Any practical way to slow down my belt grinder?

O

Omega Leather Works

Joined
Jun 13, 2007
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Hey guys. I have the venerable 1"x30" HF grinder and I'm wondering if any of you have any ideas about how to slow it down.

I'm not gonna buy a vfd for it any time soon. :D

Thanks!
 
Slowing down a motor will need rewiring, depending if it's a fixed current or fixed voltage type.
Firstly, you'll need to find the wiring schematic and re-wire a controller there, some mulpi steps transformer or resistors.
IIRC, one of the type cannot be slowed down without decreasing the torque output, which makes it almost impractical to do so.
This is as much as my 20 yrs old memory of elementary electrical motor class .. Not much help, I guess.:p
 
Google and Amazon "variable speed motor controller". They're not expensive but take some knowledge of basic wiring and proper (safe) connecting. I had one and hooked it to my belt sander placed in a vise. I saw the potential for knife work but I just got into free hand sharpening and didn't go further with it. Again though, if you don't have some electrical skill and knowledge I wouldn't recommend using this as a "learning curve" project.
 
Make sure to do your research. Some types of motors don't respond well to electronic means for slowing them down, as they are literally wired to run at a given speed and/or torque (as mentioned).

There's a device known as a 'variable frequency device' (VFD) that's been mentioned and/or recommended in the Makers' forum (yours would be a good question to post in there, in fact). As I understand it, it's an electronic controller that alters either/both the voltage and the input frequency (of the line voltage) to the motor, which in turn controls the speed and torque at which the motor runs. BUT, it's primarily designed for 3-phase induction (AC) motors. Many motors used in bench/belt grinders I believe are single-phase (AC), so there may be some limitations or compatibility issues.

https://en.wikipedia.org/wiki/Variable-frequency_drive

Other 'cheaper' options, like resistive speed controls (rheostat-controlled), I think are usually designed for DC motors (a.k.a. 'universal motors'), and will dramatically reduce torque as well as speed. This is even noticeable on some tools, like a Dremel, with built-in speed controls. As the speed is dialed up, the motor's torque drops WAY off (it's much easier to slow/stall the motor when under a load, which reduces or negates it's effectiveness as a grinding tool at high speed).

First thing I'd do is contact the manufacturer of your grinder, and ask them if there are reliable and safe ways to install variable speed control on your grinder. Might also be warranty issues, as some attempts at slowing down a motor that's not designed for it may actually burn up the motor.

Only other way I know of, to change the speed of a motor-driven device, is to change the size/diameter of the driving pulleys, as with most belt-driven drill presses, for example. Speed is changed by repositioning the belt between larger/smaller drive pulleys; smaller diameter will reduce the effective linear speed (in terms of how fast the abrasive moves past the item being ground upon). The upside there is, the motor itself continues to operate at the same rpm as always, which obviously minimizes the risk of damage to it. If it were me, this is what I'd be looking into; it's likely the safest adjustment, and possibly not very expensive either. Might need to purchase a tool for removing/installing a different pulley, but there might even be a way around that, I don't know.


David
 
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Step one: Remove single phase AC motor.

Step two: Install comparable DC motor.

Step three: Apply voltage :D

But in all seriousness, programming a variable freq drive is a real pain in the buns. I've never seen small scale ones but the ones I worked with required PLC wiring and programming. Probably better to just wire in a potentiometer to reduce the voltage going to your drive.
 
Yes, David is on the trail. I have one of the devices on my 5hp. Westinghouse motor, a 3 phase motor. Installed in 2000 and has worked like a jewel ever since. Still, it's a high end motor and the unit was not cheap. (maybe the price has gone down) Plus, it's not a item any weekend computer guy can install. He needs a lot of electric experience as well. It's programmable with a high end and low setting and I've been running mine from around 400rpm to 3100rpm, a joy to run and work. It can lop off metal or polish. Runs a 2X48" belt around a 9" knobby wheel with idler pulley and pillow blocks. My motor does not run hot.
Unit was purchased from a mining supply house, brand is: Reliance Electric SP200 Md.#S20-X06P0A100 Ser.#MEAD3PU6 . Perhaps others make them by now. Good luck, DM
 
(...) "Probably better to just wire in a potentiometer to reduce the voltage going to your drive.
Click to expand...

Therein lies the danger.

