Question for the electricity boffins
philip
| Posted in Power Tools and Machinery on
One of my bench grinders has a problem: if the rotor is in a certain position e.g between 12 o’clock and two o’clock it will not start- just hums. If rotated a little it starts as normal. In other words there is a dead spot- if it came to rest when switched off in that spot it won’t start.
It is a simple induction motor- capacitor start. No centrifugal starting gear. Not a brush type motor. 220volt, single phase,3/4 horse.
So what is the story?
Philip Marcou
Replies
Between twelve and two? Give it time to digest those burritos.
Check for a faulty capacitor.
"Check for faulty capacitor"- that is what I don't understand- if capacitor is faulty surely it would hum at any start position?Philip Marcou
How about loose terminal screws?
If it's a squirrel cage motor, you could have lost a pole in the motor, Philip, but I'm no electrician.If you take the thing into a local motor repairer/rewinder, they'll be able to tell you on the spot, and, if they're anything like the ones I deal with, it will more than likely cost you nothing if it's a simple fix.They'll have a capacitance meter, so they can check the capacitor on the spot as well.Cheers,eddie
Howdy,
Caveat -- the coffee hasn't kicked in yet. So if my reply is complete crap then please consider this reply automatically retracted.
I suspect there's a problem with either the windings or the brushes of your motor. If memory serves, they need to be within fairly close proximity of each other in order to generate the magnetic force which is spinning the motor. I think something is worn and you're not getting enough force to start up the motor.
As someone else suggested, take it in to a shop. It might require a new motor which may or may not be worth it to you.
If the motor has field serviceable brushes (eg. by you) you might CYOM and see if they provide instructions on replacing the brushes. I had a dremel go out that way once after some super heavy grinding on a 4x4. But that's (the 4x4) a memory I choose not to revisit. OTW the shop should be able to change 'em out for you.
Glen
OK, better wait for the coffee to kick in. ; -)
A bench grinder is likely to have an induction motor, which doesn't have brushes. (Although it does have windings, one of which could be defective in the OP's case.)
Mike HennessyPittsburgh, PA
reply retracted then.;-)
A single phase capacitor start motor will have a centrifugal switch, and this is what's wrong with your grinder. The bobbin is pushing the switch open in a certain position, which disconnects the starting winding. When you turn the shaft slightly, the bobbin moves away, the switch closes, and the motor starts.
I suggest that you open the motor and inspect/clean/lubricate the switch and bobbin......Regards, Rod.
Come on , guys, you can do better than that- I did describe the motor type clearly- induction motor, no brushes, capacitor start, single phase.
I know this because when I got it originally, second hand, I pulled it apart , replaced bearings ,checked the simple electric stuff like switch and connections and re-sprayed: that was years ago. It has been fine until recently.
I have examined the on-off switch and the connections, and measured current under load.Philip Marcou
HI Philip,I'll admit I don't know electrical circuitry that well. However, with the squirrel cage motors, I do know that you can lose a pole occasionally, even though I don't think that's the case here.I'd swing past the local motor rewinder (having worked in NZ, I know how uncommon these are, so I'm assuming that it's a wee drive away,) and get their opinion. It may be a simple fix on the spot there, and save hours of mucking around.Cheers,eddie
Hi Philip, I am not sure that you have answered my question about the centrifugal starting switch. Have you checked it to determine that it closes when the motor is not rotating?
Regards, Rod.
Philip:RodSheridan could be correct. At the moment, I have a similar problem with a similar motor. I took the motor to a shop today and my analysis was confirmed: bad starting switch. Just to clear things up, my switch is closed at startup. The centrifugal device opens it, taking the capacitor out of the circuit. My problem differed from yours in that my switch would stick closed which fried it. Furthermore, my centrifugal device can be adjusted on the shaft to release quickly or slowly. Too much either way and it is either open or closed all the time. You should be hoping that Rod IS correct. My money says that he is or else the switch contact is burned.Cadiddlehopper
Caddy- that is a mechanical centrifugal starting switch that you are referring to-I am very familiar with those. However this motor is of the type that uses a capacitor for initial start up-there is no centrifugal switch.Having dismantled that motor in the past I know that there is no centrifugal switch gear-apart from the fact that one can always hear this type of switch in operation.
