Variable Speed for Jet Lathe

[Terry Fox]

I have a Jet 1442 with belt and pulley drive - six speeds, starting at 500 rpm. Is it possible to add a variable speed controller? I'd like to be able to slow the lathe down for large and out of balance work, and for deep hole drilling. Having five set speeds above 500 rpm in not a problem. The motor runs on a 30 Amp, 110 volt circuit. Maybe there's a different way to achieve the same result?

Thanks for your advice.

Terry

 

[Bill Boehme]

By far, the simplest way would be to just modify the speed control lever to add additional detents since the actual drive system is a continuously variable Reeves pulley system. This path would mainly involve time to modify the control arm. Other changes such as using an electronic variable speed drive system would cost hundreds of dollars and not be significantly different from the cheap mod.

Bill

 

[ssinner]

Yes, it is possible. But expensive, if you want a good system. That's one of the reasons high end lathes are so costly.

See Craft Supplies' catalog for variable speed drives. They have a couple. If you look in the American Woodturner and other mags, you may find ads leading you to other suppliers of such systems.

You'll find AC and DC systems. In general, the AC systems employing an inverter to change 110 or 220 single phase to 220 three phase are the best. They provide a great deal of torque even at low speeds.

Three phase motors are easy to find at pretty decent prices, but the inverters cost quite a bit. You can put your own system together if you wish to search out some bargains at surplus outlets, etc. You may need the advice of an electrician friend, though. The motor and inverter have to be matched within specifications.

 

[Bill Boehme]

Three phase motors are easy to find at pretty decent prices, but the inverters cost quite a bit. You can put your own system together if you wish to search out some bargains at surplus outlets, etc. You may need the advice of an electrician friend, though. The motor and inverter have to be matched within specifications.

In general, I agree with the above information, but with some caveats. A typical general purpose three-phase motor is not satisfactory to use in inverter duty applications because of the following reasons:

1. Inverter duty motors require a special class rating for the insulation on the motor windings. Inverters stress the insulation of the winding wires to an extreme level as a result of the high speed switching action of the driver transistors in the inverter operating into a large inductive load reactance (the motor). The faster the rise and fall times of the switching circuit, the higher the peak transient voltages which are due to the collapsing magnetic field in the windings. Adding series resistance, would reduce the peak transient voltage levels, but it has the unfortunate effect of lowering the motor efficiency, so that idea is out the window. The solution is to wind the motor with a thicker and more breakdown resistant insulation. You won't normally see a sudden failure in a motor due to this phenomenon -- its performance will just slowly degrade over time until problem number 2 below rears its ugly head.
2. Overheating is the ultimate failure mode seen in motors being used inverter duty when they are not rated for that type of service. One of the causes of overheating is loss of efficiency caused by insulation breakdown described above. The average three-phase motor is not a super high efficiency motor to start with so it will run hotter than a motor rater for inverter duty. Inverter duty rated motors also must be able to withstand much higher than normal temperatures encountered by a single speed motor. The reason is that most motors are TEFC and when the motor is operated at slower speeds, it gets less cooling air because the fan is direct coupled to the motor shaft.
3. The bearings and grease used on single speed motors will run into problems when operated at very low speed because the oil and soap can separate and allow the oil to run out of the bearings. At higher than base speed, the bearings may also run hot and lead to the same problem.
4. The thick iron laminations in a motor intended for 60 Hz only operation become very lossy at higher frequencies and at low frequencies, the iron may magnetically saturate at at well below FLC limiting the torque output and adding to the heating problem.

Bill