I’m renovating a very solid very old grindstone. I have broken the centrifugal switch that switches from starting winding to running. Before I attempt micro surgery could I bypass this method and use a capacitor instead? If so how to wire it ?
it needs greater brains than I possess to figure this one out so all input will be very much appreciated.

 

I think not. Because the start winding is different in a centrifugal switch type motor compared to a capacitor start one.
That picture of the snapped off carbon brush is the switch I presume? Shorts the two rings together until up to speed?
Another way to do it might be to have an external timer relay that connects the start winding for 1 or 2 seconds or so when the power is switched on and hope that the motor is running by then. Get an adjustable timer so that you can set-up for the time required.
The start winding often takes a lot of current so make sure that the relay contacts can handle the current.

 

While you don't say what voltage, I'm going to assume it is for a mains circuit. There is something called a "potential starting relay", used in refrigerator compressors. The compressor is a sealed unit so the centrifugal switch can't be change or serviced, so they came to this idea. It is outside and does the same thing that a normal centrifugal switch does, takes out the start winding when up to speed. Here is a link explaining them, and then you can look for one that fits your local voltage ratings.
https://www.google.com/search?client=firefox-b-1-d&q=potential+switch

 

I think not. Because the start winding is different in a centrifugal switch type motor compared to a capacitor start one.
That picture of the snapped off carbon brush is the switch I presume? Shorts the two rings together until up to speed?
Another way to do it might be to have an external timer relay that connects the start winding for 1 or 2 seconds or so when the power is switched on and hope that the motor is running by then. Get an adjustable timer so that you can set-up for the time required.
The start winding often takes a lot of current so make sure that the relay contacts can handle the current.

That sounds like a good idea ... but no need for a timer , the switch over time is probably not that critical . Just have the ON switch with two positions (3 including OFF) ... When turning on first throw switch to position 1 .... count one two three and switch to position 2 .... How easy is that???

 

Got to be careful that the start winding does not stay connected for very long as it will take a large current and burn out.
If you get a timer relay with change-over contacts (NC / NO). Use the normally closed contacts to supply the start winding. After a short time the timer will energise and break the contacts. Only a few seconds should be enough. So your timer is the delay-to-make type but you will use the NC contacts.

I have seen the starting relays Shortbus talks about. I think these monitor the current the running winding takes and cuts the start winding when the current in the running winding drops to its normal value. Thus automatically cutting the start winding when the motor is up to speed. Also has a time-out (possibly thermal) to protect the motor if it is jammed on start or doesn't reach normal speed. These starting relays are matched to the motor so will be hard to size.

 

1) optical segment sensor from each old-type of computer mice.
2) much better except the heavy loaded kW scale motor environment where are rather large magnetic leaks. Thats the Hall sensor (0,1 USD) plus opamp plus Schmitt trigger.
3) simplificated version: take a kitchen refrigerator motor startup relay (electromagnetic or piezo - better EM), make shunting the primary until current is passed with those You need, and contacts fortificate via triac or magnetic launcher of corresponding Amperage. Simply, at start the motor is spending far much higher current in work coil, and if that current is falling beyound the target, means the motor is spinning well, thus the surplus capacitor must be disconnected.
4) method as at russia: apply the separate switcher, one is labeled "normal work on", another is named "surplus capacitor on for startup". First is lever, second is push-button.
5) May integrate within the magnetic launcher, launcher plays the first switcher role, and button "ON" is firing the own-keeping launcher AND non-keeping other launcher for startup - as long one will keep this button, as long the motor will starting, after release of button motor keeps running on normal capacitor. Next button is "STOP" which interrupts the first relay coil circuit and nothing else.
6) With time-relay, for example capacitor and resistor via thyristor or triac or diac. When circuit is charged-up, then startup relay releases.

If there are resistive launching instead of capacitative, all said is in force except the capacitors itself.

 

I have seen the starting relays Shortbus talks about. I think these monitor the current the running winding takes and cuts the start winding when the current in the running winding drops to its normal value. Thus automatically cutting the start winding when the motor is up to speed. Also has a time-out (possibly thermal) to protect the motor if it is jammed on start or doesn't reach normal speed. These starting relays are matched to the motor so will be hard to size.

Only reason I didn't link to one is the lack of information from the OP. They go by voltage and HP or motor amperage. Not really that many to choose from since they are pretty universal.

I think not. Because the start winding is different in a centrifugal switch type motor compared to a capacitor start one.

