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Im looking at pancake motors, wondering how they work.

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LAN_403

BigBad

Fri Nov 14 2014, 06:07PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

I think pancake motors generally give better torque at low RPMs because the electromagnets are further from the axis. They also often seem to use neodymiums, which again helps give plenty of torque at low rpm.

...

Fri Nov 14 2014, 07:36PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Worth mentioning-
The correct name of the 'pancake' motor shown in post 3 is 'outrunner brushless dc (BLDC)', searching this term may help you find info. (ie,

and

, compared to an 'inrunner' which has the magnets inside the coils. The pancake motor links above are for much more advanced motor designs (not really useful for multirotor designs IMHO)

Outrunners do have a number of advantages, which can mostly be summed up in that they can generate enormous amounts of power for their size/weight, even at modest RPMs, in particular their low RPM performance is better than inrunner BLDC motors. In general however, there is not much of a fundamental difference between an outrunner and inrunner, and it is mostly practical concerns that determine which one will fit your application better. Also, note that it is possible to get outrunners of different dimensions, you can get them in a more normal motor shape like

with powers up to about 10kw.

Ash Small

Fri Nov 14 2014, 08:17PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Ok, so now it's 'pancake outrunner BLDC' as opposed to 'cylindrical outrunner BLDC', or any other type of 'pancake' motor.

In my opinion, I think this is probably ideal for this application (large prop, with large 'disc area' and relatively low RPM).

The prop 'hub' itself could house the neodymiums, as the centre of the prop itself contrubutes very little to lift anyway.

This minimizes overall mass (I think) and results in a very simple design, with lots of torque at low RPM.

EDIT: As I mentioned (often) in the 'other' thread, the trade off is between efficiency and manouuverability, as disc area increases, so lots of torque at low RPM is exactly what's required here, as this will obviously aid manouverability.

Obviously, you don't want to run at 'high torque' all the time, as this adversely affects efficiency, Smaller props have much more losses, and therefore require high torque at high RPM, in order to produce the extra power that is then wasted using a smaller 'disc area'.

Patrick

Fri Nov 14 2014, 09:09PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

well i meant both cases being neodynium brushless, outrunner. that was my bad.

ill start a kick starter soon, and try for 3,000 ish $ for a agricultural / pollution drone.
it looks like ill go with a quad of 26 inch props, for 13 lbs.

Ash Small

Fri Nov 14 2014, 09:38PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

well i meant both cases being neodynium brushless, outrunner. that was my bad.

ill start a kick starter soon, and try for 3,000 ish $ for a agricultural / pollution drone.
it looks like ill go with a quad of 26 inch props, for 13 lbs.

I'd do the VABS simulations first. You may well decide to go a bit bigger than 26".

26" does sound big, but increasing by just a bit will significvantly improve flight time, which was your original priority. I'm not certain exactly where the manouverability will 'fall off', it depends on several factors.

Also, VABS may help decide whether three or four is better, for tha same total disc area. I suspect three for efficiency, and 'maybe' four for manouverability. There are other advantages to three as well, one less motor, for a start, and lower overall weight for another.

Sulaiman

Fri Nov 14 2014, 11:31PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A 'pancake' motor to me is the old flat ironless wire rotor with multiple magnets
(high temperature samarium cobalt) and carbon brushes running off a d.c. supply.
lots of low speed torque but poor power/weight ratio.
Gearboxes are surprisinly inefficient.

my 2 cents worth;
If you're selling to the military then performance is the main criterion,
for agriculture/environment I expect that reliability is more important
so I would urge you to consider the hex-copter approach
as a single prop failure is not catastrophic
and six rotors can be very similar to one large rotor in terms of airflow pattern/efficiency.

I'd aim for the largest heaviest (with payload) copter that is legal/allowed,
your main aim should be for others to find new uses for your dependable platform,
so reliable mechanicals/electronics/communications will in the end be a long term seller.
not glamorous but hopefully profitable.

Ash Small

Fri Nov 14 2014, 11:46PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

for agriculture/environment I expect that reliability is more important
so I would urge you to consider the hex-copter approach
as a single prop failure is not catastrophic
and six rotors can be very similar to one large rotor in terms of airflow pattern/efficiency.

I'd aim for the largest heaviest (with payload) copter that is legal/allowed,
your main aim should be for others to find new uses for your dependable platform,
so reliable mechanicals/electronics/communications will in the end be a long term seller.
not glamorous but hopefully profitable.

Pretty much what I said some time ago, however, I think that now Patrick has the VABS software, he should simulate these things first, before comitting himself. VABS should tell us whether 3, 4 or 6 is best.

By the way, there is a high temp. samarium cobalt magnet factory not far from you, Sulaiman (opposite Chesterfield on the M1) at a place called Barlborough Links. I stopped by there once, chatted to the bloke in the security office, and they brought me out some samples. I may use some in the pickup mechanism for the electronic ign. system I'm working on. They are currently stuck to my fridge

EDIT: My memory's returning, It's Arnold Magnetics UK, but it looks like they may have moved, or maybe this is just a 'head office', or something:

Patrick

Sat Nov 15 2014, 03:47AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i just wanted to so pics of my motors for clarity, the blue one is not really a pancake.

Chris Cristini

Tue Nov 18 2014, 06:47PM

Registered Member #1749 Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497

Well I have never seen this thread and turns out I'm trying to figure out the same thing hmm? Well its like henry ford and mitsubishi I think comming up with the petrol car around the same time but they did not have 4hv at that time haha.

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