Relay construction
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, all other factors being equal:
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 field may only consume fractions
of a watt of power, while the contacts closed or opened by that magnetic
field may be able to conduct hundreds of times that amount of power to a
load. In effect, a relay acts as a binary (on or off) amplifier.
Just as with transistors, the relay's ability to control one electrical
signal with another finds application in the construction of logic
functions. This topic will be covered in greater detail in another
lesson. For now, the relay's "amplifying" ability will be explored.
In the above schematic, the relay's coil is energized by the low-voltage
(12 VDC) source, while the single-pole, single-throw (SPST) contact
interrupts the high-voltage (480 VAC) circuit. It is quite likely that
the current required to energize the relay coil will be hundreds of
times less than the current rating of the contact. Typical relay coil
currents are well below 1 amp, while typical contact ratings for
industrial relays are at least 10 amps.
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 sparkless and won't corrode, but they tend to be limited in
current-carrying capacity.
Shown here are three small relays (about two inches in height, each),
installed on a panel as part of an electrical control system at a
municipal water treatment plant:
The relay units shown here are called "octal-base," because they plug
into matching sockets, the electrical connections secured via eight
metal pins on the relay bottom. The screw terminal connections you see
in the photograph where wires connect to the relays are actually part of
the socket assembly, into which each relay is plugged. This type of
construction facilitates easy removal and replacement of the relay(s) in
the event of failure.
Aside from the ability to allow a relatively small electric signal to
switch a relatively large electric signal, relays also offer electrical
isolation between coil and contact circuits. This means that the coil
circuit and contact circuit(s) are electrically insulated from one
another. One circuit may be DC and the other AC (such as in the example
circuit shown earlier), and/or they may be at completely different
voltage levels, across the connections or from connections to ground.
While relays are essentially binary devices, either being completely on
or completely off, there are operating conditions where their state may
be indeterminate, just as with semiconductor logic gates. In order for a
relay to positively "pull in" the armature to actuate the contact(s),
there must be a certain minimum amount of current through the coil. This
minimum amount is called the pull-in current, and it is
analogous to the minimum input voltage that a logic gate requires to
guarantee a "high" state (typically 2 Volts for TTL, 3.5 Volts for
CMOS). Once the armature is pulled closer to the coil's center, however,
it takes less magnetic field flux (less coil current) to hold it there.
Therefore, the coil current must drop below a value significantly lower
than the pull-in current before the armature "drops out" to its
spring-loaded position and the contacts resume their normal state. This
current level is called the drop-out current, and it is analogous
to the maximum input voltage that a logic gate input will allow to
guarantee a "low" state (typically 0.8 Volts for TTL, 1.5 Volts for
CMOS).
The hysteresis, or difference between pull-in and drop-out currents,
results in operation that is similar to a Schmitt trigger logic gate.
Pull-in and drop-out currents (and voltages) vary widely from relay to
relay, and are specified by the manufacturer.
- REVIEW:
- A solenoid is a device that produces mechanical motion from the energization of an electromagnet coil. The movable portion of a solenoid is called an armature.
- A relay is a solenoid set up to actuate switch contacts when its coil is energized.
- Pull-in current is the minimum amount of coil current needed to actuate a solenoid or relay from its "normal" (de-energized) position.
- Drop-out current is the maximum coil current below which an energized relay will return to its "normal" state.
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.
The three "opposed-question-mark" shaped devices in series with each phase going to the motor are called overload heaters.
Each "heater" element is a low-resistance strip of metal intended to
heat up as the motor draws current. If the temperature of any of these
heater elements reaches a critical point (equivalent to a moderate
overloading of the motor), a normally-closed switch contact (not shown
in the diagram) will spring open. This normally-closed contact is
usually connected in series with the relay coil, so that when it opens
the relay will automatically de-energize, thereby shutting off power to
the motor. We will see more of this overload protection wiring in the
next chapter. Overload heaters are intended to provide overcurrent
protection for large electric motors, unlike circuit breakers and fuses
which serve the primary purpose of providing overcurrent protection for
power conductors.
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 specific 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
thermo-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.
