This is interesting information.?? I am NOT an electrical engineer and claim no expertise in motor protection.? Certain things jump out:
Many cheap Asian motors don't have enough information on the data plate to select protection.? Or the claimed horsepower doesn't match the current rating, or.....
The internal "klixon" type protectors are somewhat responsive to both motor temperature and current draw.? A simple external? "starter"--be it melting alloy or bimetal-- doesn't know how hot the motor is, but only how much current it is drawing.? So current
draw could be within ratings but if the fan is loose or the air passages clogged with debris, the motor could be cooking.
But, I would think that if the motor is stalled it would draw "locked rotor amps" and this ought to trip a properly selected overload or fuse pretty much immediately.? Not so?
My 4x6 gets only light use and I don't let it run unattended.? If I were to use it more heavily I think I would install an IEC type bimetal starter and back off the current adjustment until it tripped in normal use, then turn it up a little.....? Is this sound
thinking?
I also have a 14" wood cutting band saw.? The base (factory) has no provision for ventilation of the motor.? This clearly would not do for heavy use.
am
At 11:38 AM 2/1/2021 -0800, you wrote:
My stock Horror Freight 4x6 is TEFC. I just went out and confirmed. The motor is finned and there is a fan shroud over the rear. When running, the fan blows air over the fins for cooling. It does not have
any overload protection.
Now for my diatribe.
The best solution to prevent damaging a motor due to stalling isto be present when operating. Leaving the machine unattended is why the motor will burn up when stalled. I've said this before, overloads do not adequately protect motors from a stalled position.
They definitely don't hurt but you cannot rely upon them to protect an unattended machine.?
I'm a retired registered electrical engineer with over 45 years of experience who taught electrical system protection and have been an expert witness in protection matters. There will always be anecdotal evidence that suggests an overload will protect against
a stalled condition but I can present you with lots of technical data to show that is not the case. The simplest explanation is that overloads are designed for when the motor is running which means they are designed to trip while the motor is still getting
fan cooling. A stalled condition has no cooling and must trip much faster because of it. There is thermal lag between the motor winding and any thermal sensing device which may be in the motor. This lag can be long enough to either damage or fail the motor's
insulation.?
There are electronic overload relays with stall protection that will protect a small motor like on a 4X6 but the cost of those relays approach the cost of a replacement motor. If you are really worried about it and are electronically inclined, you could make
your? own stall protection using something like an Arduino and mechanically sense a stalled condition and will disconnect the motor. There is more to it since you actually have to get the motor to start since it is stalled at start which requires some type
of controller beyond a toggle switch. Personally, I will attend the operation of the machine since that is easier and more likely foolproof.
On Mon, Feb 1, 2021 at 11:03 AM John Vreede <vreededesign@... > wrote:
Good point Alan, the TE motors at least restrict the access to oxygen to start a fire and the?? '....molten metal, burning insulation etc' bit Jim quoted would be covered.
However I don't know of any OEM TEFC motors on 4x6's, and do know that at least one major manfr. is still using open motors.?? Has someone got a new HF saw (HF is probably the biggest seller??), that can say whether
they use TEFC motors?
This would be an additional thing to look for when buying - jv?
On Tue, Feb 2, 2021 at 7:40 AM Alan Muller <alan@...> wrote:
Do you consider that totally enclosed motors (TEFC) pose less of a safety hazard than open (actually shielded) motors??? It seems likely that an enclosed motor would be less likely to spew sparks and hot metal than a motor with ventilation openings.
I think I have noticed that equipment from Asia, even low priced stuff, is likely to have a TEFC motor where conventional domestic production (if there still were any) would not.
am
At 05:40 PM 2/1/2021 +0000, you wrote:
John Wrote
??�?Don't think you should be concerned with UL certitification and such.?? Induction motors have been around for 100yrs or so and the things that make them fail aren't tested for in UL anyway ??????
I would not go that far? ?? Safety certifications UL and other wise (UL isn??�?t the only game in town)?? isn?sn??�?t so much about t testing for things that make things fail, as testing to ensure that WHEN they do fail, the fail ??�?relatively??�??????? safely.
This relatively safety includes not starting fires {??�??? … molten ten metal, burning insulation, fflaming particles, or other ignited material could fall onto flammable material, or through which a flame could be projected….??�???}
> and not creating a shock hazard.
?
A ??�?Listed??�??? or ??� ??????Recognized??�??? motor (as compared to an unlisted/recognized one)?? are far less likely to start a fire or catch fire, when given the same amount of abuse.
They are also less like to fail to live up to expectations as a consequence of having to meet listing requirements.
? In particular a motor needs to fall within ~10% of the NEC values for current for that size motor, at rated nominal voltage.
There are also temperature rise tests, sheet metal thickness, and grounding resistance tests.
There is also a locked rotor test, For motors without a thermal cutout this can run for 18 days, or until the motor fails. When it fails (or at the end of the test if it doesn??�?t) there are temperperature limits,
and it still has to pass insulation resistance tests to demonstrate that it has not created a shock hazard. For motors with a thermal cutout, the test runs until the cutout operates. If the motor has an auto-reset cutoff, then the test runs for 18 days.
?
I have replaced the motor on my saw with a UL listed one. BUT I did not do so until it failed.
A saw that comes with a UL listed motor, is likely to be significantly more expensive. If you are not using a flammable cutting fluid, and make sure that both the motor and saw frame are grounded (and that your
plug has all 3 prongs.) I would not be worried. Just recognize that the motor likely will die on you the moment you turn your back on it
?
Also recognize that a ??�?CE??�??? markmark is in NO WAY a safety certification mark like UL, CSA, ETL, etc
?
R James (Jim) Klessig P.E. | Senior Power Systems Engineer |
Electrical Reliability Services, VERTIV
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