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Need help fixing this motor
Posted by
mortos360
on 11 May, 2014 10:40
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I got an old milling machine and it came with a motor to move the table.. how ever, the motor doesn't work as it should.
I took it apart and it looks like the whole system works on AC... and I really don't feel like poking around while its live, so I took a few pictures and maybe some of you are familiar with this type of systems and can give me a hint
what to check.
The behaviour is following:
When the motor is running for longer then 3-4 seconds, the fuse inside the unit jumps out.
When I turn the motor on for a few seconds I can see and feel that its not running smoth its like 1 of this pager motors with some balance offset, but its not a mechanical problem. when I spin it by hand its perfectly fine and in balance.
Its more like the current flow is interrupted every second turn or something.
Also.. when the motor runs clockwise, it starts every time.. but when I run it CCW it often doesn't even start, it just twitches a bit till the fuse jumps out.
here are some pictures of the system, hope some if you have an idea what could be wrong here.
Thanks!!
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#1 Reply
Posted by
Jeroen3
on 11 May, 2014 11:23
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Try to measure the rotor. If the "current flow is interrupted every second turn" there is most likely something wrong there. Isolation failure, wire break or a bad contact to the rotor.
You can measure ohms with your multimeter, and high voltage ohms to test isolation with your megger. (
http://en.wikipedia.org/wiki/Megohmmeter)
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#2 Reply
Posted by
David_AVD
on 11 May, 2014 11:33
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Looks like the neon lamp (top right in picture) on the control PCB has blackened? I'd probably replace that as a matter of course.
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#3 Reply
Posted by
Bryan
on 11 May, 2014 11:40
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Should be fairly easy to test the components if the mechanicals test ok. Looks like a simple motor control circuit using a SCR.
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#4 Reply
Posted by
mortos360
on 11 May, 2014 11:55
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Looks like the neon lamp (top right in picture) on the control PCB has blackened? I'd probably replace that as a matter of course.
for cosmetic reason or does the function of that lamp has an effect on the circuit?
the lamp does work, but it is very black...
I did test the components on the PCB, they look all ok. The 2 blue devise there I didn't check sind I didn't know what they are // how to test them.
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It's been a long time , use to use a growlier to test rotors .
Not enough light to see any bad windings but , from memory if you put a multi-meter on audio / continuity with one lead on one of the transformer laminations then slowly take the other probe and roll around the brush contact segments , then do with all of each of the laminated sections , what your looking for is shorts .
Then the next , is to at the base of the area that the brushes ride on testing each segment to wire connection for continuity - sometimes these may heat up the solder & loose connection .
Then some of the same with the brush holders in windings in the body .
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This is a universal motor, the same type in cordless drills and corded drills. They run on AC or DC.
-Check the resistance between each commutator bar. If there is a big difference in resistance the rotor coils are NFG.
-Press down hard on the bearing race and turn it to make sure there are no bad spots.
-Check the brush connections and wear limit, if they are too worn then the pre-load spring won't provide the correct force to keep the brush in contact with the commutator bar
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further,
Check the mill table gibs, maybe too tight/damaged and stalling the motor?
Check the rotor shaft, feed screw and mechanics to ensure nothing damaged/bent.
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#8 Reply
Posted by
mortos360
on 11 May, 2014 15:26
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further,
Check the mill table gibs, maybe too tight/damaged and stalling the motor?
Check the rotor shaft, feed screw and mechanics to ensure nothing damaged/bent.
mill is fine and the motor behaves like that when it free of all gears and stuff.
I wonder if windings on the motor can burn if somebody has try to move the table while it was locked?
It does have some kind of mechanical fuse that pops out all the time now, which I think would have prevent too much current... but who knows.
Thanks for all your feedback guys.. ill try all the recommendations on monday and see if I get something new.
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#9 Reply
Posted by
SeanB
on 11 May, 2014 16:23
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Simplest and most likely is the rotor has shorted windings. Easy to test for with a growler, or the longer way using a DVM on ohms range and seeing resistance between opposite ends as you go around the commutator. If there is a more than 10% change it is faulty. As well check stator resistance, and if the stator or rotor is shorted to the metal of the unit.
If you want to run it you can use a 12V DC battery to power it, it should turn if no load is applied to the shaft. If it turns apply a load to the shaft ( light load with fingers) and see if there is a position where it stops. That will be the shorted rotor.
I have rewound these motors, but it is a difficult job if you do not know how. If you cannot get a rotor you will need to find a rewinder and have them rebuild it. Probably will take about 2 weeks and cost around $200-$300, depending where you are. Not cheap, but often the only option.
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#10 Reply
Posted by
David_AVD
on 11 May, 2014 21:12
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Looks like the neon lamp (top right in picture) on the control PCB has blackened? I'd probably replace that as a matter of course.
for cosmetic reason or does the function of that lamp has an effect on the circuit?
the lamp does work, but it is very black...
I did test the components on the PCB, they look all ok. The 2 blue devise there I didn't check sind I didn't know what they are // how to test them.
The neon lamp could be part of the speed control but hard to say for sure.
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#11 Reply
Posted by
miguelvp
on 11 May, 2014 21:25
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It might have a temperature switch to prevent it for running too hot
http://www.pepiusa.com/modb.htmlBut if that's the case, it's just doing its job.
Like others suggested, have you check the resistance to see if the readings are way off?
You could have a short in one winding, making the coil heat up then the thermal switch will shut it off.
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#12 Reply
Posted by
Strada916
on 11 May, 2014 21:41
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sounds like one of the windings is shorted. You'll need an armature winding tester (growler). Or your ohms meter to check resistance or lack of it across the bushes.
