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I just installed a hanging fixture for two 40W T-12 fluorescent bulbs over my table saw. If I power up the saw when the lights are on, they flash off and come right back on (after the usual flickering). The saw is 1.75HP, 110V, 14A, and the circuit is 20A.

The receptacle currently powers the table saw from one outlet and a 1.5HP dust collector and the light fixture plugged into a power strip from the other outlet.

I know there is a big draw on the house's power when I turn on the saw, but this is a pain. Can I plug a UPS or other voltage-stabilizing device in front of the lights and dust collector to make this go away? Should I plug all the devices into a UPS? Or, should I deal with it and stop whining?

Thanks!

  • UPS systems don't like surge current, keep motors off of them. – Fiasco Labs Mar 11 '14 at 23:28
  • Also motors don't like the rough output of a UPS, unless you get a high-end sine wave one (and even a lot of those are more triangular than sine). – Hank Mar 12 '14 at 2:25
  • Yep, the most important part of your solar power system is an inverter that will run motors without shortening their lifespans. – Fiasco Labs Mar 12 '14 at 5:03
  • You could however plug the lights into a UPS. Preferably one where you can turn off the beeping when it dwitches to battery. Might require a more expensive online/souble conversion UPS to eliminate any flicker. – Grant Mar 13 '14 at 21:38
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When an electric motor turns on, there is a thing called "inductive kick". What this means is that for a second at start up, the motor looks like a short circuit and draws more than the rated amperage. Voltage is dropped as well. This is corrected by huge capacitors in an industrial situation. In your case, the combination of large current drain and a resulting voltage drop causes the ballast for the florescent lights to fail to deliver proper voltage to the bulbs. This is common. I would move the plug for the lights to another circuit. The only other thing that will cure the problem is a motor starting capacitor.

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1.75hp saw, 1.5 hp dust collector and lights all on the same 20 A circuit? Way too much before you even get to the lights.

Not only should the lights be on a completely separate circuit, the saw and dust collector should not be on the same 20A circuit - you are overloading that just with those (there are no 6 amp 1.5 hp 120VAC motors without using "magic"...an impossible 100% efficient 1.5HP 120V motor will draw 9.25 A, and real motors draw more since they are not 100% efficient.)

If you only add one circuit, put the lights with the dust collector. Better to provide a lights only circuit, then when you manage to trip the outlet breakers there's still light. Even better to switch the dust collector and saw over to 240V, if possible.

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A low cost UPS in front of the lights would fix the problem, cheaply and easily. I'd use it just for the lights not the dust collector. The only potential problem is the UPS might just alarm during the transient, which may be worse than the flicker.


Since the lights are on the same 20A circuit, moving them to another circuit would probably help a lot. You can extend a nearby circuit if permitted by code, or run a new 15 or 20 amp service.

A whole house surge protector might take away some of the transients that are giving the lights fits.

A different brand of light might perform better under the same circumstances.

Power filters (different from surge protectors) might also help, but they're pricey. Soft start motors or motor capacitors are a possibility, but too much trouble unless adding a new circuit is really impossible.

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While I think either shirlock or ecnerwal's answers are more likely to be the case here, another possibility is a loose connection somewhere along the circuit.

I would avoid a UPS, you would just be masking the problem, and they are typically not designed for repetitive short bursts of power loss - you'll likely wear down the battery and need to replace them frequently.. Depending on how much you use your saw, you could possibly drain the battery and not give it an opportunity to charge and you'd end up with the same problem.

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UPS's here and there are interesting options and may even work. But that's just addressing the symptom, not the disease.

The disease here is too much on one circuit.

The canonical and safest way to fix this is to create a circuit at the breaker panel for for the table saw. Nominal cost: breaker, cable, outlet, electrician (though many can do this by themselves).

End result: peace of mind.

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I believe that the lights are cutting off because the motor is drawing all of the available current when it starts. In other words, the lights which are on the same cicuit are being deprived of power. If you use a variable frequency drive (VFD) to control your motor, then the motor will only be allowed to draw a limited amount of current.

From wikipedia: In starting a motor, a VFD initially applies a low frequency and voltage, thus avoiding high inrush current associated with direct-on-line starting.

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