Crydom has been well known for over 40 years as a
supplier of Solid State Relays (SSRs). However, Crydom
also designs, manufactures and markets Solid State
Contactors (SSCs). What is the difference between
SSRs and SSCs?
Remarkably, there is very little actual difference. They
use similar power semiconductors and control circuits,
and in some cases, even the same housings. SSRs, being
considered as components, are applied in a large variety
of applications and uses. SSCs, are generally applied in 3
phase AC heater and motor control applications although
the SSCs themselves can be used successfully in almost
any load control application. Why then are they viewed
and applied differently?
There are two main reasons: Tradition and Ratings.
Tradition is that for most AC power control
applications utilizing 3 phase AC power and some DC
applications, traditional mechanical contactors are
employed. (Note: mechanical contactors rated to switch
AC loads are quite different from those rated for DC
loads of similar currents due to the arcing and contact
degradation associated with making and breaking a DC
circuit). Therefore when the need arises to use solid state
technology in these type applications rather than EMRs,
engineers immediately think of Solid State “Contactors”,
not Solid State “Relays”. So they are disposed to consider
SSCs rather than SSRs despite the fact that SSRs can
perform exactly the same switching function as a
Ratings of contactors, whether Solid State or
Mechanical, always include allowed motor load ratings
and allowed resistive load ratings. The reason for this is
again tradition because for most mechanical contactors,
the switching capabilities and life expectancy vary
significantly for each type of load. Further, motor control
requires consideration of such aspects as Locked Rotor
Rating, Full Load Current Ratings and Horse Power Rating,
while resistive load ratings must account for significant
inrush current that also degrades mechanical contacts.
SSRs and SSCs don’t suffer the same type degradation
due to load characteristics as mechanical contacts do
and therefore the motor and resistive load ratings are
not as widely different. However the one significant
differentiator is that to be considered a contactor, the
SSR or SSC must be evaluated to and carry ratings
appropriate for motor control.
So in summary, the major technical difference between
an SSR and SSC has to do with the mandatory motor
ratings required to be defined as a “Contactor”.
Solid State Relays versus Solid State Contactors