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Magnetic Switch Benefits ?

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Forum topic by Eric posted 01-14-2018 05:12 PM 522 views 0 times favorited 16 replies Add to Favorites Watch
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Eric

221 posts in 2539 days


01-14-2018 05:12 PM

I’m reconditioning a 1966 Powermatic 60 with a re-placement 3hp 220v motor. Are there any motor life expectancy benefits to using a magnetic switch? It came with an old aftermarket mag switch, but I think I would prefer a paddle switch that is easier to get to on the front of the machine assuming the powerups with a simple switch are not damaging to the motor. Thanks

-- Eric


16 replies so far

View Carloz's profile

Carloz

1147 posts in 619 days


#1 posted 01-14-2018 05:26 PM

No difference for the motor. For yourself: With magnetic switch the saw does not start by itself after a power failure.

View TheFridge's profile

TheFridge

9608 posts in 1513 days


#2 posted 01-14-2018 05:26 PM

Nope. As long as the switch is rated for the HP motor you out on it it’s fine.

Worst case is motor startup/shutdown arcing kills the contacts on the switch a lot sooner than expected.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View MrUnix's profile

MrUnix

6767 posts in 2226 days


#3 posted 01-14-2018 05:46 PM

Motor have overload protection built-in?

-- Brad in FL - In Dog I trust... everything else is questionable

View msinc's profile

msinc

430 posts in 531 days


#4 posted 01-14-2018 06:09 PM

Well, first off do you have a simple magnetic switch or is it a magnetic starter??? There is a difference. A magnetic switch {which you probably don’t have} is simply a switch that makes contact {closes the circuit} when a magnetic field is passed across it. A magnetic starter is different in that it does offer some protection, such as low voltage start up or motor overload. It will also “kick out” {open the circuit} in the event of a power failure. A magnetic switch will not open until another magnetic field is passed across it.
Magnetic starters are commonly used on electric motors rated with more than one horsepower and since it is what you push to turn the motor on it is commonly referred to as a “magnetic switch”. In terms of the motor wearing out, no, it wont prevent or lessen that. But, as above it does offer some protection and that is why they are commonly used on larger motors that sometimes run with a heavy load.
The latest types, which have been around for quite some time now are “digital” and employ a microprocessor for control. I would suggest you install some type of magnetic starter on your saw for the types of protection it does offer as opposed to a simple mechanical on/off contactor {which is what it sounds like is your candidate for another option.}

Edit: I guess I should offer some examples of the protection a magnetic starter does afford…imagine a welder that is off but has the working electrodes shorted. The typical metal welding table having the one clamp affixed and a welding rod clamped in the other laying right on the table so as to complete a dead short. Turn on the welder with a simple contactor type switch that has no overload protection and you could quickly burn up a winding in the transformer or in the commutator {if it’s a that type welder}. With your table saw, a piece of wood or other object could have been inadvertently placed on the table in such a way as to interfere with the blades easy rotation on start up…”throw the switch Igor” and you could again quickly burn up a winding. With a magnetic starter it would simply trip out and not allow the motor to attempt a dangerous {to the motor at least} overload startup.
I know these are somewhat stupid and “rare” scenarios, but because of the possibility…...thus we have the NEC.

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TheFridge

9608 posts in 1513 days


#5 posted 01-14-2018 06:19 PM


Motor have overload protection built-in?

- MrUnix

Forgot about that.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View William Shelley's profile

William Shelley

572 posts in 1497 days


#6 posted 01-14-2018 06:43 PM

The answer, as it always is: Replace the motor with a 3-phase motor with equivalent RPM and frame size, and use a VFD to run the 3ph motor. Then you get soft-start (thus prolonging the life of the motor), and a MUCH more accurate overload protection provided by the VFD, because it’s microprocessor constantly monitors the electrical load.

-- Woodworking from an engineer's perspective

View Eric's profile

Eric

221 posts in 2539 days


#7 posted 01-14-2018 07:11 PM

I’ll have to check but, yes, its probably a mag starter. The machine is a 1966 Powermatic 8” jointer. It had a 2hp 3 phase motor with the mag starter, which I believe was already a replacement motor. I put in a single phase. To get the correct shaft and rpm, I purchased 3hp. Motor has a reset switch so I assume that’s for overload protection. Its a Smith & Jones 3hp with dual capacitor start… whatever that means.

-- Eric

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MrUnix

6767 posts in 2226 days


#8 posted 01-14-2018 07:28 PM

If it has a reset switch on it (Klixon w/little round red button), then it’s got built in overload protection. A simple magnetic paddle switch will work in that case. Just make sure it’s rated for your motor. They are not as robust as a starter and the contacts will fry quicker… but they are also significantly cheaper :)

Oh, in answer to your original question – no, there are no motor life benefits to using a magnetic switch other than if external overload protection is needed, which a starter provides (IEC or NEMA).

