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Reducing electric motor speed

A

Anonymous-0

Well-known Member
I'm working with a wood lathe, and would like to know if anyone has used a current limiter/rheostat to slow the rpm rate, without harmful effects to the motor. 2hp, 1725rpm The lowest speed is 550 rpm, and would like to get down to 100rpm. the motor is bolted to the lathe frame, and uses a variable speed 4" pulley. Granger sells a current limiter, but cannot find an answer if it will have an adverse affect on the motor. any help will be appreciated!
 
I tried one of those $30 boxes on a AC motor on a test stand machine but it had too much rpm fluctuation to suit me and no low end torque. Very low speeds with a load made it blow the fuse on the box. Either build another jack shaft and step pulley setup on it or go with a variable speed DC motor setup. The DC is not cheap but won't have the rpm fluctuation and give more torque on low speeds. Metal lathes step speeds down that slow with a gear box or open gear setup.
 
You cannot slow a single phase motor without destroying it.

3 Phase motors you can use a variable frequency drive to control the speed.

Or go with DC as described below.

You may be best to add a jackshaft and a few pulleys. Cheapest option just will take some time and beer to engineer.
 
Hi bruster,

I'm interested to find out the answer to this question, too. I suspect that the rheostat will reduce rpm at the cost of reducing power proportionally (this is what FixaLinc already said, if I understand his reply.) Aside from that, my reason for thinking so is that I have a 1/2" electric drill motor from Harbor Freight which apparently uses a rheostat to attain variable speed. At the slowest speed, the drill has almost no torque whatsoever. It has always seemed to me to be an extrememly poor way to accomplish variable speed.

All the best, Stan
 
An AC motor is controlled by the frequency, not by voltage.
VFD's "variable frequency drives" are dirt cheap on ebay. You can set them up for soft start, any rpms from a crawl to double the motor's nameplate rpm's. You can set them for torque limiting too, if something jams the will stop.
Running reduced voltage on an AC motor is a quick way to let the smoke leak out of the wires.
 
If you intend to run it at a slower speed in most all instances, I would just reduce the motor pully by 1/2 (4" make it a 2")
That way each step on your four setp is cut in half.
Spinning big bowls are you :)
 
BuickandDeere's VFD route is the way I'd go.
You can use it on other equipment as well.

100 RPM's is pretty slow perhaps you could rig up a 2nd motor with a gear reduction on it. Ebay or McMaster Carr. I've made pulleys out of wood in the past...low RPM stuff. Have seen 32" wood construction band saw wheels.
 
I put a treadmill motor on my band saw. Just used the existing controls on the treadmill to set it all up. That saw is sure nice to use with just the turning of a knob for any speed I need.

It is one of the much older 1 hp rated motors, but most later treadmills went with the 2 and 3 hp motors.
I left it belted through the existing jack shaft that my dad had rigged up for speed reduction back in the early 60s. He had used an old Bendix washing machine motor on the saw. Bendix had a motor with a high /low machanical transmission on it for spin and wash that was activated with a solinoid arm. Pop just flipped a switch to go from cutting wood to cutting metal.
Seems like a treadmill motor would work pretty well on a wood lathe also.

John
 
Rheostats will work on small brush motors like sewing machines.Your drill uses an scr for speed control again only usefull on small motors.You can adjust speed with pulleys on your lathe.1200 rpm motors are available.
 
VFDs are for three-phase motors only. It is possible to build a single-phase VFD, but single-phase induction motors do not run well at all at speeds that differ much from the design RPM.
 
Years ago I traded for an electric motor like I'd never seen before or since. It had a lever on one end that rotated a ring holding a set of four (I think) brushes. I think the spec plate said, "RPM: 400 to 4000". It worked, but the brushes made it noisier that a normal electric motor. I let it get away from me in another trade. Always wondered what it was used for and if it was an efficient idea. Guess not, cause they'd be everywhere if they were any good. . .
 
If you want speed control on single phase you have to use a DC motor and a common speed control with enough amp rateing. Good controls are compensated for changes in torque, but available torque does not increase with lower speed.
 
Most VFDs will accept single-phase INPUT, but the output is three-phase only. Single phase induction motors do not run well at speeds very different from the design speed.Check the VFD specs carefully for the HP output rating with single-phase input.
 
Yup! I'm beginning to think that it would be easier to build a heavy, solid one with 12, 10, 8, and 6" pulleys from TSC. I can throw a 2" one a spare motor, and use its weight to keep the belt tight
 
I like the treadmill idea! My dad rigged up his old Oliver band saw with the same 2 speed idea your dad did.....works great when switching from wood to metal
 
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