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Pdelcast
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08.22.2008, 09:18 PM

Well, my $.02:

There are electromagnet-electromagnet motors today -- just open up a plug-in hand drill and you'll see one. They need a lot of cooling because they just aren't all that efficient (double the I^2R losses.)

Induction motors are great, and can be very efficient, but they are expensive to control, and don't scale DOWN very well -- and they are VERY heavy compared to DC brushless (which is really a three phase AC permanent magnet motor) for the same power output. And they don't scale well for high RPM applications -- the rotor losses get really high at high RPMs.

So for the hobby, and for any application where highest power to weight ratio is required, you just can't beat a DC brushless motor.


Patrick


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Pdelcast
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08.22.2008, 09:31 PM

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Originally Posted by Metallover View Post
hmm, yes I was thinking brushed, but induction would work too. But it all comes down to if a neodymium magnet is stronger or weaker then an electromagnet of the same size. If you could get an all electriC motor to work, you wouldn't have to worry about temps as much because there are no magnets to demagnatize. You would have to worry about things melting like plastic and insulation. That means in theory if you have a very powerful esc then you could put as many volts as you want and gear as highas you want until to either reach the escs limitations or you start melting stuff...

It whole probably have to be induction because brushes would wear out fast. Another peopem would be efficiency. It would have to be very powerful to be as efficient as two motors.
Neodymium is actually stronger than an EQUAL size electromagnet. Remember, most of the magnets in the motors we use are really small. You couldn't pack much electromagnet in that size. :)

Electromagnets have a limitation based on the characteristics of the steel that is used to carry and concentrate the field. There is a point at which the steel carries the maximum amount of magnetic flux that it can carry, and it won't carry any more. This is called "magnetic saturation", and when that point is hit, the electromagnet doesn't get any stronger.

So, you could make a pretty powerful electromagnet-electromagnet motor, BUT, it would be larger than an equal power permanent magnet motor and would be a little less efficient. What would happen at maximum output (instead of demagging) would be that it would get hotter and hotter (very quickly) when the steel saturated, until the windings failed. This is what happens with Cobalt magnet motors -- the cobalt magnets won't fail until over 1000 degrees (F) -- but the windings fail long before that. Plus, a much larger magnet is required because cobalt doesn't have the power of neodymium -- so the motor is larger and heavier for the same output power.

SO, therefore, today you just can't make a more powerful motor than a DC brushless in the size we use (sub 5 horsepower.)

The maximum power output of a motor ALWAYS comes down to efficiency... efficiency is really the "holy grail" of motor design, because the losses always become heat in the motor, and you must shed that heat to produce power efficiently ... And if you think about it, you can output twice as much OUTPUT power on a 90% efficient motor as you can on an 80% efficient motor.

We in the industry are always looking to increase the efficiency -- to keep the temperature down when running huge amounts of power into the motors.


Patrick


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BrianG
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08.22.2008, 10:42 PM

Speaking about motor efficiency, I've heard it said that a slotless stator design (basically an air core) is generally more efficient, but the Neus are slotted. I would think because of the slotted nature (windings wrapped around a "core") that the magnetic field would be much more focused, and wouldn't have as much flux loss. I can see that the width of the magnetic field would be a lot less in a slotted vs slotless. What are your thoughts on this?
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Pdelcast
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08.22.2008, 11:51 PM

Quote:
Originally Posted by BrianG View Post
Speaking about motor efficiency, I've heard it said that a slotless stator design (basically an air core) is generally more efficient, but the Neus are slotted. I would think because of the slotted nature (windings wrapped around a "core") that the magnetic field would be much more focused, and wouldn't have as much flux loss. I can see that the width of the magnetic field would be a lot less in a slotted vs slotless. What are your thoughts on this?
Depends on who you talk to -- there are those who think that slotless is better, and those who think that slotted is better. I just think you should use whatever motor design fits your application...

Slotless motors have a larger air gap, and so, are generally less torquey than slotted motors. But they also have more space for copper, so typically have lower copper losses. Slotless motors have less inductance, so they switch at high frequency (higher RPM) better than slotted motors. Slotless motors also have virtually no torque ripple, so deliver power smoother than a slotted motor. The CM20 and CM36 motors (our standard Mamba and Mamba Max motors) are slotless because we wanted maximum spool-up speed, fast response, and moderate torque. IMO, Slotted motors don't do as well as slotless in 1/10 scale applications because the buggies and cars are lightweight, and accelerate very quickly without high torque -- and the high RPM performance and quick response make up for the lack of low-end torque (although our slotless motors generate a LOT more torque than our competitor's slotted 1/10 scale motors -- -but that's due to poor rotor design, rather than a slotted vrs slotless tradeoff.)

Slotted motors can have very small air gaps, and so are very torquey. They can generate much higher peak torques as well, but have the advantage of higher inductance at low RPM, which helps to limit peak currents and keep temperatures lower at low RPM. Our 1515/1Y and 1512/1Y Monster motors are slotted because we wanted maximum low RPM grunt and shaft twisting torque. And IMO, slotless motors just don't do as well in a Monster Truck or big buggy as a slotted motor. Once you get above about 3 pounds or so, the slotted motors have a slight advantage.

So, for smaller, higher RPM motors (low torque, high horsepower), slotless motors usually have the advantage. For larger, lower RPM motors, slotted motors usually have the advantage.

So you can think of it this way: Slotless motors are like motorcycle engines -- high RPM, lower torque, high horsepower. Slotted motors are like automobile engines, low to medium RPM, high torque, high horsepower. Outrunners are like Diesel engines... lower RPM, lower horsepower, very high torque.

