Induction motor winding questions

I am experimenting with various schemes to rewind AC induction motors to change their characteristics for special purposes. I have successfully rewound several single phase motors for three phase, and I have changed their speed by varying the number of poles. I am winding them with much heavier wire than originally used for the 120 and 240 VAC machines, as I want to use them for low voltage battery powered applications.

The most successful motor so far is my first motor, although I rewound it several times before I got it to work properly. It was a 1/2 HP capacitor start pump motor with a 24 slot stator. I wound it using #14 AWG THHN wire, which required it to be threaded through the slots rather than prewound and pushed in. The winding scheme I used was a serpentine path which went in slot 1, out slot 7, in slot 13, and out slot 19 (for phase A), with a total of 8 turns. The next slot consisted of 4 turns of phase A, and 4 turns of reverse polarity C (C-). Then 8 turns of C-, and then 4t C- 4t B, etc. Thus I had two pole pairs and the motor runs at 1740 RPM with about 4 VAC phase to phase and a current draw of about 5 to 10 amps per phase. The motor seems to have good torque and runs fairly quiet, even though my three phase source is imperfect.

I also built a modified sine wave generator using a PIC and a six-MOSFET bridge, and was able to run the motor at several speeds, with current draw of only 1 or 2 amps on a 12 VDC battery.

My next attempts used 36 slot stators, and I tried to get the highest number of poles possible for slowest speed and highest torque. I wound the stator also in a serpentine pattern but with only A, -C, B, -A, C, -B windings, for a total of 12 pole pairs. I expected it to run at about 550 RPM, and it actually runs about 510 RPM. However, it does not want to start itself, and it draws about 25 A on two phases, and only about 2 amps on the third phase, at about 5-8 V/phase. I think this imbalance is because of my test setup, but it does seem like this motor is much less efficient, and seems to have much less torque than the previous motor.

For my third attempt, I got some #15 AWG magnet wire and wound the motor in the more conventional and faster way, although it was hard to work with the stiff wire, and I had to force it into some of the slots. The pattern I tried here was 8 turns completely around pole piece 1, 7, 13, 19, 25, and 31 for phase A, then 8 turns on 3, 9, etc for B, and the same for C on 5, 11, etc. Then I wound 8 turns on the even numbered pole pieces with opposite polarities -C on 2, 8, 14, etc, -A on 4, 10, 16, etc, and -B on 6, 12, 18, etc. I had a total of 12 wires out of the motor, labeled A1, -A1, B1, -B1, C1, -C1, A2, -A2, B2, -B2, and C2, -C2. I connected -A1, -B1, and -C1 together, joined -A2 to A1, -B2 to B1, and -C2 to C1, and hooked power to A2, B2, and C2. I was surprised that the motor ran at about 235 RPM, rather than about 550 as I had expected for what I thought was a 12 pole motor. It also produced an impressive display of internal sparks cause by damaged insulation on the magnet wire. Then I connected the power only to A1, B1, and C1, and the motor ran at about 115 RPM. In this case, I was only energizing the positive polarity (North) poles, so that the South poles were created by the consequent pole phenomenon that is used in some multi-speed motors.

I made one more attempt on this same motor, after removing the damaged wire and removing sharp edges so I could rewind more safely. Here, I wound 14 turns completely around the odd pole pieces 1, 3, 5, etc, in a pattern A, B, C, in an attempt to make a 12 pole motor, but I got about the same result as my first attempt, with about 234 RPM and very poor torque.

I have done some analysis of what might be happening, although my knowledge of electromagnetic fields is rudimentary. It seems that, in this last motor, the vector sum of currents in the slots next to the pole pieces that were not explicitly wound, makes a pole of positive polarity, so the actual sequence of poles is A, C, B, etc., for 36 pole pieces, so it is essentially a 24 pole motor that would be expected to run at 3600/12 = 300 RPM, so 234 is reasonable. I suppose the negative polarity consequent poles do not exist in the pole pieces, but perhaps deep in the slots, which makes for a very wide magnetic gap and consequently very poor torque.

The purpose of this long discussion is to solicit ideas and recommendations so that I can design a practical, efficient, high torque, low speed motor to run on about 6 to 12 VAC at 60 Hz, and be capable of running up to 3600 RPM (or higher) by means of a V/Hz PWM drive. I have some ideas for a 12 pole motor where I may wind pole pieces 1 and 2 with phase A, 3 and 4 with phase B, and 5 and 6 with phase C. Then I will wind 2 and 3 with -C, 4 and 5 with -A, and 6 and 7 with -B. If I am correct, pole piece 1 will have a vector sum of (A-B), which will be at -30 degrees to A, with amplitude about

0.86 compared to purely A, and the pole sequence will be correct for a rotating field. If this is correct I may rewind a 1 HP 8 pole or 2 HP 2 pole motor (both 3 phase). I have one of each, and they are almost identical in size and weight, yet the HP is double in the faster motor. I am curious why this is so, and I will pay special attention to the winding scheme. Then I will concentrate on a PWM controller.