I think your mixing two different motor types here. A cap start motor uses a centrifugal switch always. But a split permanent capacitor motor doesn't, the cap is always in the circuit, and protects the start winding from burning out.

 

Thinking of a timer relay like this. Contacts rated at 8A which may be enough. Might be a good idea to connect a suppressor circuit across the contacts to prevent excessive sparking when they open. Something like this.

 

That sounds like a good idea ... but no need for a timer , the switch over time is probably not that critical . Just have the ON switch with two positions (3 including OFF) ... When turning on first throw switch to position 1 .... count one two three and switch to position 2 .... How easy is that???

Of course if you want to get really simple you could go with this but I would arrange the 3-position switch so that:

• Position 1 - OFF
• Position 2 - Running winding only
• Position 3 - Running winding + start winding

Position 3 needs to be spring return to position 2. Other positions stick.
Trouble is, a fancy switch like this will be more expensive than the timer, and if incorrectly operated risks burning out the motor!

 

In the majority of cases 3 secs is plenty long enough to start most motors, and not enough time to burn it out.
Max.

 

Of course if you want to get really simple you could go with this but I would arrange the 3-position switch so that:

• Position 1 - OFF
• Position 2 - Running winding only
• Position 3 - Running winding + start winding

Position 3 needs to be spring return to position 2. Other positions stick.
Trouble is, a fancy switch like this will be more expensive than the timer, and if incorrectly operated risks burning out the motor!

I’ve had a fantastic response thanks to all its very encouraging. This simple three position switch appeals to my Neanderthal brain and will be my preferred first attempt. I will go for 1 off, 2 start position for 3 secs, 3 on. Flash! Bang! Blackened face? Time will tell. Thanks everyone x

 

Of course if you want to get really simple you could go with this but I would arrange the 3-position switch so that:

• Position 1 - OFF
• Position 2 - Running winding only
• Position 3 - Running winding + start winding

Position 3 needs to be spring return to position 2. Other positions stick.
Trouble is, a fancy switch like this will be more expensive than the timer, and if incorrectly operated risks burning out the motor!

Hello, I’m going with this method thanks for your input. Just a thought but would these parts still be made nowadays? or is this mechanical method outmoded now?

 

You can still get them. Might take some time looking through manuals and manufacturers datasheets to find exactly what you want. For your machine you need a nice industrial style rotary switch.
You could start here and browse. Good luck!

 

Thanks once again I’ll start looking

 

I found this thread and am having a very similar issue with an old bench grinder, I was wondering If I can just place another toggle switch in place of the cent. switch, I was thinking something like a off-momentary on switch and just hold it on and flick the normal power toggle while holding it for a second or two to get the motor up to speed then release it to the off position, I am weary of doing this because I cant find anyone that has done this before so Im thinking its because it could either be dangerous or just wont work

 

It should work, the problem is with start capacitors that are generally. or can be of the bi-polar electrolytic type, if you leave them connected a little too long they can over heat and fail.
If the oil paper filled, then usually no problem.
Max.

 

My problem is the centrifugal switch doesnt open, it allows the motor to start but fails to open therefor blowing my cap, this happened twice, the first time I thought I had the wiring wrong however I now know its right after some process of elimination (the wiring out of the grinder is all the same color) replaced the cap and after about 40 seconds of grinding it blew again, I inspected the switch and one of the contacts has a big hole in it but still makes contact with the plate, I cant find a replacement anywhere so have been looking for alternatives and I though a momentary toggle would be the easiest way, thank you for the help, just wanted to make sure it was ok to do this because like I said I havnt seen anyone try it this way

 

a off-momentary on switch and just hold it on and flick the normal power toggle while holding it for a second or two to get the motor up to speed then release it to the off position

I have been doing that for decades with a lathe. You will get used to it soon. The motor growls if you hold the start switch too long.

 

Only reason I didn't link to one is the lack of information from the OP. They go by voltage and HP or motor amperage. Not really that many to choose from since they are pretty universal.



I think your mixing two different motor types here. A cap start motor uses a centrifugal switch always. But a split permanent capacitor motor doesn't, the cap is always in the circuit, and protects the start winding from burning out.

Sortbus called it right, the solution is to use a refrigeration type of starter relay. Any sort of timer is inviting a failure, and far more complex than required. Many grinders do use capacitor motors, but those motors are built for that application, meaning that they are different. Just be sure to get the relay connections right.

 

This is my crude wiring diagram that I dont think is super accurate but it is what I have currently, at the moment the grinder just makes a loud hum and the momentary switch I have placed in place of the centrifugal doesnt do anything