I like to convert a AC motor to DC so I can adjust motor speeds for use on a wood lathe. I will incorporate a floor/foot switch to activate the motor. Refer to a photo of the motor nameplate data below.
What would i need to do to accomplish this task?
Are there other inexpensive options that can be done to vary motor speed?
I have 2 varying diameter pulleys, but there is danger of the belt slipping into a different diameter thereby increasing lathe speed. This occurred to me once and the work piece shattered into several pieces. Fortunately, i was not injured. This was really a scary event!
I prefer to have control of motor speed to eliminate any hazards.
You are trying to swing the stick by the short end.
A 3600 RPM base speed motor is quite fast for a lathe.
It is very difficult and expensive to run a single speed motor of that type at varying speeds.
Your best bet is an 1800 RPM base speed, three phase motor and a variable frequency drive.
The poor mans version and what was once common was opposed multi-pitch pulleys.
If economics dictate using the motor that you have, you may have to use a jack-shaft to drop the speed.
A pair of stepped pulleys should give you a good range of speed but consider an intermediate jack-shaft to drop the speed. Using stepped pulleys without other reduction may result in a speed of 7000 RPM or more.
Too fast.
There are other solutions. How much do you want to spend?
And, if you are looking at this as a challenge, that is one thing, but if you just want an economical and fairly dependable and usable drive for your lathe, go with the pulleys.
@WaRoss Thank you!
i currently have the poor mans version, the opposed multi-pitch pulleys, which i do not like at all. somehow, someway, the belt on drive pulley went to a smaller diameter which resulted in too high rotational speed and the wood piece shattered. Scary event! hence, my desire for a DC motor where speed can be controlled. something more safe than what i currently have. I do not like the idea of exposed pulleys and belt. yes, i could cover them, but my preference is to eliminate the risk and go with something more safe.
Agree that 3600-rpm is too high for base speed.
i see that McMaster Carr has several options for $500-$700 which i may consider. A simple 0.5hp DC motor w/ 1800-rpm max speed.
And these drives don’t include a heat-sink, but the MC text indicates that you will probably need one of those, too.
A couple of other issues.
Decreasing the power of the drive will limit the work you can do. The speed sags as you do work, and sags more as the load increases.
Switching to DC offers speed control, but the power at the shaft equals the rated power only at the rated speed. At all other speeds the power decreases. I expect you to see a net decrease in torque at lower speeds - possibly bad enough to be unusable.
Compare that to belts/pulleys/gears, where you only change ratios, thereby exchanging torque for speed at constant power. You get more torque at low speed.
The cost of using 3-phase + VFD seems about the same as doing the DC motor option, and avoids the low-speed power problem. I haven’t checked all the details of this - I only mean to give you a starting point if you want to consider it.
Actually the characteristics of a VFD drive and a DC drive are similar.
In both types, the torque is related to the current, so;
Maximum available torque remains relatively constant and maximum available power varies as the speed.
This holds true for both types for overspeed if overspeed is achieved by applying higher than rated voltage.
If overspeed is achieved by simply increasing the frequency with a VFD or by field weakening with a DC drive then torque will drop as the speed is increased above rated speed and the power will remain constant.
single phase power is not a problem.
There are VFDs available that take single phase in and output three phase.
Changing from 3600 to 1800 is probably not feasible.
Or you can replace the motor with a universal motor. These motors operate on Dc or low frequency AC, and I recall they can be controled with a reostat.
I shopped a 3-phase drive from Automation Direct - plugs into 115/120v AC, single phase, and gives 0.5 hp 3-phase output, for about a hundred bucks. Lovely little unit, controls speeds down into the 1-2hz range or roughly 1800-30 rpm. My application was for a hydraulic gear pump, which needs relatively constant torque to push oil at a given pressure (speed just varies the flow rate, to first order), it works just fine.
In the documentation there will be a diagram for a single phase connection.
Often one of the three three phase inputs is ignored.
The input supply, whether single phase or three phase will be rectified to DC to feed the DC bus.
Two issues with single phase;
More ripple.
Less power supplied to the DC bus with the same current through the rectifier.
Both these issues may be mitigated by derating the device for single phase use.
While, the basics indicate that any VFD may be supplied with single phase if suitably derated, there may device specific reasons why some VFDsare not suitable for a single phase supply.
If you are going to use a VFD, select a drive that is rated for single phase input.
There are lots of them available.
