# 3 phase a/c motor, a rotating transformer?



## NintendoKD (Apr 29, 2012)

I have a couple of questions, I understand little, in relation to some of the electrical engineers who moonlight here. I am trying to understand more about transformers, the thought being, that armed with this knowledge, I could create a better, more efficient motor. As I understand it, the induction motor is nothing more than a rotating transformer. I have seen guys use a BIG baldor, with 220v in and a pull-string, power 3-phase to their whole shop "motors cannot exceed a certain output however, due to the limitation of the big motor being able to produce X current/voltage". I have been thinking, isn't there a way to build a more efficient motor? Why is there such a lack of information on winding, formulas for windings and output, as well as materials to produce motors on the world wide web? or maybe I just cannot find it. using current technology I have some ideas that could make better, more efficient motors...... I think, but I need more information to prove my ideas. I need a lot more information to prove my ideas. I am not looking to make a cool million here, I just want to learn more. For example, how are common rotors constructed? Why? are they constructed with manufacturing efficiency in mind? or with power efficiency in mind? why is such low voltage used? theoretically if a motor is rated at 4kw I should be able to run it at [email protected] right? the same as running it at [email protected], or [email protected] rps are regulated by frequency in hz, so as long as I keep the current down, i can make the motor spin faster without the motor seizing, this also allows for greater magnetic control, as the voltage drops much faster than current. I also have some interesting ideas about mechanical switching using a single 12v output, to power a 3-phase motor. how many stator windings does what? @ what gauge wire? why? is this different from a traditional transformer? If matter and energy cannot be created or destroyed, then where does the electricity go after making the rotor move? what is back emf, and why? can it be harnessed, and re-used? why/why not? I am not a college student nor am I writing a college paper.

thank you,

Nintendo


----------



## subcooledheatpump (Mar 5, 2012)

Induction motors (thats what you're talking about, 3 phase AC motors that are essentially rotating transformers) are limited in efficiency because of limits in 

Copper
Steel 
Power Factor
Rotor Balance
Bearings
Inverters for motor control

You can attack any one of those areas and if you can improve it, you can make a more efficient motor. 

I'll go down the list 

Copper is used in the windings, but it's not a perfect conductor, therefore there is some resistance and there is some heat produced when current flows through it. 

Steel is used to make the stator, it creates a path for the magnetic fields, or flux to flow through. Again it's not a perfect conductor of flux, heat is produced when the magnetic fields flow through it. The steel also has a very specific cut-off point where it won't be magnetic if too much current flows, or the frequency is too high. 

Power Factor, thats when the voltage and current don't peak at the same time. The induction motor is an inductor by nature, therefore it tends to store current, not voltage. So the current will "lag" the voltage. Meaning peak power won't be possible because you never get maximum voltage and maximum current at the same time. 

Rotor balance. Specifically for EVs, rotor balance is critical but a rotor cannot be balanced to work at all speeds. It can only be balanced at one speed and it will eventually go out of balance as the bearings wear, causing vibrations and more energy to be consumed. 

Bearings require lubrication, that means energy must be consumed to move the bearings, and their lubricant. 

Inverters used with induction motors in EVs are not without losses. The switching elements, be them MOSFETs or IGBTs are not perfect conductors when they are in their ON state. What's more, when they turn off, an excess voltage is produced due to V=L*DI/DT. Lowering the DC bus inductance helps, but there will always be some inductance within the package of the element itself. 

So again, Those are the reasons why motors consume energy, and if you can improve in those areas, you can improve the motor overall, efficiency, power, and lifetime.


----------



## NintendoKD (Apr 29, 2012)

subcooledheatpump said:


> Induction motors (thats what you're talking about, 3 phase AC motors that are essentially rotating transformers) are limited in efficiency because of limits in
> 
> Copper
> Steel
> ...



