Need help fixing this motor - Page 1
Forum section: electronics / beginners

author: mortos360
author_rank: Regular Contributor
timestamp: May 11, 2014, 10:40:43 am
content: 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 hintwhat 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!!

author: Jeroen3
author_rank: Super Contributor
timestamp: May 11, 2014, 11:23:20 am
content: 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)

author: David_AVD
author_rank: Super Contributor
timestamp: May 11, 2014, 11:33:11 am
content: 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.

author: Bryan
author_rank: Frequent Contributor
timestamp: May 11, 2014, 11:40:19 am
content: Should be fairly easy to test the components if the mechanicals test ok. Looks like a simple motor control circuit using a SCR.

author: mortos360
author_rank: Regular Contributor
timestamp: May 11, 2014, 11:55:56 am
content: Quote from: David_AVD on May 11, 2014, 11:33:11 amfor 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.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: May 11, 2014, 01:02:29 pm
content: 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 .

author: superUnknown
author_rank: Regular Contributor
timestamp: May 11, 2014, 02:51:30 pm
content: 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

author: superUnknown
author_rank: Regular Contributor
timestamp: May 11, 2014, 03:14:42 pm
content: 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.

author: mortos360
author_rank: Regular Contributor
timestamp: May 11, 2014, 03:26:34 pm
content: Quote from: superUnknown on May 11, 2014, 03:14:42 pmmill 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.

author: SeanB
author_rank: Super Contributor
timestamp: May 11, 2014, 04:23:36 pm
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: May 11, 2014, 09:12:36 pm
content: Quote from: mortos360 on May 11, 2014, 11:55:56 amThe neon lamp could be part of the speed control but hard to say for sure.

author: miguelvp
author_rank: Super Contributor
timestamp: May 11, 2014, 09:25:31 pm
content: It might have a temperature switch to prevent it for running too hothttp://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.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 11, 2014, 09:41:50 pm
content: 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.

author: madires
author_rank: Super Contributor
timestamp: May 11, 2014, 09:59:45 pm
content: Doesn't the resistor in series with the neon lamp look burnt?

author: Fank1
author_rank: Regular Contributor
timestamp: May 12, 2014, 01:01:00 am
content: The first test for a brush motor is to check and see if the brushes are good and have spring tension on them.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: May 12, 2014, 02:35:09 am
content: 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

author: mortos360
author_rank: Regular Contributor
timestamp: May 13, 2014, 04:02:48 pm
content: 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.@madiresits just oil on the resistor, I did check it and its fine.@miguelvpI 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.

author: SeanB
author_rank: Super Contributor
timestamp: May 13, 2014, 04:56:20 pm
content: 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.

author: mortos360
author_rank: Regular Contributor
timestamp: May 13, 2014, 04:59:35 pm
content: Quote from: SeanB on May 13, 2014, 04:56:20 pmhmmm... but the measurements looked ok?!I will check tomorrow what parts get hot.. didn't think about checking that myself

author: M0BSW
author_rank: Frequent Contributor
timestamp: May 13, 2014, 05:10:35 pm
content: 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

author: AlfBaz
author_rank: Super Contributor
timestamp: May 13, 2014, 11:13:56 pm
content: 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

author: mortos360
author_rank: Regular Contributor
timestamp: May 14, 2014, 04:57:35 pm
content: 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!

author: G7PSK
author_rank: Super Contributor
timestamp: May 14, 2014, 05:29:02 pm
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2014, 05:40:54 pm
content: 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.

author: madires
author_rank: Super Contributor
timestamp: May 14, 2014, 08:46:47 pm
content: Quote from: SeanB on May 14, 2014, 05:40:54 pmIt 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.

author: mortos360
author_rank: Regular Contributor
timestamp: May 14, 2014, 09:55:00 pm
content: Quote from: madires on May 14, 2014, 08:46:47 pmBut the resistance that I measure seem wrong accruing to colorcode... not?

author: calexanian
author_rank: Super Contributor
timestamp: May 15, 2014, 05:38:55 am
content: You can take that motor to a electric motor service shop and they can test it with the afore mentioned growler. Motor rewinding is done. I can't comment on your location but we have a few firms that do it here in my local area. I would think you would too.

author: G7PSK
author_rank: Super Contributor
timestamp: May 15, 2014, 11:07:27 am
content: Looking at the photo of the field coils I can see that the brush at the top of the photo shows signs of burning, could be just a sticky brush but more likely due to armature fault.

author: madires
author_rank: Super Contributor
timestamp: May 15, 2014, 11:10:02 am
content: Quote from: mortos360 on May 14, 2014, 09:55:00 pmYep, 33k should be 3 orange rings but the rings in the picture look more like 3 times red (2.2k). And as SeanB already mentioned, 33k would be quite low for a neon lamp used as indicator. But please check the motor first, then the PCB.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 15, 2014, 11:25:40 am
content: Quote from: mortos360 on May 11, 2014, 03:26:34 pmFree of its own bearings? As a quick check I would re-assemble the motor without it brushes and check that it spins freely (by hand) without any undue roughness.

author: mortos360
author_rank: Regular Contributor
timestamp: May 15, 2014, 05:25:50 pm
content: Quote from: Andy Watson on May 15, 2014, 11:25:40 amNah.. just the gears that connect the motor shaft to the table.It seems that 1 of the problems was with the brushes... they are long and have good pressure to the pads.. but they are also what gets very hot.As "G7PSK" suggested, I watched the brushes while the motor was running and the sparks looked really bad.. especially when I try to run it in reverse, since the motor gets stuck, it forms an arc that almost welds the brush to the pads of the commutator.After some sanding with very fine paper, the motor runs now much more smother in both directions and the spark are a lot smaller on the brushes... but its still not perfect and the fuse is still kicking in after 5-6 sec. running.I will order a new bulb and brushes and see if that helps. The cable that goes to the fuse comes straight from the neon bulb... maybe the bulb causes some strange behaviour.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2014, 06:30:38 pm
content: Quote from: mortos360 on May 14, 2014, 09:55:00 pmWere you touching the resistor's leads when you measured it with the meter? If so you would've measured your body's resistance in parallel with the resistor.

author: SeanB
author_rank: Super Contributor
timestamp: May 17, 2014, 06:44:26 pm
content: Sparking hot brushes are a symptom, not a cause. That says that you have a shorted rotor winding, and it is drawing excessive current. Growler time and then a new motor or rewind.

author: Strada916
author_rank: Frequent Contributor
timestamp: August 11, 2014, 11:34:40 am
content: The motor is toast mate. Shorted turn for sure. Rewind or replace.