RC brushless motors: Should I use a low or high Kv?

It's better to use a brush motor rather than using a brushless although for low voltages.if you are restricted to brushless motor ,taking the resistance of armature conductors and commutator select either low or high kv brushless motor.

Hi sita rama.

I am restricted to brushless and there is no commutator. But that is the question: due to the low speed application I have (direct drive), which RC motor will deliver more voltage: a high or a low Kv (= rpm/V for motor regime)? My guess is a low Kv, but I'm looking for a second opinion from someone who has practical knowledge on brushless RC motors used as generators. I know that the longer the magnet and the more turns/pole make more voltage and power. Usually these variables are associated with lower Kv in a series of motors from the same manufacturer.

A brushless motor that is capable of being used as a generator or alternator (some have built-in commutating electronics and must be taken apart to get to the motor guts) will have a power rating -- e. g. 12V @ 2A, 10,000 RPM @ 12 V. In order to get 6 volts, the RPM needs to be 5,000.

So the question is, how fast are you going to turn the generator?

Is it possible to use a car Alternator? or is the RPM too low?

Only if it has permanent magnets as excitation - but I never saw one.

Thanks anyway.

As a bit of trivia, there are car-style PM alternators used in the wind power world. They would, of course, be far too large for your application.

I don't think you can say whether a higher or lower turns-per-volt constant is better or worse. I think you'd want to go for the greater efficiency motor(generator), and then design the drive to suit it. This process could take several iterations, I'd guess, because you may find that the ideal motor cannot be turned with the available torque.

It would be more efficient to generate current from an engine driven alternator than to use any method to extract the energy from the airstream. A disc with a couple tiny rare earth magnets could be mounted directly behind the prop, with pickup coils in close proximity. The disc (alu) would be drilled with holes to fit the magnets, so that you wouldn't be relying entirely on glue to provide the centriptal force. Although this could be three phase, it's possible that a single phase though a single diode would suffice -- basically a miniturized version of the lighting coils used on old mopeds.

Thanks Blink.

I would still like to have a practical comparison between similarly power-rated RC brushless motors with different Kvs when used as generators - before buying any, as they don't come cheap.

For starters.. If you run the generator speed at about 5 percent above its no load speed, as a motor, you can expect to get very nearly the original terminal voltage out.

At the no load speed, the counter emfs (internal generated voltage) are just about equal to the input terminal voltage so what ever speed it is turning plus about 5% extra (estimate to cover the wind-age and bearing friction losses) will be the speed you will need to turn it as a generator. This will give you a speed benchmark for your design.

The wattage will be the product of the current you draw from it.

The power required to turn it (hp) will be determined by the watts you draw from it Plus the friction losses (see above)

The required shaft torque will be a function of the power drawn and shaft speed.

You can find the constants in the handbook i.e. 746 watts per hp. 33,000 pound feet etc. As others have commented, I think you major problem, calculation of the energy extraction from the prime mover, propellor, turbine etc.

Remember the wind drag from a propellor is a function of the area swept by the blades.

On a crop sprayer type of aircraft, the 6 inch propeller blade on the spray pump reduces the aircraft speed by as much a 15 miles per hour, when its turning a swept area.

Why do you refer to the KV term. This is a killovolt term and I suspect you are working with voltage well below the 1000 volt level.

I am also greatly interested in how you are going to build an Ironless generatior. The designs willl have to be pretty unique.

Why do you refer to the KV term.

He was using Kv, (sometimes seen as Ke) with the v being a subscript to mean voltage. It's read voltage constant, and is usually in rpm/volt.

Thank you Ken. The K_e constant makes sense. In this case the comments that I made provide a means to calculate this K_e factor and as can be noted above, the program provides a means for the printing of subscripts. Had this been used, the question would have been much easier to understand and saved the confusion.

Often in these comments, many foreign engineering terms are used, not common to American engineering and I jumped the gun. The author seems to know what he is looking for. I think, perhaps, more concentration on determining the available prime mover power, and then sizing the generator to fit it might be a better engineering approach.

Don

The Kv (not KV) term, as I explained earlier, is specifically used in the RC world and means rpm/V at no shaft load. For example, from a Kv=3000 motor you get 3000rpm for each volt you supply to it.

As the voltage vs speed graph of such a motor used as a generator is linear, for a constant electrical load the shaft resistant torque is highly predictable. I have no problem in calculating the available wind power (drag) as a primary mover.

As you said, the design of an ironless generator is pretty unique and proprietary, so please understand I cannot say more on this.

Thank you for your thoughts.

The situation you are facing is much like the situation I faced in designing a generator/ICE pairing for a hybrid vehicle. The engine has a particular speed and torque at which it is most efficient. Most generators also have an efficiency curve, although it is generally much flatter than that for an ICE. One generator possibility could provide the desired voltage at 2800 rpm. Another could produce the desired voltage at 3600 rpm. The engine peak efficiency was at 2500 rpm for the high load condition under which the batteries would be recharged.

