Small motor wiring question.

Thread Starter

MonteYou

Joined Nov 13, 2020
10
I have a 3 speed fan motor salvaged from an old box fan. I want to repurpose it to power a small homemade disk sander. The motor wires had been disconnected from the little rotary switch, and I don't have enough electrical savvy to know how to rewire them to a NEW switch (The old switch is falling apart). The motor wires are white, black, blue, and red. I don't really need any speed except high for the sander, but I'm puzzled about how to achieve the highest possible speed. I connected the neutral ac line to the white motor wire, and the hot ac line to each of the other motor wires in turn, and all 3 seem to produce about the same shaft speeds; around 1140 rpm. If all 3 produce about the same speed, that must mean the fastest speed is achieved by connecting, in the switch, 2 or more of the motor wires together. That sounds good in theory, but I'm reluctant to test it for fear of damaging the motor. Any help greatly appreciated.
 

shortbus

Joined Sep 30, 2009
8,429
I have a 3 speed fan motor salvaged from an old box fan.
Probably one of the worst motors to re-purpose for that application. Not counting the very low horsepower of them, ~1/20HP, they are open frame, and that will let it fill up with saw dust or what ever your sanding fast.
 

Thread Starter

MonteYou

Joined Nov 13, 2020
10
Probably one of the worst motors to re-purpose for that application. Not counting the very low horsepower of them, ~1/20HP, they are open frame, and that will let it fill up with saw dust or what ever your sanding fast.
I have taken all that into consideration:
1. It's only a 5 inch disc, so the low hp won't be a problem.
2. I am housing the motor, and collecting the sawdust with a vacuum attachment.
 

MisterBill2

Joined Jan 23, 2018
8,213
The three connections all will deliver the same unloaded speed, but with a load there will be differentr levels of torque and so different speeds. The switch would select on;y one wire at a time. So you can do a trial connection when sanding and see which wire provides the highest speed under load. My guess is the red wire.
If you have access to an om meter it will be the connection with the lowest resistance thyat should deliver the highest speed.
 

MisterBill2

Joined Jan 23, 2018
8,213
My really cheap disk grinder/sander claims 5000RPM no load , and it drops to about 4500 under load. BUT for light sanding and polishing and buffing you may be satisfied.
I do suggest that you consider safety issues because any spinning disc tool can cause problems, especially if the disc gets even a bit off center. Also, holding and controlling a thing shaped like that will be a challenge.
 

Thread Starter

MonteYou

Joined Nov 13, 2020
10
I think you'll find it will be a problem :(. My hand-held electric drill rated at 350W (getting on for 1/2HP) slows noticeably when powering a 5-inch disc sanding attachment.
Alec, thanks for the input. I'm going to try it anyway, just for grins. I won't be doing any heavy sanding, just smoothing and rounding corners mostly. And the motor's free! If it proves to bog down too much, I have a fallback motor; a 3/4 hp blower motor that was never used. I'll use it to build a 12 inch sander. Only problem: the Damned thing weighs a ton.
 

Thread Starter

MonteYou

Joined Nov 13, 2020
10
How do you know those RPM's? I'm betting you are guessing at the load one. Or your not using it to the full capacity.
shortbus, thanks for the feedback. I used a small wire attached to the shaft. When the shaft spins the wire strikes a piece of plastic and makes a sharp click. I record that and use computer software to read the waveform and measure the interval between clicks in microseconds, then compute the rpms from that. By the way, I didn't measure the speed under a load, my measurements were of an unloaded shaft.
 

Thread Starter

MonteYou

Joined Nov 13, 2020
10
shortbus, thanks for the feedback. I used a small wire attached to the shaft. When the shaft spins the wire strikes a piece of plastic and makes a sharp click. I record that and use computer software to read the waveform and measure the interval between clicks in microseconds, then compute the rpms from that. By the way, I didn't measure the speed under a load, my measurements were of an unloaded shaft.
I think I mean milliseconds, not microseconds. My bad.
 

shortbus

Joined Sep 30, 2009
8,429
shortbus, thanks for the feedback.
The post your answering was for mrbill. He makes some very outlandish comment here and I call him on them. But you do bring up another problem with your fan motor, they usually don't have enough RPM to sand effectively.
 

