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Old 11-13-2021 | 07:01 PM
  #31  
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Originally Posted by gigaplex
Agreed. Stator variance is far easier to keep consistent than rotor variance. You'd need a hugely defective stator to increase the current by 50%. The two components with the highest variance are the rotor or sensor board. The latter can actually be checked by an analyser.
not in my experience. I’ve tested the individual components (stator, rotor and timing board) of motors of 2 reputable brands for well over 100 “identical” samples of both groups when brand new and then for a very high portion of them, as used units as well.

Stator resistance can vary HEAPS out of the factory. Easy to see 1milliohm per phase (tab to collector) between two samples.

Rotors I have found to be much closer out of the same batch. Typically it’s 25gauss or so variance.

25 gauss is around 1.5% typically, 1milliohm is more like 5% on a 17.5 - and as said, the latter is more common to see.

Sensors vary a heap too - but you can “tune” around it.
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Old 11-13-2021 | 10:15 PM
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Originally Posted by cplus
not in my experience. I’ve tested the individual components (stator, rotor and timing board) of motors of 2 reputable brands for well over 100 “identical” samples of both groups when brand new and then for a very high portion of them, as used units as well.

Stator resistance can vary HEAPS out of the factory. Easy to see 1milliohm per phase (tab to collector) between two samples.

Rotors I have found to be much closer out of the same batch. Typically it’s 25gauss or so variance.

25 gauss is around 1.5% typically, 1milliohm is more like 5% on a 17.5 - and as said, the latter is more common to see.

Sensors vary a heap too - but you can “tune” around it.
I'm surprised to see you're only getting 1.5% variance in rotors, I've seen much larger variance. However, even with 5% stator IR variance, I wouldn't expect that to lead to a 50% higher current draw.
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Old 11-14-2021 | 01:49 AM
  #33  
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Sometimes just a simple re-shimming of te rotor can make a huge difference.
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Old 11-14-2021 | 07:41 AM
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Originally Posted by waitwhat
I just got off the phone with Ron Schuur of Schuur Speed and he said there is no way two brand new motors would pull those different amps because of the slight manufacturing differences between rotors. He said the only explanation is that the stator in the one pulling less amps has a lower internal resistance. He stated the motor pulling less amps is the better motor. I trust what he says as he literally sets up locked timing motor batches for spec classes at big races.

If you want his phone number I'm sure he'd love to talk to you about it.
Ron makes a good argument, but I challenge the possibility of a poorly shimmed motor could lose power either yielding too low of a power draw starving the motor from peak power or causing inefficiency which could generate excessive draw... it's a double edged sword.

I mentioned above that I would go with the motor which offered the closest gap between the timing across all 3 phases which can be measured with the analyzer, that's the motor I would pick regardless of the amp draw. The closer the gap on timing is the motor with better shim setup and will theoretically offer the best efficiency/power ratio.
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