Electric motor sometimes goes one way sometimes oppositeway why?

I have just obtained a small electric motor about 100 or 150 watts on a small tool and cutter grinder (the name plate has faded )the name on the casing is Newream with a old Corby telephone number 0536 i assume at least 20 years old when i switch it on it sometimes goes clockwise sometimes anticlockwise . I sometimes have to give it a flick with my finger to get it to start,is it kaput or just need a new capacitor.thanks

On Saturday, December 1, 2012 10:18:01 PM UTC, Graham wrote:
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mall tool and cutter grinder (the name plate has faded )the name on the cas ing is Newream with a old Corby telephone number 0536 i assume at least 20 years old when i switch it on it sometimes goes clockwise sometimes anticlo ckwise . I sometimes have to give it a flick with my finger to get it to st art,is it kaput or just need a new capacitor.thanks -- posted from http://w ww.polytechforum.com/modelengineering/electric-motor-sometimes-goes-one-way -sometimes-oppositeway-53355-.htm using PolytechForum's Web, RSS and Social Media Interface to uk.rec.models.engineering and other engineering groups
Try a new capacitor.
The symptoms you describe are the same as I had on my sawbench. That was cu red by replacing the capacitor.
John H

On Saturday, 1 December 2012 22:18:01 UTC, Graham wrote:
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http://www.polytechforum.com/modelengineering/electric-motor-sometimes-goes-one-way-sometimes-oppositeway-53355-.htm
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Thanks John i will get get a new one and try.Graham

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Single phase AC electric motors, like single cylinder steam engines, have no inherent preferred rotation direction built into the armature, or the piston and crankshaft. They depend on inertia to keep going in the direction they were started in. Without a kick both can hang up at top or bottom dead center. The engine gets its direction from valve timing, the motor from a second winding placed at an angle to the main one, often through electrical wizardry that makes the start winding appear to be where it isn't, physically. The capacitor performs this wizardry unless it's gone bad.
There is often a centrifugally-opened starting switch that deactivates the start winding as the motor gets up to speed, since the extra winding tends to be too thin (for cost) to survive continuous running. this switch makes the click when the motor coasts down, like a bench grinder. On the grinder you may see and hear the difference after the switch closes and increases the drag that slows down the wheel.
I replaced the 3-phase motor on my lathe with a single-phase one and rewired the switch to start it in either forward or reverse. If I shift the switch rapidly while the motor is at full speed it just continues in the same direction because it hasn't slowed down enough to let the starting switch close. jsw

On Sun, 2 Dec 2012 13:18:36 -0500, "Jim Wilkins"
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If I remember my motor theory correctly the higher resistance of the thinner starting winding helps to put the starting winding electrically in quadrature to the running winding. Eric