Shown here is a contactor for a three-phase electric motor, installed on
a panel as part of an electrical control system at a municipal water
treatment plant:
Three-phase, 480 volt AC power comes in to the three normally-open
contacts at the top of the contactor via screw terminals labeled "L1,"
"L2," and "L3" (The "L2" terminal is hidden behind a square-shaped
"snubber" circuit connected across the contactor's coil terminals).
Power to the motor exits the overload heater assembly at the bottom of
this device via screw terminals labeled "T1," "T2," and "T3."
The overload heater units themselves are black, square-shaped blocks
with the label "W34," indicating a particular thermal response for a
certain horsepower and temperature rating of electric motor. If an
electric motor of differing power and/or temperature ratings were to be
substituted for the one presently in service, the overload heater units
would have to be replaced with units having a thermal response suitable
for the new motor. The motor manufacturer can provide information on the
appropriate heater units to use.
A white pushbutton located between the "T1" and "T2" line heaters serves
as a way to manually re-set the normally-closed switch contact back to
its normal state after having been tripped by excessive heater
temperature. Wire connections to the "overload" switch contact may be
seen at the lower-right of the photograph, near a label reading "NC"
(normally-closed). On this particular overload unit, a small "window"
with the label "Tripped" indicates a tripped condition by means of a
colored flag. In this photograph, there is no "tripped" condition, and
the indicator appears clear.
As a footnote, heater elements may be used as a crude current shunt
resistor for determining whether or not a motor is drawing current when
the contactor is closed. There may be times when you're working on a
motor control circuit, where the contactor is located far away from the
motor itself.
How do you know if the motor is consuming power when the
contactor coil is energized and the armature has been pulled in? If the
motor's windings are burnt open, you could be sending voltage to the
motor through the contactor contacts, but still have zero current, and
thus no motion from the motor shaft. If a clamp-on ammeter isn't
available to measure line current, you can take your multimeter and
measure millivoltage across each heater element: if the current is zero,
the voltage across the heater will be zero (unless the heater element
itself is open, in which case the voltage across it will be large); if
there is current going to the motor through that phase of the contactor,
you will read a definite millivoltage across that heater:
This is an especially useful trick to use for troubleshooting 3-phase AC
motors, to see if one phase winding is burnt open or disconnected,
which will result in a rapidly destructive condition known as
"single-phasing." If one of the lines carrying power to the motor is
open, it will not have any current through it (as indicated by a 0.00 mV
reading across its heater), although the other two lines will (as
indicated by small amounts of voltage dropped across the respective
heaters).
- REVIEW:
- A contactor is a large relay, usually used to switch current to an electric motor or other high-power load.
- Large electric motors can be protected from overcurrent damage through the use of overload heaters and overload contacts. If the series-connected heaters get too hot from excessive current, the normally-closed overload contact will open, de-energizing the contactor sending power to the motor.
Time-delay relays
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 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 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:
The following is a timing diagram of this relay contact's operation:
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:
The following is a timing diagram of this relay contact's operation:
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:
The following is a timing diagram of this relay contact's operation:
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:
The following is a timing diagram of this relay contact's operation:
Time-delay relays are very important for use in industrial control logic circuits. Some examples of their use include:
- Flashing light control (time on, time off): two time-delay relays are used in conjunction with one another to provide a constant-frequency on/off pulsing of contacts for sending intermittent power to a lamp.
- Engine autostart control: Engines that are used to power emergency generators are often equipped with "autostart" controls that allow for automatic start-up if the main electric power fails. To properly start a large engine, certain auxiliary devices must be started first and allowed some brief time to stabilize (fuel pumps, pre-lubrication oil pumps) before the engine's starter motor is energized. Time-delay relays help sequence these events for proper start-up of the engine.
- Furnace safety purge control: Before a combustion-type furnace can be safely lit, the air fan must be run for a specified amount of time to "purge" the furnace chamber of any potentially flammable or explosive vapors. A time-delay relay provides the furnace control logic with this necessary time element.