I look forward to your comments. Thanks.

Paul E. Schoen, President P S Technology, Inc.

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Paul E. Schoen
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Paul E. Schoen wrote: (snip)

Wi9thout commenting on your specific experiments, I have some generalities for you. You are altering the number of poles on the stator without regard to the number of poles on the rotor. Rotoe poles are not fixed on the rotor, but slide along the circumference of the rotor as the motor drives a torque load. The rotor poles are a consequendce of currents circulating in the squirrel cage conductor pattern cast into the slots in the rotor laminations. The motor needs several rotor shorting bars per pole to allow the rotor magnitization to slip smoothly along the surface of the rotor, while maintainimng an essentially stable field to produce torque. I think you are trying to magnetize and react against areas on the rotor that are too small relative to the spacing of the rotor shorting bars.

A better solution might be to start with a magnetic design that is optimized for a large number of poles. Hybrid stepper motors are essentially high pole, permanent magnet multi phase (usually 2) synchronous AC motors, and are often low voltage designs, to start with. Since their rotors are permanently magnetized, they don't deal with rotor shorting bars. 50 poles is a very common number.

Otherwise, I suggest you go with a straight low voltage rewind of an induction motor (without changing the number of poles) and a gear box to lower the motor speed.

Reply to
John Popelish

Take a look at what folks are doing to CD ROM motors for 3D flying. It generally involves regluing magnets, and rewinding. Here is a link: (in german)

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You can also look around on

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for information on this. Some of it is even in english... ;)

  Bob Monsen
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Bob Monsen































I think you need to check the inductance of each winding as you finish it to check for shorted turns, also as someone pointed out you need to consider the rotor too, I think the problem might be not enough slots per pole in the rotor, or even in the stator with the non sinewave supply, maybe you might consider a hysteresis type rotor ? it has a flat torque/speed charecterstic.

I dont think you can have only positive poles on the stator unless i misunderstand how youve wound it, the negative ones will be there too, deep bar rotors are used for higher torque at lower speeds.

Colin =^.^=

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For best efficiency, power rating and size its very hard to beat a geared 2 or 4 pole design. Your rotor and stator are already optimised for this solution and it's a straightforward rewind for your new operating voltage.

It's a real pain trying to thread heavy wire through stampings intended for use with relatively thin wire. It's much easier to use multiple parallel strands of wire of roughly the same thickness as the original high voltage wind.


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I appreciate the comments and they raise valid points. However, the suggestions to use reduction gearing miss the objective, and I want to use an induction design rather than a PM rotor. What I am trying to accomplish is to make an inexpensive, rugged, and simple motor with maximum horsepower for a given frame size. I would like to use a standard frame for simplicity of mounting and coupling to sprockets and chains or other means to obtain the desired speed range. I want to make the basic motor with the slowest speed possible (at 60 Hz), which means maximizing the number of poles. I will then use PWM techniques to raise the frequency to a multiple of 60 Hz, to a maximum determined by the highest safe speed of the motor, as well as any limitations of the magnetic material. So far, the motors I have taken apart have 24 or 36 slots, and that appears to limit the number of poles to

8 or 12, but my experiments and analysis so far indicate that there may be problems with those numbers, so 4 or 6 poles may be a more reasonable limit.

I have tried to come up with various winding schemes, but it appears that when poles are closer together, the windings interfere with each other due to overlap and cancellation of effective number of turns. I'm not sure I am doing this correctly, but for each pole piece, I do a vector addition of the currents in the windings of the adjacent slots. Ideally, if all the windings (assume 4 in one slot and 4 out the other slot) are the same polarity (A), the total effective turns would be 8A. However, if the slots have overlapped windings from another polarity, the total effective turns will be less. The vector addition shows that crowding poles so close that adjacent pole pieces have a polarity 60 degrees apart (which creates a 12 pole motor with 36 slots), the effective winding is reduced to 1/4 of the ideal. I have not done all of the calculations, but the 2 pole and 4 pole designs should be close to ideal, but the 6 pole is probably reduced to about 50%, and it drops to 25% for 12 pole. This closely correlates to what I have seen on the size and weight of motors with slower speeds at the same HP. However, for some very large motors, this effect is much less, which indicates that they must have many more poles and will not have the same problems with overlap.