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#13 Reply
Posted by
madires
on 11 May, 2014 21:59
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Doesn't the resistor in series with the neon lamp look burnt?
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#14 Reply
Posted by
Fank1
on 12 May, 2014 01:01
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The first test for a brush motor is to check and see if the brushes are good and have spring tension on them.
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Personally, I usually have a quick poke with a meter, or an insulation tester, from the commutator to the shaft or laminations to test for a short to frame. It is best to poke a few places around the commutator in case there is a short to frame and an open winding too.
Testing the continuity of the windings with an ohm meter is easy but a bit fiddly (a simple V-notch cradle made of ply and a bit of timber helps...). I do have a 'Growler' to test for shorted windings but find it easier (if I haven't yet dismantled the motor) to connect the motor to a current limited PSU and check that it rotates evenly. The current can be observed on the PSU meter, and a small load put on the motor by hand to test further. 30 volts and 1 amp is about maximum necessary. Regarding using a 12V battery- I wrecked a motor many years ago using this method. At low to stall speed the current needs to be limited (no back EMF...)
Regards, BT
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#16 Reply
Posted by
mortos360
on 13 May, 2014 16:02
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Update:
I did some measurements today of the coils and to me they all look ok.
Commutator shows 0.7 - 0.8 Ohm pad to pad and 12-13 Ohms 180° across.
I did test all pads for shots to the laminates and the shaft.. all seem fine.
The field coils measured 14 Ohms and I did (very carefully) scape off a tiny bit of the isolation from the wire that bridges to the second coil and measured 7 Ohms from each wire to that center part... so to me that looks ok too.
The brushes look also OK.. they are big enough and have good pressure to the commutator pads.
I also measured the voltage to the brushes while the motor was running (a few sec. till the fuse kicked out) and its 30VAC in both direction.
@madires
its just oil on the resistor, I did check it and its fine.
@miguelvp
I didn't see such a device, but it could be a temp. switch since the motor does get very hot for that short running time.
I will check the pads tomorrow again and see if they have some play or something that causes them to lose contact while the motor is running. While I was turning it by hand and measuring, I couldn't feel any problems and all the measurements looked clean... but maybe the problem happens when there is more vibrations on faster speed.
Other then that, I could just try to replace all the components on the board if thats the problem... no idea really.
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#17 Reply
Posted by
SeanB
on 13 May, 2014 16:56
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Motor gets very hot after a few seconds means the rotor or armature has a single shorted coil turn. This means a rewind or a new one unfortunately. If the case gets hot and the rotor is a lot cooler then it is one of the stator coils, and as such it is a lot easier to rewind yourself, you just need a reel of wire ( about 100m) and a little thin card insulation, some sleeving and a little patience. If it is the rotor getting hotter than the stator then it is only doable if you are experienced in doing it.
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#18 Reply
Posted by
mortos360
on 13 May, 2014 16:59
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Motor gets very hot after a few seconds means the rotor or armature has a single shorted coil turn. This means a rewind or a new one unfortunately. If the case gets hot and the rotor is a lot cooler then it is one of the stator coils, and as such it is a lot easier to rewind yourself, you just need a reel of wire ( about 100m) and a little thin card insulation, some sleeving and a little patience. If it is the rotor getting hotter than the stator then it is only doable if you are experienced in doing it.
hmmm... but the measurements looked ok?!
I will check tomorrow what parts get hot.. didn't think about checking that myself
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#19 Reply
Posted by
M0BSW
on 13 May, 2014 17:10
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R3 & L1 , would that be expected to get hot, because both have burnt look on the PCB, When I was an apprentice it was my job to clean the commutator, where the brush tracks seem very deep, I would have to clean them and check there was no contact, between the copper contacts. memories
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#20 Reply
Posted by
AlfBaz
on 13 May, 2014 23:13
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To test the armature you can try and "drop test" it.
Place a dc voltage across a section of the commutator and measure the voltage drop across each segment. If you find on that's lower than the others you have a short if it's higher than the others you have either an open cct or a high resistance joint, usually where the winding is soldered to the comm.
Apply a DC voltage that produces a current the armature can handle. Ignore differences in seg voltage differences when you move the applied voltage to another section of the com as you may have included more or less segs
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#21 Reply
Posted by
mortos360
on 14 May, 2014 16:57
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What would you say is this colorcode?
The resistor measures 34k and is the R3, in series with the neon bulb... but for me this looks clearly like red brown red or even red red red... I tryed to clean it with alcohol, but that stuff is not really going down. how ever... 34k looks just wrong to me!
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#22 Reply
Posted by
G7PSK
on 14 May, 2014 17:29
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With universal type motors you will normally get a lot of arc over on the commutator segments when there is a short in one or morn of the windings, I see no signs of this on the side shown in the photograph, if there is an inspection cover on the brushes it could be worth while running the motor with the cover off or trying to watch though any ventilation slots. The problem could also be the thermal cut out switch as that keeps popping out, seen people chase all sorts of things when all that was wrong is an over sensitive trip.
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#23 Reply
Posted by
SeanB
on 14 May, 2014 17:40
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33k would be low end for use with a neon as power on indication, you would be better off replacing it with a 100k 0.5W resistor.
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#24 Reply
Posted by
madires
on 14 May, 2014 20:46
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33k would be low end for use with a neon as power on indication, you would be better off replacing it with a 100k 0.5W resistor.
It could be also part of a regulation. At around 90-110V the neon lamp starts conducting, a feature that was used by old stuff to regulate voltages for example.