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Alistair Hosie
09-23-2014, 04:13 PM
I have a small twelve inch bench mounted disc sander.In the last two days it needs to be spun by hand to get it to start,otherwise it just hums.I thought right away it must be the capacitor so I took off the big cover to expose the capacitor and a switch on top of the motor.Here is my problem The capacitor on this sander has no markings writing of any kind It is single phase 240 volts.Is this likely to be a start capacitor only or a start and run and what else should I look for ??? anyone able to help? Alistair

MaxHeadRoom
09-23-2014, 04:29 PM
If it is a start capacitor you normally would hear the centrifugal switch click as it slowed down after switching off, It sounds like it may be a Chinese unit? they are famous for cap failures.
Get hold of a 4f to 6f make sure it is a AC Motor Rated capacitor 350vac rated.
The one other thing is common on the imports is bearings go dry, and the motor doesn't have a enough torque to over come it.
Max.

MaxHeadRoom
09-23-2014, 04:49 PM
The other thing to try if a start circuit is suspected, is spin it in the opposite direction, if it runs backwards just as easy, then it IS the start circuit.
Max.

goose
09-23-2014, 05:33 PM
It's always the centrifugal switch, in my experience. Unless your capacitor has disintegrated into oily sludge I wouldn't go there first. First dismantle the end bell with the switch, clean and polish the contacts with 400 grit silicon carbide paper. Being it's a sander, it may have gotten packed with dust over the years. Even if it looks clean in there, polish the copper/brass contacts.

MaxHeadRoom
09-23-2014, 05:45 PM
If it is anything like the Belt sander I have, it does not have a start switch, just a single start/run cap, I have had to clean the dust out of the bearings on more than one occasion where it gave the exact same symptoms.
Especially if an open frame motor like mine.
Max.

shoeboxpaul
09-23-2014, 05:55 PM
I purchased a HF reciprocating saw a few years back. It worked great and then it would not start and I had to give the blade a push to get started (put blade on solid object and push down on the saw). I noticed that the new saw included carbon brushes. Just for a thrill, I removed the 2 plastic screw caps that holds the brushes and springs against the armature and one brush, somehow, was lodged in crooked. I pulled it out, set it back in place, reassembled, plugged it in and it hasn't quit since. Hopefully, you will be as lucky.
Paul

boslab
09-23-2014, 10:26 PM
The cap is normally spaded onto the wires, just swap it with another to try, just a suggestion
Mark

darryl
09-24-2014, 01:35 AM
I'm thinking that there will likely be a centrifugal switch cutting the cap in and out. It's probably more work getting to the contacts to clean them, but a blast from an air nozzle might convince them to close if they aren't closing currently. If you even get a momentary attempt at the motor starting after doing this, then most likely this is the problem area. If nothing changes, then you still don't know if it's the switch or the cap. Usually it's easier to get at the cap, and you can substitute just about anything with a high enough voltage rating and a capacity value between about 50 and 220 or so uf. If it begins to start but is slow starting, the value is too low, but also this should tell you if the capacitor is the problem. Because the value is unknown, I'd ask for something like 150 uf. Even if this isn't the original value it will probably be fine. If the startup is too slow, then maybe you can trade it up for a higher value, say double that. If startup is really snappy with that value, then it probably had a smaller one originally, but you may be ok keeping the one you got.

If there is no centrifugal switch, it will be a start/run cap and you might have to come close to the original value so the motor starts fairly quickly and runs fairly quietly. A value too far off for a run cap will probably have the motor overheating or humming more than it should.

Alistair Hosie
09-24-2014, 09:06 AM
Just to be clear the sander is covered i.e the motor and swich capacitor with a metal shroud removed with three srews .This exposes the machine it is brushless and the capacitor which I have not removed is hard wired at the cap end and this goes into a unit with a plug and seems definitely to have a swich on top when I try it with the switch one way Nothing happens the other way just a hum.The whole thing sits on a cabinet with a space underneath for a dust extractor this cabinet contains the normal red green switches for on and off.If I remember correctly when you switch from the front of the cabinet the dust extractor remains on for a few more seconds but I will check this later as I normally switch both off from the plug socket switches above the machine .I don't know why it also has this switch on the top as it is normally covered with the metal half round casing..Thanks so far guys I will check it and write more regarding my findings later.in the day. Alistair

flylo
09-24-2014, 09:51 AM
A Dodge truck has the fuel pump in the tank will do the same thing. My last gas Dodge a 2000 stopped running & a mechanis told me to "beat the h*ll" out of the bottom of the tank with a rubber mallet. Never acted up again.


