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Unread 21-05-2014, 09:48
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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

Spec Free Speed: Wfree = 1466 radians/sec

Spec Free Current: Ifree = 1.8 A

Spec Stall Torque: Tstall = 0.403 Nm

Spec Stall Current: Istall = 41 A

Calculate Ke and Kt from the above data, and discuss possible reasons why these values are not equal.


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Unread 21-05-2014, 10:02
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by Ether View Post

Let's try a different question.

Here are the specs for the 2014 BAG motor:

Spec Voltage: Vspec = 12

Spec Free Speed: Wfree = 1466 radians/sec

Spec Free Current: Ifree = 1.8 A

Spec Stall Torque: Tstall = 0.403 Nm

Spec Stall Current: Istall = 41 A

Calculate Ke and Kt from the above data, and discuss possible reasons why these values are not equal.


Do I get to answer or is that cheating?
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Unread 21-05-2014, 10:31
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by Paul Copioli View Post
Do I get to answer or is that cheating?
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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Unread 21-05-2014, 13:07
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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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Unread 21-05-2014, 13:27
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by Richard Wallace View Post
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.
Paul, did you design this motor for FRC? or was it a motor already in production (or not in production but available as a configurable item) that you picked out?

If you designed it for FRC, I'd love to hear more about the process.
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Unread 21-05-2014, 15:59
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by AdamHeard View Post
Paul, did you design this motor for FRC? or was it a motor already in production (or not in production but available as a configurable item) that you picked out?

If you designed it for FRC, I'd love to hear more about the process.
A little bit of both. CCL, the manufacturer of the CIM motor, did all the detail design work. They did not have any motor like the BAG. In our VEXpro product discussions, before we even made one product, we all agreed that losing the Globe motor from the KoP was a big loss to FRC teams. We then finalized what we wanted from a power, torque at max power, and packaging perspective. I ran some quick numbers based on DC motor design formulas (I will get the exact book details that I prefer at another time b/c I loaned it out to someone recently) to get a rough idea of what was possible.

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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Unread 21-05-2014, 16:15
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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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Unread 21-05-2014, 16:35
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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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Unread 21-05-2014, 16:36
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by Paul Copioli View Post
at high current (aka, near stall), the magnetic field saturates producing less actual torque than ideal...However, it is not the only difference.
Thanks Paul. I think that completes the explanation.

"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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Unread 21-05-2014, 20:34
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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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Unread 21-05-2014, 22:17
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by Rauhul Varma View Post
we have come to the conclusion that the real world Kt is always less than Ke
In the real world, Kt and Ke are not constant. So without enough data to compute them both at the same operating point, comparing them is like comparing apples and oranges.



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Name:	Kt & Ke at same operating point.png
Views:	66
Size:	16.9 KB
ID:	17048  

Last edited by Ether : 21-05-2014 at 22:39. Reason: added sketch
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Unread 21-05-2014, 14:22
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by Ether View Post
By all means Paul, please do jump in. I think there may be at least a small audience of interested students following the thread.

There is.
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Unread 21-05-2014, 16:13
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Re: motor Ke and Kt Quiz

Quote:
Originally Posted by Gregor View Post
There is.
OK, I'll toss this out for discussion


Attached Thumbnails
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Name:	Ke.png
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ID:	17043  Click image for larger version

Name:	Kt.png
Views:	93
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ID:	17044  Click image for larger version

Name:	Ke Kt power equation.png
Views:	51
Size:	16.4 KB
ID:	17046  

Last edited by Ether : 21-05-2014 at 18:02. Reason: corrected power equation
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