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:
http://www.aaaim.com/u/web/aaaimc/cgi-local/shop991/shop.pl/SID=59872674/page=RELS.htm#RL2052
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

 

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

 

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?

 

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

 

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

Ed