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View msinc's profile

msinc

430 posts in 531 days


#9 posted 01-15-2018 05:10 AM


I ll have to check but, yes, its probably a mag starter. The machine is a 1966 Powermatic 8” jointer. It had a 2hp 3 phase motor with the mag starter, which I believe was already a replacement motor. I put in a single phase. To get the correct shaft and rpm, I purchased 3hp. Motor has a reset switch so I assume that s for overload protection. Its a Smith & Jones 3hp with dual capacitor start… whatever that means.

- Eric

Most reset switches found on power tools, probably better than 95% or so, are nothing more than simple heat protection, they are not NOT overload in a sense of true “overload” which is really too much or high amp draw caused by bogging the machine down. They are nothing more than a bi-metallic strip that kicks out at a certain temperature. Think about the last time you tripped one…it will not reset immediately because the proximity of the strip has to have time to cool down. If wired properly for the machine you have a form of load protection, although not “ideal” in the breaker panel…it’s called a breaker. I mean, I guess if you want to over heat the motor until it trips out and call that overload protection then okay. Most, just about all in fact, magnetic starters offer true load protection.
As far as 3 phase power/motors, there is no question it is a better way to go…..if you have it. These people post as if “all ya gotta do” is pick which one you prefer and swap it on over, like it’s a simple matter of throwing a switch or getting a different $20.00 breaker to just slap in the panel. If you don’t have 3 phase power in the building already it can be way too cost prohibitive. I believe that many folks don’t even know you have to have a whole separate meter base, meter and panel installed for 3 phase power. Phase converters work, but they too are not exactly stellar. The best use of a phase converter is for when you have a machine that is only available in 3 phase or the one you have is already 3 phase, but you don’t have 3 phase power in the shop. If you are not too far from a transformer having 3 phase power ran to your shop is nothing about $2-$3000 wont take care of.
While 3 phase power is costly to install, it does save on electric bills if you use a heavy machine a lot. Unfortunately, a phase converter does the opposite and adds to the electric bill.

View MrUnix's profile

MrUnix

6767 posts in 2226 days


#10 posted 01-15-2018 05:24 AM

Most reset switches found on power tools, probably better than 95% or so, are nothing more than simple heat protection, they are not NOT overload in a sense of too much amp draw. They are nothing more than a bi-metallic strip that kicks out at a certain temperature.
- msinc

Guess how the overload protection in a NEMA starter works :)
(Hint: They are called ‘heaters’ or more precisely, ‘heater elements’ and are sized based on current)

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View msinc's profile

msinc

430 posts in 531 days


#11 posted 01-15-2018 05:41 AM


Most reset switches found on power tools, probably better than 95% or so, are nothing more than simple heat protection, they are not NOT overload in a sense of too much amp draw. They are nothing more than a bi-metallic strip that kicks out at a certain temperature.
- msinc

Guess how the overload protection in a NEMA starter works :)
(Hint: They are called heaters or more precisely, heater elements and are sized based on current)

Cheers,
Brad

- MrUnix

And again our resident electronics genius has spoken…....I guess. So, just to be clear, you actually believe that a motor being allowed to run over loaded until it gets too hot to operate and eventually trips out via an unreliable bi-metallic strip is a good way to protect it from overload???? Okay, so don’t use a “NEMA” starter, use the one with the microprocessor that affords real time load monitoring both under and over. Never mind…cancel that last sentence…please stay exactly as you are now.

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MrUnix

6767 posts in 2226 days


#12 posted 01-15-2018 05:53 AM

So, just to be clear, you actually believe that a motor being allowed to run over loaded until it gets too hot to operate and eventually trips out via an unreliable bi-metallic strip is a good way to protect it from overload????

Ummm, no – that isn’t how it works. Motor temperature does not factor into the equation.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View TheFridge's profile

TheFridge

9608 posts in 1513 days


#13 posted 01-15-2018 06:30 AM

It’s an actual heating element rated in a specific amperage size to the fla of the motor. The reset springloads a sprocket set in solder or the like and will only turn to trip when the heater gets hot enough. They are a size very specific with a narrow range.

Fla 3.6?

Pick a heater in the 3.52 – 3.66 range (not actual spec but basically)

I’m sure there are others with bimetallic strips and gadgets but for many it’s very similar but maybe with a different type of mechanism.

Edit: I’ve haven’t learned anything new about their construction since I learned about them in class.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View PPK's profile

PPK

1047 posts in 837 days


#14 posted 01-16-2018 03:40 PM

I’m real fond of mag switches for the added safety.

-- Pete

View teejk02's profile

teejk02

481 posts in 1153 days


#15 posted 01-17-2018 12:28 AM



So, just to be clear, you actually believe that a motor being allowed to run over loaded until it gets too hot to operate and eventually trips out via an unreliable bi-metallic strip is a good way to protect it from overload????

Ummm, no – that isn t how it works. Motor temperature does not factor into the equation.

Cheers,
Brad

- MrUnix

Looks like somebody wants a fight. My money’s on you. But I have to ask…you seem to know so much about just about any machinery related topic ever posted here (some brands that I have never heard of). Care to share your background?

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