Can someone educate me on start capacitors?

[1950ChevySuburban]

Hello all! My classic heavy duty bench grinder died, smells burnt. Found a wire fell off the start capacitor. I re-connect it, it's dead.
There's a relay in there too, part of the circuit. How does all this work?
I'm pretty savvy with electrical things, but never got a grasp on start caps.


Some enlightenment please? I did a search first. THANKS!

John

 

[1950ChevySuburban]

I forgot to mention the burning smell is in the motor part, not the external box with the start cap and relay.

I really like this grinder, and hope I can save it. It's very stout.

 

[Gooch]

The start cap is used to cause a lag in current between the start windings and run windings(or vice-versa) to cause rotation, as in a single phase motor there is no rotation of current. the relay is most likely controlled by a centrifugal switch(as it spins it opens the circuit) to take the cap and start windings out of use(once the motor is spinning, these windings are not needed to maintain rotation.) best bet would be to take the grinder to a motor shop and have them rebuild it.

 

[eriksalo]

I had a similar failure on my old red Milwaukee bench grinder. I really liked that grinder so I tried to fix it. I bought a new start capacitor ($20) and that didn't solve the problem. The voltage sensing switch that cuts out the start system when the motor is spinning was the problem and a new one cost $40. Instead, I went to Home Depot and brought a brand new 6" grinder for about $40 on sale.

The new grinder is bigger, much more powerful and has these things called "safety features." Sadly, I don't like it near as well as the old one. It looks much nicer but doesn't spin up as fast and for some reason I just don't like it. You have motivated me to go and get that old grinder going!

 

[Greatbear]

The relay you see in your grinder is a starting relay. Some motors use a centrifugal switch to remove the starting capacitor from the circuit once the motor comes up to speed, others use a relay that sense current in the running windings which drops considerably once the motor has reached speed, other sense the potential (voltage) across the starting windings that rises when up to speed. This relay could be bad. Also, the relay has to be mounted and used in a certain orientation. If you are trying to test the motor with the grinder upside down or otherwise not on the base, the relay that has an internal plunger/pole piece that is disengaged by its weight (gravity) will remain engaged, leaving the start windings energized, overloading the motor and burning the windings.

 

[70redbee]

I have an old grinder also. When I turn it on it does not start right away and sometimes runs in reverse. Is this a bad capacitor?

 

[frankzlt1]

There should be a certain amount of resistance you should read on the slip rings of the rotor, I don't know what. Exactly but shouldn't read a very high or 0 ohms. You want to make sure the rotor is not shorted buy reading the resistance from the slip rings of the rotor to ground if you have a 0 reading or if the meter you have has audible sound you will know

 

[1950ChevySuburban]

All great info, thanks! I'll take it apart and see what happened.
I didn't know about the relay orientation, I'll keep an eye on that.