I have thought this already, and have continued thinking this, why not use higher voltage? This would improve efficiency through peak current and voltage. I get it, 100k watts is 100k watts, "looking for ~135 hp in my build" regardless of how it is achieved. as long as sufficient high voltage and low amperage can be realistically achieved and controlled at low frequencies, right? using an inverter with many mosfets in parallel, would lower overall resistance and make the circuit more efficient, this is excellent because lower currents are used, and less heat is produced as waste heat through radiation simply because less current is used because of the high voltage circuit. right? I am not an electrical engineer, but I am looking to make this whole process more efficient. dc to ac to drive induction, I mean this is essentially a rather complicated lc circuit right? the key is resonance and crafty circuit building for purpose. So re-wind the motor with smaller dia. wire to utilize smaller currents, and higher voltages. flux density would be greater, and overall efficiency would be improved through lowering the impedance to the mosfets, improving peak current and voltage. I have an old Navy friend of mine "funny, all my best friends are no where near my own age" he was telling me about the drive systems inside submarines, and how the rotors are very special, in fact, highly classified. I have also been playing around with the idea of using more phases, I mean, multiphase is a great gift from Dr. Tesla, why not use it, much like pushing a kid on a swing, if you could time it correctly, and had enough people, the force would be much greater and effort per person would be much less. What I mean is use instead of three phases, use many more, like, oh..... 9 maybe, utilize 18 poles on the rotor, and you have easy starts, high staring torque, and smoother operation, as well as increased efficiency. I understand that this makes the circuit more complicated, which is like some kind of sacrilege here but I really am excited about this, I want to start planning a circuit. I need to find a re-winding place first. Before I put the cart before the horse, I want to learn more. Thank you for the insight so far.

Nintendo


----------



## subcooledheatpump (Mar 5, 2012)

Well I do like your thinking... but 

Just consider this 

In a motor, the more phases/pole pairs you add, the more reactive power the motor will consume. It's better to size a motor specifically for what you need. Ideally, a high gear ratio and a high speed, low pole count motor works the best. Larger motors with more pole pairs or more phases, again consume reactive power. They push the current back even further along the phase, the inverter will have to make up the difference, by supplying more current into the motor (for no real reason, you won't be getting more Actual power/true power)

Also, adding semiconductors in parallel to a controller for increased conductivity does work, and it's a good idea to help dissipate heat more effectively. At the same time though, you have twice the number (or however many, n*how many you added) of gates to switch, which then means more current flowing through the gate drivers. That also means a much higher inductance which effectively means more voltage buildup (V=L*DI/DT) That means more turn off losses, and more heat to deal with.

Of course, you can always lower the switching frequency, but then you'd have a noisy motor. Don't know if that bothers you or not but just something to consider.

The analogy: "more people pushing a swing means more force" Well yes but it would be eaiser to get another motor rather than put more poles into the same one, or get a larger one. Which brings me to my next point. 

Flux density can only increase so much before the stator saturates. Smaller high voltage windings do help, but only to a certain extent. Keep in mind insulation and breakdown voltage too. What you really need for more true power is a larger stator. The ability for a motor to produce torque is based on it's size. Reason is, more steel= more magnetizable material. You can only put so much magnetic field into a given piece of steel before it gives up. Think induction heater. 

So you'll either end up with a small motor which spins fast and gives little torque, or a larger one with alot of torque that spins slowly. There is no such thing as a free lunch. But I do wish you luck and I hope you do find a way to make the whole system more efficient


----------



## NintendoKD (Apr 29, 2012)

let me lay a design on you:
dc source or batteries, to high voltage low frequency lc circuit, the high voltages that are current regulated are sent to the inverter circuit that does all of the switching, multiphase is pumped out to the motor, and Viola! high voltage low current multiphase powered ev. You could also call it a Tesla powered ev, as the lc circuit is often called the Tesla coil. Since there is no fear of high voltage sparks jumping from the secondary to the primary, due to the secondary being attached to the inverter, a more efficient design can be utilized and more thorough saturation to the secondary from the primary can be achieved. I have also contemplated using three or more such driven circuits, in close "but not too close" proximity to one another to help in absorption of inductive energy that isn't transferred initially. a rotary style gap could be used to regulate and theoretically create any number of phases as necessary. I have even thought of a simpler way to do this that involves using the windings within the motor as the secondaries, and using a variable capacitance setup to vary the frequency up or down. the primary energizes the secondary in the stator, inducing a flux, with no wires, through the air, and further regulation can be performed through a much simpler circuit attached to the windings. What this will do, is currents and therefore flux will "rise" in the stator, as opposed to being at peak power instantly, and then slowly falling back off until the phase begins again. current will therefore peak as voltage is induced, making it more efficient. right?


----------



## NintendoKD (Apr 29, 2012)

subcooledheatpump said:


> Well I do like your thinking... but
> 
> Just consider this
> 
> ...



THAT WAS FAST, you Clark Kent or something? thanks this helps substantially. I like the way I think too.


----------



## NintendoKD (Apr 29, 2012)

Can you expound more on reactive power and why more poles are bad? I want a cheap build, but curiosity got the better of me *scratches new "bug" bite* I get it buy a purpose built motor, install, drive, be happy. I am just spitballin here. I would like to pursue this as a viable idea for more efficient ev tech. though.


