Brushed DC motor not rotating - Page 1
Forum section: electronics / repair

author: timkoers
author_rank: Contributor
timestamp: May 06, 2023, 02:41:50 pm
content: I got two of the same brushed DC motors, one was running fine and the other one wasn't.To confirm my suspicion of the not running one, having bad brushes, I swapped the brushes from the working one onto the non working one.That didn't work and now the one that was working, also stopped working.I'm unfamiliair with any theory on why they might stop working, but it sounds like I've messed something up with the brushes.Currently, when running 12V through the wires, it shorts both wires together.The commutator looks fine on both motors, I've removed the carbon buildup between the commutator segments to prevent shorting, could it be that there was somekind of material in between the segments that lifted the brushes the slightest amount, causing only one segment to actually touch the brush?If the above is true, I've removed that material as I thought it was carbon build-up

author: timkoers
author_rank: Contributor
timestamp: May 06, 2023, 03:01:06 pm
content: I do notice that whenever I leave some space between one brush and a stator, that the motor wants to turn. But it creates alot of sparking, which is bad for the stator

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 06, 2023, 11:15:18 pm
content: If the brushes are mounted at an angle other than 90° to the commuter make sure to take the hat into account when swapping them.Even with 90° mounted brushes, if one commuter had a lot more wear than the other the reduced radius could cause brushes to have edges sharp enough to jam between commuter contacts.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: May 06, 2023, 11:23:27 pm
content: Let's see all these here crystal clearly

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 06, 2023, 11:38:47 pm
content: There are a lot of things that can go wrong doing a brush swap.First is brush geometry, with two types mentioned above. If the motors have run for a while the brush will be conformed to the rotor, in the way that the brush holder held it. In my experience this almost always means wear that isn't centered on the brush, and sometimes even ends up with the center of the wear arc almost at the edge of the brush. Least sparking will occur if you try to duplicate the brush geometry, but you may find that the brushes were near end of life and just barely making contact. Any slight change in spring tension or friction with the holder can push the system into non-function.Second is loss of electrical contact. It is very easy to break the current path. In some motors there is a separate lead that must be connected to the supply, in others the electrical path is through the spring or the brush holder, or some combination of these and other methods. Make sure all contact surfaces are clean, and that there is enough spring tension to get contact. If the spring is the current path don't overtension the spring, it will cause excessive wear. But check and see if overcurrent, or resistance heating at the contact points has raised the temperature enough to soften the spring. Replacement of the spring is about the only practical option when this occurs, but you can try work hardening and stretching it.

author: james_s
author_rank: Super Contributor
timestamp: May 06, 2023, 11:45:58 pm
content: From what you describe it sounds to me like the brushes are somehow shorting to the motor body. Can you post some pictures of the motors?

author: timkoers
author_rank: Contributor
timestamp: May 07, 2023, 07:55:45 am
content: Here are some pictures of the motor.These springs are very strong, so I'm doubting if this is a bad contact case.It looks like the motor has some active magnatic coils, which control the speed of the motor based on the strength of their magnetic fields,by holding back the motor.Quote from: james_s on May 06, 2023, 11:45:58 pmI think the only posibility of the brushes shorting to the motor body, is the wires to the brushes touching the top cover.The top cover also connects the motor body to groundQuote from: CatalinaWOW on May 06, 2023, 11:38:47 pmThe brushes will not work in the motor they came from in the first place. Sometimes I can get one motor to fire up, but it doesn't run for very long (2 or 3 seconds).It does seem like the brushes might indeed need to wear into the new contact pattern, which could have changed the slightest millimeter during the swapQuote from: shakalnokturn on May 06, 2023, 11:15:18 pmShould I try and unsharpen the edges of the brushes a little?

