16/32 sander potentiometer question

[david brum]

Hi all

I have an old Performax sander with a 1 hp drum motor and a DC motor which runs the belt. Like the rest of this family of sanders, the drum motor stalls really easily if I'm not careful. Last year, I rigged an ammeter which I can watch. When the needle approaches 10 amps, I can turn the belt speed down in order to decrease the motor load before it pops a breaker. That works OK, but in a moment of distraction, I can easily burn a divot in my project.

I see that the newer generation of Jet sanders have something called "Smart Sand" which automatically reduces the belt speed when the motor is reaching its limits. It is possible to buy a new DC belt drive motor from Jet which already has this feature installed. Unfortunately, these are designed to work with the current 1.5 hp motors, so won't work with my motor. The cut-out aperage is too high. Even if they would work, I'm too cheap to buy the whole DC motor just to get the potentiometer.

So here's the question: Is it possible to add a potentiometer ( if I'm even using the correct term) to my DC motor? If so, I would love some advice and direction.

[Jerome Hanby]

Reducing the current might well reduce the RPMs but it also reduces power. Most "electronically" adjustable speed controls are controlling frequency, either of the AC wave form or pulses of DC. If you have an oscilloscope you could watch the power leads on the belt motor as you turn your speed control. If you see a wave form that expands and contracts as you turn the control, then it is a frequency controlled RPM system. If I understand what you actually want, you want the RPMs to drop once you exceed a certain amount of current draw. Without getting too detailed, seems like you could use an SCR biased correctly to turn on at your trigger current and use that to activate a circuit to decrease the RPMs. Assuming your speed control adjustment is a POT, you could probably contrive some transistor driven replacement for it.

With all that said, not sure I'd want to crack open a working piece of equipment to rig the feedback and control modifications. Would be akin to going into the engine to add a racing cam in a car where you are still making payments!

[David Kumm]

There is a place on ebay that rehabs old performax machines. They do some retrofitting of sandsmart units so it might be worth a call. Look under performax and you should see the seller on ebay. I have had both with and without SS and while the SS prevents stalling the idea is to not relay on it and try to adjust just under the limit as having it kick in adds some inconsistency to the finish. Dave

[david brum]

Reducing the current might well reduce the RPMs but it also reduces power. Most "electronically" adjustable speed controls are controlling frequency, either of the AC wave form or pulses of DC. If you have an oscilloscope you could watch the power leads on the belt motor as you turn your speed control. If you see a wave form that expands and contracts as you turn the control, then it is a frequency controlled RPM system. If I understand what you actually want, you want the RPMs to drop once you exceed a certain amount of current draw. Without getting too detailed, seems like you could use an SCR biased correctly to turn on at your trigger current and use that to activate a circuit to decrease the RPMs. Assuming your speed control adjustment is a POT, you could probably contrive some transistor driven replacement for it.

With all that said, not sure I'd want to crack open a working piece of equipment to rig the feedback and control modifications. Would be akin to going into the engine to add a racing cam in a car where you are still making payments!

Wow Jerome, thanks for the explanation and words of caution. Of course, that's Greek to me and beyond the scope of my capabilities. Maybe I'll ask some electronics buddies if they're interested in tackling it. More likely, I will just stand in front of my machine with my fingers on the control knob.

[david brum]

There is a place on ebay that rehabs old performax machines. They do some retrofitting of sandsmart units so it might be worth a call. Look under performax and you should see the seller on ebay. I have had both with and without SS and while the SS prevents stalling the idea is to not relay on it and try to adjust just under the limit as having it kick in adds some inconsistency to the finish. Dave

Thanks David. I searched for the seller to no avail. I'll keep looking, though. Heck, maybe I'll even call Jet.

Edit--I found the seller. I'll call him tomorrow. Thanks!

