Post by thread:[EE]: Is empirical determination of dc motor specs

Hi all I have a largish dc servo motor (approx. 5" x 10.5") - specs unknown - for which I would like to determine voltage rating, peak current, max rpm, etc. I have been told this is possible, but the person who furnished that information didn't know how to do it. So, can anyone tell me of a methodology to do this without producing smoke and flames - preferably using a 180V 2hp scr controller with adj max current or a variac (240V 15A), caps and a diode bridge, and relevant meters. I am something of a neophyte re. electronics/mechatronics, so if what I have asked doesn't make a lot of sense, or if more information is needed, then please email me personally. I know this is not really [EE] (and definitely not [PIC]), but after lurking on this list, and a few other places, for quite a while, reckon the knowledge and helpfulness here makes it the best place to ask. TIA Pete S. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Yes ! But you need to know at least the voltage or the current, and you need a adjustable mechanical brake (Prony or other). If you know the voltage, then apply it and measure rpm w/o load. There is no rule of thumb but it is mostly so that a series wound (or permanent magnet) motor will give max output input power/(torque x rpm) at about 1/2 to 2/3 of max unloaded rpm. So use the brake to get that rpm and compute current and torque at the same time, then derate a little bit and check for long term effects (heat). Of course the right way to do this is to plot the motor characteristic (torque vs I and rpm vs torque and voltage). You will quickly find the relevant peaks. The absolute max. current can be determined from the wire gauge used. Note that servos for temporary (low duty cycle) operation will totally mislead you when plotted like this. You will know that something strange is on when the max output does not tally with the wire gauge and size of motor and the thing gets very hot very fast. Characteristic plotting works for any electrical machine and for thermal (combustion) engines. You can improvise a Prony brake using a smooth drum wheel, a leather belt and a weight table. However this may be dangerous depending on the specs of the motor. This is all based on the idea that the original designers went for best efficiency. I don't know what happens if you plot machines designed in certain countries where efficiency is not paramount for certain reasons ... ;) Peter -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Get "The Motor Handbook" from http://www.astroflight.com ALso available from Amazon if you care to wait. Andy "Peter L. Peres" <spam_OUTplpTakeThisOuTACTCOM.CO.IL> on 08/04/2000 12:36:16 PM Please respond to pic microcontroller discussion list <.....PICLISTKILLspam@spam@MITVMA.MIT.EDU> To: PICLISTKILLspamMITVMA.MIT.EDU cc: (bcc: Andrew Kunz/TDI_NOTES) Subject: [EE]: Is empirical determination of dc motor specs possible? Yes ! But you need to know at least the voltage or the current, and you need a adjustable mechanical brake (Prony or other). If you know the voltage, then apply it and measure rpm w/o load. There is no rule of thumb but it is mostly so that a series wound (or permanent magnet) motor will give max output input power/(torque x rpm) at about 1/2 to 2/3 of max unloaded rpm. So use the brake to get that rpm and compute current and torque at the same time, then derate a little bit and check for long term effects (heat). Of course the right way to do this is to plot the motor characteristic (torque vs I and rpm vs torque and voltage). You will quickly find the relevant peaks. The absolute max. current can be determined from the wire gauge used. Note that servos for temporary (low duty cycle) operation will totally mislead you when plotted like this. You will know that something strange is on when the max output does not tally with the wire gauge and size of motor and the thing gets very hot very fast. Characteristic plotting works for any electrical machine and for thermal (combustion) engines. You can improvise a Prony brake using a smooth drum wheel, a leather belt and a weight table. However this may be dangerous depending on the specs of the motor. This is all based on the idea that the original designers went for best efficiency. I don't know what happens if you plot machines designed in certain countries where efficiency is not paramount for certain reasons ... ;) Peter -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.