author: Gyro
author_rank: Super Contributor
timestamp: May 07, 2023, 12:37:35 pm
content: Seems like a silly question, but youarere-fitting the motor end bell and bearing each time you test it?The uniform matt finish of the commutator makes me think that you have used some sort of abrasive on it too, this may have left embedded particles or binder (if it's a rubber block abrasive) in / on the segments that may be affecting brush contact.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 07, 2023, 03:46:25 pm
content: This appears to be a universal motor. The external coils (the field coils) do not hold back the rotor, they provide the magnetic field that the rotor fields react with. They provide the same function that permanent magnets perform in a DC motor. By changing the current in these coils torque (and thus speed) can be controlled. Be sure that they are connected appropriately when you are testing.The woven wire leads for the brushes is more frayed than I usually see. You might look for broken bits of the wire in inappropriate places.A couple of questions which may help in diagnosis.Are you operating this motor on AC or DC?When changing brushes between motors are you swapping the entire brush block assembly (the plastic plate with brushes mounted), the brush assembly (brush, brush holder and spring), or the actual brushes (carbon block with woven wire lead)?

author: james_s
author_rank: Super Contributor
timestamp: May 07, 2023, 06:30:02 pm
content: Quote from: Gyro on May 07, 2023, 12:37:35 pmOne should never use an abrasive on a commutator, the rough finish it creates will destroy brushes quickly and cause a large amount of carbon to clog up the segments. It's sufficient to just wipe it off with some solvent and scrape any dust out of the gaps.

author: floobydust
author_rank: Super Contributor
timestamp: May 07, 2023, 06:42:58 pm
content: I find it essential to use fine sandpaper on the commutator and leave a rough finish. I use 400-600 grit because any (new) brushes are not fully mating yet, and refitted old brushes are can be flipped or not matching original position, be glazed polished smooth from wear yet not mate in their new position so a roughened commutator helps break-in that is needed. It is customary to run a motor at lower voltage and load to let the brushes wear in and make good contact.I use a scribe to lightly scrape out and debris and deposits between commutator leafs.Not sure what is going on for OP, it would be good to rule out an electrical vs mechanical problem.Either the brushes are way off in phase or there is a mistake refitting the brushes.One stator winding is a different colour which seems like different varnish.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: May 07, 2023, 11:41:57 pm
content: not a manufacturer's product is it, instead a handmade one, incl. the brush?then carbon brush house materialmust notconductive

author: themadhippy
author_rank: Super Contributor
timestamp: May 07, 2023, 11:49:13 pm
content: QuoteWhat? theres plenty of motors were the brushes sit in a metal tube

author: james_s
author_rank: Super Contributor
timestamp: May 08, 2023, 01:53:42 am
content: The brush tubes can certainly be metal but they have to be mounted on something non conductive.

author: floobydust
author_rank: Super Contributor
timestamp: May 08, 2023, 04:01:52 am
content: Just grasping at a straw, for some reason why the brushes changed between running on the two motors.One commutator looks turned down in a lathe, diameter reduced maybe 0.75mm yet the brushes are worn on a larger diameter commutator.So it looks like the curvature difference means the brush contacts more on the middle now. Wouldn't it still would have to be run for quite a while to cause the wear, unless the rough surface is doing it.Also, the rotor connection looks a bit broken? at where the commutator crimps to the rotor wire, where the lathe went too far and hit it.We don't know the motor's history here but my eyes tell me someone has been in here before...

author: andy3055
author_rank: Super Contributor
timestamp: May 08, 2023, 04:47:05 am
content: Looking at the 2nd photo, I feel the "pig tail" from the brush holder is broken where it is supposed to connect to that field coil.

author: floobydust
author_rank: Super Contributor
timestamp: May 08, 2023, 06:34:27 pm
content: It looks like pics from two different setups. One motor has a brown phenolic brush plate, the other is gray? and a bit warped from the field wire tugging against it.

author: timkoers
author_rank: Contributor
timestamp: May 09, 2023, 04:41:42 pm
content: Quote from: Gyro on May 07, 2023, 12:37:35 pmI did notQuote from: Gyro on May 07, 2023, 12:37:35 pmYes I did, 1600 grid sandpaper.Quote from: floobydust on May 08, 2023, 04:01:52 amWhat makes you think it has been lathed down?Quote from: floobydust on May 08, 2023, 04:01:52 amCould you highlight where you think it's broken? I don't think it's broken, but I might miss somethingQuote from: floobydust on May 08, 2023, 06:34:27 pmAll pictures are of the same motorAny ideas?I don't get them to move at all now

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2023, 06:51:01 pm
content: Quote from: timkoers on May 09, 2023, 04:41:42 pmThat's probably your problem then. Without both end bearings in place to align the armature, it will bind against the stator. There's no chance of it rotatingproperlyconsistently.

author: timkoers
author_rank: Contributor
timestamp: June 07, 2023, 05:25:26 pm
content: Both motors are working again! I needed to give it a spin to get the brushes to align properly and once that happened the motor speed gradually started to increase up to full speed.Many thanks!

