BLDC motor - Torque and speed... How they behave ?

[giovaniluigi]

Hey guys,

This question is for electric motor experts:



I'm here with a BLDC motor. (No brand... Sorry)
The motor is rated for 100W.
It is a 4 pole (2 pairs) rotor with 12 teeth stator.
The nominal voltage is 24V.
The no load speed is 8000 RPM
The nominal speed is about 6000 RPM.
The magnets are ferrite.
WYE coil connection.
Hall sensor commutation.

I developed the controller with a PID closed loop to keep constant SPEED.
The power stage uses 3 Half bridges made with 6 N-channel MOSFETS. I'm using gate drivers and PWM @ 20000 KHz.

Who wants more details about FETs: https://www.mouser.com/ds/2/149/FDD13AN06A0-103315.pdf

So here we go...

1. First question: At higher speeds, the motor should produce higher Back EMF. This Back EMF will affect the torque ? If yes, I assume that this Back EMF is going through the diode inside the FET, so how it can affect the motor torque ?
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2. Second question: Assuming that I change the magnets to neodymium, increasing the motor efficiency, using a stator with same diameter, same number of turns per coil but higher lenght will result in more torque ? One guy said "yes", because with a larger (in lenght) stator I'll have more magnetic flux, thus having more torque. Of course, assuming that I'll compensate the higher lenght of wire to keep it with same current as before.
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3. As the torque is proportional to the motor current, how the size of the motor affects the torque ? Probably there are many things to consider, but let's suppose that you're challenged to explain in a quick way, how would you explain the impact of motor stator and rotor to the torque and speed ?
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4. Following the same question above, increasing the motor size in 50% of diameter, would result in the same thing than increasing in 50% in lenght ? Assuming that the coils are going to be adjusted to keep same current at the nominal voltage.

Please, be nice :-D

I'm here to learn and if I wrote something stupid I'm sorry.
If the answer for some question is too relative and depends of many parameters, please try to explain me the basic about it, just to know how it works and then I'll be able to filter stupid ideas.

Thanks in advance.