----------



## subcooledheatpump (Mar 5, 2012)

Pole pairs in an induction motor are really just electromagnets with alternating current flowing through them. Each pole pair has a magnetic polarity the opposite of the other pole pairs when the motor is in operation, thats what makes the rotor turn. The more pole pairs you try to get into an induction motor, the more the pole pairs tend to fight against one another. Energy is dissipated in the stator because the magnetic fields are bypassing the rotor, since the pole pairs will be closer to each other in a high pole count motor. You can always look at a standard industrial motor and see this. 

For example, most 2 or 4 pole motors come with a power factor rating of .88 or .90, where as a 6 pole motor has a power factor rating of .83. So the manufactures have done the homework for you in a sense as to how winding patterns affect efficiency


----------



## PStechPaul (May 1, 2012)

There was another rather long and recent thread about induction motor design, where I posted some links about motors with 6, 12, or even more phases. There is some improvement in efficiency and smooth operation, but greater complexity due to having more half-bridge elements in the controller, or multiple synchronized controllers.

There are motors with high efficiency ratings, but they require more expensive materials, such as thin silicon steel laminations, and they may also be larger and heavier to accommodate heavier copper windings for lower resistive losses. Smaller motors are generally 75% to 85% efficient, but very large motors can approach 95%. This may also require liquid cooling to reduce copper resistance losses.

600 VAC is a limitation imposed by insulation class of wiring and protective devices, which are still considered "low voltage". Above 600V, up to about 5000V, is medium voltage, and there are many large industrial motors and generators running on 4160 VAC. But there is also a practical limitation to the IGBTs used in the VFDs, so a 600 VAC motor requires a 900 VDC bus, and most IGBTs are limited to 1200 VDC, which includes some safety factor. Higher voltages may require connecting several switching semiconductors in series, which involves voltage balancing and accurate synchronization of gate drives. Very expensive, complex, and risky.

A very real way to get more power out of a given size motor is to run it at a higher frequency. If the stator is wound for, say, 120 VAC 60 Hz, you might be able to run it at 480 VAC and 240 Hz, at the same current, and obtain as much as 4x the power. But you might need an 8 or 12 pole motor so the speed will not exceed a safe value of 3600 RPM. Even that may be risky if the motor was originally designed for 600 or 900 RPM.

Induction motors are also limited by needing "slip", where they run slower than synchronous speed, and at some point will stall at about 3x rated torque. This may be partially a function of the size of the magnetic gap betwen the stator and the rotor, which makes it resemble a transformer with a high degree of reactance, and hence a limited ability to transfer power to the rotor. A tighter gap may help, but increases the danger of contact between the moving parts, which can be catastrophic or at least will add a lot of friction. There is also the phenomenon of "windage", which is friction caused by the air in the motor.

Practically speaking, 90% is probably a good target for motor efficiency. VF drives are typically 95%. The main source of inefficiency (and cost and weight) remains the batteries. And for EVs, higher efficiency might be better obtained by eliminating rolling friction and aerodynamic resistance at highway speeds. TANSTAAFL!


----------



## NintendoKD (Apr 29, 2012)

I just keep learning here, thanks. found another useful, and interesting site.
the largest determining factor in output is sheer size, or volume of material right? so, I could re-wind a "honkin" 20 HP BALDOR, this baby is heavy, I could achieve the same results with different voltages, 100kw is 100kw right? now at 60 hz this motor does 1760rpms so increasing the frequency is not useful, and due to sheer weight using lower friction bearings is out of the question? what about separating the rotor sections to prevent overstauration? I know I may seem annoying, but all input is very useful, if not for me, then for someone else. I used to teach for the Corps, and the only stupid question is the one you don't ask. I also apologize for any redundant questions.
http://www.animations.physics.unsw.edu.au/jw/electricmotors.html


----------



## NintendoKD (Apr 29, 2012)

how hard would this be to do?
If one puts a permanent magnet in such a set of stators, it becomes a *synchronous three phase motor*. The animation shows a squirrel cage, in which for simplicity only one of the many induced current loops is shown. With no mechanical load, it is turning virtually in phase with the rotating field. The rotor need not be a squirrel cage: in fact any conductor that will carry eddy currents will rotate, tending to follow the rotating field. This arrangement can give an *induction motor* capable of high efficiency, high power and high torques over a range of rotation rates.
would it be worth it? I would have the added benefit of an already existing magnetic field, I would therefore require less energy to keep it going, and it now doubles as an alternator.


