Time-delay relays

Time-delay relays are very important for use in industrial control logic circuits.


Some relays are constructed with a kind of "shock absorber" mechanism attached to the armature
which prevents immediate, full motion when the coil is either energized or de-energized. This addition
gives the relay the property of time-delay actuation. Time-delay relays can be constructed to delay
armature motion on coil energization, de-energization, or both.

Time-delay relay contacts must be specified not only as either normally-open or normally-closed,
but whether the delay operates in the direction of closing or in the direction of opening. The
following is a description of the four basic types of time-delay relay contacts.

First we have the normally-open, timed-closed (NOTC) contact. This type of contact is normally
open when the coil is unpowered (de-energized). The contact is closed by the application of power
to the relay coil, but only after the coil has been continuously powered for the speci¯ed amount of
time. In other words, the direction of the contact's motion (either to close or to open) is identical
to a regular NO contact, but there is a delay in closing direction. Because the delay occurs in
the direction of coil energization, this type of contact is alternatively known as a normally-open,
on-delay.

Next we have the normally-open, timed-open (NOTO) contact. Like the NOTC contact, this
type of contact is normally open when the coil is unpowered (de-energized), and closed by the
application of power to the relay coil. However, unlike the NOTC contact, the timing action occurs
upon de-energization of the coil rather than upon energization. Because the delay occurs in the
direction of coil de-energization, this type of contact is alternatively known as a normally-open,
off -delay.

Next we have the normally-closed, timed-open (NCTO) contact. This type of contact is normally
closed when the coil is unpowered (de-energized). The contact is opened with the application of power
to the relay coil, but only after the coil has been continuously powered for the specified amount of
time. In other words, the direction of the contact's motion (either to close or to open) is identical
to a regular NC contact, but there is a delay in the opening direction. Because the delay occurs in
the direction of coil energization, this type of contact is alternatively known as a normally-closed,
on-delay.

Finally we have the normally-closed, timed-closed (NCTC) contact. Like the NCTO contact,
this type of contact is normally closed when the coil is unpowered (de-energized), and opened by
the application of power to the relay coil. However, unlike the NCTO contact, the timing action
occurs upon de-energization of the coil rather than upon energization. Because the delay occurs in
the direction of coil de-energization, this type of contact is alternatively known as a normally-closed,
off -delay.

Contactors

When a relay is used to switch a large amount of electrical power through its contacts, it is designated
by a special name: contactor. Contactors typically have multiple contacts, and those contacts are
usually (but not always) normally-open, so that power to the load is shut off when the coil is de-energized. Perhaps the most common industrial use for contactors is the control of electric motors.

The top three contacts switch the respective phases of the incoming 3-phase AC power, typically
at least 480 Volts for motors 1 horsepower or greater. The lowest contact is an "auxiliary" contact
which has a current rating much lower than that of the large motor power contacts, but is actuated by
the same armature as the power contacts. The auxiliary contact is often used in a relay logic circuit,
or for some other part of the motor control scheme, typically switching 120 Volt AC power instead
of the motor voltage. One contactor may have several auxiliary contacts, either normally-open or
normally-closed, if required.

Overload heater function is often misunderstood. They are not fuses; that is, it is not their
function to burn open and directly break the circuit as a fuse is designed to do. Rather, overload
heaters are designed to thermally mimic the heating characteristic of the particular electric motor
to be protected. All motors have thermal characteristics, including the amount of heat energy
generated by resistive dissipation (I2R), the thermal transfer characteristics of heat "conducted" to
the cooling medium through the metal frame of the motor, the physical mass and speci¯c heat of
the materials constituting the motor, etc. These characteristics are mimicked by the overload heater
on a miniature scale: when the motor heats up toward its critical temperature, so will the heater
toward its critical temperature, ideally at the same rate and approach curve. Thus, the overload
contact, in sensing heater temperature with a therm-mechanical mechanism, will sense an analogue
of the real motor. If the overload contact trips due to excessive heater temperature, it will be an
indication that the real motor has reached its critical temperature (or, would have done so in a short
while). After tripping, the heaters are supposed to cool down at the same rate and approach curve
as the real motor, so that they indicate an accurate proportion of the motor's thermal condition,
and will not allow power to be re-applied until the motor is truly ready for start-up again.

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.

