Thread: whats wrong with my table saw motor?

I think its a technician job. From your description the thermal overload may be faulty, just a guess.

Originally Posted by Albert

I was using the table saw to do some ripping. I didnt change the blade, should have used the ripping blade, but I just used a finish blade, thought its only 30 cuts of 2.4m so should be alright. The motor would trip out from time to time due to maybe over loading - it will restart after a short rest

30 cuts of 2.4m (what cut depth?) anything more that 12 mm with a finishing blade may easily over load a TS.

On my box brownie 3HP TS, about 1m of 25mm deep ripping with a finishing blade will trip the motor.

Before I bought a dedicated ripping blade this happened maybe half a dozen times to my TS.
When it happened I'd let the motor cool down to room temperature so that was at least half an hour before a restart..

Given you could feel the warmth on the table top the motor must have been VERY hot.
I'd say you have repeatedly overloaded and damaged the saw motor and he RCD tripping suggest a short in the saw motor.
This has probably registered in the contractor which then locks out all of the motors.

Bob, it is at depth of 90mm.

I have done this many times, this is the first time it went completely dead. I thought I use the table for a change instead of the bandsaw...

looks like a change of a motor is to be done.

Originally Posted by Albert

Bob, it is at depth of 90mm.

I have done this many times, this is the first time it went completely dead. I thought I use the table for a change instead of the bandsaw....

In that case your'e probably lucky it has not gone completely dead sooner.

It surprises me that more expensive gear does not have an ammeter display built into them so one can see what current is being drawn so that one does not have to rely solely on thermal overload or over-current cutouts.

These cutouts are really only designed to protect gear on an occasional basis and it is assumed that the operator notices this and adjusts their work pattern accordingly.
It does not mean that machines can be continually driven to overload conditions.

Most of my machines have a $15 ammeter fitted to them so I can see what is going on. For those that don't I occasionally use a meter inline with a short extension cord. After a bit of your it's possible to get a feel for and hear what sort of load approximates to what sort of current and I have learned not to exceed this load

Couple of things to understand about motors and thermal trips in them.

Generally the thermal is mounted remotely from the motor windings, so is not directly sensing the winding temperature, so the windings will generally be significantly hotter than the cutout.

Generally the thermal will reset if it cools as little as 10 degrees C. It is often attached to the motor frame which acts as a heatsink to cool and reset the thermal. Conversely, the windings are restrained in the iron laminations of the motor which represent a significant heat bank, retaining the heat from the windings and dissipating it much more slowly. Repeatedly overloading the motor till the thermal trips and commencing to overload again as soon as the thermal resets will result in the copper windings and iron laminations reaching extreme temperatures, likely to either crack the winding wire varnish due to expansion and contraction through the heat cycles, or out and out burn the insulation on the winding wires.

Damage to the insulation of the winding wires can be cumulative, if as you say, you have snuck a set of cuts through quite a few times in the past, tripping the thermal and starting again as soon as it resets, each of these sequences can weaken the insulation somewhat, until ultimately it is waiting to fail next time it happens. It only needs a very small crack in the insulation at any point in the winding to make contact with the laminations and you have a short to earth that will trip the RCD.

If the motor is in a combi machine, as suggested by it having other motors that work still, it may be cheaper to have the motor rewound if it can be done fairly locally, rather than purchase and install a new motor from the machine's agent. It would be fairly unlikely that the motor would be an easy swap for a commonly available generic motor. You need to exactly match the motor's power rating, duty cycle, frame size, mounting system, shaft diameter and length, and keying arrangement to find a generic motor that is a straight swap, and I would rate the odds as fairly poor.

Originally Posted by BobL

In that case your'e probably lucky it has not gone completely dead sooner.

It surprises me that more expensive gear does not have an ammeter display built into them so one can see what current is being drawn so that one does not have to rely solely on thermal overload or over-current cutouts.

72m of 90mm deep rip with a finish blade?? I don't know how much power you have, but I wouldn't even attempt that with the 10hp saw we have. The rule of thumb for re-sawing with a table saw is 1hp per tooth, but you get away with about 0.25-0.33hp per tooth going slowly.

The only machines I've seen with built-in ammeters are wide-belt sanders and that's the only one where I really feel it's useful, more for not destroying the paper than the motor. For everything else it's much like driving a manual car without a tachometer, you get to know how much load the motor is under by sound.

Interesting side-note on overloading saw motors... The fancy Altendorf infinitely variable speed motors are actually designed for bursts of 120% OVER the rated output, so you can squeeze 11kw out of a 5kw motor when needed. I've got no idea how they do it, it'd be interesting to see the duty-cycle numbers.

Originally Posted by elanjacobs

72m of 90mm deep rip with a finish blade?? I don't know how much power you have, but I wouldn't even attempt that with the 10hp saw we have. The rule of thumb for re-sawing with a table saw is 1hp per tooth, but you get away with about 0.25-0.33hp per tooth going slowly.

