2. What are Relays?
•Relays are electrical switches that
open or close another circuit under
certain conditions.
Relay Purpose
Isolate controlling circuit from
controlled circuit.
Control high voltage system with
low voltage.
Control high current system with
low current.
Logic Functions
•
3. Relay Types
• Electromagnetic Relays (EMRs)
– EMRs consist of an input coil that's wound to accept a particular voltage
signal, plus a set of one or more contacts that rely on an armature (or lever)
activated by the energized coil to open or close an electrical circuit.
• Solid-state Relays (SSRs)
– SSRs use semiconductor output instead of mechanical contacts to switch the
circuit. The output device 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.
• Microprocessor Based Relays
– Use microprocessor for switching mechanism. Commonly used in power
system monitoring and protection.
5. DC Relay (Solid State)
Image Source:http://www.sparkfun.com/products/10213
DC Relays utilize transistors and
MOSFETS to switch larger circuits. When
a voltage is applied to the Gate on one of
these components, current is allowed to
pass through them.
Pictured on the left is a MOSFET that
tolerates up to 60 volts at 30 amps.
Solid state relays are advantageous to
mechanical relays because they have no
moving parts and thus never wear out.
Mechanical relays have a finite number of
switches.
6. AC Relay
Image Source: http://www.allaboutcircuits.com/vol_3/chpt_7/6.html
AC Relays are another solid
state relay that are built primarily
from a Triac circuit. A Triac is a
component made of two
"Thyristors" which are voltage
controlled unidirectional switches.
When a voltage is applied to a thyristor, the switch will remain
on until there is no more current passing through. By placing two
thyristors in opposite directions, AC current can be controlled as
each thyristor turns on and off due to the change in current
direction.
As seen in the picture above, the two thyristors are placed in
opposite directions. AC Relays with Triacs are often used in
lighting applications where dimming is involved.
7. Advantages/Disadvantages
• Electromagnetic Relays (EMRs)
– Simplicity
– Not expensive
– Mechanical Wear
• Solid-state Relays (SSRs)
– No Mechanical movements
– Faster than EMR
– No sparking between contacts
• Microprocessor-based Relay
– Much higher precision and more reliable and durable.
– Improve the reliability and power quality of electrical power systems
before, during and after faults occur.
– Capable of both digital and analog I/O.
– Higher cost
8. Why A System Needs Protection?
• There is no ‘fault free’ system.
• It is neither practical nor economical to build a ‘fault free’
system.
• Electrical system shall tolerate certain degree of faults.
• Usually faults are caused by breakdown of insulation due to
various reasons: system aging, lighting, etc.
9. Protection and Relay Schemes
• Motor Protection
• Transformer Protection
• Generator Protection
10. Conclusion
• Relays control output circuits of a much
higher power.
• Safety is increased
• Protective relays are essential for
keeping faults in the system isolated and
keep equipment from being damaged.
