brianhabby":2tsnw0lw said:
Does anyone know if it's feasible/possible to obtain such a thing as an aftermarket device that can slow a lathe down? I would like to try my hand at threading but not at the minimum 500rpm of my Draper lathe.
Any ideas?
regards
Brian
You cannot reduce the speed of a single phase induction motor in any meaningful way and maintain useful power.
An induction motor is tied to the A/C mains frequency; its speed is the result of how many field poles are in its construction. (normally 2 or 4 poles, occasionally as on the poolwood 28-40 mechanical drive lathe 6 poles)
As soon as you reduce the power to cause the tie between the poles and the rotor to break, it ends up in limbo land with the magnetic links just slipping, and in most cases the power input will increase considerably with no useful work being done.
Some ceiling and desk fans have different speed settings achieved by switching between different multiple pole configurations, 2-4-6 or 8.
In theory a frequency controller reducing the supply frequency will reduce the speed and in a three phase motor this is effective because of the design it can maintain the magnetic lock with the poles and provides useful power over a considerable range.
A single phase motor however rapidly loses the magnetic lock with the poles and once slipping occurs the rotor does not have sufficient inertia to carry it forward to the next pole. Power consumption will increase considerably once the motor enters this ‘stalled’ condition.
A universal motor such as fitted to the old Axminster pen lathe or your hand drills will run on both A/C and D/C and can be controlled via thyristors or triacs (or even a simple resistor as in old sewing machine controllers) to reduce power and hence speed, there will be a trade-off of output power though as power out cannot be more than power in.
Lathes fitted with DC or permanent magnet motors will in most cases already be fitted with appropriate speed controllers.
just went from off to on at about 3/4 'full'