I assume this means "Tooth in the cut"?
The loads are also non linear with depth of cut on a circular blade.
On 50mm deep cut a 300 mm diameter 24 tooth blade has 2 teeth in the cut so 2HP should be able to make this cut.
Because the blade is circular, on 100 mm deep cuts ~5 teeth are in the cut which is why 3HP saws struggle to make this sort of cut.

The only machines I've seen with built-in ammeters are wide-belt sanders and that's the only one where I really feel it's useful, more for not destroying the paper than the motor. For everything else it's much like driving a manual car without a tachometer, you get to know how much load the motor is under by sound.

Sure, but especially on a machine where an operator has little or no experience, unless they know the load, how do they know the sound at which to ease off ?
The way the cut speed slows is a handy indicator for an experienced person, but probably the best way is get the low down from an experienced operator of that specific machine.
Unfortunately many DIY folks lack the experience and work alone so they wouldn't know what is an acceptable cutting speed or cutting sound.

Interesting side-note on overloading saw motors... The fancy Altendorf infinitely variable speed motors are actually designed for bursts of 120% OVER the rated output, so you can squeeze 11kw out of a 5kw motor when needed. I've got no idea how they do it, it'd be interesting to see the duty-cycle numbers.

That would be interesting.
While I believe these new Altendorf motor have significant smarts they don't tell us about, to some extent motor rating is a nominal motor parameter so it would be easy enough to just underrate a motors to begin with. When testing electric motors I load them up until they stall and most decent 3P motors can generate around 50% more than their rated power. If a motor that can overload by 50% is derated by 30% then any manufacturer can claim 120% overload.

Originally Posted by BobL

I assume this means "Tooth in the cut"?
The loads are also non linear with depth of cut on a circular blade.
On 50mm deep cut a 300 mm diameter 24 tooth blade has 2 teeth in the cut so 2HP should be able to make this cut.
Because the blade is circular, on 100 mm deep cuts ~5 teeth are in the cut which is why 3HP saws struggle to make this sort of cut.

No, it was per tooth on the blade; 48T blade, roughly 48hp. That was referring to big mills, so it may not scale directly to furniture-industry sized saws.

Originally Posted by elanjacobs

No, it was per tooth on the blade; 48T blade, roughly 48hp. That was referring to big mills, so it may not scale directly to furniture-industry sized saws.

It makes some sense if they are talking about a fixed TPI, or in the case of bigger circulars, IPT, and cutting to full depth since fixed TPI then determines max cutting depth.

In most situations the depth of the cut, or the number of loaded teeth, must impact on motor power requirements.
More teeth means more load, and as more TPI usually means smaller teeth these cannot carry much sawdust in their gullets so they stop cutting.
The lack of gullet capacity can be made up somewhat by faster tooth speed although that brings with it complications of extra friction/heat and sharpening.

On bandsaws and chainsaws it's also loaded teeth in the cut per HP.
The very rough ROT used in wider materials is about 1 loaded tooth per inch of cut per HP.
The Bandsaw mill I have access to uses a 27 HP motor a 1 TPI blade and has a max width of cut of 36"
In chainsaws most folks don't realise that when end grain and cross cutting only every 3rd of 4th cutter is fully loaded in the cut.
On a 3/8 chain there will be ~2 loaded teeth cutting per ft so a ~2HP saw is needed per ft of cut width.
For a 3ft wide log that means 6HP, and for an 8ft wide log 8HP.
Since very few chainsaws are more than 8HP, to keep cutting in wider logs skip chain is sometimes used which uses about 1 cutting tooth per ft of cut.

oh yes, 72m of 90mm, and a finish blade. I was doing it very slowly.... I always cut slow. maybe 10mm per sec?

1hp per tooth. thats a lot of hp! I think you meant tooth in contact with the timber - so if its 5 tooth in contact with at anytime, you need 5hp. if 10, then 10hp?

Going very slowly is also not particularly healthy, you generate a lot of heat and cutting edges die faster the hotter they get. It can also mess with the tension of the blade, our panel saw has scars in the sliding table about 10mm to the side of where the blade should be from overheated blades losing tension and flapping around like they're made of paper.

Originally Posted by elanjacobs

Going very slowly is also not particularly healthy, you generate a lot of heat and cutting edges die faster the hotter they get. It can also mess with the tension of the blade, our panel saw has scars in the sliding table about 10mm to the side of where the blade should be from overheated blades losing tension and flapping around like they're made of paper.

Eek!

Originally Posted by aldav

Eek!

They do snap back to flat after a couple of seconds cool-down with no apparent permanent damage, but it's a bit unnerving to see

Originally Posted by elanjacobs

They do snap back to flat after a couple of seconds cool-down with no apparent permanent damage, but it's a bit unnerving to see

Elan, my first response is maintained. I am totally unconvinced by your reply.