[RE-wrenches] Solid state relay (SSR) questions

[Mick Abraham]

Hello, Wrenchers and special greetings to the silicon gurus to whom I
address these questions about solid state relays.

A client wants a gizmo to connect grid "AC In" to their inverter/charger
when the battery voltage is low...resembling the LBX function on the good
ol' Trace SW inverter. The grid power would feed into a 120/240 Magnum
inverter/charger which is rated for 30 amps pass through on each hot leg.

I may use a solid state relay for the heavy lifting: double-pole,
single-throw, normally open...with DC for the control voltage. Using a
voltage controlled switch, I'll send battery DC to the SSR when the battery
voltage goes too low for too long; then I'll remove that DC trigger when the
battery voltage recovers.

Question 1: Are SSR's suitable for the variety of loads (including motors)
that this circuit might carry?

Question 2: Should the SSR be massively over-rated relative to the expected
load (as with typical electronic design practice)?

McMaster-Carr stocks solid state relays with built in heat sinks; model
7456K43 is published for 30 amps @ 230 volts AC. If protected with a 30 amp
double-pole breaker, would this relay suffice long term or would it instead
produce expensive smoke? (Let's assume free air conditions around the heat
sink and a 90 degree F ambient temperature.)

Question 3: Should I instead bump up to model 7456K44 with a 50 amp rating
for this 30 amp pass through situation?

I like the idea of no moving parts/no arcs/no clunks compared to a
mechanical power relay, but I also want no customer callbacks. High speed
switching (pulse width modulation) is not one of our requirements, so...

Question 4: ...is it bad strategy to even consider using an SSR for this
application?

Thanks in advance,

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675
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