Rewinding coffee grinder motor armature. - Page 1
Forum section: electronics / repair

author: mitchjkjkjk
author_rank: Newbie
timestamp: March 20, 2022, 09:52:41 pm
content: Hi everyone, new to the forum. Figured this might be a good place to find some help.I have a coffee machine grinder motor, the coils in the armature had shorted either to themselves or to the body or the armature (it was a little while ago that I checked). So, I've removed the old windings, and would like to try rewinding the armature. I ordered some copper wire which has arrived and am ready to go, however I'm not 100% how to go about it. It is a universal motor, so my understanding is that it would use a wave winding pattern, however from the diagrams I see that illustrate this, I don't follow. I also tried looking at some youtube videos on how to do the winding but they are typically not in English and I am not able to follow just by looking. The armature is an 8 slot, 8 piece commutator, if that helps, and I took a not so great photo before I took the windings off.Could someone explain or redirect me to a good resource that bridges the gap between diagrams you'd see and a real armature? I am not experienced enough and need something a little more formulaic to follow.Thanks very much, let me know if I've left out any important information.

author: IanB
author_rank: Super Contributor
timestamp: March 20, 2022, 10:19:15 pm
content: Before you take the old winding off, you need to carefully count the number of turns as you unwind, and also observe the winding pattern, so you can rewind with the same number of turns. You also need to measure the wire diameter with a micrometer, so you can rewind using the same size wire.If you didn't count the turns on the old windings, I think it will be difficult to rewind it.I think it is also important to use the correct kind of insulating spacers and formers, so the bare enameled wire does not come directly in contact with the metal armature.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: March 20, 2022, 10:24:01 pm
content: Hello, it is a high RPM motor ? how do you plan to perform a dynamic balancing of the rotor ?

author: mitchjkjkjk
author_rank: Newbie
timestamp: March 20, 2022, 11:40:32 pm
content: Are you only asking that to point out that you think I shouldn't attempt this repair? I wouldn't know how to balance the armature nor have any associated equipment. It is a coffee grinder that operates for about 10 seconds at a time and under an inconsistent load; I wouldn't have thought it necessary.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 20, 2022, 11:50:27 pm
content: Buy a new coffee grinder of the same type and swap motors. That way, you will have the satisfaction of making the repair. And, it will be safe.

author: mitchjkjkjk
author_rank: Newbie
timestamp: March 21, 2022, 12:21:59 am
content: I really don't care about the coffee grinder, the aim is to learn how to rewind the armature.

author: amyk
author_rank: Super Contributor
timestamp: March 21, 2022, 12:51:16 am
content: If you didn't take any notes when you were removing the windings, you'll need to read some motor theory textbooks and learn how to do the calculations to end up with the right parameters for the voltage and intended power.This century-old book might be a good starting point...https://archive.org/download/armaturewindingm00brayrich/armaturewindingm00brayrich.pdf

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2022, 03:09:51 am
content: I have done this many times. However, if you have already removed the windings without taking notes, it might be a lost cause.1. The armature is balanced after the winding is completed by either inserting "weights" in the slots or grinding off parts of the pole faces. If balancing equipment is not within your reach, you need to note down which winding was at the bottom and which is at the top. Basically, the starting pair of coils, the next pair... etc. When you rewind it, if you follow this sequence, it will be fairly well balanced, provided you wind it nice and tight.2. To count the number of turns, you simply cut one coil (usually the top most coil) across at a convenient place and simply count the turns. A few (5-10)turns +/- will not make a difference.3. You need to note down the coil pitch. I think it is 4 in the picture.4. You need to note down the commutator pitch-how many bars apart are the ends of one coil.5. You need thin insulating material to lineup the slots-Leatheroid and Milinex.6. Make sure the commutator bars do not short to each other. If there is too much carbon from the brushes between the bars and they are shorting, you can carefully remove them with a knife blade.You would start by winding (make sure to use the same gauge of wire) the diametrically opposite coils. Then follow with the next pair etc., as it was in the original winding. Once everything is done, you can use twine to bind everything down. Then, re-assemble the motor and give it a short run to make sure it works without any sparking and arcing. Take it apart and heat the whole thing to get rid of the moisture, dip it in shellac and bake it with a high wattage bulb. Put the thing in a tin can with the bulb so that the heat will cure the shellac.

author: mitchjkjkjk
author_rank: Newbie
timestamp: March 21, 2022, 04:48:32 am
content: Thank you for the replies,So as far as parameters go, there are 40 turns per coil, with 0.2mm wire, that much I kept track of. I have some slightly larger wire (0.25mm) but the windings weren't super tightly packed into the slots so I think there will be room, if not i'll get some 0.2. I have fish paper, although the old stuff stayed in and I doubt I could get it out anyways. Getting the old windings off was hard due to all the shellac, so it was really impossible to see which commutator bars were connected to which coils. The pitch in mine is either 3 or 4, and I would have though it needed to be a multiple of 8, but perhaps not (see below). The main thing I can't tell is the commutator pitch I guess, and I'm trying to make an educated guess.My understanding of the wave winding is that the commutator pitch is greater than the coil pitch, here for example it would be a coil pitch of 5 and commutator pitch of 9if I watch this guy for examplehe seems to mark a commutator bar, and then moves 3 slots (inclusive). It is (as best I can tell) a 12 slot armature, and if the commutator is lets say 11, then he winds 1 and 6 so a coil pitch of 6 and commutator pitch of 10, assuming he then goes to 8, but I can't see which he goes to, there is a cut in the video, he goes on to wind 7 and 12, I am assuming because this is the parallel path but now there is another commutator bar connected.anyways, based on the fact that I have an 8 slot, and the pitch appears to be 3 or 4, is there a way to deduce what the commutator pitch would be? I will be hitting the books (thank you amyk) to try and answer this question but if you know, by all means jump in.also, to point 6, I have check that the bars are not shorted.

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2022, 06:13:56 am
content: How many bars are there in the comutator and how many slots are there on the armature?The balance will be completely out of wack if you don't start as I explained earlier.

author: amyk
author_rank: Super Contributor
timestamp: March 23, 2022, 12:19:19 am
content: 8 slots, 8 bars. The OP has already provided that information. What's left is how many poles the field has, and where the brushes are relative to them.