There is a permanent-magnet synchronous motor with an external rotor that I want to explore and possibly modify. To do this I need some answers to my simple question:

For a given shape of stator lamination (in my case a 24 "teeth" one), what is the effect of increasing the length of the stator lamination stack (and larger magnets in the rotor accordingly)?


closed as unclear what you're asking by Eugene Sh., Daniel Grillo, uint128_t, placeholder, Dmitry Grigoryev May 23 '16 at 11:32

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  • \$\begingroup\$ The question is tagged "induction motor," but the phrase "magnets in the rotor" implies that this is a permanent-magnet synchronous motor. The rotor length needs to match the stack length of the stator. Is this really a motor modification or a new motor constructed using some salvaged parts? Someone has voted to close the question. That may be because it is not clear. Please edit the question to make it clear. \$\endgroup\$ – Charles Cowie May 16 '16 at 17:10
  • \$\begingroup\$ I have taken out the "induction" tag if it's so misleading. It is a 3phase AC motor with an external rotor that has a ring magnet firmly attached to it. \$\endgroup\$ – J.K. May 17 '16 at 17:54

If the stator stack length is increased, the windings are changed to accommodate additional current required for the torque increase, and the number of rotor magnets are also increased to maintain the flux density, the torque capability will be increased in proportion to the increase in length. The length of the rotor body must match the length of the stator stack such that the flux in the air gap is uniform and full use is made of the rotor and stator iron.

Additional rotor weight and torque will put additional stress on the shaft and bearings, but a small motor may have enough safety factor to accommodate that. Given that the motor has an external rotor, the shaft and bearing design might be questionable.

  • \$\begingroup\$ Thank you! That is really helpful! So is it safe to say that if the parameters you have pointed out are met (length of ring magnet is increased accordingly), then an identical motor with X1.5 or X2 core length will have X1.5 or X2 torque with regards to the original? \$\endgroup\$ – J.K. May 19 '16 at 7:15
  • \$\begingroup\$ Yes. The electromagnetic design will allow X1.5 or 2 torque. I neglected to mention the mechanical design. There could be a problem with the shaft and bearings. In a small motor, I would expect that there would be a large safety factor, but the safety factor would be reduced accordingly. I did mention the current. It will increase X1.5 or X2, but I didn't state that correctly; see revision. \$\endgroup\$ – Charles Cowie May 19 '16 at 13:43
  • \$\begingroup\$ Thanks for the clarification. I am going to use a longer shaft cartridge with longer self lubricated bushings. Next question:the copper wire needed for this type of modification will have to be thicker (currently 0.3mm diam) or it can be kept the same? The goal is not to make a faster spinning motor but one with more torque at same (very low) RPM. \$\endgroup\$ – J.K. May 19 '16 at 15:46
  • \$\begingroup\$ The current drawn will be directly proportional to torque, so X1.5 or 2. The cross sectional area of the wire needs to be increased by that amount. \$\endgroup\$ – Charles Cowie May 20 '16 at 13:17
  • \$\begingroup\$ I see. But isn't this (thicker wire) going to reduce the number of turns that can be wound on the stator slots? Less turns won't have an effect on the characteristics of the motor? \$\endgroup\$ – J.K. May 20 '16 at 17:32

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