Are all start capacitors equal ?

[JacquesBBB]

Thanks  for your message

I would say, that there is a chance, that those blown caps took away the aux coil and it's now shorted (polyurethane insulation melted and there is contact between turns).

I doubt this because the old capacitor was not shorted.  On the opposite, its resistance was a few Mohms.

I have dismounted the motor.  It is an induction AC motor with no other capacitors nor centrifugal switch.

I have cleaned everything, remove most of the rust, cleaned the bearings, which are just bronze olives, oiled them.
Remounted everything.
 
Again it starts when I hand start it, but not with the 4.7 uF cap, not with a similar 10 uF cap.

I  tested it out from its shell, and I found that it was heating a lot.

[wraper]

OK, those caps look like they can handle this for some time. I was afraid you are using something less robust.

Guess this some time will not be long at all. Even 400V DC capacitors do fail very often when exposed to 230V AC. Here is only 160V DC x2, however, because of the non perfect voltage distribution which happens because of the capacitance tolerance, it is even worse than that. Likely will last for the test time though.

[wraper]

Capacitors for connection to the mains (230V) usually have a rating of 250V~ or 400V.

Nope, that ~250V is not the same as the rating for usual capacitors. This is very tough load condition. Just some example from the datasheet of X2 ~250V rated capacitor: 800 VDC at 85 °C, 630 VDC at 110 °C.

[mrkev]

Thanks  for your message

I would say, that there is a chance, that those blown caps took away the aux coil and it's now shorted (polyurethane insulation melted and there is contact between turns).

I doubt this because the old capacitor was not shorted.  On the opposite, its resistance was a few Mohms.

That doesn't really mean anything... It could short out for a few seconds and than  disconnect due to vaporization of the metal inside that cap. Or the wiring is just old and mechanically stressed and the cap died because of it being shorted...

Again it starts when I hand start it, but not with the 4.7 uF cap, not with a similar 10 uF cap.
I  tested it out from its shell, and I found that it was heating a lot.

That sadly supports my previous theory, that the aux wiring is gone and you will have to rewind... Btw, does it heat up without that aux connected - running only at main coil?

[JacquesBBB]

That sadly supports my previous theory, that the aux wiring is gone and you will have to rewind... Btw, does it heat up without that aux connected - running only at main coil?

I think you are right !  Thanks !

I tried again without the capacitor, and this time the motor did not heat.

How difficult is it to rewind a motor ?

It there now  another way to make a motor start ?  For example with some electronic device in order to shift
the phases as in a DC brushless motor ?

[BradC]

How difficult is it to rewind a motor ?

It's not easy, but it's not particularly difficult, just time consuming and fussy.

It there now  another way to make a motor start ?  For example with some electronic device in order to shift
the phases as in a DC brushless motor ?

No, a DC brushless motor works because it has 3 identical windings 120 degrees out of phase. A PSC motor has two windings with the start winding usually 90 degrees out of phase. Without that winding you have exactly one winding and no way to generate an apparent phase shift.

Since the motor is toast, you could either take it to a motor re-winder and have it done, or get dirty and strip the windings to figure out how to wind new ones.
If you are lucky you should be able to just get the start winding out and re-construct that.

The other option is to get used to giving it a flick to start and just using it until it dies.

[mrkev]

How difficult is it to rewind a motor ?

Just look for motor rewinding on youtube, there is plenty of stuff. It's not that hard, but a really fiddly job requiring a lot of patience (especially getting those coils in the stator). And I don't know if you'll be able to impregnate it. Anyway, I would recommend to rewire whole thing, after all, it's old motor...
There could be some local company that does this, question is for how much, but I would rather use them...

It there now  another way to make a motor start ?  For example with some electronic device in order to shift
the phases as in a DC brushless motor ?

Not really. Main wiring of 1F motor doesn't make rotational magnetic field (f.e. it's going only up and down), the aux coil creates 90deg shifted field (sideways), without operational aux coil, you can't produce that effect.
On the other hand, this kind of operation isn't really damaging for the motor as long as it's spinning (you'll get slightly less power), in contrast with 3F motors, which produce magic smoke if you let them run at 2 phases...
You can even spin it by hand and then press the start switch (if you are not using it daily and don't wanna spend time/money, that's IMHO the best solution)...

[bills]

If the values on the schematic you posted on page 2 are correct you need to try real 4.7 uf  +/ - 20 %   run cap@ 370v check the windings to ground before you waist any more time.

[SeanB]

Start winding is cooked, you can see the discolouration in the photo under the varnish. Rewinding will cost around $100 ( price here for a small motor, the rewinder is near me in a small shop in a back alley, near the stadia) so if the motor is worth it to you, for sentimental or other reasons, take it to a local rewinder ( just the stator only) and ask for a quote.

On a positive note the rewind will come back with class H insulation, as opposed to the old stuff which dies at 130C. It will now survive another few decades.

[Circlotron]

It there now  another way to make a motor start ?  For example with some electronic device in order to shift
the phases as in a DC brushless motor ?

You really need both windings to function for this motor to run properly. In this particular motor the second winding that is powered through the capacitor is not just a starting winding, it is the second phase because the machine is actually a 2-phase motor. if you were to run it electronically you need to give it two sinewaves separated by 90 degrees, both windings always powered.

[Circlotron]

You can even spin it by hand and then press the start switch (if you are not using it daily and don't wanna spend time/money, that's IMHO the best solution)...

With the capacitor in circuit the current sent to the bad winding was limited. If you now use a start switch into what is now a near short circuit I think you might completely blow up the rest of the windings. 

Edit -> I thought you meant a "start" switch wired where the capacitor was.
Now I realise you mean the normal on-off switch.
But putting a switch in place of a cap and feeding a shorted winding would be a disaster.

[mrkev]

You can even spin it by hand and then press the start switch (if you are not using it daily and don't wanna spend time/money, that's IMHO the best solution)...

With the capacitor in circuit the current sent to the bad winding was limited. If you now use a start switch into what is now a near short circuit I think you might completely blow up the rest of the windings. 

Edit -> I thought you meant a "start" switch wired where the capacitor was.
Now I realise you mean the normal on-off switch.
But putting a switch in place of a cap and feeding a shorted winding would be a disaster.

Duh, I meant that it would run only on main coil. And OP wrote that he would start the motor by giving it a notch, which is stressful for wiring, if you wanna start motor like this with aux coil disconnected, it's better to spin it first, not after you switch it on...

[JacquesBBB]

Thanks all for your concern.

Once I realized, as mrkev said, that the aux was shorted, I disconnected the cap,  as indeed, this was much worse than with the old cap that had a high resistor value.

Unfortunately,  in my tests  the motor heated a lot,  and maybe now it is worse than in the original state.
I  could start it manually, but it would stop, and now does not want to start again.

So now I understand that the only way would be to rewind it,  but  it  is too much work. I will just leave it like that,
and provide to my friend my own grinder  which is a cheap chinese one on which he could put his stone.
And if I manage some day to find a electric motor of the same kind, I  will replace it,
as    I find the  casing very nice.

So it is an example of total failure in searching to fix something,  but at least I learned something.

Thanks to all for your contributions !