SSR or Mechanical relay for a low power heater and a motor?

Inductive load: Vacuum Pump motor 115VAC 0.9Watts
Resistive load: Heater 115VAC 0.75 Watts

What is the best way to control these from a PLC output? SSR or a mechanical relay? Control as in turning ON/OFF 115VAC to Pump and heater. Neither will be toggled ON/OFF rapidly. At least 30 or more minutes in between an “ON to OFF to ON” again cycle. Reliability of the relay over cost is of utmost importance. Willing to use a relay rated for 2 or three times required current rating for the sake of reliability in the long run over cost. Vacuum pump being on is not a concern. Heater not getting turned off is. There is a secondary temperature monitoring scheme that will e-stop the system and cut-off power. Want to avoid this if at all possible. This will be a 24/7 in use system. Heard of mechanical relay’s getting fused in ON position and SSR’s failing in ON position. Which one is the lesser of two evils for inductive and resistive loads? As in what is the best and safest option? SSR (provided correct type is used for inductive and resistive loads) or, a Mechanical relay?


Thoughts? Suggestions?


Thanks.

Use an 8-pin ice cube relay. It has two sets of NO/NC contacts that should handle this just fine.

I assume you meant kilowatts, not watts. I would use a mechanical contactor (motor starter) for the motor, and a mechanical contactor for the heater WITH a solid state relay for modulation. If you close the contactor a half second before firing the SSR, the contactor will have almost no wear. Also, the contactor provides a positive disconnect to ensure that the heater doesn't get stuck on if the SSR fails "ON".

Thanks Sparkie/Rupej,


My mistake. I meant to say kW instead of Watts.

Np. Also, if you're not familiar, the amp rating of contactors vary by the type of load. Normally, their nameplate rating is in AC-3, which is for squirrel cage motors. Typically, their rating for AC-1, which is for resistive loads like your heater, will be much higher than that.

rupej said:

I assume you meant kilowatts, not watts. I would use a mechanical contactor (motor starter) for the motor, and a mechanical contactor for the heater WITH a solid state relay for modulation. If you close the contactor a half second before firing the SSR, the contactor will have almost no wear. Also, the contactor provides a positive disconnect to ensure that the heater doesn't get stuck on if the SSR fails "ON".

Click to expand...

I agree here. Based on the ratings, I suggested an ice cube relay, as with that small amount of current it is no big deal.

Now, you really need to determine the temperature range of your process to see if you really *need* a SSR to control this process. Typically, when heating tanks and such you have a pretty wild swing temp, so it isn't a big deal. If you are trying to keep a forming mold at a certain temperature (this screams packaging equipment), then you would want to use a 4-20mA input SSR rated for well over your load, a contactor for safety and then you can use a PID to hold the molds within a much tighter range.

I would use a 2 pole mercury contactor and mov terminal block for the relay output.

james