Not sure if this is the right place to ask this questions, but the Mods can move the thread if needed.
My electric 220V 1.3kW lawn mower's asynchronous mono-phase AC motor burned out and I wanted to rewind it. While cutting and removing the copper winding I have counted the number of turns and made a drawing of the circuit diagram.

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This is where I got stuck, because the diagram is not the standard single phase AC motor diagram what I expected. The main winding is made of two 0.6mm thick wires running in parallel (bifilar coil), which would be understandable if both terminal endings would be soldered together to form two coils in parallel, with the intent to increase the effective wire cross-section area (compared to a single 0.6mm diameter wire). In that case the two wires could be replaced with a single wire of about 0.85mm diameter.

However, the two coils are connected only at one point, at the ground. One wire of the other end goes to the AC phase input, and the other wire end goes to one terminal of the capacitor as shown on the attached diagram. In this setup the second wire of the main winding works as the secondary of a transformer feeding the starter coil through the capacitor, which does not make much sense to me. The motor did work for 6 years for about 1h every two weeks in the spring-autumn season. I still wonder if this is how the motor was designed, or rather a construction fault caused by a rookie worker. I am hesitant to rebuild the motor as it was unless someone can convince me that this is a correct desing and better than soldering both terminals of the main winding together in parallel.

My question is whether the diagram represents a properly desinged and constructed motor, or it is the result of a construction mistake?
Thanks for any input in advance.

 

Could L1 act like a Transformer Primary and L2 act like a Transformer Secondary?

 

Could L1 act like a Transformer Primary and L2 act like a Transformer Secondary?

Apparently they do, because the flux through them is identical, and the induced voltages are the same as well. But from the point of view of making an efficient single phase AC motor, it does not make any sense to me why they are not connected in parallel.

 

Is it an Induction Motor ?
How is the rotor constructed?
What brand?

 

Is it an Induction Motor ?
How is the rotor constructed?
What brand?

Yes, it is an asynchronous motor, or with other words an induction motor. The rotor is of the usual aluminum squirrel cage construction.
It is a no-name brand, or at least I could not find any factory markings on it. The lawn mower was sold by a local agricultural company in Serbia.

 

Here is an update about the issue. After rewinding the motor with new copper wires and connecting it the correct way the lawn mower works much better than the original. It is stronger, it's break stops the blade faster, and it generates less heat.

It turned out that the wrong wiring was not the only weird feature of this motor. It took me a while to realize that the winding was made of copper-coated aluminum! Aluminum wire has got about 1.5 times greater resistance than copper. If we add to this the effect of not connecting both main windings in parallel, which doubles the input resistance, then the original motor was dissipating about 1.5*2=3 times more heat than it supposed to (assuming identical current intensity). So to settle the issue, L1 and L2 indeed must be connected in parallel, and use copper wire instead of aluminum.

I am sure now that this was not a mistake, but a designed "feature" to cheat the customers without getting exposed. I doubt that anyone else besides me who used one of these lawn mowers ventured on figuring all this out to rewind a burned out motor. This is a nifty way of forcing the people to throw away their machines much sooner than necessary, by building a motor that works when new, but since it generates 3 times more heat than it should, it will burn out much faster. This is good for the factories because they use cheap materials, increase their profits, and make sure the customers come back soon to buy new products, because the old burns out... The copper coated aluminum wire is advertised by Chinese companies on the net, so it is highly probable that at least the motor, or perhaps the whole lawn mower was made in China (which would explain a lot).

 

I have seen and even tried to repair an electric lawnmower with the copper colored aluminum wires. It went to the scrap man.
And why was there no mention at all of L3 and C1?

 

I have seen and even tried to repair an electric lawnmower with the copper colored aluminum wires. It went to the scrap man.

This is exactly what the factories count on, namely that even the electronics experts or electricians who pick the lawn mower apart and try to fix it, will stop at the motor, as if it would be a red line. Burned out motor? Well, then that goes to the scrap, and since it is almost impossible to find replacement motor that fits a specific lawn mower (definitely no spare parts for those made in China), the whole lawnmower gets scraped. Customer buys a new one.