----------



## subcooledheatpump (Mar 5, 2012)

You can rewind it and stuff in more turns per slot to get a greater power density. The stator and rotor core would sitll be unchanged though. So you'd essentially still get the same flux density, though it may take slightly less current to get it

Also a 20 HP motor isn't really that big. If you do it right, you should be able to get out 80 HP max. You can certainly replace the bearings with a lower friction design, and you can use external cooling (cooling provided from a seperate fan, not one attached to the motors' shaft) to reduce windage losses.

Adding permanent magnets to an induction motor essentially gives you a brushless DC motor. 

Several advantages and disadvantages. But mainly it does increase efficiency, but also makes developing low end torque a little bit more tricky for the inverter running it, since it has to know which phase to excite first. It also creates a very specific current limit, because if the limit is exceeded, the permanent magnets will become demagnetized, and never work again.


----------



## NintendoKD (Apr 29, 2012)

subcooledheatpump said:


> You can rewind it and stuff in more turns per slot to get a greater power density. The stator and rotor core would sitll be unchanged though. So you'd essentially still get the same flux density, though it may take slightly less current to get it
> 
> Also a 20 HP motor isn't really that big. If you do it right, you should be able to get out 80 HP max. You can certainly replace the bearings with a lower friction design, and you can use external cooling (cooling provided from a seperate fan, not one attached to the motors' shaft) to reduce windage losses.
> 
> ...



I do not understand, there is a lot of material, a rewind and adding magnets to make a BLDC three phase motor to the existing motor should make higher power right? the 150HP motors are smaller and more lightweight than the motor I have, I don't know anything about this, but it doesn't seem to add up. I understand that I am working with a single speed setup, but a re-wind should fix this right? heat is universally bad for perm. magnets, This I understand very well, BTW exploding perm.mags are rather dangerous FTW Low end torque eh? well, I guess you can't have your cake and eat it too. It doesn't change the fact that I still want a slice.

thanks,

Nintendo


----------



## subcooledheatpump (Mar 5, 2012)

It's like I said, you can add more power but only to a certain extent. I think the motors you are thinking of are specialized motors that use low loss laminations and low loss rotors or permanent magnets. 

Keep in mind smaller motors with more power are usually liquid cooled so they don't need to have large casings to act as heatsinks, and they are generally rated for intermittent use, not continuous use. 

If you can keep the stator from saturation, then you can increase the frequency and retain torque, so you increase speed and the torque stays constant, therefore you do get more power in a smaller space, although less torque and more speed. That usually means you need a gearbox for good torque. 

Take for example the motor in a Tesla Roadster, which is an induction motor. 

It does output about 280 HP, but it spins at 15,000 RPM and it requires an 8.28:1 ratio gearbox. Whats more, it cannot perform continuously, only in a burst.


----------



## NintendoKD (Apr 29, 2012)

subcooledheatpump said:


> It's like I said, you can add more power but only to a certain extent. I think the motors you are thinking of are specialized motors that use low loss laminations and low loss rotors or permanent magnets.
> 
> Keep in mind smaller motors with more power are usually liquid cooled so they don't need to have large casings to act as heatsinks, and they are generally rated for intermittent use, not continuous use.
> 
> ...


aha! now I'm beginning to understand, thanks


----------



## NintendoKD (Apr 29, 2012)

so the goal is, to 1: source, or build a high voltage controller 2:re-wind the motor for higher voltage using multiple turns of smaller guage wire 3:change the rotor to a reluctance style with 9 points and a slight twist 4:rebuild the motor with goodies, sensors, low friction bearings, etc. 5: order appropriate batteries with an inverter. Install, and enjoy.


----------



## PStechPaul (May 1, 2012)

I'm not sure I understand what you are trying to accomplish. 

In an EV, especially one that uses a transmission, you can generally get the same performance with a motor that is about 1/3 the HP rating of the ICE, and it is also quite a bit smaller and lighter. Standard motors offer efficiencies of 85-90%, compared to 30-40% for an ICE. The overall performance (in terms of efficiency) of an EV is mostly determined by the batteries and mechanical factors, so adding a few percent to the motor efficiency or getting more power from a smaller size or weight does not add much to the overall equation.

I can understand your desire to try things to see if you can make improvements to existing technology, but I don't think there are are any ideas that have not been tried and tested, and there are many other factors to consider in your choice of motors than getting state-of-the-art performance. If you factor in cost, ruggedness, maintenance, and ease of control, your choices become easier, and a custom-made motor is usually not the way to go.

Your idea of rewinding a motor for higher voltage does not really make sense for an EV. Even the usual 144 VDC to 720 VDC or 480 VAC systems are on the edge of what standard wiring and protection components can handle. Motors operate on a combination of voltage and current for power, and torque and speed are dependent on geometry and number of poles. As long as you stay within reasonable limits of conductor size and insulation class, you can get whatever performance you need. There are always trade-offs when you push the limits of high current or high voltage.

