Alternative to centrifugal switch

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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.2776D203-2E1D-4942-BD9F-018EBD6FF9CB.jpegC781F96D-9E34-403E-8527-B8938104F493.jpeg2776D203-2E1D-4942-BD9F-018EBD6FF9CB.jpeg2DD3AA62-2144-4E41-A76B-B0D92B5CFB17.jpegC781F96D-9E34-403E-8527-B8938104F493.jpeg2776D203-2E1D-4942-BD9F-018EBD6FF9CB.jpeg2DD3AA62-2144-4E41-A76B-B0D92B5CFB17.jpegC781F96D-9E34-403E-8527-B8938104F493.jpeg
 

Marley

Joined Apr 4, 2016
334
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.
 

shortbus

Joined Sep 30, 2009
7,359
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
 

oz93666

Joined Sep 7, 2010
731
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???
 

Marley

Joined Apr 4, 2016
334
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.
 

Janis59

Joined Aug 21, 2017
984
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.
 
Last edited:

shortbus

Joined Sep 30, 2009
7,359
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.
 

Marley

Joined Apr 4, 2016
334
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.
 

Marley

Joined Apr 4, 2016
334
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!
 

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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
 

Thread Starter

Keith Robinson

Joined Nov 30, 2014
53
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?
 

Marley

Joined Apr 4, 2016
334
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!
 
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