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Oldneweng
7th Jun 2016, 11:03 PM
I recently bought a secondhand wood cutting bandsaw. The motor needs a new capacitor. The seller told me this when I was speaking to him on the phone. It was not mentioned in the Gumtree ad. This is a simple fix tho and may have put off at least one potential buyer.

I pulled the motor out today to look at the capacitor, and because I was going to look at new bearings as I had heard a squeaking sound when turning the saw over by hand. It turns out that the rotating part of the centrifugal switch is loose on the shaft. It is tightened on to the shaft with a screw. There is no damage on the shaft.

My question is how to reset this in the right position? It obviously needs to disconnect well before the motor reaches full speed, but other than that I have no idea. Can anyone offer any suggestions?

Dean

BobL
8th Jun 2016, 09:59 AM
Dean, These switches are usually centrifugal and self disconnect when the rotor gets fast enough so there is no right position

Oldneweng
8th Jun 2016, 10:24 AM
Maybe this one is different. There is a section mounted to the motor frame and a section mounted to the shaft. The section mounted to the shaft is free to move around, and up and down the shaft at the moment. It contains the centrifugal mechanism, of course. It needs to apply pressure, axially to the other, fixed section (contacts) to do its job. It cannot do this when it is free to go where it likes. They are two completely seperate pieces and have no physical attachment to each other except via the shaft/bearing/frame of the motor.


These switches are usually centrifugal and self disconnect when the rotor gets fast enough so there is no right position

It can self disconnect to its hearts content and achieve absolutely nothing at the moment.

I have pulled motors apart and seen the switch before, but not in recent years and I did not pay any specific attention to the switch and how it is mounted.

Dean

kwijibo99
8th Jun 2016, 12:01 PM
G'day Dean,
When you say there is no damage to the shaft I presume you mean the locking screw has not left a mark where it was originally tightened. If that's not what you meant then have a look for that mark and set the rotating part there.
With these centrifugal switches typically the weights will pull the ring away from the contacts which will allow them to open. As an initial setting I would set the rotating part such that the ring just holds the contacts closed. Verify that the mechanism has enough clearance to operate by moving the weights out by hand and check the ring withdraws enough to let the contacts open.
Put the motor back together and see how it runs.
If all is good the motor should spin up to its rated speed and run smoothly, you may or may not hear a click as the switch opens.
If the motor runs lumpy and doesn't reach its full speed the switch is not opening correctly so you will need to re adjust it by moving it away from the contact plate.
If the motor fails to spin up and pulses or surges the switch is opening too soon and will need to be adjusted towards the contact plate. This condition is highly unlikely.
Switch the motor off and as it spins down you should hear a click and a rubbing sound which is the switch resetting.
Additionally, lightly lubricate between the motor shaft and the ring and the weight pivot points. Do not put any oil on the ring where it bears against the contact spring as this may contaminate the contacts themselves which need to be clean and dry.
Good luck.
Cheers,
Greg.

Oldneweng
8th Jun 2016, 09:22 PM
Thanks Greg. That is great. There is a slight rub mark from where the screw was. I will have a look and see if It is enough to locate it. Now I know what to look for when trying the motor.

Dean

Oldneweng
16th Jun 2016, 09:21 PM
I Reassembled the motor today. I have been trying to sort out a capacitor. There was no discernable spot on the shaft where the centrifugal switch screw was. I attached the it as Greg said. When I tried it, the cover and fan were off so I could see the switch. It worked fine.

Dean

sossity
20th Jun 2016, 03:08 PM
I did one a while ago and seem to remember the switch will pull two contacts apart (for the one circuit). I presume this is to increase the air gap quicker and faster. At first i didn't bother with where the two contacts were opening and I think it was causing more flashing as it switched out. I went back in and bent things so the contacts opened as closely together as I could. I think that helped with reducing the arcing between the contacts. Of course, mine might be completely different to yours.

Oldneweng
20th Jun 2016, 11:36 PM
I did one a while ago and seem to remember the switch will pull two contacts apart (for the one circuit). I presume this is to increase the air gap quicker and faster. At first i didn't bother with where the two contacts were opening and I think it was causing more flashing as it switched out. I went back in and bent things so the contacts opened as closely together as I could. I think that helped with reducing the arcing between the contacts. Of course, mine might be completely different to yours.

I think it only had 1 contact. It wasn't easy to see tho as the contact was behind a large disk that the centrifugal mechanism pushed against. When I said switch in my last post, I meant the centrifugal mechanism.

It just happens that I decided to finish connecting the motor tonight. I had already put the lower half of the saw back together, and bolted the motor on. The capacitor I bought is much bigger than the old one and so it won't fit in the connection housing. I have added some wires and brought them outside. I need to find a housing to hold the capacitor now. Maybe Bob can make me one. :D I was thinking pvc pipe and caps but it won't fit in 50mm so I am looking at big pipe. Easier to get buy a housing from Jaycar.

Dean

sossity
21st Jun 2016, 12:53 PM
Yes, that's the switch I meant too. on mine the centrifugal part caused two copper contacts to break contact with a solid copper (or carbon?) disk that spun with the motor (something like that, it was a while ago). So the circuit was made by going through one contact, round the disk and out the other contact. On mine, you could bend things to make the contacts pull off the disk at the same time as each other when the centripetal force caused it to operate . When I did this, I'm sure the arcing wasn't as bad as before.