Even though it does not stand to reason that the capacitor could be faulty I am going on to plan B- which is substitute another capacitor if I find one in the Drawer For Things Collected for possible use one day....Philip Marcou
"uses a capacitor for initial start up-there is no centrifugal switch."I believe that it uses the capacitor for running also if there is no switch. Sorry! My suggestion cannot be correct.Cadid
Caddy, I will post some pictures of various single phase induction motors- it is not surprising that the average person is confused .Philip Marcou
Philip,I read the section in my old edition of "Machinery's Handbook" about motors. It wasn't very informative, but single phase capacitor motors come in several varieties internally. Not knowing which one you have makes remote troubleshooting impossible. The symptom you describe sounds like one caused by an intermittent connection. Replacing the capacitor may have fixed such a connection also. Anyway, I hope so. Should the problem recur, a trip to a shop may be wise.Cadiddlehopper
Plan B has worked- I had a suitable capacitor, connected it and all is sweet.So now the question for the electrickery folk is "How can a capacitor act like that i.e not start the motor when the rotor is in a certain position each time?" I vaguely recall something in school physics to do with sine curves, phases, emf's and not learning my definitions....
Anyway, for those interested, I have pictures of various types of single phase induction motors in my workshop- and no wonder there is confusion out there.
The green grinder is capacitor start-the one in question.The capacitor is in the base.Observe the solid tool posts-made in Zimbabwe.
The silver Millers Falls grinder is centrifugal start-old American iron-ditto the tool posts.
The red motor is with brushes-drives a buffing machine- 1/2 real horses.
The black one is with centrifugal start, capacitor run combined with magnetic brake-in the houseing opposite pulley. 1 horse British-good for small jointer etc.
The silver one is the familiar old Rockwell Delta-has brushes and grunt.
The gray one is a sewing machine motor, with clutch and capacitor start in end houseing. Good for lathes.
The other silver one is a capacitor start and capacitor run- see the two capacitors.
The green one is a General Electric centrifugal start, capacitor run. The black box is a relay. So this can be switched on and off repeatedly without too much worry-unlike ordinary ones.
How about that then?Philip Marcou
Edited 3/1/2007 12:58 am by philip
Dang Mook, I thought we had a lot in common, but after seeing how clean your shop is, I think I must have been delusional. It would take me a week to get my shop looking that clean.Next time before you post any photos, you should go to the pet store and buy some wood shavings / bedding, and spread it around so it will look like you actually do some work there. Ha K
Ha ha Keith , I have been wandering when somebody would comment....Fact is that not much woodworkery is going on at all, and also you may notice that only a small area of the bench is in the picture-all the "stuff" is on the other side of that piece of pine going across the bench top.
As for engineering stuff the dozens of small parts and tools make it necessary to be reasonably clean and tidy-anyway, swarf and chippings are easily swept under the carpet in order to take pictures (,)
Next time I'll take some dirty pictures.
So what is the explanation re that bench grinder?Philip Marcou
How sure are you that it is only in that small window of space in the rotation? Are you sure that it is sitting in a dead spot? or did you give it a spin by hand, and flip the switch on, and just make that assumption.My guess is that it is just the switch contacts are getting pitted. sometimes you flip it on, and you get enough juice to make it hum, but not enough amps to start.Next time it does this, don't flip the switch off, but be ready to unplug it or flip the breaker. This will allow you to then go into it and put a tester on the lines out of the switch, when you flip the breaker, or plug it back in. I think you can expect a pretty good drop in volts it this is the cause.If this is not the cause, you may have to leave the switch on the next time it hums until it smokes. The smoke will lead you right to the problem. Ha K
Philip,
It's either them bloody electrical gremlins....or your motor was manufactured by the Prince of Darkness.
Used to have an air-cooled VW vanagon (newer style bus) in which the starter motor would do the same thing a couple of times per year: it would stop in a dead spot and not start until it was rocked back & forth in gear to turn the starter motor a little bit; then it would start fine, just like nothing had happened. My son's '91 Volvo wagon just did the same thing about a month ago; the same fix cured the problem. Don't know whether it will help you any, but it might be of some relief to know that you're not the only one (who has been) plagued by evil electrical motor pixies....
;-)
.
Tschüß!
James
"I'm sorry, Dave, I'm afraid I can't do that...."
--A.C. Clarke
Chames- that is understandable, because starter motors have brushes- so if the commutator has a "dead" segment in contact with a brush it will not turn.
Can't blame the Dark Prince either- it has been working fine for about ten years....
