The perfect ebike motor!?

[CNCAddict]

Hey guys. I'm working on a new inrunner motor design for high powered electric bikes. This will be a suggestion thread. Some things we need to settle on are...

Motor OD (80mm stator, 86mm-88mm case)
Motor Length (40mm short motor to ??? for long motor)
Shaft Length (30-40mm)
Shaft OD (12mm)
Motor Kv (~130, ~200 versions)
Max RPM (15,000)

Motor OD is the most critical since it's very expensive to make a new die set. The other parameters can be changed after the fact without much trouble. I'm also planning on making the motor with a stub shaft at the back for an encoder. This way the motor will work with any drive you can put your hands on.

Response to http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=7180&p=121900#p121900
This motor will be way more powerful than the Maxcim at a smaller OD and bit more weight. The gearbox is just an idea...nothing too serious. Also, 100mm OD might be possible, but a short large diameter motor is not the best configuration for a radial flux motor. They really need to be close to square (stator length=diameter) to keep things efficient. Also, I want to try to keep the weight pretty low, we're using some high end materials on these motors so costs will start getting out of hand. It's better to use steeper gearing with a small motor than go with a huge motor with low gear ratio.

http://www.xeramotors.com <-my webpage...has a few pictures of my 6050 motor..one of 2 that will ever be made

 

[Drunkskunk]

Just curious why an in runner? the in runner design is generaly higher RPM than the equivilent outrunner, meaning it needs to be geard down more.

 

[recumpence]

80 to 90mm OD
2.5 inch can length
1.5 inch shaft length for an overall 4 inch motor length
8mm shaft OD
120 to 160 KV



Matt

 

[CNCAddict]

-outrunners can't be sealed effectively and have good cooling at the same time
-Inrunners have a higher natural frequency in the rotor dynamics so they are much less susceptible to damaging vibrations no matter what the rpm
-It's impossible to add an encoder to an outrunner without a complex mounting system that's prone to failure
-an inrunner can have just as much torque as the outrunner...the dimensions might be a bit different however.


Matt, thanks for the suggestion. I was hoping for a longer length...I may have to go to a very high pole count to get the power up on a design like this...that might increase costs and the no load current a lot. Going to an Axial flux motor like the mars motor would make this possible...but they have their own set of problems.

P.S. 8mm shaft might not like the torque output that much. I was thinking 12mm??

 

[Miles]

I think I'd go for an even larger diameter - something like 100mm dia, 65mm length, 10mm shaft.

 

[GGoodrum]

Just curious why an in runner? the in runner design is generaly higher RPM than the equivilent outrunner, meaning it needs to be geard down more.

That is not really true. Most existing inrunners have small diameters, which is what limits torque. Steve Neu has done largeer diamete inrunners, with as many as 8-poles. These were very torque-y, and had similar lower kVs, like an outrunner. Even Steve's largest series, the 2200s, which are very powerful, only have 2.2" stators, so something in the 80-90mm range, or about 3.5", will mean these are going to be real beasts. We are talking better than Etek/Mars-like power levels, in a fraction of the form factor and weight.

My "dream" specs pretty much mirror Matt's, but I think the more poles, the better, like 8 or 12 maybe, and I'd like the kV closer to 100-120, if possible. I'm thinking with Matt's eDrive, it would be better to keep the ratio down to something like 3-4:1, so that we can use a larger sprocket on the motor. Like my NM 2215/P62 setup, this beast will probably have too much torque for an HTD belt drive, so the chain will be better. The bigger the motor sprocket, the more motor torque it can handle. Anyway, with a lower kV, the single stage will work well, I think.

-- Gary

 

[GGoodrum]

I think I'd go for an even larger diameter - something like 100mm dia, 65mm length, 10mm shaft.

There's lots of motor sprocket choices if an 8mm shaft is used, but I suppose these can be drilled out to 10mm easy enough.

 

[Miles]

There's lots of motor sprocket choices if an 8mm shaft is used...

I'd say 8mm was a bit on the small side - 12mm would preclude some of the small 5M pulleys, though.

 

[CNCAddict]

the external shaft size can always be made smaller...but it can't easily go bigger without some redneck engineering. So if I go with a 12mm internal shaft...anything smaller than that is ok.

 

[GGoodrum]

the external shaft size can always be made smaller...but it can't easily go bigger without some redneck engineering. So if I go with a 12mm internal shaft...anything smaller than that is ok.

Yep, much easier going that way.

How many poles are you thinking?

 

[CNCAddict]

I was going with an 8 pole for small diameters...but if you guys want a pancake motor..then that's not going to be best. I was also going with a distributed wind motor...but I may do a concentrated pole setup for such a flat motor to reduce copper losses.

