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I am creating follow focus system (motor with gears that moves camera lens ring).

Vast majority of such systems use low Kv high torque BLDC motors - similar to the ones used in gimbals. I think it is mostly because higher Kv motors require gearing to provide enough torque which introduce noise.

I bought such module from FeyiuTech to reverse engineer it. The BLDC motor is very small (28x8mm stator) but has 5ohms resistance and about 150Kv. At the same time It is impressively small. It has no symbols.

From my research the torque approximation can be calculated as follows (simplified):

T = I/Kv = (U/R)/Kv

Therefore my understanding is I need to buy motor with as small resistance and Kv as possible. The best motor I could find was emax 87Kv. It has 6.5omh resistance and 87Kv so it should have more torque at the same voltage compared to 5ohm 150Kv FeyiuTech, right? I measured it and turns out it is actually slightly lower. Dissapointing. On top of that it is twice as big as FeyiuTech.

My questions are therefore:

  1. Why would 150Kv 5ohm motor have more torque than 87Kv 6.5ohm?
  2. Where can I buy such motor? I could not find similar motors anywhere. They are either too big or have insufficient ratings. Where do companies get such custom motors?
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4 Answers 4

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Why would 150Kv 5ohm motor have more torque than 87Kv 6.5ohm?

As your research showed, Kt (torque constant) is the inverse of Kv (velocity constant). Therefore a 150Kv motor should have 150/87 = 1.72 times less torque than an 87kV motor at the same current. However at 5 Ω vs 6.5 Ω it should draw 6.5/5 = 1.3 times more current at stall, so at the same voltage it should have 1.72 / 1.3 = 1.32 times less torque, or 76% of the 87Kv motor's torque.

I measured it and turns out it is actually slightly lower. Dissapointing.

There could be several reasons for that, eg. Kv specification error, torque ripple (varies with rotor angle), resistance error, different drive voltage or waveform, torque measurement error. With only 24% difference it wouldn't take much to wipe out that theoretical advantage (some of the small 'hobby' motors I have tested had measured Kv up to 18% higher or lower than spec).

On top of that it is twice as big as FeyiuTech.

One problem with gimbal motors is that they 'run' at 0% efficiency, so all the power they use has to be dissipated as heat. A physically larger motor should stay cooler and maintain full torque better because the windings don't heat up as much (copper has a positive temperature coefficient, so resistance increases and maximum torque decreases as the temperature rises). If the windings get too hot they may burn off their insulation and short out, or cook the magnets and reduce them to junk.

A motor with a 28 mm diameter stator probably won't be able to handle nearly as much continuous power as one with a 40 mm stator. On top of that the FeyiuTech is (theoretically) consuming 30% more power. At 11.1V it should be drawing ~25 Watts, which is a lot for a small motor.

I could not find similar motors anywhere. They are either too big or have insufficient ratings.

Probably because your motor is actually insufficiently rated for what you are expecting it to do. There is only so much that can be done to get more torque out of a motor. More turns produces more magnetic force, but increases resistance. More poles reduce the magnetic distance, but makes the stator arms smaller so they can't take as many turns. Gimbal motors try to have the greatest number of poles practicable. After that the main limiting factor is power dissipation, so the ultimate solution for more torque is a larger motor.

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Why would 150Kv 5ohm motor have more torque than 87kV 6.5ohm?

Well, the 87Kv motor has got more resistance, and, for a given supply voltage, the current must be less than the 5 ohm version and therefore, the torque would be naturally less. Torque is proportional to current taken. More torque means more current taken. Measure the current and then compare torques.

Where can I buy such motor?

That's an off-topic question but I wish you luck on your quest.

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    \$\begingroup\$ Torque is proportional to current * torque constant, (Which is inversely proportional to the speed constant Kv). Thus each motor ought to produce torque V/R/Kv (all in SI units) and the R*Kv product is higher (lower torque) in the 150Kv motor. So, something else is going on ... datasheet lies? frictional losses? \$\endgroup\$
    – user16324
    Jan 19, 2020 at 16:24
  • \$\begingroup\$ @BrianDrummond - Thanks for the tip. I tested 87Kv motor and the resistance is correct (6.5ohm). Maybe it is friction losses. 87Kv is twice as heavy too. \$\endgroup\$ Jan 19, 2020 at 17:30
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150Kv means 150 rpm/V. 87Kv means 87 rpm/v . It's a voltage constant of electric motor. The torque constant Kt[Nm/A], is also important , it tells you how many torque the motor will output for certain amount of current. These two constants (voltage and torque), like mentioned here in this answer, with their respective units, are inversely proportional. Higher Kv means lower torque per ampere.

It shall be noted, that these constants are regardless of the motor size.

The nominal torque is a product of nominal current and Kt. The nominal current is object to motor size and cooling capability.

Why would 150Kv 5ohm motor have more torque than 87Kv 6.5ohm?

This could be partially true, when you have measured the torque at standstill. You need to increase voltage, so that current is the same in both motors, to be able to determine the correct Kt.

Where can I buy such motor? I could not find similar motors anywhere. They are either too big or have insufficient ratings. Where do companies get such custom motors?

Have no idea, but I would advise you to buy only motors that have a datasheet. Here is an example of one (Courtesy of Ali express): enter image description here See, the most valuable information is nominal current, Kv,Kt, inertia, max voltage, max current.

Sample image of the motor:

enter image description here

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Basically, if you have a motor that is the right size, physically and power-wise, but you want to optimize it to put out more torque at lower current, you would rewind the stator with more turns of smaller diameter wire. This would lower the Kv of the motor. The stator resistance will go up also (longer and narrower wire will have more resistance).

In a given motor, there is only so much room for copper wire. Regardless of wire diameter, you will always want to fill all available space with copper to keep current density in the wire as low as possible. Thus changing the Kv will not have much affect on motor efficiency, assuming the total copper in the winding is about the same. Obviously there will be limits to how far you can go, because at some point, the wire diameter will become too small to be practical.

By itself, Kv does not tell you much about a motor. You can build very different motors that have the same Kv. But if you have two motors with identical rotors, identical stator geometry and identical copper fill factor, then comparing different Kv tells you a lot.

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