Rotor KV on a TP

[dasboata]

Is there a certain KV rotor say on there 40x110 mm motor ,, for a 2080-2200?1700?etc ,, thanks


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[donhuff]

I don't quite understand your question "KV rotor"

The rotors are exactly the same for all the 40mm diameter motors, except for the length of the different sizes, ei 40 50 60 70. And BTW that number is the actual length of the rotor magnets.

What determines the different KVs is how many times the wire goes through the slots, making a loop/turn (coil) in the stator.

[dasboata]

I don't quite understand your question "KV rotor"

The rotors are exactly the same for all the 40mm diameter motors, except for the length of the different sizes, ei 40 50 60 70. And BTW that number is the actual length of the rotor magnets.

What determines the different KVs is how many times the wire goes through the slots, making a loop/turn (coil) in the stator.

Hi Don I think you answered me by telling me the rotors are all the same KV on the 40MM motors as long as so is the lengths thank you !

[donhuff]

??? your reply still makes me think you don't understand it just right.

The rotor has nothing to do with kv. the number of turns, of the wire, going through the stator, is the only thing that determines the kv.

[dasboata]

??? your reply still makes me think you don't understand it just right.

The rotor has nothing to do with kv. the number of turns, of the wire, going through the stator, is the only thing that determines the kv.

I understand, the KV is determined by the winding,,,,, not the rotor Don thank you

[Alfa Spirit]

Woow, very interesting ..... LOL

[donhuff]

I wasn't trying to be a smarty pants. It's just that your wording sez "the rotors are all the same KV on the 40MM motors".

The rotor CAN have an effect on the kv of a motor. But it is a big job to alter the rotor . A smaller rotor will raise the kv, IF, the stators rotor hole stays the same. A larger rotor will lower the kv. A tighter air gap between stator/rotor will lower kv, AND increase torque a lot, a whole lot! Stronger magnets will lower kv and increase torque, and so weaker ones will raise kv.

And that last one is a scenario that all of us have experienced, but most of us didn't know what was happening. When neodiddyum magnets (that's what I call them) get hot, they lose some of their strength. This raises the kv of your now hot motor. It probably got hot in the first place because let's say you were using a prop one size to big for the power of the motor at the rpm it is turning (KV). So NOW, with the hot magnets your KV increases, and within a second your prop is 2 sizes to big! Heat is starting to build rapidly at this point. Give it a few more seconds and your 3 sizes to big!!! It doesn't take but a very few seconds running like this and you will reach the temp where the wire insulation breaks down and wires short out.

I bet that this is one of the reasons that NEU motors use samarium cobalt magnets. Even tho they are a little weaker than Neo magnets, they do not lose but very little strength when they get hot. But NEU makes up for the lower strength by using BIG honking magnet in all their motors.

[RaceMechaniX]

Hey Don,

I don't think Neu use SmCo magnets unless they recently changed magnets. Based on the Gauss measurements I have taken on 15 and 22 series rotors, they are beyond the magnetic field strength of SmCo magnets even the best ones I test regularly. They might be using a lower grade N42 or N45 magnet instead of N52 which is pretty common. As you point out heat has a lot to due with performance and it is why it's so important to keep the motor temps (both stator and rotor) are reasonably levels under 200DegF or 90DegC. This chart below the different grades or Neo and SmCo magnets versus their performance at temperature.

NeoEnvelope.jpg

[T.S.Davis]

When neodiddyum magnets

Please tell me I'm not the only one reading that in Bill Murrays voice. Neeeeo ditty diddydum diddy doo. RrrrrAZZZLE DAZZZLE!

We don't always get rain at the Michigan Cup race been when we do.....we make Don dance in it. There may well be singing this time.

[donhuff]

You would know better than I do Tyler. I thought that they were the cobalts, based on the way they look. They sure do not look like all the other neos that I have seen. Square corners and black coating, not the nickel coating like most neos.

I read up on the neos that most manufacturers use, and seems like the normal was N38s. Reason for doing so was that the 52s while the strongest, they lost power with heat quicker than the lower powered ones do. And the 38s were about the best comproise.

[donhuff]

??????? did I dance in the rain? It must have been after Miller time had started!

Yeah, you don't want to hear me sing. That's not pretty.