 

[GGoodrum]

Well, we don't have to go to extremes. If a 90mm 8-pole motor will work for some of your optimization ideas, what sort of length do think is best? We should start with what works more efficiently, and is easy enough to fabricate that won't excessively to the costs

Also, do you have idea about what kVs might be doable, or is this putting the cart before the horse.

 

[CNCAddict]

Right now my simulation is showing right at 200rpm/v with the 2y winding and an 80x60 stator. The motor needs another 10mm or so on each end so the external dimensions would be close to 80x80 or a bit longer. I can also do a 3y winding for 130rpm/v.

This motor would have a weight of around 4lb which is getting on the high side. But power should be spectacular with a winding resistance of about 0.006ohm for the 2 turn.

 

[Miles]

Possibility for a 40mm stator length option, at higher kV?

 

[GGoodrum]

Actually, that is not bad at all. 80mm long is only about 15-16mm longer than Matt's "ideal" can length. I could certainly live with 80mm.

Even at 4 pounds, this will be a ton lighter than anything with anywhere near as much power potential. I can live with the weight.

The 3Y sounds good, at a kV of 130, but if there was something like a 3.5Y, or maybe a slightly larger diameter, that might get the kV even lower, I'd be very happy. I can certainly live with 130, though.

 

[CNCAddict]

Hmm, I can maybe do a 3.5y winding depending on my winding method. I haven't built the motor yet...and I'm most familiar with winding outrunners...so I don't want to make any promises. I can also add 10mm onto the motor and it would drop the kv to 110rpm/v for the 3y 80x70.

 

[etard]

I don't think weight (1-2 lbs.) is that much of an issue, especially if you can do away with the extra stage of reduction that smaller motors require. I would rather spend more money on a motor rather than deal with the hassle, space, complexity of a 2 stage drive. I think a 10mm shaft should be sufficient while the length of the motor would be the greatest concern to most ebikers, cuz alot of us would like to be able to fit the motor between our pedal stroke, or inside the mainframe triangle. I am not sure what you mean by the stub coming out the back, what will that be used for? Can you put a hall sensor on it?

 

[GGoodrum]

Hmm, I can maybe do a 3.5y winding depending on my winding method. I haven't built the motor yet...and I'm most familiar with winding outrunners...so I don't want to make any promises. I can also add 10mm onto the motor and it would drop the kv to 110rpm/v for the 3y 80x70.

Okay, that's great. I'd definitely trade a bit longer length for a lower kV. That will add to the torque as well, I think, right? A kV of 110 would be great for a single stage setup, to a driven crank.

 

[John in CR]

Since you put ebike in the title, then whatever you need to do to get half the power of an Etek or Mars into a package that is 1/3 to 1/4 the weight, while maintaining the efficiency and durability. eg something about like the little Kollmorgens, but which can handle 1.5kw continuous and won't fly apart at 60v.

John

 

[CNCAddict]

Possibility for a 40mm stator length option, at higher kV?

Sure, I can make a short lower powered motor no problem.

alot of us would like to be able to fit the motor between our pedal stroke, or inside the mainframe triangle. I am not sure what you mean by the stub coming out the back, what will that be used for? Can you put a hall sensor on it?

Hmmm, a short motor won't have the same torque as a long one. So doing a short motor combined with a single stage reduction would work fine, but would be alot less "aggressive" than the high reductions. The stub shaft could be an encoder. I am fiddling with an idea to make the shaft easily replaceable (a first for inrunners), so that would make the stub shaft optional.

Since you put ebike in the title, then whatever you need to do to get half the power of an Etek or Mars into a package that is 1/3 to 1/4 the weight, while maintaining the efficiency and durability. eg something about like the little Kollmorgens, but which can handle 1.5kw continuous and won't fly apart at 60v.

Yeah, it will definitely do all of that with ease.

 

[fechter]

Looks like a 90 degree gear reducer would be the hot ticket. Motor can be amost any length.

 

[MitchJi]

Hi David,

I really like the inrunner idea.

My wish list is:

• 
  1. Works with Matt's Drive
  
  2. Quiet - (First Stage Belt)
  
  3. 3kw continuos (12s)
  
  4. Lightweight

Single stage would be nice but this is not a big deal. The extra cost, weight and space (not very much) required by Matt's 2 stage isn't a big deal (to me).

How big an efficiency hit and how noisy would 90 degree gear reducer be?


EDIT ADDITION: Would a 90 degree 2:1, 3:1 or 4:1 reducer be compatible with Matt's drive with a first stage belt or would the power be too much?

A reducer of 2:1-4:1 and a kv of 200 would be equivalent to a kv of 50 or 100. If the efficiency and noise are not issues that could be a good match for the single stage version of Matt's drive.


Not working with Matt's drive, a big efficiency hit or substantial additional noise would probably rule out the motor for me.

Maybe one diameter with more than one length/ kv option is the best way to satisfy the most people.