MisterBill2

Joined Jan 23, 2018
8,213
The disc tool makes a loud whine as it drives and the change in pitch is quite obvious. The 5000 rpm is from the label on the body of the motor housing. I could use a microphone and frequency counter because the sound is loud enough to have the dynamic microphone drive the counter without any amplification. But it is not worth the bother. The thing does slow down as the load increases..
 

MaxHeadRoom

Joined Jul 18, 2013
22,639
Probably one of the worst motors to re-purpose for that application. Not counting the very low horsepower of them, ~1/20HP, they are open frame, and that will let it fill up with saw dust or what ever your sanding fast.
Agree, that is about the worst motor you could select for that application, or any fan motor come to that.
There is a considerable difference in the torque required between the two applications.
Max.
 

Tonyr1084

Joined Sep 24, 2015
5,803
Here's what I know about those fan motors: First, you're converting electrical energy into mechanical energy then into wind energy. With no load you're not converting anything into anything, just spinning a motor. I know. I've tried myself. Long time ago.

The RPM with no load will be virtually the same because the motor will spin according to the frequency (60 Hz) at a given speed. Whether it's on low, medium or high. You won't see any difference. But when loaded with a fan blade, the low will produce enough energy to rotate the fan and move a given volume of air. On medium the fan will move more air because it is converting more energy into wind energy. On high you get the most air movement because you're using the most energy to convert to wind.

Trying to use "Low" and "Medium" (or any combination of two different motor phases will result in a locked rotor. The fan WON'T spin.

You say you want to turn this into a sander. For that I assume you mean a motor with a disk with sanding medium on it; something like this. Given the low horse power, it will not prove to be a very good sander. Even on its highest setting, the motor won't provide much power for sanding. But if you want to move forward with this project - it will be a good learning experience for you.

[edit] you may also notice the pulley sizes of the motor and the sanding disk - there is an upping of the RPM, meaning there will be even more significant losses of power. The disk should spin a lot faster than the fan motor you want to use. Given the low torque or horsepower of your motor - it's probably not going to work out for you.
 

Thread Starter

MonteYou

Joined Nov 13, 2020
10
The three connections all will deliver the same unloaded speed, but with a load there will be differentr levels of torque and so different speeds. The switch would select on;y one wire at a time. So you can do a trial connection when sanding and see which wire provides the highest speed under load. My guess is the red wire.
If you have access to an om meter it will be the connection with the lowest resistance thyat should deliver the highest speed.
I do have an old analog multimeter that I use mostly to test for continuity on switches, etc. So to measure the resistance on the 3 wires, do I put the 2nd lead on the neutral (white) wire?
 

Thread Starter

MonteYou

Joined Nov 13, 2020
10
My really cheap disk grinder/sander claims 5000RPM no load , and it drops to about 4500 under load. BUT for light sanding and polishing and buffing you may be satisfied.
I do suggest that you consider safety issues because any spinning disc tool can cause problems, especially if the disc gets even a bit off center. Also, holding and controlling a thing shaped like that will be a challenge.
This will not be a hand tool; it will be clamped to a work table.
 

Tonyr1084

Joined Sep 24, 2015
5,803
Measure resistance between:
White & Black
White & Red
White & Blue

White is the common point of all three motor core windings. Each core is different in terms of how much power is applied. The lowest resistance will be the highest RPM, the highest will be the lowest RPM. Medium is in between both.
 

Tonyr1084

Joined Sep 24, 2015
5,803
This will not be a hand tool; it will be clamped to a work table.
Some time back I had a dish washer motor and a flat disk I thought about doing something similar to what you're planning. It had much higher RPM and much more power. I never built it, or at least - not yet. I don't even know if I still have that motor around anymore.
 
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