1. Home
  2. Hobby Electronics Basics
  3. Is this relay appropriate

Is this relay appropriate

I have a 120 VAC 1 hp motor that I want to control with a solid state relay because they are sealed. Looking at solid state relays with 3 to

32 VDC control voltage I found a couple that might work. 1 is rated 240 VAC 25 amps. The other is rated 240 VAC, 40 amps, and is zero crossing. And twice as much as the 25 amp model. Since the load is an AC induction motor does the zero crossing relay need to be used? Is either relay appropriate? Will these relays rated at 240 volts be OK switching 120 volts as long as the amperage is within spec?Here's a link to the relays:
formatting link
The stock numbers are RL9801and RL2052. They are located near the bottom of the page. One other thing comes to mind. Since these relays are solid state devices and they may be sensitive to voltage spikes (I don't know if they are or not). If they are, and fail in the ON mode, it won't be a safety hazard because the motor can run continuously without burning up. The motor runs a circulating pump so the water would just get circulated more. Thank You, Eric R Snow
Reply to
Eric R Snow
Loading thread data ...

--
If that\'s true, why can\'t you just let the pump motor run all the
time and blow off all the control crap?
Reply to
John Fields

Greetings John, I should have described the situation better. I'd make a lousy teacher. The pump only needs to run about 30 minutes 5 or 6 times a day. So, while running the motor non-stop won't hurt the system or be a safety hazard, it will use electricity. Eric

Reply to
Eric R Snow

formatting link

The problem running motors from any switching mechanism is the locked rotor current that occurs for a few cycles at start up, and lots longer if anything actually jams the rotation. Have you compared the locked rotor current rating of the motor to the surge current rating of the relay? Otherwise, the only problem you may have is that the TRIAC switches in many solid state relays don't handle inductive loads, like motors very well. The inductance causes the current to shift phase relative to the voltage. This causes a problem at turn off. When the current goes through zero and the TRIAC switches off, there is significant line voltage at that moment that appears across the TRIAC, almost instantaneously, and tries to turn it back on by capacitive coupling of the rising voltage to the internal gate. So you may need a pretty big capacitor in series with a current limiting resistor connected across the TRIAC or motor. Maybe a 1uF or 2.2uF line rated cap in series with a 10 to 22 ohm 1 watt resistor.

One other question: A 1HP motor seems pretty big just to move water around in a circle. Must it be so large? Is the circulation being used as a heat source?

Reply to
John Popelish

formatting link

Yes, it is. And the relay it currently uses has failed a couple times. The contacts fail. Everything else has been checked and it seems that the relay it came with is undersized. No wonder the company that built it is now out of business. I had a friend who installs and repairs HVAC units looked at the control and said the relay current capability was too small for the surge. Since the relay is in a humid enviroment I thought a nice sealed solid state relay would be best. My friend who does the HVAC stuff doesn't know enough about SSRs to tell me if this application is OK. I guess I'll just get a machanical relay with the proper rating. Thanks, Eric

Reply to
Eric R Snow

Good move - it will be cheaper. And add an R/C snubber across the contacts to prevent/lessen arcing and contact burning.

Ed

Reply to
ehsjr

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.