How Relays Work
Relays
are switches that open and close circuits electromechanically or
electronically. Relays control one electrical circuit by opening and closing
contacts in another circuit. As relay diagrams show, when a relay contact is
normally open (NO), there is an open contact when the relay is not energized.
When a relay contact is Normally Closed (NC), there is a closed contact when
the relay is not energized. In either case, applying electrical current to the
contacts will change their state.
Relays are
generally used to switch smaller currents in a control circuit and do not
usually control power consuming devices except for small motors and Solenoids
that draw low amps. Nonetheless, relays can "control" larger voltages
and amperes by having an amplifying effect because a small voltage applied to a
relays coil can result in a large voltage being switched by the contacts.
Protective
relays can prevent equipment damage by detecting electrical abnormalities,
including overcurrent, undercurrent, overloads and reverse currents. In
addition, relays are also widely used to switch starting coils, heating
elements, pilot lights and audible alarms.
Electromechanical
Relays.
Basic parts
and functions of electromechanical relays include:
Frame:
Heavy-duty frame that contains and supports the parts of the relay.
Coil:
Wire is wound around a metal core. The coil of wire causes an electromagnetic
field.
Armature: A relays moving part. The armature opens and closes the contacts. An
attached spring returns the armature to its original position.
Contacts: The conducting part of the switch that makes (closes) or breaks
(opens) a circuit.
Relays involve two circuits: the energizing circuit and the contact circuit.
The coil is on the energizing side; and the relays contacts are on the contact
side. When a relays coil is energized, current flow through the coil creates a
magnetic field. Whether in a DC unit where the polarity is fixed, or in an AC
unit where the polarity changes 120 times per second, the basic function
remains the same: the magnetic coil attracts a ferrous plate, which is part of
the armature. One end of the armature is attached to the metal frame, which is
formed so that the armature can pivot, while the other end opens and closes the
contacts. Contacts come in a number of different configurations, depending on
the number of Breaks, poles and Throws that make up the relay. For instance,
relays might be described as Single-Pole, Single-Throw (SPST), or Double-Pole,
Single-Throw (DPST). These terms will give an instant indication of the design
and function of different types of relays.
Break -This
is the number of separate places or contacts that a switch uses to open or
close a single electrical circuit. All contacts are either single break or
double break. A single break (SB) contact breaks an electrical circuit in one
place, while a double break (DB) contact breaks it in two places. Single break
contacts are normally used when switching lower power devices such as
indicating lights. Double break contacts are used when switching high-power
devices such as solenoids.
Pole
-This is the number of completely isolated circuits that relays can pass
through a switch. A single-pole contact (SP) can carry current through only one
circuit at a time. A double-pole contact (DP) can carry current through two
isolated circuits simultaneously. The maximum number of poles is 12, depending
upon a relays design.
Throw -This
is the number of closed contact positions per pole that are available on a
switch. A switch with a single throw contact can control only one circuit,
while a double-throw contact can control two.
Types of Relyas:
Electromechanical.
General
Purpose Relays are electromechanical switches, usually operated by a magnetic
coil. General purpose relays operate with AC or DC current, at common voltages
such as 12V, 24V, 48V, 120V and 230V, and they can control currents ranging
from 2A-30A. These relays are economical, easy to replace and allow a wide
range of switch configuration.
Machine
Control Relays are also operated by a magnetic coil. They are heavy-duty relays
used to control starters and other industrial components. Although they are
more expensive than general purpose relays, they are generally more durable.
The biggest advantage of machine control relays over general purpose relays is
the expandable functionality of Machine Control Relays by the adding of
accessories. A wide selection of accessories is available for machine control
relays, including additional poles, convertible contacts, transient suppression
of electrical noise, latching control and timing attachments.
