start capacitor open circuit ? disconnected? or start winding open circuit?

forgot – also check the centrifugal switch on the motor that switches the start winding out of circuit – if jammed open then start winding not powered – generally the motor will start rather slowly as the start winding is there to give a pulse of drive in the desired direction.

Is it single or three phase?

A motor fitted with a capacitor is almost certainly single phase. I sometimes convert 3 phase motors to single phase operation by using a capacitor but only on lightly loaded things like my little drill grinder.

Does your switch have the facility to reverse the motor? No reason why it should on a shaper but you never know it might be faulty or falling apart. Otherwise, it’ll be the start circuit in the motor itself, most likely the capacitor. Could be you have a wiee afloat in the junction box but that’s a long stretch.

Small motors sometimes have a single stator winding with a permanent magnet rotor. They start in either direction. I had such a shoe polisher, a good feature to keep the brush effective.

With a single phase motor with a failed open circuit capacitor, if the remnance in the rotor were sufficient, it might start in either direction. The poorer quality the rotor iron, the more likely this would be.

The assumption is it is a cap start induction motor – the OP has not said anything to tell us what type of motor he has ? If it is indeed an induction motor and the start windings or the start circuit is faulty then the motor IF it can start to spin will run either way. It is odd and I’ve never come across such a situation BUT that does not mean that under  strange circumstances it is impossible.

The OP must tell us what he has, Ideally with a picture of the data plate, then maybe we can help.  Where is the OP ?    Noel.

A normal cap start motor cannot change it’s mind on which way it wants to run without the start coil being rewired. A failed start cap normally presents its self as a stationary hum as above.

If it’s very old as in Great War there may be brushes which are very obvious, but these reverse by rotating the brush carrier. These have no cap at all.

Could a Stienmetz modded 3 phase motor behave like this?

That’s very clean for 1959 vintage.

On 3 January 2026 at 10:56 Macolm Said:

…

With a single phase motor with a failed open circuit capacitor, if the remnance in the rotor were sufficient, it might start in either direction. The poorer quality the rotor iron, the more likely this would be.

Macolm’s suggestion seems likely to me.

Electric motors need some arrangement by which the magnetic fields in the stator and rotor push/pull each other so the rotor turns.   There are several arrangements, but, bearing in mind this is a simplified generalisation, once the rotor of a single-phase is turning there is no need for a start winding or capacitors. But something has to get the rotor moving in the first place.

Early single-phase motors were pull-started with a rope, but apart from lots of dislocated shoulders and fatalities, the method had a nasty habit of burning out motors if it didn’t start first time.   Automatic starting systems were soon added.  Usually start and run windings phased 90° apart convert the motor temporarily into two-phase.One type is fitted with a centrifugal switch that disconnects the run winding once the motor is turning; if the switch fails the lightly wound run winding might burn out.  Other types limit current through the run winding with a capacitor, or two, which also provide the phase shift needed to generate push/pull magnetic fields.

Single-phase motors are not simple.  Conceptually more difficult than 3-phase, as anyone who tries to design one will find out!  They’re complicated electrically and mechanically, vibrate, and have poor starting power and torque.  Unreliable compared with 3-phase, and hard to reverse too.

I think remanence explains what’s happening.  The start winding, or start capacitor or the centrifugal switch has failed, but there’s enough residual magnetism left in the rotor to move it when the run winding is energised.  If the rotor moves at all, the motor starts.

At restart the way the rotor turns depends on how it was magnetised when the power was turned off and it alternates 50 times per second.

More information needed to fix it.

Dave

If it has a capacitor, they are pretty cheap, just change it!  If you switch it on (as it is) and let it run up to speed then switch it off, as it slows down do you hear a click?  That would be the centrifugal switch operating which would indicate a cap start unless the switch is kaput as well.  If the motor runs fairly quietly then no need to change it once the start direction is fixed.