Question about relays...

[John]

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?

 

[Andrew Gabriel]

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.

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.

 

[Tim Perry]

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.

is this a smallish relay inside a clear plastic case?

 

[Joe Waag]

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?

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...

 

[PCK]

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. not likely at over 1 hp

 

[Bob Peterson]

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.

 

[daestrom]

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.

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.

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?

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