Relays and Contactors

Question 1

What are the four basic electromagnetic relay categories?

Answer 1

Plug in (general purpose)
Industrial control
Power
Low-voltage lighting

Question 2

What is the major difference between a contactor and a relay?

Answer 2

A contactor is constructed to handle larger power loads.

Question 3

Must the rated voltage of the coil and contacts of a relay or contactor be the same value?

Answer 3

It is not required to be.

Question 4

What are the maximum ratings of plug in relays?

Answer 4

10-15 Amps and 1/2 hP.

Question 5

For every set of NO/NC contacts in a general purpose relay, how many pins are there?

Answer 5

3 pins.

1 common, 1 NC, 1 NO.

Question 6

What device is attached to an industrial control relay to increase the number of contacts available?

Answer 6

Adder decks.

Question 7

What kind of contacts are normally found in industrial control relays?

Answer 7

Bridge contacts. (double make/double break).

Question 8

What are the maximum ratings of power relays?

Answer 8

30 Amps and 2 hP.

Question 9

What is a significant hazard associated with power relays

Answer 9

They have exposed contacts.

Question 10

What classification of voltage do low-voltage lighting relays operate under?

Answer 10

Extra-low voltage.

Less than 30 V.

Question 11

Low-voltage lighting relays utilize which mechanical feature to only require momentary energization?

Answer 11

Latching.

Question 12

Does a contactor contain overloads?

Answer 12

No.

An overload relay is a separate device.

Question 13

What are the two main classifications for contactors?

Answer 13

NEMA (American) and IEC (European).

Question 14

What are important factors when sizing an IEC contactor?

Answer 14

Duty cycle and whether it is used for jogging or reversing

Question 15

What is a reversing contactor?

Answer 15

Two regular contactors mechanically and electrically interlocked by a block device, so that only one contactor may operate at one time.

Question 16

What is the primary purpose of mechanically held contactors?

Answer 16

To reduce coil power consumption as it only requires momentary energization.
This is used in large office or retail buildings.

Question 17

What specific ratings would a contactor coil have?

Answer 17

Voltage to be applied.

Question 18

What specific ratings would contactor contacts have?

Answer 18

Horsepower
Voltage (frequency)
Current (continuous)
NEMA pilot duty
IEC control circuit classifications.

Question 19

What is meant by form x, when related to contact arrangement?

Answer 19

Single pole, single throw normally open double make contacts.

Question 20

What is meant by form C, when related to contact arrangement?

Answer 20

Single pole, double throw contacts.

Question 21

What are bifurcated contacts?

Answer 21

A contact offering parallel current paths, with twice as many contact points.

Question 22

What are the three main parts of a contactor?

Answer 22

Coil
Armature
Core (high permeability material).

Question 23

What is the purpose of “laminating” the core and/or the armature of a contactor?

Answer 23

To reduce magnetically induced eddy currents.

Question 24

If a contactor core was without “shading coils”, what would be the expected result when energizing that coil?

Answer 24

The contacts would hum or chatter at the frequency of the source current.

Question 25

What useful function do “shading coils” provide?

Answer 25

An out of phase magnetic flux, which helps seal in the armature during times of 0 normal flux.

Question 26

What are silver contacts used for?

Answer 26

Low current, low voltage, frequent switching.

Question 27

What are copper contacts used for?

Answer 27

High current, high voltage, infrequent switching.

Question 28

What are Cadmium plated contacts used for, and what is the major disadvantage of these?

Answer 28

High current, high voltage, frequent switching.

They do not conduct as well as silver or copper contacts.

Question 29

What is the purpose of contact springs?

Answer 29

To absorb the shock of the armature pulling in, thereby reducing contact bouncing. They also provide firm pressure to keep resistance low.

Question 30

Incorrect spring tension may result in…?

Answer 30

Increased resistance at the contact face or excessive arcing (possibly welding) due to bounce.

Question 31

What are two primary benefits of “wiping action”?

Answer 31

Self cleaning.

Arcing confined to the tips (prolonged contact life).

Question 32

What are two primary benefits of bridge type contacts?

Answer 32

Two points of contact splits the arc voltage.
Provide good contact alignment (seating).
Bridge contacts also typically utilize a wiping action.

Question 33

How are seal-in and drop-out voltages related to a coil’s rated voltage?

Answer 33

Seal-in = approx. 85% of rated voltage.
Drop-out = approx. 50% of rated voltage.

Question 34

May a contactor ever be operated at 120% of it’s rated voltage?

Answer 34

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.)

Question 35

When an armature seals in, what happens to the current within the coil?

Answer 35

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.

Question 36

If a lower than rated voltage is applied to a coil, can the coil burn out?

Answer 36

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.

Question 37

Why might an armature fail to drop out after it has been de-energized?

Answer 37

Residual magnetism in the core.
Welded contacts.
Jammed armature.