Capi's Brushless Motor and ESC 101

[Pinecone]

Originally Posted by r40734

I can't really go into detail, but outrunner motors have several military applications as well.

Hmm, I will have to think about that.

[helikirk]

Thanks for the great writeup. It's been awhile since 'basic electric machines' class at school.

I have a question. If the ESC needs reserve power to cover additional torque demands, that means that anyone who uses governor mode should NOT run a flat 100% -- 100% -- !00% curve. Is this correct?

[DominicD]

Absolutely correct, this is why when in gov mode you should not be above 75-85%.

[BL]

Thanks for the good writeup.

Would you like to talk about motor timing?
I think quite a number of ESC has built-in timing adjustment.

Most common timing are low, mid and high, and some higher end ESC allow us to set it from 0deg to 32deg(I guess).

[DominicD]

Maybe at some future date i will cover some more of the advanced topics. Currently iam finishing up some school stuff then i will do a Gyro 101.

[Pinecone]

I talked to Patrick of Castle at Toledo and he explained the various timing, WPM rate, and such choices very succintly. He said he would put it up on the website.

IIRC Timing is a power versus efficiency issue. Higher timing, more power, but less efficient. So motor gets hotter.

PWM rate I forget. And it was so simple nad made so much sense standing there.

[DominicD]

PWM rate trades off ripple for efficiency. I have no clue about timing. I would love to read what they have to say and maybe summarize it on here.

[r40734]

Hey Pinecone, what site was he going to put it up on? This one or somewhere on castlecreations.com?

[Pinecone]

Castle site. Several of us standing around asked for that, and he said he would. Of course, does he remember?

[Pinecone]

Originally Posted by HFG

PWM rate trades off ripple for efficiency. I have no clue about timing. I would love to read what they have to say and maybe summarize it on here.

That rings the bell. Higher PWM rates are harder on the ESC, but smooth the ripple to the motor, making it more efficient. But ESC efficiency falls.

Timing is higher timing is more power, but less efficiency so more heat.

His recommendation was leave them alone unless you need them. He said that low timing is fine for everyone except those guys doing timed motor runs that are trying to get every last inch of altitude.

As For PWM, he pointed out that Castle's lowest PWM is higher than most other ESCs, and as high as their max. So again, for most uses, on a Castle, stock setting is fine.

[HeliDude36]

I have nowhere near the education, or the tools or software that you guys have. But I started building my own CDrom motors a couple of years ago for my indoor foamie's. I started learning because damage from a crash ruined my best motor. I built motor kits from GoBrushless and SloFly for a while before I started to make my own HyBrid motors. I am able to build motors that out perform $60 off the shelf motors for the foamies.
It is very interesting to read this thread, because all the info I have read, is about what I have been doing for a long while now. It is information that just "worked" but was unknown to me.

Just recently,, I found that the popular Scorpion 2221-8 motor is hard to get hold of... while doing some searching I found a place that is selling a DIY Scorpion 2221-8 kit. I have been wanting to order it,, and wind the motor myself,,, but I also wanted a prebuilt -8 to compair it to inflight after I had finished. Hand wound motors generally end up having more power because of the amount of detail and care put into the windings buy the user who is only building one motor over a couple of days, compared to a company that wants to produce a hundred motors a day.
I think I just might order the DIY Scorpion for fun,, and then compair it to a prebuilt Scorpion after a couple of months of flying. The flight performance being the only tool I would have to determine the results.
The whole point of any hobby is to have fun,,, and building motors is one way I have found to have fun. The tools you guys have, takes that to an elevated level.

[DominicD]

If iam not mistaken all scorpion motors are sold as DIY kits, innov8tive assembles these kits and sells them as complete motors.

[uneverno]

Back when dinosaurs roamed the earth, I used to race electric cars. Brushless motors did not exist (at least for hobby applications) at that time.

I had a case with a dozen or so hand wound motors so I could select a motor based on track conditions. Part of the reason hand wound have more power and greater efficiency is that care is taken to balance the rotors. Machine wound rotors aren't as carefully balanced.