My conclusion is that I should probably use no more than 6 poles for a 36 slot stator. This will probably give me only a 50 % increase in HP when I run it at 3x (180 Hz), and perhaps three times rated HP if I dare run it at

7200 RPM. I may be able to find some commercially available motors with 48 or even 72 slots in the lower HP types I am looking at (up to 5 HP).

If any of you have any old motors you may wish to give (or sell) to me, it may help with my experimentation. I will of course pay for any reasonable shipping and handling. You could first remove the copper windings and sell them for scrap and reduce the weight. I have some new motors I will be stripping and rewinding, but I'd rather play with an old junker.

BTW, one of these motors is a three phase 1 HP unit which runs at 850 RPM, which implies 8 poles. However, it has a 36 slot stator, so I assume it is wound with considerable overlap and a 50% reduction of the torque it would have with 2 or 4 poles. Interestingly, it is the same size and weight as a 2 HP motor rated 3450 RPM.

< Snip - details of experimental motors>




Reply to
Paul E. Schoen

Well, for Good Grief's sakes, haven't they been working on that already for almost a hundred years? From the rest of your explanation, it sounds like you're trying to determine by experiment, stuff that somebody else already has the numbers on. What a waste of time!

Or are you trying to, in your garage, come up with the magical design that will blow the experienced motor makers out of the water? ?:-|

Good Luck! Rich

Reply to
Rich Grise

What I am trying to accomplish is a low cost and higher power version of the tiny but powerful 400 Hz aviation motors that I found on a website

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I also want the motors to be powered directly (through a PWM inverter) from fairly low voltage battery supplies of 12-72 VDC or so, and see about what frequency limits may show up as reduced performance due to magnetic losses. The inexpensive implementations of the concept of Volts/Hz motor control are fairly recent, and as far as I know have been applied only to standard 240/480 VAC type motors. I have not seen anything about increasing HP by overdriving the frequency above 60 Hz, mostly because the DC bus is generated from the same original AC line voltage as the motor rating, and also because it can be unsafe to drive a motor much over its nameplate RPM and voltage.

I like the simplicity and ruggedness of AC induction motors (all hail Tesla!), as well as their well-documented speed and torque control. I am looking forward to making a motor that I can hold in my hand, connect to a

12 VDC battery, control speed with a pot or keypad, produce 1 HP, and cost under $100. If nothing else, I will learn a lot and be able to build a hell of an electric drill!





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Paul E. Schoen

















I know some washing machine motors used induction motors for quietness and they were rather large (compared to unversal ac/dc type), also they were dual wound for low speed high torque/high speed low torque. im not sure of the power in each mode.

so theres 36 slots in the stator but how many slots are there in the rotor of these motors ?

Usualy with 3 phase overlapping windings, when one winding is peak energised in the positive direction the other two phases wil be energised in the oposite direction becuase the sum of the all the 3 phase currents is 0.

energised in the negative direction thus in the center of the first pole the other two windings actualy add rather than subtract to the field, maybe you need to study some winding schemes, its been a while since I messed about, I remember re winding the stator of a large dc motor and it never seemd as powerfull again, I was convinced i rewound it exactly the same, its probably easy to miss something and be 1 slot away from a much more powerfull motor.

Maybe it would be helpfull to look at a book on the subject, I found a book called 'electric machines' very usefull, cant remember the author.

let me see if I can draw it in ascii ...

____________________ \\ / Ia-> \\ / X X / \\____________________/ \\ / \\ | pole A | \\ ____________________ / \\ / Ib-> \\ / X pole a+b X / \\____________________/ \\ / \\ / X pole b+c X / \\____________________/ \\ / \\ / X pole c+a X / \\____________________/ \\ / \\ / X pole a+b X / \\____________________/ \\ / \\ /

... not sure if this is corect as its a bit hard to draw/folow ...

Ia+Ib+Ic always = 0, so when Ib is at its positive peak :- Ia and Ic will be

0.5 negative ... therefore if you look at pole B it is imediatly surounded by a clockwise direction curent from winding b plus clockwise current from half of winding a and c b + a/2 + c/2 = 1.5

If you look at pole c+a then it is surounded by an anticlockwise curent of a/2 + c/2 + b = 1.5 here is where you wil find the oposite pole to b.

when there are many more slots the windings are just spread out, the overlap ocurs gradualy, but at the pole centers the same rules should apply, any 1 pole should never be completly overlaped by adjacent pole windings at its center.

hope this helps

Colin =^.^=

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