Reducing voltage to a motor that draws a fixed load (in watts or HP), will cause an increase in current to the motor. That's what sometimes burns them up. If the motor is spec'd to actually handle the increased current, might be OK. But if not, bad things sometimes happen (involving smoke). That's why it's real important to know the type and limitations of the motor, before attempting to mess with input voltage/current.

This is a pic & description of a 'variable speed motor controller' found on Amazon. Note the comment 'not for use with brushless motors', which is the warning not to use them on induction (AC) motors, as most-often found on bench belt grinders. 'Universal' (DC) motors use brushes, which essentially makes the DC winding of the motor able to function on an AC power source. Most portable power tools, like hand-held drills, routers and sanders (orbital/rotary type) use this type of motor, which is generally smaller/lighter (hence the portability). Also vacuum cleaners, blenders, etc. That's what this particular controller was made for. Most bench power tools, like belt grinders/wheel grinders/buffers use an AC induction motor. Easiest way to differentiate between them: induction motors run QUIET, and brushed 'universal' motors are NOISY (think of a portable electric drill or circular saw; all that noise is due to the brushes in the motor).

41e9yf2ho1L._SX385_.jpg


"Electronic speed controller for power tools and appliances
Superior speed control with step-less adjustment knob for infinite settings
Rocker on/off switch allows unit to remember last setting
72" UL insulated 16AWG 3 prong power cord; not for use with brushless motors or slow start routers
Circuit protected with 15 amp fuse; 120VAC 15 amp capacity"
Click to expand...


David
 
Obsessed with Edges said:
Reducing voltage to a motor that draws a fixed load (in watts or HP), will cause an increase in current to the motor.
Click to expand...

I apologize, after writing about changing to a DC motor I had DC motors in my head. Yeah, don't wire in a pot to an AC motor :eek:
 
No - for AC motor. Reduce voltage or current only lead to motor burn out.
You can add some sort of step pulley to change rpm but won't change how this grinder won't track & tension worth darn. Yes, I've one too :D
 
David, you missed my comment about the vfd. ;)

I should have pointed out from the git that I have very little electrical knowledge. :o

David, that router controller was exactly what I was going to buy...
 
Strigamort said:
David, you missed my comment about the vfd. ;)

I should have pointed out from the git that I have very little electrical knowledge. :o

David, that router controller was exactly what I was going to buy...
Click to expand...

Yeah, I had noticed your mention of the VFD, but of course after I'd posted. This topic had come up a time or two in the past, and I'd seen references to the VFD as being likely the best way, though certainly not the easiest or least expensive. That's why I was pretty quick on the trigger, in bringing that up. :)

Some have claimed to succcessfully use the other controller on their grinders (not clear what types exactly), but I think it's a huge gamble. Within certain limits (current-wise), they may work. But I'd not want to risk it myself.

If you're mainly wanting just to slow your grinder down (only), as opposed to making it variable-speed through a wider range, I'd still investigate the possibility of changing out the drive pulley for a smaller diameter. I don't have or use a grinder, but it seems on the surface like it might be the simplest, safest and least technical route to go, and maybe not so costly either.


David
 
Since the drive wheel (it might be possible to switch this out for a smaller one as suggested earlier, not holding my breath on this though) is attached directly to the motor, and the motor can't use a speed controller, the only answer is to replace the motor either with a same size, but slower (1700-1800 rpm) motor, or a DC motor and controller.

Since this tool is going for $40 @ HF these days, both solutions seem $$$$ expensive given the minimal initial investment you have. Kinda like dropping a bored and stroked V-Twin in a moped. Bad idea.

There are other sharpening systems that will give you way more bang for the buck then trying to retrofit the HF I'm afraid. Not any good answers to this one.

Sorry

Syn
 
Oh I'm not using it to sharpen. I'll be using it to make small wood carving knife blades. I can get it to work as it sits but it would be a lot nicer and safer if it were slower. I don't know why they make them so dang fast.

Changing pulleys may or may not be possible I'll have to look into it more. Without having it in front of me I'm going to say that it would be difficult. Aside from the fact that the main pulley is directly mounted to the motor, all of them are incased in a metal frame. That may not be an issue though.

Thanks for the ideas guys. If I figure something out I'll post about it.
 
