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May be safer to use a bleeder on the hose...many vac, especially inexpensive ones, rely on the air flow from the hose for cooling. Slowing the motor slows the air flow. With a bleeder, you reduce "suction" at the tool but still have full air flow through the vac.Originally Posted by Mike Henderson
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The most expensive tool is the one you buy "cheaply" and often...
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Several people have pointed to this objection in the past. The reason it's not a problem is that the electrical power required to drive a fan (which a vacuum is) varies with the third power of the RPM. So when you slow down a fan (vacuum) you greatly reduce the current flowing through the motor and the heat generated.
And for a real world experience, I've used a "router speed control" on my shop vac for years connected to my sanders. Never a hint of a problem.
Mike
[Someone should check me but I think the equation is W2=W1(RPM2/RPM1)^3 Where W=Watts. I'm going from memory here. Can also be written as W1/W2=(RPM1/RPM2)^3]
Last edited by Mike Henderson; 05-20-2020 at 1:13 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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The complete question is not whether the power dissipation reduces, but does the power dissipation reduce as fast or faster than the cooling system's reduced ability to remove waste heat from the motor.
The above equation should not be extrapolated to indicate that anything that causes RPM to go up, also increases power dissipation, or vice versa.
-- Andy - Arlington TX
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Hi Clark,
I'm a little late to the tread, but wanted offer my two cents worth. I went from my old PC RO sander to a Festool ETS EC 125 and a Festool dust extractor. Although pricey, it was more than a game changer, it was a life changer. Sanding went from something I dreaded to something I almost enjoy now.
Cheers,
Mark
Contributor
I suppose we could have a fan system where the motor is only cooled adequately by it's internal fan at the standard operating RPM, and when you slow it down it begins to overheat because the fan doesn't cool it adequately. But that would be counter to experience with fans that have speed controls built into them.
From actual experience I can say that years of use of a shop vac at reduced RPM has not caused any noticeable harm to the unit. Given that the input power is reduced by the third power of the reduction in RPMs I'd highly doubt if the heat generated could outrace the fan cooling.
Can you point to a fan system that will overheat when it is slowed down?
Mike
[I would expect the opposite to be true: When you overspeed a fan the motor may get excessively hot because the input power is increasing with the third power of the RPMs.]
Let me run some numbers. Let's take a shop vac operating at 120 volts and 10 amps. Ignoring power factor the input power is 1,200 watts. Universal motors are not very efficient - let's assume 50% efficiency. Using that the watts going to heat is 600 watts per second. When you turn the vac on, the motor will begin heating until it reaches a temperature that allows 600 Watts per second to be conducted to the airflow around the motor. At that point, the temperature of the motor will stabilize.
Now, let's slow the motor down by 50%. The third power of 0.5 is 0.125 so the input power to the motor will be 150 watts. If the efficiency remains the same, the watts of heat will be 75 watts per second. The motor will stabilize at a temperature that allows those 75 watts per second to be conducted to the new airflow.
To show that the motor will overheat in this situation you need to show that the 75 watts per second cannot be conducted into the airflow unless the motor becomes excessively hot.
I would expect the motor to run cooler under half speed.
Now let's look at overspeeding. If the RPMS are raised by 20%, the factor is 1.2 which raised to the third power is 1.728. So the input power would have to be 2,073 Watts. If the efficiency remains at 50%, the heat is now 1,037 (rounded) watts per second. A very significant increase for only a 20% increase in RPMs.]
Last edited by Mike Henderson; 05-20-2020 at 7:30 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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I have two Bosch sanders with vibration control that replaced an old PC ROS. The difference is night and day regarding the comfort and lack of numbness. I'm very pleased with dust control, for which I use a Fein Turbo II with a dust deputy. The hose on the Fein has a bleeder built in and the vac has an outlet for auto-on and off when the sander is turned on and off. I'm not sure how much better this setup could be, but I've never used a festool sander, so...
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Mike,
I'm not saying that it will overheat, but I am saying that neither you nor I know that it won't overheat for an arbitrary shop vac and application.
I have no experience with operating shop vacs with after-market PWM variable speed controls. I do have 34 years of electrical engineering experience to understand whether all potential impacts of a design change are adequately addressed.
I would not use one of the typical aftermarket PWM variable speed controls for universal motors, but I know many people have used them without issue, particularly for routers (not necessarily shop vacs). I do not know how much they stressed their equipment otherwise.
I also know that, to the extent that slower cooling airflow becomes less turbulent over the motor, heat transfer from the motor to the cooling air will be reduced significantly (beyond proportionately to airflow). How much? I don't know; that's why I'm suggesting caution.
-- Andy - Arlington TX
Contributor
I understand your concerns. It's impossible to calculate the heat flow for a shop vac because we don't know all the parameters required to do the calculations. But sometimes you just have to make a decision based on imperfect information.
