All domestic sewing machines uses universal motors. They can be overhauled in less than an hour by following a simple procedure. Carbon accumulation and brushes sparkling are the main causes of motors low performance.
This guide will help you to overhaul any universal motor. The term "universal" means such a motor can work either on AC or DC but nowadays, most electrical appliances works on domestic household AC current.
Overhauling a motor is VERY easy and take LESS than an hour ! All you will need is an electric drill or a press drill and a few strips of 400 and 1500 grit sandpaper. This procedure (used in motor rebuilding shops) apply also to ANY carbon brushes motors.
Lets begin with a very popular "universal" sewing machine motor. I choosed this particular model on which the carbon brushes are not accessible from the outside. This means in other words, if your actual motor has carbon brushes accessible from the outside (by removing the 2 brushes caps), you will save time by removing the carbon brushes instantly. Basically, any motor is made of 2 principal parts: the ROTOR (or Armature) and the STATOR (or Field Core). The stator consist of fine insulated windings creating a magnetic field when electricity is applied. The rotor, which is in the center of the motor and is turning, consist of multiple very fine insulated windings (named also "sectors") which have a minimum of 110 turns of wire per winding. Some expensive motors worth to have their armature rewired. Some common motor troubles are what we call "dead spots" when 50% of the time the motor will not start by itself. Also another common trouble is a "boiling armature" causing the motor to overheat dramatically after a few minutes of operation. On these two extreme problems, armature rewinding is the only solution. Here is the motor we will overhaul:
The first thing to do is to remove the bracket. Some motors have a "L" bracket like this one, retained by 2 screws. Others have a "K" bracket which is usually retained by 2 nuts. Next, remove the motor pulley which is retained by a small set screw. Some motors may have special brackets but all of them can be dismantled, observe carefully how their bracket was installed. On some other motors, the bracket was soldered in place (spot welding). On such motors, you will have to dismantle the casting only, thus leaving the bracket on.
Since the bracket and pulley are removed, the next step is to separate the casting (2 parts). Most of them are retained by 2 long screws with nuts at both ends. Some models have only one nut at the end of each screw. Before separating the casting, check the electrical wire. Is it retained by a strain relief like the one I am showing? If so, remove this plastic strain relief and avoid to damage the wire. If the wire pass through a groomet then no need to remove it. If your motor is equipped with 2 carbon caps, unscrew them, remove the spring and carbon brush in each channel. If your motor has hidden brushes like the one I am showing, proceed to separate the casting. Note the small washers which can stick to the inside of the removed casting part. It is very important to place these thin washers in the same order they were removed. You can tape them on a piece of paper with masking tape.
The next step consist of removing the other casting part. In this case (hidden carbon brushes), we will have to remove the 2 nuts retaining the field coil (big winding inside the casting) and the brush plate assembly. Once removed, slip off gently the field core (wired to the brush plate). You will be able to release the armature (often named "rotor"), the center part of the motor on which the pulley is held.
Remember the thin washers are found also on the other extremity of the armature. Place them in the same order you found them on a piece of paper. Here I was holding the sandpaper with my left hand and the camera with the right hand. I recommend you to hold the sandpaper with both hands to apply a small pressure on the sandpaper.
The next step is to clean the armature commutator. This is where the carbon brushes make contact. 90% of motor's troubles comes from a bad commutator. Heat, oil, carbon accumulation and dirt are the ideal mix to provocate sparkling when electricity is applied to the motor. Under load, the sparkling will increase and some motors will start smoking. The main thing is to clean the copper contacts of the armature commutator. To do so, install the armature on a press drill or on a hand drill. You will need 2 narrow (1/4") strip of 400 grit and 1500 grit sandpaper. If you can't find a drill, you can clean the commutator with a pencil eraser but this is very long compared to a drill.
By applying the 400 grit sandpaper on the commutator while it turns, you will get a shiny finish but that's not all ! Such a finish remains abrasive for the carbon brushes. Repeat the same procedure by applying the 1500 grit sandpaper to obtain a super smooth finish. Do not touch the commutator with your fingers. This would leaves oil on the surface, thus contaminating the carbons...
On some older motors, the carbon brushes contact can wear out the commutator, showing a deep groove in it. The sandpaper will smoothen it and clean it. After turning the commutator, you should be able to see a small gap between the commutator contact plates. If the surface is flush with the insulator (mica), this is what we call a "high mica" commutator. You will have to pass the edge of a sharp instrument such as a needle, X-Acto knife, etc. to "scrape" about 1/32" of mica between each contact. After doing so, you will turn again the commutator on the drill with a new 1500 grit sandpaper.
Before reassembling everything, try to clean all the parts (casting parts, screws, nuts, etc.) either in a bath of mild water with laundry soap or by means of a popular degreaser and paper towels. Use nothing else than rubbing alcohol to clean both carbon brushes. Avoid to touch the contact curved edges with your fingers.
You are now ready to reassemble everything! Start with the armature. Install all the washers respectively in the same order they were before dismantling. Then slide the armature inside one of the casting (usually the opposite of pulley). Then put in place the field coil with its attached brush plate.
Be sure the electrical wiring will exit on the same side your other casting is fitting. Also on a hidden brushes motor, you will have to install the 2 carbons before sliding back the field coil/brush plate assembly. To do so, it is wise to install both carbons and their springs inside the channels and to retain them with a rubber band. Simply cut the rubber bands when they are in place. Do not forget to install the 2 nuts retaining the brush plate. You are now ready to put back the second casting part, nuts and pulley. When reassembly is completed, turn the motor pulley by hand. It should turn freely whitout any kind of friction inside. In the case of an outside brushes motor, put back in place both brushes and their springs with the brush caps. You are now ready to lubricate your motor. Apply only one drop of oil at both ends. You should be able to locate a very small hole at each end. You can reach also the armature shaft on which the pulley is attached. You can apply one drop of oil on the shaft near the bushing. Wipe any excess of oil. Before testing the motor on electricity, try to rotate again the armature and shake the motor a few times. This will put the small washers in place and will already provide the needed armature end play gap. Just like it was set at the factory !! Happy testing !! Note: If nothing happens when plugging the motor, the carbon brushes may not be well seated in the channel. If some smoke appear, some oil contaminated the commutator or the armature has bad windings (sectors). If smoke persist, you will have to remove half of the casting and clean the commutator with rubbing alcohol.
If people find this guide helpful, perhaps I’ll write a few more focusing on other elements of sewing machines. Thank you for your time and please do not hesitate to e-mail me for any particular question. I will gladly put over 30 years of sewing machine experience at your service!
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Happy Sewing !!