Electromechanical Relays

An electric current through a conductor will produce a magnetic field at right angles to the direction
of electron flow. If that conductor is wrapped into a coil shape, the magnetic field produced will
be oriented along the length of the coil. The greater the current, the greater the strength of the
magnetic field.

Inductors react against changes in current because of the energy stored in this magnetic field.
When we construct a transformer from two inductor coils around a common iron core, we use this field to transfer energy from one coil to the other. However, there are simpler and more direct uses
for electromagnetic fields than the applications we've seen with inductors and transformers. The
magnetic field produced by a coil of current-carrying wire can be used to exert a mechanical force
on any magnetic object, just as we can use a permanent magnet to attract magnetic objects, except
that this magnet (formed by the coil) can be turned on or off by switching the current on or off through the coil.

If we place a magnetic object near such a coil for the purpose of making that object move when
we energize the coil with electric current, we have what is called a solenoid. The movable magnetic
object is called an armature, and most armatures can be moved with either direct current (DC)
or alternating current (AC) energizing the coil. The polarity of the magnetic field is irrelevant for
the purpose of attracting an iron armature. Solenoids can be used to electrically open door latches,
open or shut valves, move robotic limbs, and even actuate electric switch mechanisms. However, if
a solenoid is used to actuate a set of switch contacts,we have a device so useful it deserves its own name( the relay).

Relays are extremely useful when we have a need to control a large amount of current and/or
voltage with a small electrical signal. The relay coil which produces the magnetic ¯eld may only
consume fractions of a watt of power, while the contacts closed or opened by that magnetic ¯eld
may be able to conduct hundreds of times that amount of power to a load. In e®ect, a relay acts as
a binary (on or off) amplifier.

One relay coil/armature assembly may be used to actuate more than one set of contacts. Those
contacts may be normally-open, normally-closed, or any combination of the two. As with switches,
the "normal" state of a relay's contacts is that state when the coil is de-energized, just as you would find the relay sitting on a shelf, not connected to any circuit.

Relay contacts may be open-air pads of metal alloy, mercury tubes, or even magnetic reeds,
just as with other types of switches. The choice of contacts in a relay depends on the same factors
which dictate contact choice in other types of switches. Open-air contacts are the best for high-
current applications, but their tendency to corrode and spark may cause problems in some industrial
environments. Mercury and reed contacts are spark-less and won't corrode, but they tend to be
limited in current-carrying capacity.

Control switches cont.

Speed switch
These switches sense the rotary speed of a shaft either by a centrifugal weight mechanism mounted
on the shaft, or by some kind of non-contact detection of shaft motion such as optical or magnetic.

Pressure switch
Gas or liquid pressure can be used to actuate a switch mechanism if that pressure is applied to
a piston, diaphragm, or bellows, which converts pressure to mechanical force.

Temperature switch
An inexpensive temperature-sensing mechanism is the "bimetallic strip:" a thin strip of two
metals, joined back-to-back, each metal having a different rate of thermal expansion. When the
strip heats or cools, differing rates of thermal expansion between the two metals causes it to bend.
The bending of the strip can then be used to actuate a switch contact mechanism. Other temperature
switches use a brass bulb filled with either a liquid or gas, with a tiny tube connecting the bulb to
a pressure-sensing switch. As the bulb is heated, the gas or liquid expands, generating a pressure
increase which then actuates the switch mechanism.

Liquid level switch
A floating object can be used to actuate a switch mechanism when the liquid level in an tank
rises past a certain point. If the liquid is electrically conductive, the liquid itself can be used as a
conductor to bridge between two metal probes inserted into the tank at the required depth. The
conductivity technique is usually implemented with a special design of relay triggered by a small
amount of current through the conductive liquid. In most cases it is impractical and dangerous to
switch the full load current of the circuit through a liquid.



Liquid flow switch
Inserted into a pipe, a flow switch will detect any gas or liquid °ow rate in excess of a certain
threshold, usually with a small paddle or vane which is pushed by the °ow. Other °ow switches are
constructed as differential pressure switches, measuring the pressure drop across a restriction built
into the pipe.
Another type of level switch, suitable for liquid or solid material detection, is the nuclear switch.
Composed of a radioactive source material and a radiation detector, the two are mounted across
the diameter of a storage vessel for either solid or liquid material. Any height of material beyond
the level of the source/detector arrangement will attenuate the strength of radiation reaching the
detector. This decrease in radiation at the detector can be used to trigger a relay mechanism to
provide a switch contact for measurement, alarm point, or even control of the vessel level.

Control switches cont.

Selector switch
Selector switches are actuated with a rotary knob or lever of some sort to select one of two or
more positions. Like the toggle switch, selector switches can either rest in any of their positions or
contain spring-return mechanisms for momentary operation.

Lever actuator limit switch
These limit switches closely resemble rugged toggle or selector hand switches fitted with a lever
pushed by the machine part. Often, the levers are tipped with a small roller bearing, preventing the
lever from being worn off by repeated contact with the machine part.

Proximity switches
Proximity switches sense the approach of a metallic machine part either by a magnetic or high-
frequency electromagnetic field. Simple proximity switches use a permanent magnet to actuate a
sealed switch mechanism whenever the machine part gets close (typically 1 inch or less). More com-
plex proximity switches work like a metal detector, energizing a coil of wire with a high-frequency
current, and electronically monitoring the magnitude of that current. If a metallic part (not nec-
essarily magnetic) gets close enough to the coil, the current will increase, and trip the monitoring
circuit. The symbol shown here for the proximity switch is of the electronic variety, as indicated by
the diamond-shaped box surrounding the switch. A non-electronic proximity switch would use the
same symbol as the lever-actuated limit switch.

Another form of proximity switch is the optical switch, comprised of a light source and photocell.
Machine position is detected by either the interruption or reflection of a light beam. Optical switches
are also useful in safety applications, where beams of light can be used to detect personnel entry
into a dangerous area.
In many industrial processes, it is necessary to monitor various physical quantities with switches.
Such switches can be used to sound alarms, indicating that a process variable has exceeded normal
parameters, or they can be used to shut down processes or equipment if those variables have reached
dangerous or destructive levels.

solenoid valves

Most solenoid valves operate on a digital principle. They therefore

possess two distinct states, which are (1) - when the coil is

activated by an electrical current, and (2) - when the valve is resting

(without electricity). Valve functions are defined from the resting

position.

The direct acting or pilot operated solenoid valves may have two

functions:

Normally closed (NC)

A solenoid valve is normally closed (abbreviated - NC) if there is no

flow across the valve in its resting position (with no current on the

solenoid contacts).

Symbol

Please note that in the case of 3-way solenoid valves, port A is

open to port R which, for example, enables the valve’s single-action

cylinder to be exhausted to atmosphere.

Normally open (NO)

A solenoid valve is said to be“normally open” (abbreviated NO)

when it enables fluid to pass in its resting position (with no current

on the solenoid contacts).

Symbol

A specific choice of entry ports can change a valve’s function.

However, since balanced-force calculations take rebound effects,

coil effects and the effects of pressure exerted on the seal into

account, the performance of an NC valve fitted in an NO position

would be reduced. In this configuration it would be better to choose

a universal solenoid valve.

Latching or Bi-stable

applications a short electrical impulse enables the solenoid valve

to be opened or closed, and thanks to the residual effects of a

permanent magnet this is sufficient for maintaining the valve in a

particular working position with no electrical energy consumption.

A short impulse of inverted polarity ensures the valve’s return to its

previous position. Electrical power consumption and heating are

almost negligible.