The thing that gives me confidence is the third power factor. As an electrical engineer, you know how quickly a factor changes when it's raised to a power, and the third power is more than we usually see. A factor raised to a third power changes significantly.
When we can't calculate all of the parameters we usually fall back on experimentation. Of course, my use of a shop vac under the conditions described is an experiment but more importantly, thousands of people have used those "router speed controls" on universal motors successfully for many, many years. If universal motors were failing when those controls were used on them we'd be getting lots of reports because failures get reported more readily than successes.
I have no problems recommending the use of a "router speed control" for slowing down a shop vac.
Mike (also an electrical engineer with a lot of years in the business - since 1968)(But credentials don't matter. What matters is making sense.)
[Also, I'm pretty sure that Festool and Fein use this system to control the speed of their vacuums. I'm pretty sure they use a universal motor in their vacuums and there are not many low cost ways of doing speed control other than pulse width modulation. In fact the speed controls built into things with universal motors (like routers) are probably PWM devices.]
Last edited by Mike Henderson; 05-21-2020 at 3:20 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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Mike,
It's not the PWM that worries me, it's whether the vendor has considered and accounted for the effects of PWM within their system, let alone whether the motor is sufficiently cooled across the manufacturer's defined range of speeds. Some 3rd party controllers allow you to vary the speed all the way down to zero. Wonder why EVS tool vendors don't do that in most cases?
And it's not just universal motors with PWM voltage control...
Even using VFDs on 3-phase motors (PWM is used to generate the variable frequency phases), without sufficient filtering, can cause motor bearing damage. The filtering need not reduce or remove the high frequency PWM power content, but it should induce such currents to return through the power wiring, instead of to ground via the structure and bearings. A simple common mode choke can do exactly that. Good VFD vendors recommend and supply such filtering. Luckily these filters are not expensive, but they are not free either, so the cheap vendors won't bother telling you that you need it, especially if they don't provide it. Without such filtering, running a VFD on a GFCI circuit can can trip the GFCI. Many VFD suppliers recommend not using them on GFCI circuits.
PWM is not inherently bad, but it does cause some unusual issues that have to be dealt with at the system level for reliable operation.
-- Andy - Arlington TX
Contributor
Well, I suppose if you put your mind to it you can find a hundred problems and reasons why something won't work - some people make a career out of doing that.
However, we have thousands of users who use those "router speed controls" successfully - they don't know of all those possible problems so they just move forward and do. While the points you made may be true, they don't appear to be serious enough - or common enough - to cause problems in the real world.
Mike
Last edited by Mike Henderson; 05-21-2020 at 5:23 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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wow - i guess it was a big side step
but interesting
jerry
jerry
Contributor
I just hook mine up, and use it.
Member
i have the Bosch Dual Mode sander coupled with the Mirca Abranet, I've been blown away at the quality of it and the lack of dust when used with my lousy vacmaster shopvac. The sander is heavy and will dive you a good workout in the turbo mode, however on the regular ROS I let it sit on the surface and guide it around the workpiece with two fingers on the hose port. When I want something lighter and quieter (it's quite loud), I use my battery powered Dewalt ROS. It's very light and I'm quite happy with the combination of sanders. My 5 year old daughter likes to help sand and has no problem using that one. That being said the smaller one is also easier to use when having to sand narrow objects or the side of things.
Last night my daughter and I sanded a rough end grain cutting board, I started with the Mirca 80 grit discs and worked my way up to 120 grit before switching to ROS mode, we continued up to 400 grit and produced the smoothest cutting board I've ever felt (end grain or not). The sanding for both sides and edges of the 12x16x2 cutting board took about 40 minutes, that includes about 10 minutes of my daughter working with the Dewalt. All in all I'd recommend having two sanders, the Bosch dual mode and a quality battery powered ROS (having no cords while doing detail and edge sanding is a huge plus!
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When I use my Bosch inside, I hook it up to a small shop vac with the 1 1/2" hose. My adapter is usually duct tape. Rockler sells a universal adapter. I got the small (1.5 gallon) shop vac at a garage sale for a few bucks, and only use it for the sander.
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I sand mostly smaller work pieces and swap grits frequently so I have several sanders set up with different grits. I know it is quick to swap disks, but i have three sanders with progressively finer grits and find it pretty handy when working on small pieces where I'd be swapping grits constantly. I find my DeWalt sanders to be not too bad vibration wise and they have held up well. I have used others that were terrible on my arthritic wrists or didn't hold up as well.
I don't usually bother to hook them up to the DC. The little bag catches a lot of the dust and I most often sand on the out feed table of the thickness sander (remember that I am working with smaller work pieces) with the DC turned on for the thickness sander it sucks up the air born dust pretty well.
Really cheap abrasives are a waste of money, but there can be decent stuff at prices that aren't terrible. In general I buy a lot of abrasives from Klingspor, but I have currently only Mirka in stock for the ROS because I stocked up with a large purchase when they were on sale.
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