- Motor soft-start delay control: Instead of starting large electric motors by switching full power from a dead stop condition, reduced voltage can be switched for a "softer" start and less inrush current. After a prescribed time delay (provided by a time-delay relay), full power is applied.
- Conveyor belt sequence delay: when multiple conveyor belts are arranged to transport material, the conveyor belts must be started in reverse sequence (the last one first and the first one last) so that material doesn't get piled on to a stopped or slow-moving conveyor. In order to get large belts up to full speed, some time may be needed (especially if soft-start motor controls are used). For this reason, there is usually a time-delay circuit arranged on each conveyor to give it adequate time to attain full belt speed before the next conveyor belt feeding it is started.
The older, mechanical time-delay relays used pneumatic dashpots or
fluid-filled piston/cylinder arrangements to provide the "shock
absorbing" needed to delay the motion of the armature. Newer designs of
time-delay relays use electronic circuits with resistor-capacitor (RC)
networks to generate a time delay, then energize a normal
(instantaneous) electromechanical relay coil with the electronic
circuit's output. The electronic-timer relays are more versatile than
the older, mechanical models, and less prone to failure. Many models
provide advanced timer features such as "one-shot" (one measured output
pulse for every transition of the input from de-energized to energized),
"recycle" (repeated on/off output cycles for as long as the input
connection is energized) and "watchdog" (changes state if the input
signal does not repeatedly cycle on and off).
The "watchdog" timer is especially useful for monitoring of computer
systems. If a computer is being used to control a critical process, it
is usually recommended to have an automatic alarm to detect computer
"lockup" (an abnormal halting of program execution due to any number of
causes).
An easy way to set up such a monitoring system is to have the
computer regularly energize and de-energize the coil of a watchdog timer
relay (similar to the output of the "recycle" timer). If the computer
execution halts for any reason, the signal it outputs to the watchdog
relay coil will stop cycling and freeze in one or the other state. A
short time thereafter, the watchdog relay will "time out" and signal a
problem.
- REVIEW:
- Time delay relays are built in these four basic modes of contact operation:
- 1: Normally-open, timed-closed. Abbreviated "NOTC", these relays open immediately upon coil de-energization and close only if the coil is continuously energized for the time duration period. Also called normally-open, on-delay relays.
- 2: Normally-open, timed-open. Abbreviated "NOTO", these relays close immediately upon coil energization and open after the coil has been de-energized for the time duration period. Also called normally-open, off delay relays.
- 3: Normally-closed, timed-open. Abbreviated "NCTO", these relays close immediately upon coil de-energization and open only if the coil is continuously energized for the time duration period. Also called normally-closed, on-delay relays.
- 4: Normally-closed, timed-closed. Abbreviated "NCTC", these relays open immediately upon coil energization and close after the coil has been de-energized for the time duration period. Also called normally-closed, off delay relays.
- One-shot timers provide a single contact pulse of specified duration for each coil energization (transition from coil off to coil on).
- Recycle timers provide a repeating sequence of on-off contact pulses as long as the coil is maintained in an energized state.
- Watchdog timers actuate their contacts only if the coil fails to be continuously sequenced on and off (energized and de-energized) at a minimum frequency.
Protective relays
A special type of relay is one which monitors the current, voltage,
frequency, or any other type of electric power measurement either from a
generating source or to a load for the purpose of triggering a circuit
breaker to open in the event of an abnormal condition. These relays are
referred to in the electrical power industry as protective relays.
The circuit breakers which are used to switch large quantities of
electric power on and off are actually electromechanical relays,
themselves. Unlike the circuit breakers found in residential and
commercial use which determine when to trip (open) by means of a
bimetallic strip inside that bends when it gets too hot from
overcurrent, large industrial circuit breakers must be "told" by an
external device when to open. Such breakers have two electromagnetic
coils inside: one to close the breaker contacts and one to open them.
The "trip" coil can be energized by one or more protective relays, as
well as by hand switches, connected to switch 125 Volt DC power. DC
power is used because it allows for a battery bank to supply close/trip
power to the breaker control circuits in the event of a complete (AC)
power failure.