If nobody bothers to rewind the motor, nobody will discover how they were built (or even designed) to overheat and fail as soon as possible. But if someone can buy magnet wire in small quantities per kg (or pound) at a reasonable price, and likes to fix electrical gadgets, then even the motor can be rewound easily for cheap. For example a new lawnmower costs around say $100 and up, while in my case the 1kg 0.6mm copper wire with the insulator sheets & sleevings cost only $15 including postage. I did have varnish that was used earlier for transformers, but even if you skip varnishing, it will still work just fine as long as the plastic frame will last. Of course there will be always people who will argue that if you count in the price of work, then it is not worth rewinding the motor, which may be true if your hourly wage is high and have lots of money to spend. But if this is not the case and you also like to do this kind of work as a hobby then it is surely worth a shot.

And why was there no mention at all of L3 and C1?

L3 is the starter winding positioned at 90 deg angle relative to the main winding. It was made of 0.6mm single wire (4 windings in 8 slots with 98 turns in each slot). C1 is a high voltage capacitor of 20uF (non-electrolytic) that serves to phase shift the current in the starter winding. These were not mentioned because I did not find anything weird or special about them; they are as expected.

 

I am quite impressed that you were able to rewind the motor and have it run well. My basis for scrapping the aluminum wound motor was that i saw that the quality of the rest of the assembly was also poor. The cheaping was not limited to the motor windings, but also included the bearings and the frame, and the fact that the assembly was welded and not bolted. And all of it was before the poor quality imports that we see now were even possible. Back then China was not even doing much, because Chairman Mao was running his revolution and being unpleasant about things.
And the varnish in an electric motor also serves to keep the wires from vibrating against each other, if they are not absolutely tight, and that vibration will lead to worn away insulation and short circuited turns and a ultimate failure, since the shorted turns will overheat and cause more shorted turns until either the whole thing burns up or the circuit fails open. So that varnish soaking is rather important.

 

Yes, I agree that varnishing is necessary if one wants to make a professional quality motor. It does not only stop vibration, but also assist in heat conduction, and keeping the enamel insulation of the wires away from the atmospheric effects like moisture, dust, chemical fumes etc.

I was aware of the vibration risks, but it is a bit moot point for me, since I don't have extensive experience in el. motor failures and their exact causes. The double insulated enamel coating rated to 200 deg C on the modern wires are quite tough (it takes a lot of hard scraping with a sharp knife to clean the ends for soldering), and in my estimation it would take a hell of a lot of soft small amplitude mechanical vibration to break it and cause electric short. If the slots are well filled, and the windings are made compact and also strongly tied together with a string, there is almost no possibility to move any of the inner wires. Any movement is restricted to the wires on the surface. It all boils down to force intensities, and momentum that the wire can acquire in a single cycle. I would be curious to read a study that quantitatively and experimentally studies this subject.

Anyway, my suggestion about the possibility of skipping the varnishing was to eliminate a possibly expensive varnish from the shopping list that is usually sold in large quantities. But perhaps it can be replaced with other high temperature resistant resins, lacquers, paints etc. that are easily available in smaller quantities at local paint stores. But the best is indeed to find a shop or rewinding service that sell the special varnish for electric windings in small quantities.

Here is a nice page demonstrating each step of the rewinding process, with the varnishing included:
https://www.instructables.com/id/Rewinding-and-Renovation-of-the-Electric-Motor/
but there are plenty of youtube videos about the subject as well.

 

Cheap varnish, and the last quarter inch left in the can , are a couple of options. So as often as not I find that somebody has something that can do the job. Even that last bit of spray paint in the can may work, if you can get it out of the can without making a horrible mess.

 

On the subject of mechanical stabilization and el. insulation of the windings here is an important factor to consider. If you want to do a high quality job, it is recommended to find out (or estimate) the highest working temperature of your motor, and use a varnish and enamel wires that can tolerate that temperature.

Otherwise even though the enamel insulation of the wires may tolerate the high temperature, but the non-heat resistant varnish may start smoking and decomposing. This may still not mean the end of your motor (as long as the enamel insulation is healthy), but it would surely make such an impression in the hands of someone not knowing about the low temperature varnish used in the rewinding process.

If you use a cheap el. lawnmower that was designed to work for only 30-60 min in a continuous run and then require a cool down period, and you intend to run it for longer than an hour, its temperature may rise well above 100C where you could cook an egg on it. In such low quality cheap motors you need more expensive heat tolerant varnish and double insulated enamel wires to make it last and tolerate the high temperature.

But if you have a well designed motor with good cooling mechanism and ventilation, and you intend to run it in cool temperatures and for short continuous time periods, then this is not an important factor, and you can use low temp. varnish as well, which is cheaper.

