Maintenance Technology September 2016 - 41
AUTOMATION
SSR or EMR?
Select the Right Relay
Solid-state and electromechanical relays are not necessarily interchangeable. Evaluate your application before deciding which to use.
SOLID-STATE RELAYS (SSRS) are replacing electromechanical relays (EMRs)
in many applications across industry. There are several reasons why, including
their long life, low noise, compact size, lack of moving parts, and total absence of
arcing. These advantages make SSRs a popular choice for applications involving
repetitive operations or fast turn-on/turn-off times, or in areas that require
minimal electrical noise.
So, what types of SSR or EMR relays are right for the various applications in
your plant? Automation professionals at Opto 22 (opto22.com), in Temecula,
CA, provide some selection guidelines.
Special SSR Concerns
According to Opto 22's (Temecula, CA) automation experts, in the use of solid-state relays
(SSRs), two factors inherent to semiconductor-based relays require special attention:
■ Leakage current. When in the "off" state, an SSR will exhibit a small amount of
leakage current, typically a few mA. It's slight, but this current can keep some
loads from turning off, especially in high-impedance applications such as small
solenoids or neon lamps, that have relatively small "hold in" currents. When SSRs
that switch high voltages are electrically open, leakage current can still cause their
circuits to produce potentially troublesome voltages on the outputs. These issues can
usually be addressed by placing a power resistor, sized for 8 to 10 times the rated
maximum leakage current for the SSR, in parallel with the load.
■ Operational-temperature limits. Semiconductor-based relays become less efficient
as their temperature increases. Thus, the current rating for an SSR is de-rated, or
reduced, based on the ambient temperature. Since SSRs also generate heat in the
"on" position, heat management is vital.
Use SSRs in applications
that require:
■ Repetitive operation cycles.
Such applications include lights
and electric heaters. SSRs have no
mechanical components to wear out
and no failure mode related to the
number of operation cycles.
■ Minimal electrical noise. SSRs
greatly minimize electrical noise
because they turn on and off when
voltage is zero in the AC cycle.
Conversely, most EMRs turn on
and off at any point in the AC cycle,
which means they can generate
significant voltage spikes, causing
electrical noise that can affect other
devices in the area.
■ High-speed timing. SSR turn-on
times are highly predictable, while
times for a mechanical relay vary
based on the nature of the device
and the environment.
Consider EMRs in
applications that require:
■ High starting loads. Such applications include motors and transformers. SSRs are more sensitive
to voltage transients than EMRs.
If a relay gets hit hard enough a
sufficient number of times, even
SSRs with good transient protection
will degrade or fail. This makes SSRs
less ideal for driving highly inductive
electromechanical loads, such as
some solenoids and motors.
■ Operation in high-temperature
environments. SSRs become less
efficient as the relay temperature
rises. The current rating for an SSR
is de-rated, or reduced, based on the
ambient temperature. EMRs are not
affected in the same way.
■ Zero leakage current. In the "off "
state, an SSR will exhibit a small
amount of leakage current-typically a few mA. Because EMRs
are mechanical, they do not leak
current. MT
-Jane Alexander, Managing Editor
For more information on various automation-related topics and technologies that link electrical, mechanical,
and electronic devices to networks and computers, visit opto22.com.
SEPTEMBER 2016
MAINTENANCETECHNOLOGY.COM | 41
http://www.opto22.com
http://www.opto22.com
http://www.MAINTENANCETECHNOLOGY.COM
Table of Contents for the Digital Edition of Maintenance Technology September 2016
Editorial
Uptime
On The Floor
Old-School Approach To New-World Technology
Art Meets Reliability Science
Commit To Safety
Robot-Human Interaction Getting Safer
Sanitary, Stainless Conveyors Not The Same
Reduce Hidden Process-Safety Costs
Pace Your Reliability Program
Win The War With 'Lubrication By Design'
SAP Tips And Tricks
Understand Pump Vibration
Select The Right Relay
Bearing Care
Clear Up HOA Confusion
Potato Waste Becomes Clean Water, Energy
Products
Showcase
Ad Index
Final Thought
Maintenance Technology September 2016 - 1
Maintenance Technology September 2016 - Cover1
Maintenance Technology September 2016 - Cover2
Maintenance Technology September 2016 - 1
Maintenance Technology September 2016 - 2
Maintenance Technology September 2016 - 3
Maintenance Technology September 2016 - Editorial
Maintenance Technology September 2016 - 5
Maintenance Technology September 2016 - Uptime
Maintenance Technology September 2016 - 7
Maintenance Technology September 2016 - On The Floor
Maintenance Technology September 2016 - 9
Maintenance Technology September 2016 - Old-School Approach To New-World Technology
Maintenance Technology September 2016 - 11
Maintenance Technology September 2016 - 12
Maintenance Technology September 2016 - 13
Maintenance Technology September 2016 - 14
Maintenance Technology September 2016 - 15
Maintenance Technology September 2016 - Art Meets Reliability Science
Maintenance Technology September 2016 - 17
Maintenance Technology September 2016 - 18
Maintenance Technology September 2016 - 19
Maintenance Technology September 2016 - Commit To Safety
Maintenance Technology September 2016 - 21
Maintenance Technology September 2016 - 22
Maintenance Technology September 2016 - 23
Maintenance Technology September 2016 - Robot-Human Interaction Getting Safer
Maintenance Technology September 2016 - 25
Maintenance Technology September 2016 - Sanitary, Stainless Conveyors Not The Same
Maintenance Technology September 2016 - 27
Maintenance Technology September 2016 - 28
Maintenance Technology September 2016 - Reduce Hidden Process-Safety Costs
Maintenance Technology September 2016 - 30
Maintenance Technology September 2016 - Pace Your Reliability Program
Maintenance Technology September 2016 - 32
Maintenance Technology September 2016 - 33
Maintenance Technology September 2016 - 34
Maintenance Technology September 2016 - 35
Maintenance Technology September 2016 - Win The War With 'Lubrication By Design'
Maintenance Technology September 2016 - 37
Maintenance Technology September 2016 - 38
Maintenance Technology September 2016 - SAP Tips And Tricks
Maintenance Technology September 2016 - Understand Pump Vibration
Maintenance Technology September 2016 - Select The Right Relay
Maintenance Technology September 2016 - Bearing Care
Maintenance Technology September 2016 - 43
Maintenance Technology September 2016 - Clear Up HOA Confusion
Maintenance Technology September 2016 - Potato Waste Becomes Clean Water, Energy
Maintenance Technology September 2016 - 46
Maintenance Technology September 2016 - 47
Maintenance Technology September 2016 - 48
Maintenance Technology September 2016 - 49
Maintenance Technology September 2016 - 50
Maintenance Technology September 2016 - 51
Maintenance Technology September 2016 - 52
Maintenance Technology September 2016 - Showcase
Maintenance Technology September 2016 - Ad Index
Maintenance Technology September 2016 - 55
Maintenance Technology September 2016 - Final Thought
Maintenance Technology September 2016 - Cover3
Maintenance Technology September 2016 - Cover4
Maintenance Technology September 2016 - 62
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