Reed Relays
are a small, compact, fast operating switch design with one contact, which is
NO. Reed Relays are hermetically sealed in a glass envelope, which makes the
contacts unaffected by contaminants, fumes or humidity, allows reliable
switching, and gives contacts a higher life expectancy. The ends of the
contact, which are often plated with gold or another low resistance material to
increase conductivity, are drawn together and closed by a magnet. Reed relays
are capable of switching industrial components such as solenoids, contactors
and starter motors. Reed relays consists of two reeds. When a magnetic force is
applied, such as an electromagnet or coil, it sets up a magnetic field in which
the end of the reeds assume opposite polarity. When the magnetic field is
strong enough, the attracting force of the opposite poles overcomes the
stiffness of the reeds and draws them together. When the magnetic force is
removed, the reeds spring back to their original, open position. These relays
work very quickly because of the short distance between the reeds.
Solid State Relays.
Solid State
Relays Solid State Relays, like the one pictured above, are capable of
switching high voltages up to 600 VACrms. These relays are designed to switch
various loads such as heating elements, motors, and transformers.
Solid state
relays consist of an input circuit, a control circuit and an output circuit.
The Input Circuit is the portion of a relays frame to which the control
component is connected. The input circuit performs the same function as the
coil of electromechanical relays. The circuit is activated when a voltage
higher than the relays specified Pickup Voltage is applied to the relays input.
The input circuit is deactivated when the voltage applied is less than the
specified minimum Dropout voltage of the relay. The voltage range of 3 VDC to
32 VDC, commonly used with most solid-state relays, makes it useful for most
electronic circuits. The Control Circuit is the part of the relay that
determines when the output component is energized or de-energized. The control
circuit functions as the coupling between the input and output circuits. In
electromechanical relays, the coil accomplishes this function. A relays Output
Circuit is the portion of the relay that switches on the load and performs the
same function as the mechanical contacts of electromechanical relays.
Solid-state relays, however, normally have only one output contact.
Types of Relays: Solid
State.
Zero-Switching
Relays - relays turns ON the load when the control (minimum operating) voltage
is applied and the voltage of the load is close to zero. Zero-Switching relays
turn OFF the load when the control voltage is removed and the current in the
load is close to zero. Zero-Switching relays are the most widely used.
Instant ON
Relays - turns ON the load immediately when the pickup voltage is present.
Instant ON Relays allow the load to be turned ON at any point in it's up and
down wave.
Peak
Switching Relays - turns ON the load when the control voltage is present, and
the voltage of the load is at its peak. Peak Switching relays turn OFF when the
control voltage is removed and the current in the load is close to zero.
Analog
Switching Relays - has an infinite number of possible output voltages within
the relays rated range. Analog switching relays have a built in synchronizing
circuit that controls the amount of output voltage as a function of the input
voltage. This allows a Ramp-Up function of time to be on the load. Analog
Switching relays turn OFF when the control voltage is removed and current in
the load is near zero
Electromechanical
Relays vs Solid State Relays.
.
Relays are
either electromechanical relays or solid-state relays. In electromechanical
relays (EMR), contacts are opened or closed by a magnetic force. With
solid-state relays (SSR), there are no contacts and switching is totally
electronic. The decision to use electromechanical or solid state relays depends
on an application's electrical requirements, cost constraints and life
expectancy. Although solid-state relays have become very popular,
electromechanical relays remain common. Many of the functions performed by heavy-duty
equipment need the switching capabilities of electromechanical relays. Solid
State Relays switche the current using non-moving electronic devices such as
silicon controlled rectifiers.
These
differences in the two types of relays result in advantages and disadvantages
with each system. Because solid state relays do not have to either energize a
coil or open contacts, less voltage is required to "turn" Solid State
Relays on or off. Similarly, Solid State Relays turn on and turn off faster
because there are no physical parts to move. Although the absence of contacts
and moving parts means that Solid State Relays are not subject to arcing and do
not wear out, contacts on Electromechanical Relays can be replaced, whereas
entire Solid State Relays must be replaced when any part becomes defective.
Because of the construction of Solid State Relays, there is residual electrical
resistance and/or current leakage whether switches are open and closed. The
small voltage drops that are created are not usually a problem; however,
Electromechanical Relays provide a cleaner ON or OFF condition because of the
relatively large distance between contacts, which acts as a form of insulation.
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