Switch contact ratings for motor?

I'm sure this is a straight forward question for someone here...Is there a rule of thumb for switch ratings when switching inductive loads?
I have a timer switch with contacts rated as 16A resistive. With a 240V single phase supply what what is the maximum power (HP or kW) motor the switch will operate.
cheers
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Yeah, usually they are called AC 3 and AC 4, IIRC. What's right for your case, is AC 3, for induction (squirrel cage) motors. Although the spec for your timer says 16 A, it would be a good idea to use your timer to operate a relay (magnetic contactor) and choose a single phase, two contact relay. This will move the (electrical)stress of opening and closing the contacts from the timer to the relay . I usually use those transparent single phase relays, which here are called valve type, because they are trasparent and look like old fashioned valves. They come with a base, that can be installed on a rail (panel) or with screws, and when the relay is toast, you just plug in a new one, instead of trashing the whole timer. BTW, the specs for AC 3 are: printed on any relay, and you can ask the vendor (where you buy it)
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If a switch is motor rated it will generally have an HP rating on it. You can usually get away with using small fractional HP motors on a switch rated at 16a resistive but I would not use more than 1/3 to 1/2HP on one. I believe what T.D is talking about is a "solid state relay" in the US and that will handle whatever the relay is rated for on a very small operating contact. They will even run directly off a CMOS device since it is just a LED doing the signaling an opto-isolator on the low current side and a big triac switching the load.
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snipped-for-privacy@aol.com submitted this idea :

I wouldn't advise using an SSR to switch motors. They are good for resistive loads and some inductive loads, but not motors due to the way they fail. If the DIAC develps a fault the relay can go "half closed" and burn out the windings in your motor.
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wrote:

I guess you never saw an IBM check sorter. They had about 50 SSRs running motors. The only real problem was that these were 3 phase and since there is never a true "zero crossing" they switched under power so they generally used something with a rating 5x-10x the expected load. They usually failed "full on". The single phase motors just ran all the time and the 3 p motors tripped the over current device.
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snipped-for-privacy@aol.com formulated on Sunday :

No, I can't say that I have, I have seen SSRs fail full on & open ciruit. That wouldn't hurt your motors, but I have replaced several that would allow "half"voltage when they were supposed to be de-energized. Thermal protection should protect the motor, but if the motor doesn't have proper protection or is running a refrigeration compressor. There is a company in Australia (who I won't name) that used ssrs to start refrigeration compressors. They went back to a coil relay after they noticed the number of compressor failures increased.
Some failures were due to burnt windings and some were caused by damaged valves from liquid floodback (caused by the compressor never turning off and icing up the evaporator coil)
When I wrote I wouldn't advise using an SSR to start a motor I didn't mean that you shouldn't ever use one. I can understand the logic of using SSRs in that sort of applicaton though, They have a higher switchlife, & no noise :)
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Arlowe expressed precisely :

my apolgies for the grammatical & spelling errors...
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wrote:

The biggest problem I have seen was when you had ripple on the DC supply (in a DC gate SSR) and it had a screwy wave form on the output side. What brand were you using? I have had pretty good luck with Crydom but I think the Opto22s are better. I have a bunch of them around the house doing different things.
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snipped-for-privacy@aol.com submitted this idea :

I've used several brands, Golink Crydom, Eurotherm, Omron... The Eurotherms are very good, our company buys 40Amp SSRs mounted on heatsinks W/DIN rail mounts. They are rated for a higher output voltage too (660VAC) and work at higher temperatures.
The only ones that have ever really been a problem (we only use them for resistive loads ie. Heaters) were Golinks. We had heaps of them fail because of faulty resistors in the optocoupler. Golink corrected the problem but we were out for alot of warranty replacements and were not game to try them again...
We use Crydom in most products, they seem to be very reliable.
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wrote:

No, it's a normal electromagnetic relay, but I know how its called only in greek. (Rele lychnias). See here, for example: http://www.alifragis.com.gr/view_cat.php?cat_idR0 They just have a transparent cover, and usually are single-phase, rated contacts for 16 A.

--
Tzortzakakis Dimitrios
major in electrical engineering
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Tzortzakakis Dimitrios laid this down on his screen :

In english, those are called plug-in relays.
The 10 amp units are easy to find and cheap, however the bigger units do start to get expensive. If the OP is looking for just a single pole then I would advise him to look at this: http://www.songchuan.com/lfpart.cfm?itemU http://www.songchuan.com/ppdf/841.pdf I have used hundreds of these and found them to be very reliable.
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Actually, it is what I visualised when you described it, though I was a little confused by your description of a "valve relay".
I would probably just describe it as a an "octal base relay" (though sometimes they may have more than 8 pins). They usually plug into a standard octal valve base which is presumably why you refer to them in the way you do.
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wrote:

Me too. I guess I am old enough to remember old tube guys who couldn't stop saying valve when we switched to silicon
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Can't say as I remember that particular problem. The biggest problem I recall, was trying to get my head round the idea that the "HT" line was negative WRT earth, especially with the early Germanium stuff.
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I'm an anal wrenching specialist!
Kirk Johnson
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