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It’s always been my understanding that when electric motors get into the 3 HP or larger size that a magnetic contactor type switch should be used. Is this the case for those of you having high power woodworking machinery?
A magnetic contactor type switch has a low voltage coil, that when activated with a small switch, pulls in a set of large contacts (usually made of a silver alloy). Around the typical home, such switches are usually only found on central air conditioners.
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OSHA requirement. When power goes off the machine won't start up when power is restored.
*I want to get a magnetic switch upgrade for my Jet Contractor's saw. I had a good scare when the motor circuit breaker tripped while bogging down in a thick rip cut. I unplugged the saw, (stupidly forgetting that the wood was still on the saw)reset the breaker, and plugged it back in. Whammo! The saw fired the wood like a missile, classic kickback. Lucky for me I wasn't in the line of fire. A magnetic switch would have prevented this act of stupidity from launching the wood.
*Sounds like good fodder for an upcoming FWW article to me. Momentary contact switches and latching circuit contactors belong on most all shop equipment, and can be had from W.W.Grainger and many other industrial supply shops (ask for motor starter switches). Most are staffed to coach you in proper sizing for overload heaters, so have the information from the motor data plate handy when you commence shopping.John Adams
*Alex, threre's product a called Start Guard that Delta and Hubbel sell that will prevent your saw from starting without hitting a reset.Hubbel-703-382-1626...ask for the safety division. Easy retrofit and cheaper than most magnetic switches.Should also be in the Delta catalog...at least I used to see them there. Over 2 hp you should have a magnetic starter as the Start Guard is only good up to 2 hp single phase.
*There's another device available that's similar to the Start Guard. It's called the Sensing Saf-Start and will work with 120v or 240 v tools, up to at least 3 hp. You wire it in between the power plug and on/off switch. I've been using one and it works fine. For more info, call the company at 916-933-2699.
*Hmmmmm.... I think I know a wee bit about this.According to companies like baldor, motors of 5 HP and larger need mag starters regardless of phase orientation. Truth is, a motor running 220 volts will be a problem regarless of amperage and here is why. During a short circuit in which you are the short circuit, you will have a 220 volt potential between you and ground and that potential has to go to zero once finished with you! Thus, according to ohms law, you become the resistive load and as such govern the amount of current passing through you. Quite likely, enough to burn off your knuckle hairs or get you into an emergency room.In three phase, you have three switches that need to close together. So its natural to have a single botton gizzmo to so this. These are called contactors or starters and use a coil to pull in the switch bar. Pull in voltages vary greatly going from line voltages in the 440 range, for example, all the way down to voltages like 24 volts AC. When I assembled my phase converter, I had to increase the pull in voltage from 110 to 220 since my converter's control voltage was line voltage. Thus, I had to recompute the contactors turn ratio and rewind the contactor pull in coil. Now, this westinghouse, slams shut on 220 volts. So, three phase contactors are the worlds first mag starters. In fact, you can use a three phase contactor on a single phase motor if you so desire leaving two sets of points open.Another advantage of a contactor or mag starter is that even in the on position, these devices depend on voltage to remain open. So, if your running a jointer and your lights go out, then on return of power, your jointer will not suddenly come to life unattended.As for a safe pull in voltage..... jury's out on this one. If I have a short circuit in my all metal machine, then its possible that the machine's frame will have a 220 potential and you get it. This assumes that I have wood or rubber mounting pads underfoot. It is nice to pokin around with a bottom that only has 110 volts behind it instead of 220 or in the case of my jointer, 440 volts. I would hate to be zapped by 440! General rule these days have mag starter pull in voltages set to about 110.Other forms of motor starters have multiple settings to allow motors to step up RPMs on start up. You see these types in places like ski resorts and so forth. We do not need these at home, but they are extensions of mag starter philosophy.I would reccommend you use these on any stationary power tools in which the switch does not already pop open on release. I am amazed that porter cable has gotten away with its 690 router switch design as long as it has! I have had my 690 come to life unexpectantly a couple of times!
*Gee, maybe I shouldn't be coming out here and running my tools while barefooted ya recon? :)The fear of amps crusing through my body is one reason why I installed an 10' copper ground rod outside my shop.Dev - if the switch on the coil is not a manual reset type, then when the power comes back on, the coil will kick in and the contactors will re-energize the motor. With my central air conditioner, there is a delay circuit built into the thermostat that prevents the coil from re-energizing for about 2 minutes. This prevents the compressor from starting up under load as it takes about 2 minutes for the high pressure side to bleed down. The point is - for human operated machines you should have a manual reset switch on the coil of magnetic contactors, not an automatic reset. The motor on my Rockwell 10" TS has a manual reset overload switch on the motor, but nothing in case there is a power outage. I can remember maybe two times in my lifetime that the power did go off while running the TS. I did remember to quickly throw the switch to the off position.
*Mike, what you really need is to be sure the starter has a set of holding contacts, there are manual (line starts) available for lower horsepowers that are mechanical and won't disengage if the power is disrupted.bake
http://www.saf-start.com/
Start Guard and Sensing Saf-Start are the same thing. Only good up to 2 hp. After that a magnetic starter is the way to go
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