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.