Comments for The Ellipsah Blog http://www.ellipsah.com/blog1 The thoughts of Ellipsah's President Steve Garfinkel Thu, 09 Jun 2011 14:04:18 +0000 hourly 1 http://wordpress.org/?v=3.0.5 Comment on Inductive effects on Motor Performance: Quantitative Analysis by sgarfinkel http://www.ellipsah.com/blog1/?p=79#comment-6 sgarfinkel Thu, 09 Jun 2011 14:04:18 +0000 http://www.ellipsah.com/blog1/?p=79#comment-6 Hamilton, For a brushless dc motor, when a phase is released it still has current flowing in it. Unless you have very sophisticated current sense, this current is not sensed by the drive circuitry. Most motor drives will either have high side or low side current sense, or phase current sense where the correct driven phase is selected and adjusted for polarity for use in the current loop. I have never considered it, but it is quite possible with phase current sense to include the inductive current flow in the sense and this may reduce some sensed current ripple. Many times when a torque speed curve is performed, the power supply current going to the drive stage is taken as the motor current and this would not measure any of the current due to phase inductance. For a generator you will have a similar effect caused by the phase inductance. The phase voltage must exceed the rectified clamping voltage for some amount of volt-seconds to get current to flow through the phase inductance. This will be related to generator commutation frequency, hence a function of speed, poles and phases. When the phase voltage decreases to below the rectified clamping voltage, current will continue to flow until the energy stored in the phase inductance is exhausted. The nonlinearity you would see is that the voltage constant of the generator will be under estimated, since voltage is used to overcome the phase inductance. In a generator all the phase current will be measured so you will not have the measurement error found in motor drives. Please note that this discussion assumes a trapezoidal back emf waveform, one where the voltage remains nearly constant during the current free-wheeling time of the phase inductance. Other waveforms, such as sinusoidal back emf, will have slightly different effects since the torque constant is instantaneously proportional to back emf. I edited the equations a bit. I hope they are clearer. Brushless Dc motors make wonderful generators. In winches, any time you lower a load energy is generated. If this energy is not properly handled the drive DC link voltage is pumped up until dramatic failure occurs. Hamilton,
For a brushless dc motor, when a phase is released it still has current flowing in it. Unless you have very sophisticated current sense, this current is not sensed by the drive circuitry. Most motor drives will either have high side or low side current sense, or phase current sense where the correct driven phase is selected and adjusted for polarity for use in the current loop. I have never considered it, but it is quite possible with phase current sense to include the inductive current flow in the sense and this may reduce some sensed current ripple. Many times when a torque speed curve is performed, the power supply current going to the drive stage is taken as the motor current and this would not measure any of the current due to phase inductance.
For a generator you will have a similar effect caused by the phase inductance. The phase voltage must exceed the rectified clamping voltage for some amount of volt-seconds to get current to flow through the phase inductance. This will be related to generator commutation frequency, hence a function of speed, poles and phases. When the phase voltage decreases to below the rectified clamping voltage, current will continue to flow until the energy stored in the phase inductance is exhausted. The nonlinearity you would see is that the voltage constant of the generator will be under estimated, since voltage is used to overcome the phase inductance. In a generator all the phase current will be measured so you will not have the measurement error found in motor drives. Please note that this discussion assumes a trapezoidal back emf waveform, one where the voltage remains nearly constant during the current free-wheeling time of the phase inductance. Other waveforms, such as sinusoidal back emf, will have slightly different effects since the torque constant is instantaneously proportional to back emf.
I edited the equations a bit. I hope they are clearer.
Brushless Dc motors make wonderful generators. In winches, any time you lower a load energy is generated. If this energy is not properly handled the drive DC link voltage is pumped up until dramatic failure occurs.

]]> Comment on Inductive effects on Motor Performance: Quantitative Analysis by hamilton8415 http://www.ellipsah.com/blog1/?p=79#comment-5 hamilton8415 Wed, 08 Jun 2011 00:08:31 +0000 http://www.ellipsah.com/blog1/?p=79#comment-5 Hi Steve, nice work here. I've been reading these last few pages carefully as I try to make sense of some testing I'd done that involve running a brushless PM motor as a generator. I was unable to get the voltage versus speed results to not depend on speed and current until I included the corrections for the switched inductance as you outline, then, perfect. The torque versus current results are still a bit non-linear though I and I don't perfectly understand the end of this page where you estimate a current that will appear in the phase windings but not in the current measured on the other side of the rectifier (which takes the role of the six-step communation in your example). Not only am I not sure how you could have currents in the phases but not in the rails, I had a hard time following your last two equations. In the second to the last for instance, you have "I" on both sides but I suspect you are referring to different currents. Also the last equation looks suspicious, the 1/4V that applies to the first term is worrisome and again the "I"'s may apply to different quantities. Thanks for your help! Hi Steve, nice work here. I’ve been reading these last few pages carefully as I try to make sense of some testing I’d done that involve running a brushless PM motor as a generator. I was unable to get the voltage versus speed results to not depend on speed and current until I included the corrections for the switched inductance as you outline, then, perfect. The torque versus current results are still a bit non-linear though I and I don’t perfectly understand the end of this page where you estimate a current that will appear in the phase windings but not in the current measured on the other side of the rectifier (which takes the role of the six-step communation in your example). Not only am I not sure how you could have currents in the phases but not in the rails, I had a hard time following your last two equations. In the second to the last for instance, you have “I” on both sides but I suspect you are referring to different currents. Also the last equation looks suspicious, the 1/4V that applies to the first term is worrisome and again the “I”‘s may apply to different quantities. Thanks for your help!

]]> Comment on The Effects of Inductance on Motor Performance: Qualitative Discussion by sgarfinkel http://www.ellipsah.com/blog1/?p=75#comment-4 sgarfinkel Sat, 16 Apr 2011 12:34:25 +0000 http://www.ellipsah.com/blog1/?p=75#comment-4 I am glad the blog has helped you. The next blog entry will cover the equations. I should be able to post it within a week. I am glad the blog has helped you.
The next blog entry will cover the equations. I should be able to post it within a week.

]]> Comment on The Effects of Inductance on Motor Performance: Qualitative Discussion by pmcgill http://www.ellipsah.com/blog1/?p=75#comment-3 pmcgill Fri, 15 Apr 2011 01:18:40 +0000 http://www.ellipsah.com/blog1/?p=75#comment-3 GREAT blog. I really appreciate your practical approach to motor performance. It has helped me understand some things that I could not find clear explanations of elsewhere. We're currently working on a DARPA project and are trying to select a BLDC to use as a generator. We're wrestling with modeling the effects of motor inductance. You show that it's much more important than Kd, which is a surprise. Others have led us to believe that core losses are important and can equal copper losses at higher speeds, while our models (which may be wrong) are not showing a big performance hit due to inductance. We'd be really interested in seeing the equations you used to generate the red lines in this post's graph. Are those coming in the next post? We're anxious to see your model. GREAT blog. I really appreciate your practical approach to motor performance. It has helped me understand some things that I could not find clear explanations of elsewhere.

We’re currently working on a DARPA project and are trying to select a BLDC to use as a generator. We’re wrestling with modeling the effects of motor inductance. You show that it’s much more important than Kd, which is a surprise. Others have led us to believe that core losses are important and can equal copper losses at higher speeds, while our models (which may be wrong) are not showing a big performance hit due to inductance.

We’d be really interested in seeing the equations you used to generate the red lines in this post’s graph. Are those coming in the next post? We’re anxious to see your model.

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