I watched a DIY video recently on building a brushless DC motor. Can someone demonstrate for me how to calculate the max torque & max rpm of that motor based on the components that are used in the video--not by using a dynamometer or any other device to measure the mechanical power/torque or rpm of such a device? There must be some EE method/s to reverse engineer & use such data to redesign a similar motor to your own specifications.

  • \$\begingroup\$ Torque is theoretically limited by heat or demagnetization, but may especially at speed be limited by the availability of voltage to push enough current through the winding inductance when commutating rapidly. \$\endgroup\$ – Chris Stratton Dec 9 '15 at 3:31

You can't "reverse engineer" something that was not engineered in the first place. From an engineering point of view, there is nothing of value here. This is a nice demonstration of how to make a motor that works using inexpensive and easily-obtained parts. You can learn something from it, but it is not a good basis for an engineered motor design.

The major flaw in the design is the lack of any effort to design a magnetic circuit. A good motor needs a closed path for magnetic flux through good magnetic materials with an air gap that is as short as possible. You can inexpensively buy used text books that were published 15 or 20 years ago and find on-line engineering course material that will provide a lot more engineering design information that anyone here is likely to post.

  • \$\begingroup\$ Thanks for responding to my question. Perhaps 'engineer' wasn't the best word to use. "design" might have been a better choice. I imagine the value might be that the motor could be made quite inexpensively. It appears to have quite a bit of torque for such a small motor. It could be used for small projects that don't require much of a motor. I also noticed that an ESC motor controller can be obtained for less than 15$. What is "magic flux"? Did you mean to type "magnetic flux? Isn't there some relatively simple method for calculating the torque & rpm for such a small motor? \$\endgroup\$ – zeffur Dec 9 '15 at 2:34
  • \$\begingroup\$ "Magic" flux is what you get when you combine fat fingers with an automagic spelling corrector. I believe that a motor like this works like a permanent magnet synchronous motor. The speed is completely determined by the overall frequency of the ESC output. It would be very difficult to calculate the torque. Look up "prony brake" to see how to measure the torque. Use some flexible steel to make a ring to connect together the external ends of the bolts holding the coils. That should increase the torque. \$\endgroup\$ – Charles Cowie Dec 9 '15 at 3:01
  • \$\begingroup\$ Thanks for the "Prony brake" technique reference. It's quite a clever technique. Do you know of any physics calculation method that uses the number of stator poles, number of wire turns per stator, stator flux, rotor magnetic flux, etc. to determine the max torque & max rpm at the shaft? \$\endgroup\$ – zeffur Dec 9 '15 at 7:21
  • \$\begingroup\$ Speed (RPM) = 120 f (Hz) / p where f is the frequency at which the ESC reverses polarity and p is the number of motor poles, 4 for the motor described. There are two components of torque. The reaction torque is generated between the rotor magnets and the electro-magnets of the stator. The reluctance torque is the result of the rotor magnets being attracted to the bolts. The dimension of the magnets, bolts and the path of the magnetic flux through the air all figure into the torque. I don't know how to calculate the torque. \$\endgroup\$ – Charles Cowie Dec 9 '15 at 22:54
  • \$\begingroup\$ Thanks for responding to my question. Why "120" in your formula if the power source (battery) provides DC power? Does "...f is the frequency at which the ESC reverses polarity" mean one full sinusoidal cycle (1 positive phase + one negative phase) or does it mean each clocked phase--as in the case of positive (non polar) square wave? \$\endgroup\$ – zeffur Dec 10 '15 at 20:57

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