3 Phase BLDC motor sizing

A motor question:

So, I asked some motor guys for a size of a 15W 3 phase BLDC motor

They came back with a 70mm diameter size motor

Then, did some googling and came up with this 35mm dia motor:

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I would like to understand the reason for the size difference. Any rules of thumb to calculate the size of a 15W motor running at about 4000 RPM?

Thanks

Klaus

Reply to
Klaus Kragelund
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What torque did you specify from "Them"? Note that the power will be proportional to torque times RPM.

Reply to
John S

Without seeing the 75mm motor it's hard to say what they may have been thinking. Most motors, brushless included, are mostly limited in continuous operation by the heat generated by the armature windings and the wear on the bearings. The armature dissipation limits current, while the bearings limit speed. In some applications you can significantly reduce the size of the motor by providing adequate airflow and, in the case of conventional brushless motors, heat sinking to the case.

Just for chuckles, consider a model airplane motor that's about 20 times smaller, yet is rated for up to 55 watts (but not at 4000 RPM).

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motor-2200kv-eflm1130

It works because it spins faster, because it's hanging out in a strong airstream, because it's for intermittent use, because pilots are willing to do maintenance on the airplane far more often per hour of running time than a copier owner would, etc..

So with adequate cooling and a scheduled maintenance program, you can get quite a bit smaller than the motor you found.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

could be speed.

motor torque scales with volume.

power is speed times torque,

(torque aslo scales with the strength of the magnets, B could have stronger magnets)

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Reply to
Jasen Betts

15W is not a lot of power. Check out Pittman 4444 series, they are NEMA 17 size, I think, so 43 mm dia. Actually, seems they are smaller than that, I'd think closer to 30mm. Depending on the stack length, you can certainly get 15 W out of one.

Jon

Reply to
Jon Elson

The Pittman motors are nice. I will give it a try. Thanks :-)

Reply to
Klaus Kragelund

Yes, but I am comparing 4500 RPM motors in both cases

True. AFAIK the calculation was base on strong rare earth magnets

Guess I need to get information on the design constraints

Cheers

Klaus

Reply to
Klaus Kragelund

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zwgearbox

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