Slow down motor with rheostat?

Brian Avila

Brian Avila

Joined
Jan 2, 2011
Messages
1,286
After looking at some mods and threads about grinders I have noticed that pulleys are the conventional method. Has anyone used a rheostat? Thoughts?
 
it has been tried but it will not work... well, it might work but only until it fries your motor due to lack of power.

A rheostat sure would be an easy and cheap method if it worked but people don't spend the extra money on VFD and pulleys just because they have it to spend. It is the only thing that works to slow down a grinder.

Jason
 
A simple dimmer like a ceiling fan, or router only works on universal ac/dc motors - Motors with brushes.
Those motors are limited in power.
You won't find one with sufficient power for a grinder
Even if it is labelled as 1 HP like some wood routers are, it's a BS rating



You can use

Step pulleys
Good pulleys like Maska are expensive $60ish each they offer a max of 5 speed steps, but usually only 3
and your design need to use bearings, shaft


A variable frequency drive to control a 3 phase motor
This is the most common and the least expensive electronic option
A NEMA 4 enclosure will deep the dust out.


A DC controller on a DC motor
The controllers are super cheap - $25- 50 for a basic board without enclosure,
but good NEMA framed motors are WAY expensive and uncommonly found used.
Some people pull them from old exercise treadmills, but to me the open framed motors, controllers, non standard mounting, small HP rating isn't worth it.


There are other ways like tapered pulleys,but it becomes more bulky and more involved.





I know you still want to try a rheostat.
go ahead, it will only cost you a motor.
How ? Maybe it pulls more current, something causes heat buildup that burns the winding insulation.




In your profile you list researcher as occupation
This book may give you the details you want.
http://www.amazon.com/gp/product/0827386400/ref=olp_product_details?ie=UTF8
 
Last edited:
Rhreostats are resistance devices, they cause a voltage drop, and therefore modulate the load by delivering a reduced voltage, in the world of motors and Electrical services, that is called a bownout condition. Supplying undervoltage to a motor under load will destroy it given just a little time and a one or two attempts. There needs to be enough power supplied to the motor to maintain its rated horsepower/ power factor, anything less than that and the motor starts to let the rotor slip and rather than going roundy roundy, the stator sits there and cooks the windings, thermal overload, and insulation like an inductor would.

VFD's employ Power Factor correcting capacitive banks to make up for these losses.

TheWeatherman said:
How does lack of power fry your motor?
Click to expand...

by overheating the motor.
 
Since we're on the subject, would a pulsed output controller like they use for many model trains and such work? Obviously there would be serious amperage issues but in concept at least? Is there an upper limit on the power output for such a method, or is that actually what the popular VFD's are doing? I ask because I remember from years back that the pulse method was a big deal for things like model trains and some RC vehicles for lowering speeds without losing power or generating a lot of heat. It let your model train work at very very low speeds to allow realistic acceleration when an older style control would require more juice to get it rolling and potentially burn up motors if you weren't careful.

I just had a thought, my dad's a retired engineer... he didn't use that degree much for most of his career since he moved into an unrelated field, but he's still got the knowledge. Maybe I'll hassle him about it and see if he wants a project. A nice turn key power setup for the KMG and NWG wouldn't really be a big profit item but it'd be something interesting for him to work on.
 
I'm willing to wager that your trains and RC cars are DC motors
DC controllers are very cheap already made, but DC motors are expensive and rare used.



VFD's are AC only.


You can get used 3 phase motors cheaply, but 2 HP DC motors are $800ish

The NEMA 1 vfd's like the are also cheap $150 ish
http://www.factorymation.com/s.nl/it.A/id.196/.f

Just create a better enclosure for it. and get a used 3 phase motor for $50 ish

For $250 or $300 you can have working VFD and motor setup
It's very little more than a good step pulley setup.




Nobody can design and build a one off VFD for less than $150
 
I agree with 1-10.

You cannot put together the parts to perform the same duty, cheaper and as reliably as a VFD will. When VFD's were in their infancy you may have had a case, but like cell phones, TV's/ LCD screens and other electronics the prices just keep falling.
 
I've been calling around town and stalking Craigslist for a motor that will run off of 110V and have 1725-1750 RPM with HP 1/2-2, for under $100. No luck, no used ones to be found in my town.
 
TheWeatherman said:
I've been calling around town and stalking Craigslist for a motor that will run off of 110V and have 1725-1750 RPM with HP 1/2-2, for under $100. No luck, no used ones to be found in my town.
Click to expand...

This is for a pulley setup?


I can buy NEW chinese ones for $100



Try used motor repair shops

They get trade ins they they may want to get rid of.

When you get higher in HP, like 1.5 or 2HP you may have to go to a larger breaker anyway - when you do 110 on a big breaker vs 220 is little difference.
so keep 220v motors in mind


I have found that a
3400 rpm 1hp 110v
is the largest single phase motor I can find at 12 amps, which will fit onto a 15 amp 110v outlet/breaker.

When you go VFD and 3 phase in a KBAC-27D VFD, you can to 1.5HP

If you use a different VFD, or go to 2 HP, you need a 220v input.
 
I have called every repair shop and electric motor repair shop and no 110V (1/2-1HP) 1750RPM motors. I will keep looking.
 
When you say "around town" you just mean Laporte?
Wiki lists that as only 3.000 ish people

Maybe you have to expand your search range, drive an hour or 2 ?

Also look at different items.
I found one good motor and controller on an industrial sewing machine.
A full sized 1 Hp DC motor and controller.

I bought the package and sold off the sewing machine.

Now I'm thinking I should have kept it and use it for sheaths, but at the time it didn't occur to me.

I see more during spring, when the spring cleaning fever is on, not Christmas time.
 
I'll keep my eyes out. I did start in Laport, then moved to the closest town Fort Collins and now I'm starting to work my way out by about an Hour. We will see what turns up...
 
Still a little high for your price point, but you can order them new from Grizzly online for around $120-$130 for a 1hp 1750 rpm single phase AC motor. Also, you can check ebay and occasionally find a great deal.

--nathan
 
I did the looking around thing as well. The local motor shops were far more expensive then online. After checking all my options and needs I went with Wayne Coe and had him include a switch kit. I got a 1.5 hp from him and it runs like a champ and I have tried to bog it down and could not. Give him a call. He is easy to work with and you will be supporting the knifemaking community as well. :)

GIBcomplete003.jpg
 
ron_m80 said:
Rhreostats are resistance devices, they cause a voltage drop, and therefore modulate the load by delivering a reduced voltage..VFD's employ Power Factor correcting capacitive banks to make up for these losses...
Click to expand...

Jeez, you guys make me feel so dumb sometimes. :(;)
 
You must log in or register to reply here.
Back
Top