Protective relays can monitor large AC currents by means of current
transformers (CT's), which encircle the current-carrying conductors
exiting a large circuit breaker, transformer, generator, or other
device. Current transformers step down the monitored current to a
secondary (output) range of 0 to 5 amps AC to power the protective
relay. The current relay uses this 0-5 amp signal to power its internal
mechanism, closing a contact to switch 125 Volt DC power to the
breaker's trip coil if the monitored current becomes excessive.
Likewise, (protective) voltage relays can monitor high AC voltages by
means of voltage, or potential, transformers (PT's) which step down the
monitored voltage to a secondary range of 0 to 120 Volts AC, typically.
Like (protective) current relays, this voltage signal powers the
internal mechanism of the relay, closing a contact to switch 125 Volt DC
power to the breaker's trip coil is the monitored voltage becomes
excessive.
There are many types of protective relays, some with highly specialized
functions. Not all monitor voltage or current, either. They all,
however, share the common feature of outputting a contact closure signal
which can be used to switch power to a breaker trip coil, close coil,
or operator alarm panel. Most protective relay functions have been
categorized into an ANSI standard number code. Here are a few examples
from that code list:
ANSI protective relay designation numbers
12 = Overspeed
24 = Overexcitation
25 = Syncrocheck
27 = Bus/Line undervoltage
32 = Reverse power (anti-motoring)
38 = Stator overtemp (RTD)
39 = Bearing vibration
40 = Loss of excitation
46 = Negative sequence undercurrent (phase current imbalance)
47 = Negative sequence undervoltage (phase voltage imbalance)
49 = Bearing overtemp (RTD)
50 = Instantaneous overcurrent
51 = Time overcurrent
51V = Time overcurrent -- voltage restrained
55 = Power factor
59 = Bus overvoltage
60FL = Voltage transformer fuse failure
67 = Phase/Ground directional current
79 = Autoreclose
81 = Bus over/underfrequency
- REVIEW:
- Large electric circuit breakers do not contain within themselves the necessary mechanisms to automatically trip (open) in the event of overcurrent conditions. They must be "told" to trip by external devices.
- Protective relays are devices built to automatically trigger the actuation coils of large electric circuit breakers under certain conditions.
Solid-state relays
As versatile as electromechanical relays can be, they do suffer many
limitations. They can be expensive to build, have a limited contact
cycle life, take up a lot of room, and switch slowly, compared to modern
semiconductor devices. These limitations are especially true for large
power contactor relays. To address these limitations, many relay
manufacturers offer "solid-state" relays, which use an SCR, TRIAC, or
transistor output instead of mechanical contacts to switch the
controlled power. The output device (SCR, TRIAC, or transistor) is
optically-coupled to an LED light source inside the relay. The relay is
turned on by energizing this LED, usually with low-voltage DC power.
This optical isolation between input to output rivals the best that
electromechanical relays can offer.
Being solid-state devices, there are no moving parts to wear out, and
they are able to switch on and off much faster than any mechanical relay
armature can move. There is no sparking between contacts, and no
problems with contact corrosion. However, solid-state relays are still
too expensive to build in very high current ratings, and so
electromechanical contactors continue to dominate that application in
industry today.
One significant advantage of a solid-state SCR or TRIAC relay over an
electromechanical device is its natural tendency to open the AC circuit
only at a point of zero load current. Because SCR's and TRIAC's are thyristors,
their inherent hysteresis maintains circuit continuity after the LED is
de-energized until the AC current falls below a threshold value (the holding current).
In practical terms what this means is the circuit will never be
interrupted in the middle of a sine wave peak. Such untimely
interruptions in a circuit containing substantial inductance would
normally produce large voltage spikes due to the sudden magnetic field
collapse around the inductance. This will not happen in a circuit broken
by an SCR or TRIAC. This feature is called zero-crossover switching.
One disadvantage of solid state relays is their tendency to fail
"shorted" on their outputs, while electromechanical relay contacts tend
to fail "open." In either case, it is possible for a relay to fail in
the other mode, but these are the most common failures. Because a
"fail-open" state is generally considered safer than a "fail-closed"
state, electromechanical relays are still favored over their solid-state
counterparts in many applications.
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