Brushless Motor Specifications

by John David Heinzmann
(Manchester, NH)

If I may comment on your "Understanding Motor Specifications" section:

Actually, Kv is exactly the amount of voltage produced for the number of rpms it is spun at.

It tells you how the motor behaves as a generator with no electrical load, thus no current flowing.

As a motor, the reason it doesn't spin at the expected number of rpms for the voltage applied is, as you stated, because of loss torques in the motor.

Current must flow to overcome these losses (friction, magnetic eddy currents and hysteresis) and thus there is a voltage drop in the resistance of the motor windings so the voltage applied is effectively lower.

It helps greatly to include units in this discussion. The units on Kv are V/krpm (or V/rpm, or V/(rad/s)).

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Brushless Motor Specifications

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Clarification
by: JD

Matt,

Ooops, I'm afraid I may be creating more confusion with my post than
clarity.

I work more on the industrial side of motors and I commented on Kv before completely understanding how the RC community uses this
spec.

They have inverted the normal industrial meaning of the back-emf constant, Kv, to use units of rpm/V instead of the more traditional
V/rpm.

And if one uses SI units, Kt (A/Nm) is numerically equal to Kv (V/(rad/s)) for brushed motors.

I am sorry if I have intruded and I
know that was not the perfectly correct place to post, but I didn't see where else to do so it on your site.

Thanks.

JD

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Appreciate the feed back
by: Matt

Hey JD,

You're not intruding at all. I welcome this kind of feedback. I want the information on this site to be factually correct. If I am wrong on something I want to know! And to be honest, I'm not exactly an expert on electrical engineering, lol.

I'm an ME actually...

It is interesting that there are two different uses of the term KV. I never knew that.

Do you fly RC?

Thanks for the feedback!

Matt

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My background
by: JD Heinzmann

Hi, Matt,

I also am an M.E., but I do not fly RC. 40 years ago, I flew U-control though.

I work with brushless DC motors at Segway Inc. and am interested in the motors you have here because they are now approaching the size to our motors. My focus is electrical propulsion so I have gained a lot of insight into the subject, mechanical electrical and thermal.

I am glad to be able to shed some light on the subject.

JD

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what's your application for small brushless motors?
by: Mattt

What are your motors used for if you don't mind me asking?

Do you design those cool gadget that you stand on and it transports you around? If so, can you please tell me what keeps those things from falling over? LOL. I've always wondered that. They look like majic...

I think you should try RC! What better way to learn study propulsion?

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Segway Propulsion, Kv and Kt
by: JD Heinzmann

Yes, the wheel motors of the Segway Personal Transporter (PT) are propelled by brushless DC motors. Balance is achieved by sensing when the platform is not level and commanding the motors to drive in the direction needed to make the platform level again. Level is sensed by accelerometers and angular rate sensors, not that different, I would imagine, from the sensors used to stabilize RC helicopters.

As to your suggestion Matt, I already learned about propulsion by developing the propulsion system for the PT. Now, maybe it's time for me to learn about RC by applying my propulsion knowledge there!

Regarding Kv and Kt, the reason they have a constant relationship for a given motor is that they are a measure of the same physical phenomenon. Kv is about the voltage generated when a wire moves through a magnetic field (generation). Kt is about the force generated when current passes through a wire that is in a magnetic field (motoring). These two effects are just complements of one another. You only need one to know the other. The value of Kv (and thus Kt) is set by the strength of the magnetic field and the number of times the wire passes through the field (the number of turns in the winding).

Do you know if there is a glossary of terms for RC propulsion? One that tells what units go with each specification? Because these are 3-phase AC motors, there are important details that are often left out of specs that are needed in using them to calculate performance. For example, are the motor voltages and currents a measure of the amplitude (0-peak) or the rms value? Are the voltages, resistances and inductances measured line-to-line or line-to-neutral. For the PT, we use Line-to-line and 0-peak in all our specs. Line-to-line is the easiest to measure (neutral terminals are often buried, or in the case of a delta-connected motor winding, non-existent) and 0-pk relates directly to the "dc" values of the current and voltage in a flux vector controller.

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Here's what I know...
by: Matt

Thanks for the explanation of the relationship between Kv and Kt.

To be honest I am pretty ignorant when it comes to the kind of technical explanations you are looking for. I just learn what I need to know to determine which motors will work best with my planes! : )

There seems to be some debate in the RC community about whether these are actually 3-phase AC motors. I guess it all depends on what you define a true 3-phase AC motor to be.

To me, I don't care what they're called. As long as my RC plane is flying I'm good to go! I let the brainiacs debate the details.

Anyway, here's how I understand it.

The batteries we use are DC batteries. The electronic speed controllers we use chops this voltage completely on or off at each pole in such a way that may simulate an AC voltage. But it's not a true sinusoidal AC current. Kinda like an invert you use in a car I guess...

So the voltage is the voltage from the battery. Which depends on what size battery back you have. A 3s lipo back has a nominal voltage of 11.1V.

When I actually check the voltage of my packs, they can be anywhere between 12.3 and 12.4 volts when I take off. By the time I land they are usually around 10 volts or so. Here is more info on the lipo packs we use.

You might find this thread on RC Universe interesting. Pay close attention to what GeraldO posts. He seems to be very knowledgeable on the subject.

Electrifly is one of the top brand brushless motor manufacturers for RC airplanes. You may want to poke around their site, maybe find a contact there that can give you the answers you are looking for.

Here is a manual for a RimFire motor amd a manual for a Great Planes electronic speed controller, Perhaps this may offer you some insight as well.

I hope this helps you figure out what you need.

If you're serious about learning to fly RC, here is a good place to start!

Let me know if I can help you in any way.

Matt

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