Control switches

An electrical switch is any device used to interrupt the flow of electrons in a circuit. Switches
are essentially binary devices: they are either completely on ("closed") or completely or ("open").
There are many different types of switches, and we will explore some of these types in this chapter.
Though it may seem strange to cover this elementary electrical topic at such a late stage in this
book series, I do so because the chapters that follow explore an older realm of digital technology based
on mechanical switch contacts rather than solid-state gate circuits, and a thorough understanding of
switch types is necessary for the undertaking. Learning the function of switch-based circuits at the
same time that you learn about solid-state logic gates makes both topics easier to grasp, and sets
the stage for an enhanced learning experience in Boolean algebra, the mathematics behind digital
logic circuits.

The simplest type of switch is one where two electrical conductors are brought in contact with
each other by the motion of an actuating mechanism. Other switches are more complex, containing
electronic circuits able to turn on or off depending on some physical stimulus (such as light or
magnetic field) sensed. In any case, the final output of any switch will be (at least) a pair of
wire-connection terminals that will either be connected together by the switch's internal contact
mechanism ("closed"), or not connected together ("open").

Any switch designed to be operated by a person is generally called a hand switch, and they are
manufactured in several varieties:

Toggle switch
Toggle switches are actuated by a lever angled in one of two or more positions. The common
light switch used in household wiring is an example of a toggle switch. Most toggle switches will
come to rest in any of their lever positions, while others have an internal spring mechanism returning
the lever to a certain normal position, allowing for what is called "momentary" operation.

Push button switch
Push button switches are two-position devices actuated with a button that is pressed and released.
Most push button switches have an internal spring mechanism returning the button to its "out," or
"unpressed," position, for momentary operation. Some push button switches will latch alternately on
or off with every push of the button. Other push button switches will stay in their "in," or "pressed,"
position until the button is pulled back out. This last type of push button switches usually have a
mushroom-shaped button for easy push-pull action.

The transformers

A transformer is a device made of two or more inductors, one of which is powered by AC,
inducing an AC voltage across the second inductor. If the second inductor is connected to a
load, power will be electromagnetically coupled from the first inductor's power source to that
load.

The powered inductor in a transformer is called the primary winding. The unpowered inductor
in a transformer is called the secondary winding.

Total primary current in an unloaded transformer is called the exciting current, and is comprised of magnetizing current plus any additional current necessary to overcome core losses. It
is never perfectly sinusoidal in a real transformer, but may be made more so if the transformer
is designed and operated so that magnetic flux density is kept to a minimum.

Core flux induces a voltage in any coil wrapped around the core. The induces voltage(s) are
ideally in phase with the primary winding source voltage and share the same wave-shape.


Any current drawn through the secondary winding by a load will be "reflected" to the primary
winding and drawn from the voltage source, as if the source were directly powering a similar
load.

Batteries

"battery" is a set of voltaic cells designed to provide
greater voltage and/or current than is possible with one cell alone.

The symbol for a cell is very simple, consisting of one long line and one short line, parallel to
each other

the voltage produced by any particular kind of cell is determined strictly
by the chemistry of that cell type. The size of the cell is irrelevant to its voltage. To obtain greater
voltage than the output of a single cell, multiple cells must be connected in series. The total voltage
of a battery is the sum of all cell voltages.

The cells in an automotive battery are contained within the same hard rubber housing, connected
together with thick, lead bars instead of wires. The electrodes and electrolyte solutions for each cell
are contained in separate, partitioned sections of the battery case. In large batteries, the electrodes
commonly take the shape of thin metal grids or plates, and are often referred to as plates instead of
electrodes.

For the sake of convenience, battery symbols are usually limited to four lines, alternating
long/short, although the real battery it represents may have many more cells than that. On occasion,
however, you might come across a symbol for a battery with unusually high voltage, intentionally
drawn with extra lines.

The ideal battery, in a short circuit with 0 ­ resistance, would be able to supply an infinite
amount of current. The real battery, on the other hand, can only supply 50 amps (10 volts / 0.2 ­)
to a short circuit of 0 ­ resistance, due to its internal resistance. The chemical reaction inside the
cell may still be providing exactly 10 volts, but voltage is dropped across that internal resistance
as electrons °ow through the battery, which reduces the amount of voltage available at the battery
terminals to the load.