I entered this in the knots forum, but thought I’d post it here also.
I trying to hook an extension switch for my Dust collector up.
I have a Berco thermostat relay rated 25 amps resistive load.
The Dust Collector motor is rated 10 amps.
I have all the wiring and such, onlt need to know if the relay can
handle the motor.
I’m not up to speed on the resistive rating verses the motor rating.
Any electricians or electronic people out there who might know the
answer.
Jeff
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Replies
I do believe that Lee Valley has a peddle operated remote switch for any type of appliance.
It will work but only for a short while.
Resistive loads just start pulling current up to their potential. Say you had a 10 amp lightbulb. It will pull 10 amps (ignoring a millisecond phenomenon peculiar to incandescent lightbulbs)
A Motor load is different. For the first few seconds of its circuit life the motor has to do several things that suck lots of amperage.
The first thing that happens is you have to establish the magnetic field in the motor. (wires wrapped around steel make a magnetic field) Then another field is generated in the rotor (the rotating part) windings. Then the motor starts to magnetically chase itself. Until the motor comes to running speed it is drawing much more current than the rated amperage. 6 times the rated amperage is not unusual for a motor starting.
So why don't you put a 60 amp circuit in for a 10 amp motor? Well it is only for a short time and the wire you use for ten amps (say 14 gauge) can carry much larger currents for short periods or even long periods of time but by code it is limited to 15 amps. The NEC 15 amp rating I believe takes into account all the uses including motor loads.
Why doesn't the breaker trip? Some might, as there are different trip curves in breakers used in the industrial world, but home load centers have a trip curve that allows them to accept motor inrush current for a short period of time. This inrush is expected in circuit breakers (I squared T) as different products manage it per your need.
Electrical contacts, your thermostat relay, are much different. A breaker just has to maintain a circuit. A contact set like your thermostat has to make and break the electrical load. The contact set you are using to make the motor run is using its contacts for connection at the worst possible time. That time is when you need the magnetizing current and the power to chase the field to running speed. What happens is the equivalent of Arc welding on the contacts. Each successive time it starts the contacts become more pitted and the resistance is greater. Eventually there is nothing but resistance and no current.
If that wasn't enough there is a phenomenon on the other end of the motor as well. rotating motors when idling to a stop generate current as well. So when you turn off the motor with this thermostat relay the contacts pull an arc that is the equivalent of a lightening bolt between the contacts. More metal is lost there.
What makes an inductive contact set? First thing is "Snap Action" contacts that don't tease open or closed. Second thing is specially alloyed cadmium contacts that can take the heat. And sometimes third there is a set of arc chutes that arrest the breaking arc of a contact.
One of the greatest concentrations of millionaires (before the computer revolution) used to be in Freeport, Illinois. Home of microswitch the inventor of the snap action contact. Furnace contactors the big brother of the snap action contact were also situated nearby in Illinois. Back in the day there were many fires and injuries from the screwy ways we connected electric motors. Remember knife switches from watching Frankensteins monster? That was dangerous stuff.
So there is a difference. You can use the thermostat if you want, but get an interposing relay (one between the thermostat and the motor load) that is inductively rated like a Potter & Brumfield PRD____ relay. Electrical supply houses will have them, not sure on the box stores.
Edited 1/28/2003 9:19:28 PM ET by Booch
Thank you for the answer.
I had liked this Berko relay because it was all self contained with
a 24 volt transformer for the switch end and relay all neatly
packaged.
I can easily put in a motor relay.
The only differance will be that I'll have to mount it in my breaker
box and run a 120 V cable not a low voltage cable.
While not that much more work, I thought I'd save myself a trip to
the electric supply house here (an hour away).
Thanks again.
Jeff
You can use the same device but get a 24 volt coil on the interposing relay, or even a contactor with a 24 volt coil. I'd hit the web and look for an online source for a P&B relay if it is that much of a poke. There are thousands of options out there. Just don't use solid state relays. Lots of issues there, not the least of which is expense.Jack of all trades and master of none - you got a problem with that?
Found the foot switch.
http://www.leevalley.com/wood/page.asp?SID=&ccurrency=1&page=30049&category=1,240,41065
Thank you
I have that switch for my router table, works great.
My original idea was for a relay for my dust collector to set up
switches at the different woodworking machines.
Now that I know the relay I have in stock isn't strong enough, I'll
probably buy one of the remotes sold for this use.
Thanks again.
Jeff