[Neil Brooks]

I bought the gizmos to wire an ammeter inline, into mine, but .... something deep within my DNA ... has me just slowing down the conveyor, and ... not over-thinking it, so much :D

That's sort of humor, but sort of true. It's like ... once we had e-mails, faxes took forever ? Well ... once I got my 16/32, I got impatient. When you realize, though, what a time saver it is ... for me ... I decided it was better just to dial it back a bit and appreciate the wonderful work IT does ;)

[Curt Harms]

Wow Jerome, thanks for the explanation and words of caution. Of course, that's Greek to me and beyond the scope of my capabilities. Maybe I'll ask some electronics buddies if they're interested in tackling it. More likely, I will just stand in front of my machine with my fingers on the control knob.

That's what I do though I use my sander infrequently. Keep us informed what you find out with the retrofit.

[david brum]

That's sort of humor, but sort of true. It's like ... once we had e-mails, faxes took forever ? Well ... once I got my 16/32, I got impatient. When you realize, though, what a time saver it is ... for me ... I decided it was better just to dial it back a bit and appreciate the wonderful work IT does

Good point, Neil. I should probably put on some classical music and take a deep breath (through my mask) and turn the belt drive down. Having a machine that doesn't work to its full potential nags at me, though. It certainly isn't a high priority project, but I'll continue to work on the idea. If I can find a relatively cheap and easy solution, I'll share it with everybody.

[Tony Zaffuto]

How do you wire in an ammeter? Is this wired in line and would it be appropriate to wire in at the reset switch on the motor?

[Jerome Hanby]

The amp meter has to be in series (not across/parallel) with the power feed. I would think the reset switch would also be in series with the power, so taking a lead off that switch and using the meter to remake the connection should work. Just be sure that your meter is rated higher than the current draw you will be measuring.

[Curt Harms]

How do you wire in an ammeter? Is this wired in line and would it be appropriate to wire in at the reset switch on the motor?
I wired mine into a 4"X4" project box. I put a heavy duty toggle switch so I could start without the inrush going through the ammeter then once running switch to the ammeter. The motor plugs into an outlet cut into the box so I didn't mess with the sander at all.
210847
The picture doesn't show it but there's a single 20 amp outlet on the far side of the box. The sander or any other 120 volt tool can be plugged in there to check amperage draw. The time consuming part for me was to find a reasonably priced direct read relatively high amperage meter. Most required a shunt arrangement that I was unfamiliar with.

[John TenEyck]

I think Neil's got it right. These drum sanders arn't meant to be used as planers, they are sanders. I can easily stall my Delta 1836 if I try to take too large a bite, but even short of that it'll leave unwanted scratches that are hard to remove. It took me awhile to realize that the best approach for me was to plane to within 0.020" before going to the drum sander. Then 2 light passes per side with 120 grit gives me a board ready for hand sanding. The only time I plane less and sand more is if I have some wood that tears out a lot on the planer. Anyway, with those light passes of only 0.005" there's never an issue of bogging down the motor or stalling the conveyor.

[Jerome Hanby]

If I recall correctly that shunt arrangement is calculated by getting a few test results dealing with the meter movement then applying either Norton's or Thevenin's theorem and solving some simultaneous equations.


I wired mine into a 4"X4" project box. I put a heavy duty toggle switch so I could start without the inrush going through the ammeter then once running switch to the ammeter. The motor plugs into an outlet cut into the box so I didn't mess with the sander at all.
210847
The picture doesn't show it but there's a single 20 amp outlet on the far side of the box. The sander or any other 120 volt tool can be plugged in there to check amperage draw. The time consuming part for me was to find a reasonably priced direct read relatively high amperage meter. Most required a shunt arrangement that I was unfamiliar with.

[Jack Ganssle]

Two points:

- The discussion about changing the frequency, using an SCR, and all that is not relevant, at least on my 16/32. The pot controls the belt drive, which runs from a DC motor. All the control does is vary the voltage to that motor.

- The easiest way to add an ammeter (this is what I did) is to use a Belkin Conserve Insight ($28 from the book site). It plugs into the 110 VAC outlet, and the drum drive motor plugs into it. No wiring needed.

[Jerome Hanby]

Two points:

- The discussion about changing the frequency, using an SCR, and all that is not relevant, at least on my 16/32. The pot controls the belt drive, which runs from a DC motor. All the control does is vary the voltage to that motor.


The OP was asking about implementing an automatic feed reduction based on the current draw. I could argue that all the other posts were the ones not relevant:p