John in CR
100 TW
One of the reasons hubmotors are so heavy is their thick back iron. Since it helps support the vehicle it makes sense that they make it stronger than the motor itself may need. RC outrunners have a relatively thin magnet backing ring, so thin that the magnet flux leaks through, which also makes sense because they're typically used on something that flies, so low weight is critical. As I understand these RC outrunners are quite efficient.
I'm using a hubmotor as a mid-drive, so I'd like to trim as much weight as practical. Zappy said once that he found that the any thicker than the magnets for the steel ring is wasted weight, which sounds like a good rule of thumb.
Can I use flux leakage as my reference? The stator will be in place while I machine the magnet backing ring, because I'm using the motor to self lathe. I'll have a CA attached, so in addition to checking for flux leakage I can also verify the point where Kv starts to change. I understand that if the Kv doesn't change and/or there's no flux leakage through to the outside of the ring, that I haven't changed the motor at all other than to reduce the weight. I have 2 questions:
1. Is this previous statement true, or at higher load and current can flux leak due to the stronger electromagnets of the stator, and I wouldn't pick it up as a change in no-load rpm or flux leakage at rest?
2. What happens if I go somewhat beyond the point where flux starts to leak thru the magnet backing ring like in an RC outrunner? I understand that the Kv will increase, so torque decreases, proportionately I think. Current handling won't go up like it would with a speedier wind, but the higher rpm would increase cooling slightly. I'm nowhere near the rpm limits of the motor where iron losses will get out of hand, so the efficiency hit due to increased iron losses with no corresponding decrease in copper losses would be slight. I'm looking at maybe allowing the Kv going from the current 16rpm/v to maybe 18 or 20 depending how much weight I save at the perimeter in the process.
Any holes in my plan or basic understanding?
John
I'm using a hubmotor as a mid-drive, so I'd like to trim as much weight as practical. Zappy said once that he found that the any thicker than the magnets for the steel ring is wasted weight, which sounds like a good rule of thumb.
Can I use flux leakage as my reference? The stator will be in place while I machine the magnet backing ring, because I'm using the motor to self lathe. I'll have a CA attached, so in addition to checking for flux leakage I can also verify the point where Kv starts to change. I understand that if the Kv doesn't change and/or there's no flux leakage through to the outside of the ring, that I haven't changed the motor at all other than to reduce the weight. I have 2 questions:
1. Is this previous statement true, or at higher load and current can flux leak due to the stronger electromagnets of the stator, and I wouldn't pick it up as a change in no-load rpm or flux leakage at rest?
2. What happens if I go somewhat beyond the point where flux starts to leak thru the magnet backing ring like in an RC outrunner? I understand that the Kv will increase, so torque decreases, proportionately I think. Current handling won't go up like it would with a speedier wind, but the higher rpm would increase cooling slightly. I'm nowhere near the rpm limits of the motor where iron losses will get out of hand, so the efficiency hit due to increased iron losses with no corresponding decrease in copper losses would be slight. I'm looking at maybe allowing the Kv going from the current 16rpm/v to maybe 18 or 20 depending how much weight I save at the perimeter in the process.
Any holes in my plan or basic understanding?
John
