Electromagnetic Relays and Selection Specifications

Electromagnetic Relay acts as a electrical switch operated
by electromagnet. The coil on the relay forms the electromagnet. When
excited by rated voltage, it pulls the moving portion of the relay contacts
towards it resulting in electrical contact. Thus it turns a load circuit ON or
OFF by energizing an electromagnet, which opens or closes contact in the
circuit. A relay has one coil, but may have many contacts. The contact that
changes is called pole and the non-moving contacts are called ways. When the
coil is off, the contact with pole is called normally closed contact. (NO) The
contact established after the coil is energized is called normally open
contact. (NC) The coil and the contacts of a relay are electrically galvanically
isolated from each other. 
 Relays finds wide use in timers, interlocks
circuits, trips and safety units, changeover systems etc.

The main parameters required for
selection of a relay are,
1. Coil
rating 
2. Contact rating. 
3 Operation time or change over time. 
4. Type of
enclosure and mounting. 
5. Relay driving circuits. 
6. Isolation voltage.
(between coil and contact)
Coil rating.
            The coils are rated for nominal
voltage rating (DC or AC) and watts or VA rating. For same contact rating, the
VA rating of the coil is higher than watts. This is because of PF of the coil.
For DC operated coils the excitation voltage and resistance are specified. For
AC operated coils the AC voltage and VA rating are specified. (See the
specification sheet attached) The minimum level of coil voltage, at which the
relay switches on, is called
Pick -up
voltage. (Normally about 80% of normal coil voltage) Once the relay is on and
the coil voltage is reduced slowly, level at which it goes off is called
Dropout voltage, or Release voltage.
(About 40 to 50% of nominal voltage) This shows
Hysterisis in its operation. Because of this hysterisis, it is a
normal practice to use comparator for its ON/OFF operation. The coil rating is
directly proportional to the contact rating. If the contact rating doubles,
coil rating also almost doubles.
             The
standard coil ratings available for DC operated coils are 6, 12, 24Vdc
(nominal). Coils are designed to take up to 20% higher excitation voltage. This
reduces the need for regulated supply. 230 Vac / 50 Hz coils are available for
AC excitation. The inrush current for AC coils is about 5 times higher than
continuous coil rating.
Contact Rating
            The most important contact
specification is its continuous current and voltage rating. The three current
ratings specified are: –
a)
Inrush or ‘make contact’ capacity.
b)
Normal or continuos carrying capacity’.
c)
The opening or breaking capacity.
 
For a specified relay, the contact rating is higher
for ac currents than dc current rating. For example a contact rated for 230 Vac
/ 5 Amps, may be used for only 24 Vdc / 1 Amp. The duty cycle and use also
decide the rating. For example the contact rating mentioned above is for
resistive loads. For inductive load or inching duty for motor, this rating comes
down considerably.
A relay may have multiple contacts and changeover
contacts.
  
Operation Time
 The switch-on (or turn on time) period
for a relay is the period required for the relay to come on and establish
contact after energizing the coil. Similarly the turn off period is the period
required for the relay contact to go back to it unenergized position after the
coil excitation is removed. The operating time (Switch on and switch off
periods) are generally between 5 to 20 msec. The ON period for relays is
dependent on coil supply voltage. At pickup voltage it is minimum. As the coil
voltage is increased, this period comes down. With further increase, the period
comes down but it starts bouncing, thus increasing the settling time.
Enclosures:
 The relays
are available in open execution or in enclosure. If the relay is to be used
inside a cabinet of an instrument or panel, then it may be used in open
execution. However if in the place of use, if there is a possibility of dust
gathering on the contacts ( and thus making bad electrical contact), then it
becomes necessary to use enclosure for the relay assembly. When the relay
contacts open or close, there is sparking between the contacts. In hazardous
areas this unacceptable and a proper enclosure takes care of this problem. For
EMI problems, the enclosure used is metallic and it is grounded.  The enclosures are available with socket and
clamps for taking care of easy of removing the relay. Clamps are used to make
sure that due to vibration in the system, the relay does not become loose.
The
relays may be mounted on base plate or on PCB directly. The electrical contact
is achieved by soldering the contacts.
Relay driving circuits.
 In most of the operations the relays are operate
from control signals. The control signal drives a transistor in saturation or
in cut off mode, switching on and off the relay. A free wheeling diode is use
for taking care of coil inductance.
 Isolation
voltage.
The
isolation resistance and voltage between contacts and coil is very high The
voltage is more than 500 V and the resistance is more than 100 M ohms. This
galvanic isolation is necessary between control and other electrical circuit
for safety reasons.

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