AC is 60 hertz, therefore = 3600 rev per minute. I sure would like it runs at 1000 rpm. Forget about changing the pulley, not worth the trouble & $. I found it's easier to cut & stretch the belt out on a 1x4 board and hand rub wood or metal work piece. Make divot if you want a manual slack belt :)
 
bluntcut said:
AC is 60 hertz, therefore = 3600 rev per minute. I sure would like it runs at 1000 rpm. Forget about changing the pulley, not worth the trouble & $. I found it's easier to cut & stretch the belt out on a 1x4 board and hand rub wood or metal work piece. Make divot if you want a manual slack belt :)
Click to expand...

Wait, wha...? Why not just use sandpaper? Do you not use your grinder at all?
 
Strigamort said:
Wait, wha...? Why not just use sandpaper? Do you not use your grinder at all?
Click to expand...

Belt is super sturdy, so better for heavy pressure work and sharpening too. I use my craftman 2x42 for most of the time. 1x30 with 36 grit for tight area work.
 
The operating rpm of the motor is dependent on how it's internally wired, so the 60 Hz input isn't the only determining factor in how fast it runs. There are a lot of 1725 (~1800) rpm motors out there too, running on the same 60 Hz input. I have a hobbyist rock saw/grinder with such a motor. The 60 Hz line frequency influences rotational speed for a given motor winding config, but doesn't limit it only to 3600 rpm. At the price point of these motors/grinders, I'd assume the 3600 rpm speed is likely the simplest/cheapest wiring config, however.

Reading a little on this, the number of 'poles' in the motor winding is the determining factor, relative to the input frequency. Just so happens, the 3600 rpm motors have the fewest number of poles (2), and lower speed motors have more poles. So I'm sure that makes the 3600 rpm motors easier to manufacture.

(Quoted text below is from a handy primer on Induction Motor math -->: http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm )
Calculating Synchronous Speed:

AC motors are considered constant speed motors. This is because the synchronous speed of an induction motor is based on the supply frequency and the number of poles in the motor winding. Motor are designed for 60 hz use have synchronous speeds of 3600, 1800, 1200, 900, 720, 600, 514, and 450 rpm.

To calculate synchronous speed of an induction motor, apply this formula:

rpmsyn = (120 x f) / Np

rpmsyn = synchronous speed (in rpm)
f = supply frequency in (cycles/sec)
Np = number of motor poles

Example: What is the synchronous speed of a four pole motor operating at 50 hz.?

rpmsyn = (120 x f) / Np

rpmsyn = (120 x 50) / 4

rpmsyn = 6000 / 4

rpmsyn = 1500 rpm
Click to expand...


This whole thing has got me curious, though. I'm tempted to visit HF, just to see how this grinder is put together.


David
 
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Obsessed with Edges said:
The operating rpm of the motor is dependent on how it's internally wired, so the 60 Hz input isn't the only determining factor in how fast it runs. There are a lot of 1725 (~1800) rpm motors out there too, running on the same 60 Hz input. I have a hobbyist rock saw/grinder with such a motor. The 60 Hz line frequency influences rotational speed for a given motor winding config, but doesn't limit it only to 3600 rpm. At the price point of these motors/grinders, I'd assume the 3600 rpm speed is likely the simplest/cheapest wiring config, however.

Reading a little on this, the number of 'poles' in the motor winding is the determining factor, relative to the input frequency. Just so happens, the 3600 rpm motors have the fewest number of poles (2), and lower speed motors have more poles. So I'm sure that makes the 3600 rpm motors easier to manufacture.

(Quoted text below is from a handy primer on Induction Motor math -->: http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm )



This whole thing has got me curious, though. I'm tempted to visit HF, just to see how this grinder is put together.


David
Click to expand...

Do it do it! :D

Thanks for the info btw, that totally makes sense.
 
David,

You made me feel like a useless engineer .. well, I haven't put the knowledge in any practical way for the past 20 years, so I deserve it :o :p

Striga,

Good luck in your search, if it's meant to be knowledge seeking, go for it! Might be cheaper buying something else though, but that's how we are (Maintenance Tinkering & Embellishment) :D
 
Chris "Anagarika";12391640 said:
David,

You made me feel like a useless engineer .. well, I haven't put the knowledge in any practical way for the past 20 years, so I deserve it :o :p

Striga,

Good luck in your search, if it's meant to be knowledge seeking, go for it! Might be cheaper buying something else though, but that's how we are (Maintenance Tinkering & Embellishment) :D
Click to expand...

Amen brother! :)

Btw Chris, I haven't forgotten about you. I've asked the store owner at that Surplus store to see if they can get more of the translucent Ark stones. She actually said that she might be able to, and if she does you're first on my list. ;)
 
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