I think you will be much happier if you choose a standard motor and a battery pack that meets your needs, and then maybe look into various controllers, where I think there is a lot of room for improvement. If you really want to try your hand at a custom motor, I would suggest that you get a junk FHP single phase induction motor, remove the windings, and then rewind it for three phase and with a different number of poles and a different voltage. Then experiment with various frequencies and voltages and drive technologies, and get a feel for what is involved.

I don't think BLDCs with exotic magnets will be practical for EVs on a large scale, because of the supply problems and expense of rare earths, as well as the inherent fragility of PMs. Continue to dream and think outside the box, but make sure your proposed designs are based on facts and experience, rather than conjecture. Good luck!


----------



## NintendoKD (Apr 29, 2012)

What I want to accomplish is ~135HP. This is indeed the same as came factory with the car that I am installing it into. I do realize that this necessitates output power of 100,000+ watts of input energy. My inverter, or controller will push out higher voltages, at lower currents, useable, workable voltages inside will not exceed industrial limits. I understand the "easy" way to go I will probably go that route to produce a running vehicle for the time being. "just as soon as I can scrounge up about 12large" I want to build and design my own from off the shelf parts. I want to do this because it is not easy. maybe, just maybe, I'll make something really unique and better for everyone. I agree that controllers and inverter tech for induction motors are behind the power curve, but I do not yet think that "everything" has been thought of yet when it comes to brushless motors. I own an IDI diesel, and am also tinkering with this technology, it is no longer in use, why? because direct injection came along, and everyone would rather have some fancy pants computer control everything for them, and somehow this is better for the environment and makes the engine better, the reality is that it doesn't, the "computer" "polishes the turd" if you follow me. I think starting with a solid initial design in terms of the motor, is the best way to go, and right now, lets face it, no one is making motors for this purpose, unless you want to spend a small darned fortune. Brushed dc motors are great......... for people who are happy just getting in their cars and driving, and eventually having to replace brushes, and other various components, magnets etc. I like induction motors, only one moving part, no magnets to lose magnetism, the added benefit of being able to climb hills well, and OH! the obvious, the MOST efficient motors in the world. The point of the thread originally was to explore the idea of the induction motor as a type of transformer, as opposed to just a motor. Electromagnetic force into electromotive force. Hopefully answer some questions about waste heat, back emf, why they are not more efficient, theoretical limits, etc. A standard transformer has no moving parts, and because of that I believe that it can never be as efficient as an induction motor. But I digress.


----------



## PStechPaul (May 1, 2012)

OK, I see a little more of your intentions. But just a few points to discuss:

1. If your ICE is 135 HP, that is an optimistic peak value, which it attains at only a narrow range of speed and load, as tested on a dynamometer (brake HP). An electric motor has a flat torque curve at a wide range of speeds, so it "feels" much more powerful and accelerates much faster. And the ratings are also usually nominal for continuous duty, which does not apply for "normal" driving, and the peak torque can be 2-3 times nominal and you can overclock to get more power at the high end. So you can use a 40-50 HP electric motor and you will be very happy.

2. Large premium efficiency transformers are typically 98-99% efficient: http://www.nema.org/gov/energy/effi...ciency_Transformer_Product_Specifications.pdf
So it is unlikely that any practical motor can achieve that. About the best that can be expected is about 95%, and at that point your losses for a 40HP motor will be less than 1500W at full output. Since losses are approximately proportional to power, and the average power consumption of an efficient EV is probably 10-20 HP, the actual losses in the motor are about 500 watts. And if you have a vehicle that will be driven in cold weather, you can use that heat for cabin heating, so it's not wasted at all.


----------



## NintendoKD (Apr 29, 2012)

PStechPaul said:


> OK, I see a little more of your intentions. But just a few points to discuss:
> 
> 1. If your ICE is 135 HP, that is an optimistic peak value, which it attains at only a narrow range of speed and load, as tested on a dynamometer (brake HP). An electric motor has a flat torque curve at a wide range of speeds, so it "feels" much more powerful and accelerates much faster. And the ratings are also usually nominal for continuous duty, which does not apply for "normal" driving, and the peak torque can be 2-3 times nominal and you can overclock to get more power at the high end. So you can use a 40-50 HP electric motor and you will be very happy.
> 
> ...



I was thinking the same thing, you don't drive your car at 5-6,000 rpm's all the time, only to get to the final gear, where you are really only using about 1/3 of that, or so. Now you are talkin.


----------