Lunch break now- I will sort that turkey out later this afternoon and let those interested know the result.Philip Marcou
Phillip, ask your local motor shop to check for cracked rotor bars. A growler and a few metal filings will find out real quick if that is the problem. Also, they probably have run into this more than once, and if the growler check doesn't turn up anything, I'm sure they have some good ideas where the problem lies.
Mighty Oak
Thanks, Oak- the problem is cured. I guess a growler is some sort of tester-can you describe how it works/does.
P.S. In New Zealand your local whatever is always in some town other than the one you live in...Philip Marcou
Philip, Glad to hear that the problem is fixed. A growler is a coil and an iron core that is in a "v" shape. When an armature or rotor is laid in the "V" and the core is energized, a magnetic field is set up in the iron core of the rotor or armature. In the case of an induction rotor, which you have , a current flows through the cast rotor bars. It is best to cover the rotor with paper, then sprinkle iron filings over the core. The rotor bars can be "seen" as the filings will stick to the paper. In reality what you are "seeing" are the magnetic lines of force along the rotor bars. A cracked rotor bar cannot carry current, therefore, there will be a "break" or "gap" in the lines of force, and the filings will not stick in this area.
Incidentally, the name "growler"comes from the sound the device makes when energized and thats why it's called a "growler"
You are now an "expert"in motor testing and problem solving.
Good Luck!!!
Philip, It's me again (mighty oak) I apologize for some of the feedback you got. I thought your question was genuine, and really deserved a well intentioned answer. I also thought this forum was for serious woodworkers trying to be of USE to one another. It's too bad that we have some smart A---s out there.
Oak, no worries, and thanks for your information on the growler tester.
I don't worry if some folk come up with seemingly daft responses-most are trying to help anyway, plus there's room for some b/s , otherwise things can get too serious-which can be boring. Also, it seems that not everybody actually reads things carefully before responding- so one can get answers to questions not asked (,)Philip Marcou
you could have a low spot or burn residue, carmelization, on your commutator at that spot.. Sounds like you need a basic tune up, new brushes, bearings and have your commutator turned.
"you could have a low spot or burn residue, carmelization, on your commutator at that spot.. Sounds like you need a basic tune up, new brushes, bearings and have your commutator turned."
Hmmm. That's probably it!
And while you are at the shop getting the commutator on this induction motor inspected and turned, and new brushes installed, why not pick up a left-handed screwdriver and a new board-stretcher!
; - )
Mike HennessyPittsburgh, PA
On the counter next to the relative bearing grease!------------------------------------
"The West won the world not by the superiority of its ideas or values or religion but rather by its superiority in applying organized violence. Westerners often forget this fact, non-Westerners never do."---------------Samuel P. Huntington
<<On the counter next to the relative bearing grease!>>
Hmmm....I thought it was down the aisle with the road wheel air pressure gauges, the lanyard lube, and the soft spot hammers. Must be thinkin' of a different place....
.<!----><!----><!---->
Tschüß!<!----><!---->
<!----><!---->James<!----><!---->
<!----> <!---->
"I'm sorry, Dave, I'm afraid I can't do that...."
-- A.C. Clarke
dg, after a long patrol our nuc-fast attack sub was preparing for a dry dock session when we were blessed with several new E-2, E-3 crew in our gang -the machinist mates- but we were up to our eyes in work so our old senior chief sent our new "greenies" on tasks right after quarters on their second day on board to kinda clear the decks till we had time to properly integrate them into the trainning sched.
I remember his instructions to one one lad from Kansas(this was the first time this 19 year old had seen salt water) at the Charleston Naval Base/Naval Shipyard, a huge complex. He was told to go to the (and only to) the Senior Chief in the ship yard's inside machinists shop and plead for a 1 pound can of relative bearing grease as we were in trouble with the XO our navigator for faulty bearings.
I will not drag this out but after seeing the Senior Chief's in the outside machinist shop, the instrument shop, the shiprights, the riggers and the supply office, he showed up back on board at 1600 all depressed in failure and the Chief told him that he had a speed letter from supply that solved the problem and gave the kid his laminated liberty card.
Now, had he sent me! I would have had a fine lunch and afternoon on the strip with the ladies! Enjoy, Paddy (E-6)
ps. It's my understanding that Sr.& Master Chiefs, Top Sargents and Gunny Sargents sit at the right hand of God or slightly higher. pfh
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