Posted by Frances IoM on 03/04/2019 23:06:32:
possibly the reference was not to synchronous motors but commutated motors common in days of dc supplies that would need some load to limit speed – eg try removing the air moving fan from a vacuum cleaner motor and see how fast it can get before some bearing decides it can't cope.
Yes indeed, Universal motors as used on sewing machines & vacuum cleaners are liable to run away and self-destruct, as are series-wound DC motors. Both types need a small load to keep them sane. I can't imagine Colchester using a Universal or series-wound DC motor in a lathe though, and the entry in lathes.co.uk definitely refers to single-phase:
'Although a 3-phase motor is very robust, and will put up with a good deal of abuse, its 1-phase cousin is a relatively delicate thing and best run near its rated capacity all the time (i.e. worked nearly flat out); if such a motor is switched on and off frequently against "no load" the windings will be damaged and, if run through a cycle where it is started, worked briefly, stopped and started again, the capacitor will fail prematurely.'
I'd agree entirely with that wording if 'against "no load"' was struck-out. I'm no expert though!
In theory I think single-phase motors are a bodge on most machine tools, not smooth running, and comparatively unreliable. Only used because 3-phase was difficult to get into a domestic workshop. Since electronics could do DC, VFD, and Brushless, I don't know of any current lathes sold with a single-phase motor. In practice, years of good work have been done on Myfords and similar, though it is true their centrifugal switches and capacitors may have been replaced a few times.
Dave