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Forum topic by MrRon posted 12-29-2017 05:53 PM 363 views 0 times favorited 4 replies Add to Favorites Watch
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MrRon

4917 posts in 3326 days


12-29-2017 05:53 PM

My 3 HP saw runs off 220 V and uses a magnetic starter. Magnetic switches are available for 110 V tools. Why are magnetic switches not provided on all 110 V tools. A magnetic switch or starter is a safety device to prevent a tool from restarting automatically once power has been interrupted. It would seem that it would be mandated by OSHA. Any 110 V power tools I know of only have a normal on/off switch. ( I’m addressing only stationary power tools like saws, jointers, planers, etc. not portable tools. )


4 replies so far

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MrUnix

6845 posts in 2281 days


#1 posted 12-29-2017 06:49 PM

My 3 HP saw runs off 220 V and uses a magnetic starter. Magnetic switches are available for 110 V tools. Why are magnetic switches not provided on all 110 V tools. A magnetic switch or starter is a safety device to prevent a tool from restarting automatically once power has been interrupted.
- MrRon

A starters primary purpose is not safety, it’s the ability to control the large voltage and current requirements of an electric motor, as well as provide overload protection – and typically to do so using a low voltage as opposed to line voltage. They use a magnetic relay (contactor) that has considerably larger contacts than what you find in simple on/off switches, which reduces arcing and will provide longer service. The power-fail safety that you mention is just a side effect ;)

Magnetic switches however will only provide the safety from a power fail that you mention. The same is true with the overload protections built into some fractional HP motors (~2hp or less), where it requires a manual reset as opposed to an automatic one, which would be a safety hazard should it reset while you are near any of the spinning bits when it does.

Cheaper tools use cheaper methods to control power, and manufacturers know that people typically shop price over all else. Those $100 plastic bench top saws for example use toggle switches that probably cost the manufacturer a couple of pennies when purchased in bulk. A magnetic switch would certainly be much more expensive and drive up the cost of the tool. It’s much cheaper to include 3-4 pages of “Warnings” in the front of the manual.

Cheers,
Brad

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

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Carloz

1147 posts in 674 days


#2 posted 12-29-2017 07:00 PM

You do not need a magnet to drive larger contacts, just use larger contacts on regular switch.
I already mentioned that magnetic switches come from the industrial environment with dizens of machined connected to thd same circuit. It would not be possible to reciver after a power failure if all induction motors would try ti start at once. A magnetic switch was introduced as a remedy as after a power failure each machine has ti be turned individually thus limiting the startung load.

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MrUnix

6845 posts in 2281 days


#3 posted 12-29-2017 08:03 PM

You do not need a magnet to drive larger contacts, just use larger contacts on regular switch.
- Carloz

The magnet is just part of the contactor (relay), which is designed specifically to control high voltages/currents of an induction load without having to deal with the line voltage (isolation between load and control). That is why you see them on not just power tools, but in material handling, HVAC, compressors, and other industrial equipment that have high loads. A manual disconnect switch for a 5HP 440V three phase motor for example would be huge, would not provide overload protection, and be really inconvenient… when a contactor controlled using 24v is a much simpler solution, and allows flexibility to control many different voltages/current applications since they are modular, and individual components can be interchanged depending on the application and its particular requirements. That also reduces costs in both manufacturing and maintenance (less down time). Power fail safety is a nice side effect, but is not the primary design goal.

Cheers,
Brad

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

View Fresch's profile

Fresch

277 posts in 2003 days


#4 posted 12-30-2017 01:44 AM

They will also turn on a light or change color in your office when the contractor changes state; if you add aux. contacts.
Tools that are low voltage and power are “cord and plug” that is your safety disconnect.

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