I need a small dc motor for a momentum wheel. To conserve power the motor will be turned off once the wheel has reached target RPMs and left off until the RPMs fall out of the target range. This will require a motor that coasts with minimal mechanical resistance when off. Brushless DC motors seem like a good fit, but the permanent magnets in most BLDC motors have a small attraction to the solid core in each of the windings. I've considered modifying a computer fan BLDC to have an excited rotor, but i would really prefer to get something off of the shelf.

Here's my setup:

Power will be supplied by four AA batteries(6V) and the momentum wheel will weigh at max 100 grams. The motor and wheel will be mounted vertically and i a am looking for RPMs between 4,000-10,000 (the higher the better). Spin up times aren't critical so i have some flexibility as far as torque is concerned.

  • \$\begingroup\$ Are you trying to conserve power or energy? That is, do you want to turn the motor off so that the power the motor is using can be used elsewhere while it is off? Or are you trying to reduce energy use over time? If it is the latter, I have my doubts as to whether that strategy would lower energy use. Also, have you thought about increasing the inertia of the load? \$\endgroup\$
    – Eric
    Dec 9, 2014 at 19:12
  • \$\begingroup\$ I'm trying to reduce energy use over time. The inertia is fixed within the parameters I specified and the wheel is already the shape of a ring. \$\endgroup\$ Dec 10, 2014 at 12:01

1 Answer 1


I would go for a SwitchedReluctance. The cogging torque of a BLDC will be a hinderance to this while coasting while an SR machine does not have this problem.

  • \$\begingroup\$ SR motors look perfect, but they don't seem to be very common. Do you have any suggestions for suppliers, or is this something I'm going to have to build myself? \$\endgroup\$ Dec 9, 2014 at 14:44
  • \$\begingroup\$ Just found out SR machines are fairly noisy, unfortunately I also need fairly quiet operation. \$\endgroup\$ Dec 9, 2014 at 21:11

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