need a two pole solid state relay for 220volt?
need a two pole solid state relay for 220volt?
(OP)
I'm designing a controller for a small process heater, switching 220volts at 20amps. Since the controller uses a ramp/hold pattern with a lot cycles, an SSR is recommended for reliability. Question is: should it have a two pole relay? the 220volt is the north american "two phase" system with 220v line to line and 110volt to ground. The risk is that with only one pole the heater has 110volts to ground at all times. Certainly mechanical contactor designs are two pole in my experience, say for HVAC compressors, but adding a pole to an relay is relatively cheap, not so for an SSR





RE: need a two pole solid state relay for 220volt?
RE: need a two pole solid state relay for 220volt?
If the heater is 240V then there is no neutral involved. A single pole SSR is normal. But as Comp mentions you should include a mechanical relay controlled by something other than the main temp controller (preferably) that is indeed latching so it requires human intervention after tripping. It usually is associated with a START button that then latches the contactor in.
Keith Cress
kcress - http://www.flaminsystems.com
RE: need a two pole solid state relay for 220volt?
RE: need a two pole solid state relay for 220volt?
Since you seem so new at this, please pay particular attention to what we said about over temperature protection. The damage caused by out-of-control and unattended heaters can be extremely serious and, eventually, pretty likely to happen.
RE: need a two pole solid state relay for 220volt?
RE: need a two pole solid state relay for 220volt?
Here's a graphic for what Compositepro is explaining about a limit controller. The safety contactor really is critical.
RE: need a two pole solid state relay for 220volt?
Dan - Owner
http://www.Hi-TecDesigns.com
RE: need a two pole solid state relay for 220volt?
RE: need a two pole solid state relay for 220volt?
There is no reason to use SSRs in both poles from a functional standpoint, the control of heat will be the same whether it is one pole or two. But it is slightly more reliable in that if one SCR shorts, you still have control. Cheap ovens for cooking pizzas or sandwiches in small restaurants will usually only use one pole control to keep costs down, because the consequences of runaway heat becoming dangerous are low since they are usually attended and designed with a separate On-Off controller which acts as that safety contactor if anything goes wrong. But if the machine is making something that generates a lot of revenue and down time is going to result in losses, that second pole of control means a continued revenue stream, so the cost of it is immaterial.
The other issue is that the fused switch or circuit breaker PROTECTING this equipment must still be 2 pole. It's OK to control with only one pole, but you must PROTECT both circuit conductors.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: need a two pole solid state relay for 220volt?
RE: need a two pole solid state relay for 220volt?
Bill
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"Why not the best?"
Jimmy Carter
RE: need a two pole solid state relay for 220volt?
A few years ago I found out that elevators that use Wye-Delta controllers routinely use only 2 contactors instead of 3, meaning one side of the motor circuit is permanently connected to the line. That means of course that one welded contact anywhere else results in unrestricted current flow to that winding and the likely loss of the motor. They don't seem to care.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington