Question about relays...

Last year I bought a table saw that had a magnetic starter. After a few months it would not turn off. The relay had 4 poles but only used three, so
I found the bad pole and moved it to the unused one. Last week it happened again, but I no fresh poles. But that's okay because I wanted to change the saw to 240v anyhow, and that requires replacing the relay.
The old relay is an Potter and Beaufield (or something like that); with a great big coil and nice solid looking contacts. It is only rated for 1hp and the motor is 2hp, so the previous owner ran the hot through two contacts figuring it was 1hp each. The repeated failure suggests that is a bad scheme.
A 240v P&B replacement is $120, however I found a little plastic box rated to 3hp for only $35 in the McMaster Carr Catalog! ( 7105K12) Considering it is a third the size and price of the old one, and the old one was inadequate, I am a little sceptical that it can actually handle 50% more power.
But I know very little about relays, so maybe it is an improved technology. Whatcha think, should I try it, or is it just throwing money out?
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It doesn't work, because one contact will make slightly before the other and take the full initial hit. Also, the two contacts are likely to have quite different contact resistances and so the current is probably nothing like equally shared, particularly if one starts going bad.
--
Andrew Gabriel

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writes:

three, so

happened
the
a
1hp
contacts
is this a smallish relay inside a clear plastic case?
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so
happened
the
contacts
it
technology.
You say you need a "starter"....that would normally include overload sensors that kill power to the motor if there is a problem. The relay you are considering (if I am reading the catalog correctly) is just that - a relay. (no overloads). If you do *not* need a starter the relay should handle the motor (it's rated 3 hp at 240 Volts).
Just my opinion, make sure you are getting the proper part for your application...
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On Fri, 20 Aug 2004 19:14:23 GMT, John put forth the notion that...

If you use the proper size starter or contactor, you should not have the problems you're experiencing. You mentioned you wanted to rewire it to operate on 240 volts, but didn't say what voltage it is now. Assuming it's a 120 volt single phase motor, keep in mind that the overload heaters in the existing starter will be the wrong size if you change the voltage. If they're sized for 120 volt operation, they'll allow twice as much current as they should for 240 volt operation. You'll need to know the running load amperage when you order a new starter, because the heaters are sold separately. If you don't have overload heaters now, you probably have a contactor rather than a starter. I use hundreds of contactors in my business, and a 30 amp definite purpose contactor costs me less than ten bucks. The easiest place to find one would be a local HVAC supply outlet. This won't give you overload protection, but a lot of table saw motors have that built right into them anyway.
--
Checkmate

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three, so

happened
the
a
1hp
contacts
rated
Considering it

technology.
not likely at over 1 hp

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On Sat, 21 Aug 2004 05:35:23 -0400, PCK put forth the notion that...

I use 2 HP motors all the time with built in thermal overloads for one of the phase converters I build.
--
Checkmate

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Air conditioning relays (used to switch home a/c units on and off) are pretty cost effective for what you are doing. You don't switch the thing on/off very much so a light duty relay like that is fine.
Putting contacts in parallel is not only a bad idea, but is generally not permitted by code since they can't open simultaneously so one of the contacts actually has to break the whole load and eventually fails.

so
happened
the
contacts
it
technology.
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so
happened
the
contacts
You probably mean "Potter & Brumfield". They make a lot of different electrical relays. The fact that it's a 1hp contactor and you're using 2hp is obviously a problem. Putting the 'hot' through two contacts in series isn't the answer though. When closed, the current is twice what they are rated for (if they are in series). So when running, they will run 'hot'. And when opening (when most arcing and damage occurs), the arc will be harder to extinquish, hastening the burning of contacts.

it
technology.
Relays have several ratings, not just hp. The operating coil voltage and current (current of the coil isn't too critical unless it's being driven by solid state device). The contacts are designed for a specific voltage and current also. (also for AC vs DC and resistive or inductive loads) Even if its rated for 3 hp, if the voltage rating is wrong, that means the current it can safely switch is wrong.
Some motor controllers (like what you have there) also include motor overload protection or other specialties (I've even seen contactors with a RTD connection to monitor motor temperature, but not in residential installations).
Best solution would be to contact [no pun intended] the manufacturer and get their recommendation for a replacement part. Not the 1hp unit that was in there, the *right* part. Using an undersized contactor could weld the contacts shut or jam mid-position with the resulting arcing causing a fire.
daestrom
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