In my case the running temperature was a dubious question, because the lawn mower was working for about 6 years, even though not more than 30 min in a single run, after which the motor had a chance to cool down before another run. But ultimately it did burn out. Therefore, I estimated the max working temperature to be around 100C (which might have been the case for the original motor, but certainly less for the rewound motor). Anyway, in such dubious case where you suspect low quality design and want to make sure your good work does not go to waste, best is to use copper wires with double enamel insulation certified for 200C, and an appropriate varnish that can tolerate at least about 130C.

Again, make sure your windings are tightly fitted inside the slots (if needed use extra heat tolerant insulation material to fill the gaps and prevent vibration), and tightly tied together on the outside all around so even without the varnish there should be no possibility of wire movements. Then soak it in varnish and bake it to solidify.

On the above posted instructable it is written to bake it at 100C for 4 hours. I my case that was not possible, because I did not use a special heat tolerant string for tying the windings, but a nylon string, which is strong at room temp. but softens and melts easily. Therefore instead of baking it at 100C I have baked it at around 50C (below the softening temp. of nylon) for about 10 hours, which did the job. (Test a piece of the string in the oven first to find the highest temp. it can take). After the baking, when the varnish holds the wires together, the nylon strings are not needed anymore, so even if they would soften or melt at 100C, the coils would remain unaffected. So basically use common sense, keep the temperature and the behavior of the used materials in mind, and things will work out just fine.

 

On the subject of mechanical stabilization and el. insulation of the windings here is an important factor to consider. If you want to do a high quality job, it is recommended to find out (or estimate) the highest working temperature of your motor, and use a varnish and enamel wires that can tolerate that temperature.

Otherwise even though the enamel insulation of the wires may tolerate the high temperature, but the non-heat resistant varnish may start smoking and decomposing. This may still not mean the end of your motor (as long as the enamel insulation is healthy), but it would surely make such an impression in the hands of someone not knowing about the low temperature varnish used in the rewinding process.

If you use a cheap el. lawnmower that was designed to work for only 30-60 min in a continuous run and then require a cool down period, and you intend to run it for longer than an hour, its temperature may rise well above 100C where you could cook an egg on it. In such low quality cheap motors you need more expensive heat tolerant varnish and double insulated enamel wires to make it last and tolerate the high temperature.

But if you have a well designed motor with good cooling mechanism and ventilation, and you intend to run it in cool temperatures and for short continuous time periods, then this is not an important factor, and you can use low temp. varnish as well, which is cheaper.

In my case the running temperature was a dubious question, because the lawn mower was working for about 6 years, even though not more than 30 min in a single run, after which the motor had a chance to cool down before another run. But ultimately it did burn out. Therefore, I estimated the max working temperature to be around 100C (which might have been the case for the original motor, but certainly less for the rewound motor). Anyway, in such dubious case where you suspect low quality design and want to make sure your good work does not go to waste, best is to use copper wires with double enamel insulation certified for 200C, and an appropriate varnish that can tolerate at least about 130C.

Again, make sure your windings are tightly fitted inside the slots (if needed use extra heat tolerant insulation material to fill the gaps and prevent vibration), and tightly tied together on the outside all around so even without the varnish there should be no possibility of wire movements. Then soak it in varnish and bake it to solidify.

On the above posted instructable it is written to bake it at 100C for 4 hours. I my case that was not possible, because I did not use a special heat tolerant string for tying the windings, but a nylon string, which is strong at room temp. but softens and melts easily. Therefore instead of baking it at 100C I have baked it at around 50C (below the softening temp. of nylon) for about 10 hours, which did the job. (Test a piece of the string in the oven first to find the highest temp. it can take). After the baking, when the varnish holds the wires together, the nylon strings are not needed anymore, so even if they would soften or melt at 100C, the coils would remain unaffected. So basically use common sense, keep the temperature and the behavior of the used materials in mind, and things will work out just fine.

I am very impressed that you were able to do a complete rewind of the motor and it seems that there were no problems. That is very good, better than I have achieved. I rewound a fan motor and the thing burned out the very first time. That was 50 years ago and since then I have not had another reason to rewind a motor. I have wound several transformers, both power and audio, and they all worked, but a motor is a lot more challenging than a transformer.

 

Thanks Bill for the kind words. It is indeed more challenging to rewind a motor than a transformer, but if one is careful and knows what to avoid, then there is a good chance for success.

