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30V or less.
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Page 7
What does a contactor need to be combined with to be used as a motor starter?
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An overload device.
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Page 8
What is a reversing contactor?
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Two non-reversing contactors with a mechanical and electrical interlock between them.
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Page 9
What are mechanically held contactors often used for?
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Provide central lighting control of large lighting installations.
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Page 11
How many coils does a mechanically held contactor have?
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Two coils.
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Page 11
What are definite-purpose contactors controlled by?
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Automatic thermostat or manual push-button interface.
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Page 11
What is the Form A contact arrangement?
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SPST-NO (Single-pole, single-throw normally open)
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Page 14
What is the Form B contact arrangement?
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SPST-NC (Single-pole, single-throw normally closed)
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Page 14
What is the Form C contact arrangement?
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SPDT (Single-pole double throw)
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Page 14
What is the Form X contact arrangement?
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SPST-NO-DM (Single-pole, single-throw normally open double make (bridge contact))
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Page 14
What is the Form Y contact arrangement?
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SPST-NC-DB (Single-pole, single-throw normally closed double break (bridge contact))
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Page 14
What information can be found on a nameplate for a relay or contactor?
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Coil Ratings.
Contact Ratings.
Contact style and arrangement.
Contact terminal numbering.
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