With brushed motors, the different # of coil turns in conjunction with wire gauge was what determined whether a motor was Hi-RPM/Low torque or Low-RPM/Hi torque, so motor "ratings" were typically something like 8/16 or 21/30 where the first number was number of turns.

I'm curious whether or not the same principle applies to brushless. I don't see where any of the mfrs. provide turn/gauge info.

[r40734]

Some of them do and some don't. Usually (but not always) if the motor is simply named as a number of some sort, then it probably has something to do with the design of the particular motor. A lot of them go with the diameter/length/number of turns designation. This means that the Scorpion 2221-8 that was mentioned has a stator that is 22mm in diameter, is 21mm thick, and has 8 winds (or turns) per pole.

Other companies come up with witty little names for their motors and it is up to the consumer to dig around for the specifics of the motor.

Most brushless motors can be opened and rewound or otherwise repaired. There are a few factors that go into determining the Kv and current specifications of the brushless motor. About the only ones of those that we can do anything about is wire gauge, number of winds, and Delta or Wye configuration. If you're really ambitious, you can play around with the number of magnet poles in the can.

[Pinecone]

Axi and clones report number of turns. But of more use to the modeler is the Kv of the motor. So others report that. Of course, having both is nice.

If you want to wind your own motors, Go Brushless has a forum with lots of info and RC Gruops has a vary active section on winding/building your own.

I have wound a few motors and have the parts to wind a few more. I got most of my stuff from Go Brushless or Strong R/C. I noticed that they now carry each others' stuff.

[kgfly]

Some ESC manuals talk about "electrical RPM" particularly with reference to high/medium/low governor modes (eg Hyperion Titan ESCs). I presume that "electrical RPM" has to do with the commutation rate and from the skimpy info in the ESC manuals I *think* that the relationship is:

motor_rpm = 2 x electrical_rpm / number_of_poles

but I cannot explain why.

Can anyone explain the concept of "electrical RPM" and how it relates to the motor shaft RPM ?

[iflymyhelishigh]

Capi, I finally understand brushless motors

[Bill Williams]

When I read that headspeed bogs down and the heli needs more power, I am wondering what needs to change. Do I want a lower KV motor and larger pinion (more torque but less gear leverage which tend to cancel each other)? I guess I am looking for something like a horsepower rating to judge which motors are more powerful for a given voltage. Would watts (volts X amps = watts) be useful here?

[DominicD]

745.6W = 1 horsepower

Wattage is a good sign of the power of a motor. When turning a faster and faster HS the motor consumes more power and thus has less wattage headroom. The higher the HS the less available power the motor has to play with, but here is where things get interesting. Due to the extreme pitch ranges we run there will never be enough power to prevent bogging with extreme pitches, but there is a work around. If the pilot stores up energy in a heavy blade then they can use that energy in the blades without demanding more power from the motor. The heavier the blade the more energy you can store in the blades but longer it takes the motor to build back the HS after a hard maneuver. Depending on your flying style you might prefer a lighter blade that recovers quickly or a heavy blade that will take more to bog initially.

[kgfly]

While not disagreeing at all with HFG's comments, I interpreted your question about bogging a little differently.

What is clear is that to hold speed under increased load the motor has to generate more torque which means it draws more current. More torque at the same speed also means more power output and hence more power input. That means that the motor's power supply (battery+ESC+leads) has to be able to deliver the increased power rapidly and efficiently.

IMO the most common source of bogging is a weak or under-specified battery. If the battery voltage sags under the increased load then not only is the input power to the motor reduced (from what it would have been) but the motor slows down too (due to the lower voltage) reducing the mechanical output power and headspeed.

All batteries sag as load increases due to at least resistive losses, but chemistry and physics come into it too. However, the difference in pack performance can be very significant. Most of the latest generation of premium LiPo packs hold voltage very well if sized appropriately for the application. Discount packs often have higher internal resistance and sag considerably more.

If the pack is performing well under load (which only a flight data recorder (FDR) can tell) then the next most likely cause is lack of headroom in the motor performance. In this case a change of motor/pinion can give good results.

Of course as HFG points out, rotor inertia plays its role as well.