A few tips for anybody who would get inspired to rewind a motor. If the new winding burns out in a few days or even hours, then that means it dissipates too much heat that the cooling system can't handle.

One possible cause for excessive heating can be faulty mechanics, meaning the rotor can't turn as freely as it supposed to, and consequently the stator current increases beyond the safe level, causing excessive power dissipation. This can be caused by rusty, dirty or damaged bearings, or a safety break that does not disengage when the rotor is spinning, etc. First step is to make sure no such mechanical faults exist, and the cooling system (like fans, ducts etc.) works well.

The other possible cause of failure is damaged wire insulation. First of all, the enamel insulation on the new wire must be faultless and of good quality. If one uses a cheaper magnet wire with single enamel coating then he has to be more careful and have more experience with this kind of work, not to damage the insulation. Therefore, the best is to use double insulated enamel wire even if it is a bit more expensive, if one is not a seasoned expert in rewinding. But even if one uses the best wire, it is still very easy to damage the insulation during the assembly, especially if the iron core has got sharp edges at the slot openings. In such cases it can help to blunt these sharp edges with a small file or sandpaper.

The most challenging part of the job is fitting the coils into the slots. This requires a bit of manual dexterity, but practice makes the master. Make sure that the wires are straightened out at the place where they need to enter the slots, otherwise they will resist your efforts to get them in. It also helps if the distance between these straight sections is increased a bit wider than the distance between the slots, so the coil would act as a weak spring naturally trying to jump into the slots without manual forcing. I was holding the coil with both hands on both sides of the slot, and flattened it out between my thumb and index finger so that only one, or a couple wires had to enter through the opening simultaneously. It also helps if the wires are not only pushed from above into the slot, but also one pulls the wires that just entered into the slot towards the bottom, to make space at the top for new wires to enter.

I have seen in some videos that people try to squeeze or even hammer the wires into the slot with a sharp metal blade. Unless one does this as his profession and knows from everyday practice and experience what kind of force can be used without damaging the insulation, I would avoid this by all means. The insulation is the 'soul' of an electric motor, once it is damaged, the end is not very far away... Therefore, the emphasis is not really on the exact number of turns or accurate length of wires, but the art of keeping the insulation undamaged. If you manage to fit the coils into the slots without scratching the insulation, the success is almost guaranteed. Even if you notice that some wires got scratched and you paint a bit of varnish on them, or if the varnish can freely flow into the slots to cover and heal these scratches, the mistake can be fixed. But it is more prudent if one does not tempt his good luck, and makes sure there are no scratches. The insulator papers, and their folded edges also play an important role to prevent the wires from touching the sharp edges of the iron core. These papers (or plastic sheets) must be sturdy and of the right shape to protect the wire insulation.

The second most 'dangerous' part of the work is when the coils are all done, tied together, and the frame is mounted on the stator over the coils for the first time. If your motor is spacious inside then this is not a problem. But if you have a cheap motor designed to be as small and compact as possible, then there is hardly any free space left between the metal frame (mostly aluminum) holding the bearings and some parts of the coil. If you just force the frame into place and bolt them on, it might fit by moving the coils a bit, but in the process they can damage and scratch the insulation. Therefore, at this step also take your time, and make sure that the frame does not touch the coils anywhere. If there are touching points, then shape the coil with a soft (perhaps even padded) wooden rod (or even with fingers), until it can fit in without touching. If the space is too narrow, then put some sturdy insulation paper between the windings and the frame to prevent direct contact. Also make sure that the coil terminal wires don't get caught under the frame and flattened into pancakes causing a short to ground

I think these are the most sensitive parts of the work where most people fail. The rest is just common sense; like copy the original windings properly; connect the coils correctly; use a good HV capacitor; don't bake the windings at a temperature that is too high for the specific varnish you use (keep a pot of water in the oven and keep the temp. below the boiling point of water), but people know these already.

Finally and most importantly enjoy the work and have fun!

 

I have seen in some videos that people try to squeeze or even hammer the wires into the slot with a sharp metal blade. Unless one does this as his profession and knows from everyday practice and experience what kind of force can be used without damaging the insulation,!

When I did my session in the winding shop as part of my training, we used a tapered wedge made from a kind of insulating material and a hide mallet to gradually fill and compress the wire coils and tap them in place.
The old standard 'Electric Motor Repair' by Robert Rosenberg was used as a reference.
Max.

 

I will stick with winding transformers. I was without clues when I rewound the one motor. I can do transformers just fine.