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From information on the Internet I understood several things:

  1. Low KV motors are better for gimbals. For lower KV thinner wire is used with more turns.

  2. Outrunner motors have more torque than inrunners.

  3. More poles give smooter rotation and finer resolution.

  4. PMSM motors are driven by sinusoidal signal and therefore are rotating smoother than BLDC.

Please correct me if I am wrong.

So, now, this is a medium sized (60mm OD) motor from a gimbal which has very high torque. The wire is thick, its inrunner, has low pole count. All these things oppose the information above. What is going on here?

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  1. largely true.
  2. Often true but not universal.
  3. Somewhat true but pole shapes and drive waveform also matter.
  4. There's not much physical difference in construction betweet PMSM and BLDC. If any, it's likely to be shapes of the pole pieces.

Now what's going on with that motor?

  1. That's pretty thin wire and quite a high turn count. Moreover with 12 coils arranged as 3 sets of 4, you can get a 4:1 variation in Kv and 16:1 variation in winding resistance by connecting each set of coils either in parallel or series. I expect these are connected in series.
  2. Given the same number and size/shape of poles, the same gap, the same magnet strength, and the same winding, you wouldn't expect any difference between inrunner and outrunner.
    The outrunner version of this motor could be more compact : turn the magnetic circuit inside out and the bulky windings would then be inside and the thin ring of magnets would be outside.
    However that would leave little or no space inside the motor for an axle and decent bearings. Looks like you could stick your thumb through that space. So I guess they chose the inrunner for mechanical reasons.

  3. 12 stator poles and presumably 14 rotor poles is pretty high, it's a common outrunner configuration.

  4. For camera gimbals you probably need closed loop positioning anyway, which will compensate for lack of smoothness.

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  • \$\begingroup\$ Do you think BLDC can be driven by sinusoidal signal like PMSM? \$\endgroup\$ – Roman Simonyan Oct 8 '16 at 12:31
  • \$\begingroup\$ Yes. However it may not give a complete absence of cogging torque, depending on the pole piece shapes. \$\endgroup\$ – Brian Drummond Oct 8 '16 at 12:33
  • \$\begingroup\$ What about the impedance of windings? Lower it will be, more current will flow and more torque can be created for a given size? \$\endgroup\$ – Roman Simonyan Oct 8 '16 at 12:56
  • \$\begingroup\$ This motor has less than 1 ohm, but the available motors for gimbals have 10-20 ohms. \$\endgroup\$ – Roman Simonyan Oct 8 '16 at 12:58
  • \$\begingroup\$ No, it doesn't magically increase the torque. More current will flow ... but through fewer turns, so the magnetic field and torque are the same, you just need less voltage. However you wind a motor, Kv*Kt=1 (ignoring power losses) - you can wind it to "gear" it to the voltages and currents you want to use. \$\endgroup\$ – Brian Drummond Oct 8 '16 at 13:28
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  • In any case , the servo motor torque needs to significantly exceed the inertia forces to attenuate vibration.
  • The inertial acceleration will be a fraction of gravitational acceleration or mass of the load , if balanced.
  • the analogy is an audio woofer amplifier must have a much lower output impedance than the load.
  • in this case bidirectional PWM is used, somewhat like a buck-boost SMPS
    • often called, "micro-stepping" between poles, such as in steppers
  • output impedance open loop is the winding resistance and closed loop is reduced by the amount of gain in the servo feedback (30~300) and thus the amount of power consumed from correcting disturbances, too much and it amplifies noise
    • This ratio ~ Zout/Zload is called "load regulation error" in power supplies and in audio, which like servos, use linear feedback
    • the inverse ~ Load/Source is called Dampening Factor, so for smooth bass, we expect a dampening factor of > 100 and poor ones are >10, while the best 1000
      • so when choosing a gimbal motor choose the best ratio of motor torque/load mass rating. This is usually >1, and more is better for rough rides. Again, camera balance is critical.
    • larger diameter circumference bearings will also improve smoothness
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  • \$\begingroup\$ I am not on a such deep level of understanding in electronics for now :) I got only last 2 :( \$\endgroup\$ – Roman Simonyan Oct 8 '16 at 12:59

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