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2023, 05:33:00 pm
content: Note on sandpaper materials:Al2O3aluminum oxide is non-conductive.SiC silicon carbide is electrically conductive, and should be avoided on commutators.

author: james_s
author_rank: Super Contributor
timestamp: June 07, 2023, 08:15:33 pm
content: Quote from: TimFox on June 07, 2023, 05:33:00 pmCarbon is electrically conductive too, regardless of whether you use sandpaper or not you will want to clean out the gaps between commutator segments, I usually use a hobby knife with an old dulled blade to to carefully scrape out any crud followed by compressed air.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 07, 2023, 08:27:47 pm
content: QuoteIf one of the brushes isn't making contact then the motor will be open-circuit. No current flow at all.The standard wiring scheme is with stator (field coils) connected in series with the armature (the rotating part which receives power through the carbon brushes.)If you are getting a short circuit perhaps there is something wrong with the brush holder assembly which permits an unwanted short to the motor's housing or else the brush holders are shorting against each other.Generally there should be infinite resistance (open circuit) from all of the motor's coils (both stator and armature) to the metal case and motor shaft.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2023, 09:23:56 pm
content: Quote from: james_s on June 07, 2023, 08:15:33 pmYou can't avoid carbon, or silver-carbon composite, but you can avoid SiC paper.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 07, 2023, 09:33:37 pm
content: QuoteThe most common configuration has the field coils (stator) in series with the brushes which supply power to the rotating armature through the commutator. This is known as a"series connected DC motor."If it is designed to operate on both AC and DC, then it is known as a"series connected universal motor."Typically the wire gauge for the field coils (stator) will be nearly as thick as the wire gauge wound on the rotating armature. This type of motor spins considerably faster without a load than it does when driving a heavy load. This style is often used in appliances like vacuum cleaners and leaf blowers where the load is relatively constant.Another configuration is a"shunt DC motor"where the field coils are in parallel with the brushes. This type of motor rotates at a nearly constant speed whether loaded or not. The wire gauge of the field coils will bemuch thinner wirecompared to the wire gauge wound on the rotating armature. Today permanent magnets are sometimes used instead of the field coils for the stator (stationary pole pieces). The power source must be DC in this case. Often a simple bridge rectifier is used for permanent magnet brushed motors which are meant to operate from AC mains. some of the most popular consumer items using permanent magnet brushed motors with a bridge rectifier are hair dryers and small air compressors.There is also a "governed" style of brushed motorwhere a set of vibrating contacts are in series with the power source. A mechanism with hinged weights rotating at the same RPM as the motor armature is configured to open the contacts above a specified RPM. The governor mechanism balances the centrifugal force derived from the rotating weights against the back tension provided by a spring. The spring holds the contacts closed until the governed RPM is reached. When RPM increases above the governed speed then the contacts open. When RPM decreases the contacts close. This open/close cycle happens at a rapid rate, keeping the motor RPM constant. The spring tension can adjusted to set the RPM rather accurately. Sometimes an external lever or knob is provided for the user to control the RPM. Prior to the development of electronic speed controls, many cake mixers for making cake and bread batter used this "governor" style design to provide a wide range of speed control. Items like movie film projectors also used these motors with weight-and-spring governors.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 07, 2023, 09:40:55 pm
content: Quote from: Gyro on May 09, 2023, 06:51:01 pmThis is correct. The magnetic attraction between armature and stator is surprisingly strong. Even when operated at low voltage, the magnetic force will firmly lock one side of the rotating armature against one of the stator pole pieces. However the motor will spin freely when itisn'tpowered up. The bearings must be snug enough to prevent metal-to-metal contact between armature and stator. In fact badly worn or loose bearings are the most common reason for a motor to "lock up" and fail to rotate.