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I have some problems with understanding the motor constants from the datasheet. I cannot verify, how they even came to this parameters. So, for line to line, the following relations hold between the BEMF constant Ke and the motor speed constant Kv:

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For the torque constant Kt, the relation to Ke/Kv looks like this:

enter image description here

Now if we consider a star connected PMSM motor, there is a factor in the relation between the torque constant and Kv:

enter image description here

Now lets have a look at a real motor datasheet:

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This is a two pole pair PMSM motor, star connected. According the calculations above, this motor should have a torque constant of 0.081Nm/A for a Ke of 9.8V/1krpm. Further, I bougth the motor and tested it, so the Kv (speed for a given nput voltage into the controller) is 72rpm/V, that leads to a Ke = 13.88V/1krpm, not as stated in the datasheet. Also the torque relation is then 0.114Nm/A, what I also could verify with measurements. So is the datasheet wrong here or do I something wrong with my calculations?

I had a look at different datasheets of different manufacturers, but somehow I don't see, how to connect to real motor parameters.

For example this one here: enter image description here

If the BEMF for a phase in star connection is 15.6V/1krpm, then how it comes, the speed is limited to 1950rpm? Shouldn't this speed be much higher for this constant, or must I first calculate the line to line BEMF, which is by a factor of 1.5 higher?

HansPeterLoft
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  • Does this answer your question? [BLDC Motor Constants: Why is my Kt and Ke different?](https://electronics.stackexchange.com/questions/399593/bldc-motor-constants-why-is-my-kt-and-ke-different) –  Jul 08 '21 at 12:42
  • @JonRB Isn't OP asking why Kv and Ke are different? – DKNguyen Jul 08 '21 at 12:50
  • @DKNguyen it doesn't appear to be. they know the relationship between Ke and Kv but are questioning why their measurement is different from the datasheet. The measurement can only be line-line (unless a virtual neutral is made, or you have electrical access to the endwinding star) and thus depending on the specific of the datasheet test there will be a sqrt(3) (ie 1.6) difference –  Jul 08 '21 at 12:57
  • Kv is not quite W(no-load)/Vpeak because the no-load condition is still loaded by losses including friction, windage, etc. You could determine these from the no-load current, and eliminate the V drop across R, etc... But it's easier to drive the motor at known RPM and measure the back EMF into a high impedance. –  Jul 08 '21 at 12:58
  • Ok, the effect on the torque constant makes sense now. What is still unclear to me is, why they state a Ke of 9.8V/1krpm, but when I apply 32V, my motor spins at 2340rpm, this means a Kv of 73 and so a Ke of 13.7V/1krpm. This seems a little bit too off for me. I second motor I have not avilable, so I cannot verify it. – HansPeterLoft Jul 08 '21 at 13:18

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