If I used belt drives, either generator could provide the required voltage at 2500 rpm engine speed. (In the same way, you could drive different motors at different speeds by using differently pitched and sized props.) In that case, the decision would be based on generator efficiency, with the lower speed one being about 90% efficient vs 88% for the faster one. However, I could run the engine at 2800 rpm for a 5% loss in engine efficiency, and drive the slower generator directly, gaining 5% efficiency by doing away with the belt drive. The efficiency would end up the same either way, but the transmission cost and complexity would be lower with the direct drive. Unfortunately, the cost of the slower generator itself was much higher than the higher speed one, more than enough to outweigh the transmission cost. Also, I happen to have on hand the higher speed generator, so that is what I am using for the prototype vehicle. For the production vehicle, I'll have a generator specially wound to permit direct drive.

I think you need not be obsessed with efficiency in your application -- 5% one way or the other would not make much difference in performance, given that you will be extracting a small amount of the total power to drive your generator. (I'm assuming this is a relatively large RC plane, with perhaps a 48 inch wing span.) So then the first issue is the power required to keep a small battery charged and to feed the average power consumed by the receiver and servos. You'd then calculate the amount of power you can get from a reasonably sized propeller to drive the generator. A larger prop could turn slower, and would be more efficient, and a smaller one would turn faster, and would be less efficient, but easier to fit on (in?) the plane. Then the rpm/volt relationship would be determined by prop size, assuming a direct drive. So if efficiency is a concern, then a larger/slower prop, and a slower generator (higher Kv) would be the way to go. But if hiding the prop is a concern, the a faster generator (lower Kv) would be the way to go.

BTW, these constants are expressed in different ways, so "lower" or "higher" can get switched around easily. (Its not unlike liters/100km vs mpg) A motor which produces 1000 rpm/volt will, as a generator, produce 1 volt/1000 rpm, so the ratios can be inverted.

I'd think it would be worth prototyping a model of this (using a cheap brushed motor) that you could mount on your car. Drive at a speed equal to your typical airspeed, and play around with prop size and pitch, and see if you can get the voltage and amperage you want.

Getting back to the hybrid, given a belt drive, I could just as easily turn a very low Kv or very high Kv generator, and the real reason for choosing one over the other would be efficiency, weight, physical size, or cost. The same can be said for your situation, I think: the size and pitch of your prop determines an rpm, so then you need a generator with the right Kv to give you the voltage you need. If you can't find an appropriate generator, then change the prop parameters to fit a generator you can get.

Thank you Blink. You have a lot of hands-on experience and knowledge.

Something in your explanation is puzzling me:

"So if efficiency is a concern, then a larger/slower prop, and a slower generator (higher Kv) would be the way to go. But if hiding the prop is a concern, the a faster generator (lower Kv) would be the way to go."

As I would like to stick with the Kv definition for motors, my understanding is that for a slower prop I would need a lower Kv motor because If I spin it I get 1V (minus losses) for less speed than a high Kv motor which would need more rpm for generating the same 1V. This is also consistent with my observation that in a series of motors having the same magnet length and diameter but different number of turns/pole, the more turns the lower the Kv! More turns mean also more induced voltage at the same speed and magnetic induction.

You're right when saying that I'm obsessed with efficiency and for me a 5% means a lot.

Another issue is the cogged torque which dictates the cut-in speed for the prop+generator. The lower the better. For this reason I said an ironless generator will have no cogging torque and will be much lighter. For a classic induction generator (and motor), the rotor-stator gap has to be as small as possible and induction as high as possible for higher efficiency (power/mass unit) but all these make the cogging torque to be cumbersome for some sensitive applications. The rotor is bond to the stator through magnetic forces when coils are not energized.

"So if efficiency is a concern, then a larger/slower prop, and a slower generator (higher Kv) would be the way to go. But if hiding the prop is a concern, the a faster generator (lower Kv) would be the way to go."

Your understanding is correct: if you look at it from the motor perspective, then the higher Kv and lower Kv would be switched around.

Lynx Motion is promoting coreless motors, using a patented SEMA technology: http://www.lynxmotiontechnology.com/e225.pdf Poking around their site and patent might give you some ideas.

I'd guess that you could avoid cogging torque problems by switching off the load at low speeds -- let the prop get up to speed, and then connect the load with a transistor. You'll need some sort of voltage regulation anyway, so you'd need at least a transistor to turn the charging off when you don't need it. That same transistor could be turned on only when the generator voltage is higher than the battery voltage.

Lynx is my competition. Just think about what you can do with a motor having a very low inductance, very low inertia and constant torque regardless the speed.

We definitely are in resonance. The load disconnect is done with a simple trigger schmitt circuit (can be done with comparators too).

Thanks

Brushless Dc motor is not going to give you the output as it contain electronics in between and it works only one way.

You should use motor with brush,

If you want to use brushless dc motor then you need to remove the electronics from it and get our put directly from winding. just like a tachogenerator,

That is my take on it too......Brushless will not generate any usable amount of power......