[RaceMechaniX]

Nothing beats showing some examples and differences so here we go. For each rotor I find the strongest north or south pole and hold the Gauss meter for the pic. The meter will also show the peak and measured value. First rotor is a Neu 1521, it shows 3624G

Neu 1521 rotor.jpg

Next rotor is a Aquacraft 2030 6-pole rotor. It measured an impressive 4441G max

AQ 6-pole rotor.jpg

Next rotor is a Lehner 2240. It measured 5228G max.

Lehner 2240.jpg

Next rotor is a Lehner 2280. It measured 4215G Max.

Lehner 2280.jpg

[RaceMechaniX]

Biggest rotor I have is a Lehner 3080. It measured 5614G max.

Lehner 3080.jpg

The last rotor is a special Titanium sleeved 2-pole rotor designed to run up to 150,000RPM. This has a SmCo Grade 28 magnet. It measures quite a bit lower around 2200G. Using the latest Grade 35 SmCo magnets you can measure around 3000G through a thick containment sleeve.

Ti Rotor.jpg

This particular rotor will produce about 12kW continuously above 120kRPM and operating at 180DegC. Very HOT!

So it's plausible that Neu are using very high grade SmCo magnets but I suspect they are using lower grade Neo magnet's.

The coating on the magnets is typical for rust prevention and in some high end assemblies to reduce the eddy current losses on the skin. Nickel is very popular, but there are quite a few types of coatings.

-Tyler

[donhuff]

So, it looks like "size matters" (pun intended) or actually mass, on the gauss readings? Since the biggest rotor, and I assume the largest magnets too, has the largest reading on the scale. And with that, I guess to get truly accurate readings for an honest comparison you'd have to have all the magnets the same size.

And one thing for sure on the Neu magnets, they are thick. About 2 to 2.5 times thicker than a typical TP or Leopard or most any China motor.

One thing I have noticed when test a motor after rewinding it is that the kv will creep up as the motor warms up. And if I hold one until the point where I can no longer stand the pain of the heat. The kv will have gone up maybe 50 to 100 rpm. Then yesterday I rewound a Neu motor and test ran it and held on to it, because of this thread, until to hot to hold again. And the kv never varied more than 5 or 15 rpm, and actually went down instead of up! There is something different inside of the Neu motors.

12kw (16HP)... 120,000 rpm... 356*F sounds like an electric turbo motor. And that rotor doesn't look to be all that big.

[RaceMechaniX]

Size and shape definitely matter. All of the Lehner inrunners use segmented disc magnets that are bonded in the unmagnetized state, ground, balanced and lastly magnetized. All of the 4-pole and higher pole count motors use pre-magnetized segments that are bonded in place. That's one of the big differences between most of the 2-pole rotors and 4+ pole motors. The two poles are assembled unmagnetized and often use a disc or tubular magnet with thick cross-section. The 2-poles often will not have any "iron" or steel underneath the magnet, they are simply bonded to the shaft or a thin spindle. The 4+ pole motors use a steel core to which the magnets are bonded to. This helps route the magnetic flux through the core of the shaft. 2 pole rotors are fairly straight forward to magnetize. They are put in a fixture surrounded by a large copper coil and "zapped" with a lot of energy from a stored capacitor bank which produces a very strong magnetic field in one direction. The processes can be repeated in till the magnet reaches saturation. Magnetizing a 4+ pole motor is very difficult in the assembled state as it requires a much more complicated coil assembly. So typical process is to magnetize the tiles in a batch all with the same orientation and then assemble the tiles onto the rotor core. The magnetic field produced by a singe tile in a 4+ pole motor is very unidirectional. i.e. the field lines all point in the same direction. If you mapped the field strength by rotating the rotor one turn you would see a nice sinus curve with a 2-pole. A 4+ pole rotor would generate a field diagram closer to a square wave with rounded corners.

Have a look at the field strength of the AQ motor, those have very powerful magnetic tiles and pretty thin. The cross-section of the magnet does help as does a good yoke underneath the magnets.

And yes, that Ti rotor is from an eTurbo. The rotor is not that big maybe 28mm x 50mm so similar to a 1521, but you have to remember power=torque x speed. So triple the speed for the same torque and you get quite a bit more power. Obviously there are losses and a lot of heat to get rid of but that's a whole different discussion.