All of these comparisons are relative -- the difference in practice is fairly small. We have run Monster trucks on slotless, slotted, and outrunner motors with good results. We really went with the Neu slotted design simply because it had the highest efficiency of any motor in that size that we had tried. And because of the high efficiency, it really performed extremely well.

There are trade offs for every type of motor design. You can build a good slotted motor and it will compare very well with a good slotless motor. Or you can build a bad slotted motor, and it will compare very well with a bad slotless motor. I've seen both good and bad motors of every type.


So the real answer is: Efficiency is the single most important thing. Efficiency is directly related to power to weight ratio. And power to weight ratio is performance. So the better the efficiency, the better the performance. Whatever motor type gives you the best efficiency in your application is the one you should use...

I'm sorry for the long ramble...


Patrick del Castillo
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TexasSP
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08.23.2008, 12:26 PM

Thanks for the explanations, that helps a lot when choosing motors.


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Metallover
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08.23.2008, 12:29 PM

Do the brushless systems today have regenerative braking? As in when you use the breaks or even just coast the motor acts as a generator?
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BrianG
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08.23.2008, 01:08 PM

Quote:
Originally Posted by Pdelcast View Post
Depends on who you talk to -- there are those who think that slotless is better, and those who think that slotted is better. I just think you should use whatever motor design fits your application...

Slotless motors have a larger air gap, and so, are generally less torquey than slotted motors. But they also have more space for copper, so typically have lower copper losses. Slotless motors have less inductance, so they switch at high frequency (higher RPM) better than slotted motors. Slotless motors also have virtually no torque ripple, so deliver power smoother than a slotted motor. The CM20 and CM36 motors (our standard Mamba and Mamba Max motors) are slotless because we wanted maximum spool-up speed, fast response, and moderate torque. IMO, Slotted motors don't do as well as slotless in 1/10 scale applications because the buggies and cars are lightweight, and accelerate very quickly without high torque -- and the high RPM performance and quick response make up for the lack of low-end torque (although our slotless motors generate a LOT more torque than our competitor's slotted 1/10 scale motors -- -but that's due to poor rotor design, rather than a slotted vrs slotless tradeoff.)

Slotted motors can have very small air gaps, and so are very torquey. They can generate much higher peak torques as well, but have the advantage of higher inductance at low RPM, which helps to limit peak currents and keep temperatures lower at low RPM. Our 1515/1Y and 1512/1Y Monster motors are slotted because we wanted maximum low RPM grunt and shaft twisting torque. And IMO, slotless motors just don't do as well in a Monster Truck or big buggy as a slotted motor. Once you get above about 3 pounds or so, the slotted motors have a slight advantage.

So, for smaller, higher RPM motors (low torque, high horsepower), slotless motors usually have the advantage. For larger, lower RPM motors, slotted motors usually have the advantage.

So you can think of it this way: Slotless motors are like motorcycle engines -- high RPM, lower torque, high horsepower. Slotted motors are like automobile engines, low to medium RPM, high torque, high horsepower. Outrunners are like Diesel engines... lower RPM, lower horsepower, very high torque.

All of these comparisons are relative -- the difference in practice is fairly small. We have run Monster trucks on slotless, slotted, and outrunner motors with good results. We really went with the Neu slotted design simply because it had the highest efficiency of any motor in that size that we had tried. And because of the high efficiency, it really performed extremely well.

There are trade offs for every type of motor design. You can build a good slotted motor and it will compare very well with a good slotless motor. Or you can build a bad slotted motor, and it will compare very well with a bad slotless motor. I've seen both good and bad motors of every type.


So the real answer is: Efficiency is the single most important thing. Efficiency is directly related to power to weight ratio. And power to weight ratio is performance. So the better the efficiency, the better the performance. Whatever motor type gives you the best efficiency in your application is the one you should use...

I'm sorry for the long ramble...
Thanks Patrick! The "long ramble" is not long if the answer is complete and thorough. Anything less is not worth saying.
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08.23.2008, 08:11 PM

im really enjoying these lessons, perhaps you could start a forum class for those interested and post something small (like this) in it? id enjoy that... looking to start something in the electrical field next year after i graduate from yr 12! electricity really entrigues me


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Pdelcast
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08.25.2008, 10:36 AM

Quote:
Originally Posted by Metallover View Post
Do the brushless systems today have regenerative braking? As in when you use the breaks or even just coast the motor acts as a generator?
Yep -- the braking does force the motor to act as a generator, and dumps the excess power back into the battery.


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BrianG
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08.25.2008, 10:48 AM

Patrick, what is your take on the safety or reliability of using motor braking with lipos? Lipos require a pretty stringent CC/CV charge profile, and depending the weight/speed/braking force used, could this excess power be unsafe or at least unrecommended? After all, it isn't regulated at all...
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08.25.2008, 11:28 AM

Quote:
Originally Posted by BrianG View Post
Patrick, what is your take on the safety or reliability of using motor braking with lipos? Lipos require a pretty stringent CC/CV charge profile, and depending the weight/speed/braking force used, could this excess power be unsafe or at least unrecommended? After all, it isn't regulated at all...
Well, you can't overcharge a Lipo during braking, because you are never putting in more energy than you have already taken out of the battery.

For the short bursts we use for braking, there is no damaging chemistry change in the battery. I've verified this with two different cell manufacturers -- the short duration charge burst doesn't have any affect on the chemistry of the Lipo. The energy put back into the Lipo just isn't high enough to cause any localized heating (the real issue) on the polymer.


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