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Re: motor Ke and Kt Quiz
Let's try a different question. Here are the specs for the 2014 BAG motor: Spec Voltage: Vspec = 12 Calculate Ke and Kt from the above data, and discuss possible reasons why these values are not equal. |
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Re: motor Ke and Kt Quiz
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Re: motor Ke and Kt Quiz
By all means Paul, please do jump in. I think there may be at least a small audience of interested students following the thread.
Adam Heard and Richard Wallace, please also feel free to post. I know you both have been holding back. |
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Re: motor Ke and Kt Quiz
Russ -- I will join the discussion when I have data to share.
Paul -- this motor is your design, and I want to go on record saying you did a fine job. It is a worthy successor to the venerable Globe, and about three orders of magnitude more flexible for a range of applications powering FRC mechanisms. You may have noticed I ordered a few of them yesterday -- will share test data (and THEN my own answer to Russ's question) when I can. |
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#5
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Re: motor Ke and Kt Quiz
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If you designed it for FRC, I'd love to hear more about the process. |
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Re: motor Ke and Kt Quiz
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I then took the rough design to CCL and said, "construct this like the CIM motor, use all ball bearings, and hit these power and torque numbers". I was fairly certain they could do it, but haven't had to design a DC motor in a long time so left it to them. But this definitely is a custom motor specific to the FRC application. The MiniCIM was a similar process except we literally took a picture of a dismantled CIM and said make it only "this" long. I gave them torque and power numbers so it would match up 1:1 with a CIM and contribute the most power possible in the meat of the CIM speed profile. |
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#7
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Re: motor Ke and Kt Quiz
So, for most small, high Flux DC motors there is this phenomenon called magnetic field saturation. It happens when the permanent magnet doesn't have enough field strength to drive the high amount of Flux through the moving parts of the motor. I am simplifying here to try to keep it short.
The bottom line is that at high current (aka, near stall), the magnetic field saturates producing less actual torque than ideal and this is one of the reasons for the difference between Kt and Ke. However, it is not the only difference. This can really only be found with motor testing. |
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Re: motor Ke and Kt Quiz
From Ether's 3rd attachment, Ke = Kt + (other losses /Iw).
If there are "other losses" these will account for the difference. Of course, there's a lifetime of study in "other losses"... |
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#9
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Re: motor Ke and Kt Quiz
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"Kt equals Ke" is a theoretical result which is only approximately true for real-world motors. Firstly, the "linearity" (straight lines) assumption (notice the asterisks) used to compute Kt would not be valid when the magnetic field saturates. Second, "other_losses" such as bearing friction, windage & viscous damping, eddy currents, and hysteresis in the power equation account for the difference between Ke and Kt. |
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#10
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Re: motor Ke and Kt Quiz
If Ke = Kt (which would be a perfect world scenario)
Then the max torque the motor could output would be: Tstall = Ke*Istall Which for a bag motor comes out to: Tstall =~ .32 Nm This is less than the .4 Nm in the motor specs, and given we have come to the conclusion that the real world Kt is always less than Ke, wouldn't the true stall torque of the bag motor be even less than .32 Nm? (I also don't understand why our calculated Kt is larger than Ke or is that a result of the curve actually being nonlinear?) |
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#11
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Re: motor Ke and Kt Quiz
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Last edited by Ether : 21-05-2014 at 22:39. Reason: added sketch |
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#12
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Re: motor Ke and Kt Quiz
There is.
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#13
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Re: motor Ke and Kt Quiz
Last edited by Ether : 21-05-2014 at 18:02. Reason: corrected power equation |
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