 

[safe]

The lower the KV the better.

It would be great to be in the 100 to 200 range and the higher the efficiency the better (of course).

-------------------------------------------------------------------------

A Delta to Star 2 Speed?

RPM per Volt
Remember in last month’s article, we referred to back-emf, being the voltage induced in the winding when a motor spins. Well this parameter rears its head once more. How you wind a motor will determine how fast it will spin and what battery pack voltage we will need to get a certain level of performance.

If you take a motor with no load attached, and apply full throttle at say 6 volts, the motor will spin up to a maximum RPM. If we are able to measure this RPM and divide it by the pack voltage, we will arrive at a figure called RPM per Volt. We could then determine how fast the motor will spin for any applied voltage.

So say our motor spins at 8000 RPM at 6 Volts.

8000 / 6 = 1333 RPM per Volt

Therefore, for a pack voltage of 10 Volts, the motor will spin to 13330 RPM.

This figure helps us determine what our motor is capable of, and whether it is suited to a particular application.

If we require a motor for a ducted fan model, then we need a motor that has a high RPM/V value. A ducted fan rotor is designed to move air at high velocity, and can only do this when spinning at high RPM.

For a 3D type aircraft, we would need a motor that can provide a fair amount of torque in order to swing the large propeller required to generate a good amount of static thrust. This would typically be a motor designed for a low RPM/V.

What we must remember though, as you load the motor, the predicted RPM figure will drop off.

Getting back to Delta and Star connections, there is a relationship between the two configurations and the RPM/Volt value. If you wire up the motor for a Star connection and measure the RPM, you can calculate what the RPM/V will be for the Delta connection and vise versa.

Converting from Star to Delta hookup, the RPM/Volt value is multiplied by 1.73.

From Delta to Star hookup, multiply the RMP/Volt value by 0.578.

Basically, you are left with a method of manipulating the motor performance by simply changing the wiring scheme. Some motor builders have gone so far as to route all six wires from the motor to a connection block that would allow them to alter the wiring scheme should they wish it.

So, how do you determine / calculate the RPM per volt value before winding a motor?

For given stator sizes and stack thickness, there are some programmes that will calculate the number of turns required to achieve certain target figures. Most of the time though, we just wind on as many turns as we can and measure the motor parameters afterwards. Using the measured figures, you can then work out what you need to do to achieve a specific goal. It is pretty much experimentation and acquiring a feel for what is going on.

The magnet strength and configuration also contribute to the RPM /volt value.

As a guideline, here are some basic points to consider —

The higher the number of turns on a tooth, the greater the magnetic field produced for a given current. Stronger magnet field results in more torque and lowers the RPM/Volt number.

For higher RPM values, fewer turns are required but results in less torque being available. The lost torque can be recovered though by pushing more current through the motor to increase the field strength.

Thicker stator packs help reduce the RPM/Volt values while thinner ones do the opposite.

http://www.gobrushless.com/kb/index.php?title=Brushless_101_-_Chapter_2

Anyone ever tried this?

(how might the controller deal with shifting the wires like this?)

 

[safe]

Inductance Motor?

The "conventional wisdom" has been that small Inductance motors are inefficient. But that may or may not be completely true in all cases. What if you applied the same high rpm mindset that you use with these RC motors and produce an Induction motor?

The advantage is that Induction motors have their highest efficiency when under load which is perfect for ebikes. And given that the laws will likely crack down on anything producing over 750 watts (can't sell it) it would be great to produce the smallest, lightest, Inductance motor that produces EXACTLY 750 watts of power from near zero rpm all the way up to some redline.

It's just a thought.... but you did ask about:

"What is the perfect ebike motor?"

...the perfect motor doesn't have the powerbands like the brushed and brushless motors do. :wink:

Otherwise, you probably just want affirmation about some parameters doing things the way you are doing and if so then "keep up the good work".

 

[johnrobholmes]

A long time ago, aveox had a 6 phase controller and motor combo for gliders. Climb up in delta termination, glide around in Wye termination. It isn't anything new to have all 6 phase ends available outside the motor, but generally is not practical for the extra complexity once you have your vehicle geardown and voltage selected. The biggest hardship is getting a controller made, or getting a (large) switch setup to change it up while riding.


I applaud your choice on an inrunner design. I think that Matts size suggestions will be the best to work with since he is producing the first gearbox system. I would choose a diameter that allowed a short can with 3kw power, and keep between the pedals. Then you can always increase the length of motor for power junkies. A 100mm diameter motor would be spiffy, but a 150mm diameter motor could still mount on most bikes.


KV, would really depend on the setup. Something around 130 that can produce great power on 12s lipo would be ideal really. Get enough poles in there and it would be pretty easy to hit. This would give us 30mph with a 16:1 reduction, easy to hit in two compact stages.