Miles said:
I thought it would be good to analyse a reasonably simple case - that we can swap speed for torque and keep motor weight constant. Is it not relevant as a first step? Any other discussion can carry on on the original thread......
Ok.
My understanding of motor design is that power production is a function of active area and magnet speed. Active area gives torque. Magnet speed supplies the necessary angular speed to produce power.
Bigger diameter: more magnet speed - more power.
Bigger active area: More torque
As in: more RPM -> more magnet speed -> more power. As in: Wider stator and magnets -> more torque -> more power.
Proposed case: Take one motor. Change one parameter, change everything else - does the results match up?
I'd like to entertain this, but I am not able to do the FEMM analysis - so I'll start with a proposed method.
1. Find a suitable hub motor.
2. Design another hub motor with different parameters, but same performance.
3. What material is needed to achieve this?
4. Does material/performance match?
A test:
1. Selecting Crystalyte HS3540 ish.
Active area: ~220mm diameter, Stator height: 40mm. Total active area ~27 646 square mm.
Max RPM: 350 RPM
Keep RPM constant.
2. New hub motor. Slimline rimalichious hubmotor. Diameter 550 mm. The question now is the new active area needed to match performance?
taking regard to T=k*D^2*L. T=Torque, k = constant, D=Diameter, L= Length; I'm going to take a (wild) guess at what new active area is needed.
550/220=2,5.
2,5^2=6,25.
27646/6,25 = 4400.
New active area: 4400 square mm.
Motor magnets and stator width: 4400/(550*pi) = 2,5 mm
Remember RPM is constant. As the magnets in the new design has a 2,5 better lever arm than the HS3540 - only 0,4x the force must be generated by the magnets.
3 and 4. "What material is needed to achieve this?" and "Does material/performance match?"
How thick must the magnets be?
How thick must the stator be?
How much copper is needed?
Will the simulation even match the original motor performance?
My intuition tells me the calculations in nr 2 is wrong - but from my moderate experience with BL motors i think that my intuition is wrong. (My intuition tells me new motor active area should be divided by 2,5). Would be cool if someone would entertain the groundwork in this post and finding the fault(s).