I'm trying to find a formula to calculate the number of turns needed in a thermoplastic bobbin to use in a electric motor. Instead of using magnet wire in the slots of a lamination stack, the bobbin can be wound on a inexpensive single flyer winder before being inserted into the lamination stack slot. Here's what I've done so far.

  1. Determined what total resistance the motor needs to have.
  2. Using the AWG table pick the diameter of a representative wire (until I know how much wire I need it will be a guess).
  3. Using the AWG determine the resistance of a certain length of this wire.
  4. I calculated using a CAD program the surface area of the bobbin that will constrain the wire.
  5. Then find the elusive formula for pulling this data together.
  • \$\begingroup\$ What specs do you have for torque thus acceleration, and inertia energy to drive current source or sink. What is Bmax for core saturation vs I to determine n turns and thus L? DCR and eddy current losses thus temp rise of windings. There are many more variables undefined in your question, losses, max power, thermal conduction, temp rise, duty factor on motor, excitation voltage, commutation RPM thus inductance, for stored electrical energy etc What other variables do you know? \$\endgroup\$ Apr 4, 2017 at 14:39

1 Answer 1


For highest efficiency and lowest loss you normally want the resistance to be as low as possible. So you start with the number of turns or length of wire required to produce the desired motor performance, then choose the largest diameter wire that will fit. If you don't know how many turns are required then just put on as many as possible and test the motor on a low voltage. Then adjust the number of turns depending on how much more or less speed/torque you want.

You can calculate the number of wires that fit in a particular area using basic trigonometry, but in practice you will probably need more space due to insulation thickness, uneven winding patterns etc. The insulation on thin wire is a larger proportion of the total area, and the wires are difficult to wind evenly without overlapping. Thick wire may be hard to bend and get in the right position for best packing.

A bobbin is easier to wind and assemble, but wastes space that could be used for windings. Therefore the resistance will be higher, leading to greater loss and more heating - and a plastic bobbin will melt if it gets too hot...


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