I purchased a HF reciprocating saw a few years back. It worked great and then it would not start and I had to give the blade a push to get started (put blade on solid object and push down on the saw). I noticed that the new saw included carbon brushes. Just for a thrill, I removed the 3 plastic screw caps that holds the brushes and springs against the armature and one brush, somehow, was lodged in crooked. I pulled it out, set it back in place, reassembled, plugged it in and it hasn't quit since. Hopefully, you will be as lucky.
Paul

MaxHeadRoom
09-24-2014, 10:31 AM
Did you try to spin in it up in the opposite direction? this will show whether it is start cap related.
It is an induction motor, brushed AC are Universal (series) motors and do not require a cap.
Max.

Alistair Hosie
09-24-2014, 03:30 PM
Yes my good friend many thanks .I went today and as you said, it spun in oposite direction with just a little push.Now you are all going to hate me when I tell you the following I loooked at it again and found that the capacitor is held in situ with two self tapper screws when I undone these the capacitor was encased in a metal sheath and had lots of writing on it.I wrote down what appeared to be the most important as follows sILLY BILLY OR WHAT? LOL SORRY PLEASE LET ME KNOW if I did ok as I ordered these both

http://www.ebay.co.uk/itm/121352432640?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

http://www.ebay.co.uk/itm/290914220286?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT


MFB 16
16/360I
16MF+- 10%
UP= 860V-8
Z560 OR THAT MIGHT BE 2560-8
-25/60/70/85 DEGREE C

MaxHeadRoom
09-24-2014, 03:39 PM
Appears that is should work, although advertised as start type, later labeled mainly run.
Aptly named LastOne.
You don't have any motor winding shops in your area to source one? Although they probably can't match the price.
I guess you didn't manage to confirm if it has a centrifugal start sw?
Max.

Forrest Addy
09-24-2014, 04:27 PM
If it is the start capacitor (they can fail overnight) you need a replacement. If the capacitor markings are obscured you have to size the replacement by rule of thumb.

My suggestion is to use 80 MFD per motor HP as a selection factor. The exact size isn't important. Naturally the exact replacement will give a snappier start but 15% one way or another causes little loss in performance.

BTW here's some handy single phase motor start and run capacitor info suitable for the shop library:

http://www.marsm-a.com/images/Capacitor_Basics-98610.pdf

MaxHeadRoom
09-24-2014, 05:03 PM
If the capacitor markings are obscured you have to size the replacement by rule of thumb.



He says his shows 16f. :cool:
Max.

Forrest Addy
09-24-2014, 05:25 PM
He says his shows 16f. :cool:
Max.

That's why they call it a "rule of thumb". I looked again. He also said 220 volts. A 220V winding has more inductance than one intended for 110. More inductive reactance less needs capacitance for a resonant curcuit at line freq. Wire gage counts as well. There's a lot of factors but the 80 MFD rule of thumb is a good stating place for a 110 volt single phase motor starting capacitor, That makes it a rule of thumb but I did miss the 220 Volt part. My bad.

16MFD. I wonder if it might be a run cap.

MaxHeadRoom
09-24-2014, 05:40 PM
Voltage does not factor into inductance?
Not even inductive reactance.
Max.

Forrest Addy
09-24-2014, 06:36 PM
Voltage does not factor into inductance?
Not even inductive reactance.
Max.

No, but the turns per volts does and more turns = more inductance. I said there were lots of factors.

I wonder if there's an induction motor design software package? Enter the parameters and it gives you the stator iron lams, the stack height, the winding schedule, gages, materials list, the connection diagram, the amrature lams. etc links to AutoCad and Catia and: Boom! Instant motor. No tedious prototyping, no UL approvals - go right into production. Good for desk fans and powerhouse generators and everything in between.

MaxHeadRoom
09-24-2014, 06:44 PM
...But the inductance/capacitance value combination could well be identical for 120 or 240v motor of the same H.P. in order to achieve the 90 phase shift of the start winding.
Max.

Forrest Addy
09-24-2014, 08:49 PM
...But the inductance/capacitance value combination could well be identical for 120 or 240v motor of the same H.P. in order to achieve the 90 phase shift of the start winding.
Max.

Sounds right but I just hit my smart limit on this topic. Maybe a EE with extensive induction motor experience will chime in.

I'm fond or remarking that induction motor calculations caused many a second year EE to change majors to fast food service.

Alistair Hosie
09-25-2014, 06:48 AM
Thanks for the download forest I willo read it tonight Just what I needed.Alistair