tag:blogger.com,1999:blog-4080979156700236346.post5718856441333594843..comments2019-09-06T02:17:23.573-07:00Comments on Things in Motion: How to estimate the torque of a BLDC (PMSM) electric motor using only its Kv and current drawRichard Parsonshttp://www.blogger.com/profile/08331478925576296508noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-4080979156700236346.post-16905025293756528472019-07-10T20:02:22.045-07:002019-07-10T20:02:22.045-07:00Oh, I think I see where I went wrong. In my exampl...Oh, I think I see where I went wrong. In my example I was using the voltage and current measured before the controller, but this torque formula uses the current after the controller. So this implies that the armature current must be 1.15x the input current, to cancel out that 0.87 factor.<br /><br />I'm still a little fuzzy on exactly how armature current works, so that would be another very useful article if you ever feel like writing about it.dekutree64https://www.blogger.com/profile/09347767142078281057noreply@blogger.comtag:blogger.com,1999:blog-4080979156700236346.post-47648999641010601172019-07-10T18:41:10.847-07:002019-07-10T18:41:10.847-07:00Hi Dekutree64
Thanks for your comment.
Electric ...Hi Dekutree64<br /><br />Thanks for your comment.<br /><br />Electric motors can indeed have an efficiency >95%.<br />I think I can see the problem with your assumptions. Kv x Supply voltage = maximum no load speed that a motor can achieve and not the motor speed at a given power output. Refer to this post for more details: https://things-in-motion.blogspot.com/2019/05/understanding-bldc-pmsm-electric-motors.html<br />Assuming a power factor of 1, the electrical power supplied to a motor is given by the voltage drop across the motor multiplied by the current supplied to the motor (P = VI). The easiest way to measure this power is before the motor controller where the current is still DC and the voltage is near constant. However, keep in mind you will also be measuring the motor controller losses. <br />As you mentioned, the power output is the angular velocity multiplied by the torque and from that you can find the efficiency.<br />The source of electric motor inefficiencies is a deep topic but one which I plan on exploring in this blog in more detail soon.<br />Richardhttp://things-in-motion.blogspot.comnoreply@blogger.comtag:blogger.com,1999:blog-4080979156700236346.post-47340323686021418442019-07-10T10:55:15.102-07:002019-07-10T10:55:15.102-07:00Thank you for this very useful formula and explana...Thank you for this very useful formula and explanation! But I do have one question... how can a BLDC motor ever achieve >87% efficiency when the torque is calculated this way?<br /><br />Conversion from RPM and Newton-meters to Watts is RPM * Nm / 9.55, so if RPM = Volts x Kv and Nm = Amps x 8.3 / Kv, then Watts = Volts x Amps x 8.3 / 9.55, therefore output power = input power * 0.87. And since motors spin slower under load, that means the real efficiency will be even lower. And is this even accounting for resistive heat loss yet? So how is it that some motors like this http://nt-power.eu/motor_m.html claim 93% peak efficiency?dekutree64https://www.blogger.com/profile/09347767142078281057noreply@blogger.comtag:blogger.com,1999:blog-4080979156700236346.post-18696785145882291242019-02-18T05:41:11.939-08:002019-02-18T05:41:11.939-08:00Trying to figure out this BLDC thing for a hobby p...Trying to figure out this BLDC thing for a hobby project and your website has been of great help! Found it through the Odrive website. Thanks a lot!Anonymousnoreply@blogger.com