This works OK but moving belts on pulleys is boring and a quick change gearbox combined with a VFD appeals. A VFD on its own just wouldn't give the low speed torque needed at the chuck or spindle.
Thanks for the heads up about VDF drives I did not know that they lost torque at low speeds so that is not viable for me and you have saved me from buying one. I am cursed with spin on chucks so sudden braking of the drive would undo them. If I fitted a braking system to the chuck itself I would have hells delight removing it from the mandrel, I have four chucks and two faceplates so they do get swapped often. I am working with three speeds direct 860-543-93 + back gear and have got used to it, I would like a faster speed but my lathe has plain cast iron bearings which I defer to as they do get warm fairly quickly at top speed.
VFDs generally adjust the voltage in proportion (or close to it) to the frequency. If they maintained the voltage at the maximum at lower speeds the motor would draw more current and produce more torque as the speed came down, but it would only do it for a limited time before it burnt out.
Making the voltage and frequency (motor speed) close to proportional maintains the current and torque at close to full speed levels. Some VFDs allow you to boost current at low speeds but this is just meant for the short term startup which sometimes needs some help depending on the type of load. This boost is only a relatively small amount.
So what can you do about it? There are two things really.
1. Adopt the approach that I did with my mill conversion and fit an over sized motor as outlined here:
2. Use mechanical gearing. If you gear down to half speed you automatically double the available torque.
PS. I should have made it clear that the torque of an electric motor is proportional to the current through it and the heating is proportional to the current.
The speed of a normal induction AC motor is dependent mainly on supply frequency and to a lessor extent the load on it. Hence VFDs. The load causes slip and reduces speed to a bit under synchronous.
Some induction motors can be switched to change the number of poles, which changes speed and torque. This a bit like electrical gearing.
There is a class of small induction motors called shaded pole which are speed controlled by supply voltage. Typically the small pump motors in washing machines are like this because they are very cheap to make.
Commutator style DC and AC motors have their speed controlled by supply voltage, as in the motors from exercise treadmills.
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