Relays and Contactors Flashcards
What are the four basic electromagnetic relay categories?
Plug in (general purpose)
Industrial control
Power
Low-voltage lighting
What is the major difference between a contactor and a relay?
A contactor is constructed to handle larger power loads.
Must the rated voltage of the coil and contacts of a relay or contactor be the same value?
It is not required to be.
What are the maximum ratings of plug in relays?
10-15 Amps and 1/2 hP.
For every set of NO/NC contacts in a general purpose relay, how many pins are there?
3 pins.
1 common, 1 NC, 1 NO.
What device is attached to an industrial control relay to increase the number of contacts available?
Adder decks.
What kind of contacts are normally found in industrial control relays?
Bridge contacts. (double make/double break).
What are the maximum ratings of power relays?
30 Amps and 2 hP.
What is a significant hazard associated with power relays
They have exposed contacts.
What classification of voltage do low-voltage lighting relays operate under?
Extra-low voltage.
Less than 30 V.
Low-voltage lighting relays utilize which mechanical feature to only require momentary energization?
Latching.
Does a contactor contain overloads?
No.
An overload relay is a separate device.
What are the two main classifications for contactors?
NEMA (American) and IEC (European).
What are important factors when sizing an IEC contactor?
Duty cycle and whether it is used for jogging or reversing
What is a reversing contactor?
Two regular contactors mechanically and electrically interlocked by a block device, so that only one contactor may operate at one time.
What is the primary purpose of mechanically held contactors?
To reduce coil power consumption as it only requires momentary energization.
This is used in large office or retail buildings.
What specific ratings would a contactor coil have?
Voltage to be applied.
What specific ratings would contactor contacts have?
Horsepower Voltage (frequency) Current (continuous) NEMA pilot duty IEC control circuit classifications.
What is meant by form x, when related to contact arrangement?
Single pole, single throw normally open double make contacts.
What is meant by form C, when related to contact arrangement?
Single pole, double throw contacts.
What are bifurcated contacts?
A contact offering parallel current paths, with twice as many contact points.
What are the three main parts of a contactor?
Coil
Armature
Core (high permeability material).
What is the purpose of “laminating” the core and/or the armature of a contactor?
To reduce magnetically induced eddy currents.
If a contactor core was without “shading coils”, what would be the expected result when energizing that coil?
The contacts would hum or chatter at the frequency of the source current.
What useful function do “shading coils” provide?
An out of phase magnetic flux, which helps seal in the armature during times of 0 normal flux.
What are silver contacts used for?
Low current, low voltage, frequent switching.
What are copper contacts used for?
High current, high voltage, infrequent switching.
What are Cadmium plated contacts used for, and what is the major disadvantage of these?
High current, high voltage, frequent switching.
They do not conduct as well as silver or copper contacts.
What is the purpose of contact springs?
To absorb the shock of the armature pulling in, thereby reducing contact bouncing. They also provide firm pressure to keep resistance low.
Incorrect spring tension may result in…?
Increased resistance at the contact face or excessive arcing (possibly welding) due to bounce.
What are two primary benefits of “wiping action”?
Self cleaning.
Arcing confined to the tips (prolonged contact life).
What are two primary benefits of bridge type contacts?
Two points of contact splits the arc voltage.
Provide good contact alignment (seating).
Bridge contacts also typically utilize a wiping action.
How are seal-in and drop-out voltages related to a coil’s rated voltage?
Seal-in = approx. 85% of rated voltage. Drop-out = approx. 50% of rated voltage.
May a contactor ever be operated at 120% of it’s rated voltage?
No.
The maximum voltage applied to a contactor coil should be 110% of its rated voltage.
(Higher values may break down the insulation of the wire or cause a violent snap of the contacts, damaging the faces.)
When an armature seals in, what happens to the current within the coil?
The current will experience a large decrease, as the impedance of the coil increases significantly when the air gap in the magnetic circuit around the coil is eliminated.
If a lower than rated voltage is applied to a coil, can the coil burn out?
Yes.
If the armature does not properly pull in, the current value in the coil may be quite high and cause excessive heating of the wire.
Why might an armature fail to drop out after it has been de-energized?
Residual magnetism in the core.
Welded contacts.
Jammed armature.