Unlike fan motors, RC brushless motors have the the electronic drive circuit separate, so the winding lead wires are accessible and usually terminated with gold-plated connectors.

So he might just be able to generate some AC voltage, its probably possible then...

That's for sure, Andy.

But it's hard to find a RC hobbyist who is in the same time an electrical engineer or have extensive knowledge in the field.

Thanks

Basic (permenant magnet field) motor design: Keep in mind that the magnetic field can be on the rotating member or the stationary member. Since you mention brushless motor, the field must be on the rotating (armature) member, and since you mention 3 phase, you are indicating that the stator has 2 or more windings. (Likely 3)

If it truly is a 3 phase motor, then the electronics must include an inverter circuit the two outputs, (one with a phase shift) for the stator winding. In either case, the original electronics need to be removed. (note: they may be internal to the motor)

Assuming that you have access to the windings and intend to use an external recitifier, you treat it as a conventional generator. With a constant field (the permenant magnet) The voltage output is directly proportional to the speed. You need to size the motor for the reuired voltage for the intended operating speed.

Note, you may have to run the generator 10 to 15 % faster than the rating to allow for machine loss.

Snakers

RC brushless motors are built like 3 phase PM alternators: permanent magnets on the rotor and windings on stator. They are sold separately from the electronic drivers.

In the RC flying world, there is an application for generating on-board electric power with a brushless motor (ram generator) and I am interested to find out which Kv is more suitable for getting a higher voltage/rpm in generator regime.

For instance, I found on a RC forum that a motor having Kv=700 generated 70W but no other data on speed/voltage.

You're right: voltage is growing linearly with speed.

Thanks

Yes, you can absolutely use the motor as a generator. I happen to be an electrical engineer and my 15 yr old son is an RC fanatic. It sounds like you would like to generate power on a nitro or gas powered aircraft to keep the receiver and servo battery charged during flight so you don't have to worry about battery power, just fuel. Virtually all RC brushless motors have external inverters so you can simply use the motor as a generator hooked up to a three-phase rectifier. You may wish to look into charging circuits used with solar panels as those are optimized to produce a constant voltage whilst drawing maximum power from the source given that a ram air driven dynamo will have varying outputs over the operating envelope of the aircraft.

In terms of the Kv rating, I think it would be more important to go to the hobby store (assuming you live close enough) and check to see which motor has the least cogging torque as that will make your dynamo difficult to start given the relatively low torque of a ram generator.

I too have given this some thought for an RC power boat project I have been toying with, and depending on the engine set-up, you might want to consider a gear driven dynamo similar to the starting/charging motors used on some nitro trucks. Those I believe are brush type motors though. I'm surprised engine manufacturers have not been providing a "lighting coil" with their engines similar to the ones used with magneto style ignition systems, or an integral charging system built right into the engine. Seems like it wouldn't be that hard to integrate some magnets into a flywheel assy. Add a couple or three coils on the periphery and presto, a charger!

Thank you, Brave.

That's exactly what I'm talking about. My application includes a 3-phase bridge as well.

In terms of cogging torque, you might not see a difference between 2 motors having different Kvs but similar power unless they have a significant difference in the number of poles/phase ( I visited two hobby stores in my area). This is an issue in my application and I am seriously thinking about building an ironless generator. But before that, I'm looking for something on the shelf.

Best regards,

Michael

You are perfectly right of course but you would also reduce the power available to turn a propeller or similar at the same time. I may be wrong, but most hobbyists that I have met want the max speed from their model, be it a boat, car or plane.....that might be part of the reason.

When I used to help prepare car engines for saloon car racing, in the days before alternators, we used to reduce the dynamo windings to a minimum, just enough to put a charge equal to the requirements of the ignition circuit to allow max power for the drive wheels at all times......

It's all about the difference between lifting a battery and lifting a fuel for the same energy capacity.

For the same reason we don't have a fully electric car yet, don't we?

To decide on low or high KV we need to know motor rating.

The class of motor I'm looking at is driven by a 90A circuit. They are so-called outrunners.

Last night I was at the Historical Electronics Museum in Linthicum, MD, http://www.hem-usa.org/ I saw a display in the Space Sensor Gallery of interest. If you navigate to http://www.hem-usa.org/growth-in-space-electronics.shtml you will see a partial recreation of a red RC airplane hanging vertically in a case in the right foreground of the picture. This is an exhibit about an RC aircraft that flew non-stop from Canada to Ireland, the TAM-5. http://tam.plannet21.com/ One thing I noticed was an RC PM motor coupled to the engine crankshaft on the other side of the "firewall". Of course the additional inertia added to the engine will slow your throttle response, but a small enough motor will still probably supply more than enough power for your loads so you can minimize the effects of the inertia. Six diodes to rectify the three-phase power and some kind of voltage regulator. A linear regulator is simple enough but wastes power, so I might suggest some kind of buck-boost converter that will minimize power losses yet provide a stable voltage over a wide operating range.

Good luck with your quest.

Thanks for the links.

You're very close to my solution for voltage regulator.