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Old Dec 16, 2002, 02:09 PM
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New England
Joined Dec 2002
47 Posts
Question on Brushless Motor Terminology

Hi all! I'm new to this forum, but have been reading it for some time now, and I have a few questions regarding brushless motors. First, is there any place where the manufacturers list torque/horsepower/max RPM for their various motors, like you find for a nitro motor? The Kv for a motor - is that rpm per volt, and does a motor with a high Kv imply that it has a lower torque than a motor with a lower Kv? Is one more efficient than the other? Is one better for 3D versus scale flying? And one more question for now - why is it that running less than, say, 70% on the ESC governor causes higher ESC temperatures (do these motors pull higher current when the voltage is low to maintain horsepower)? I know there's a lot here, but these are the missing links for me right now! Thanks to all who can help.

Dave
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Old Dec 16, 2002, 02:33 PM
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St. Mary, Maryland, United States
Joined Dec 1996
8,565 Posts
Re: Question on Brushless Motor Terminology

>>Hi all! I'm new to this forum, but have been reading it for some time now, and I have a few questions regarding brushless motors. First, is there any place where the manufacturers list torque/horsepower/max RPM for their various motors, like you find for a nitro motor? <<

No, electrics are different. You can usually find Kv which is RPM/volt, Io which is no load current and Rm which is armature resitance. If the manufacturer doesn't list it often a program like Motocalc will - they advertise here and you can download a 30 day trial copy.


>>The Kv for a motor - is that rpm per volt, and does a motor with a high Kv imply that it has a lower torque than a motor with a lower Kv?<<

Yes, if it the same motor in the series. For example a Kontronik 600-15 and 600-18 and the same motor frame with different #'s of winds and different Kv's. 1500 rppm / volt and 1800 rpm / volt.

>>Is one more efficient than the other? Is one better for 3D versus scale flying?<<

Yes, assuming you can gear the motor appropriately the lower Kv motor will have more torque, a lower Io (no load current) and therfore be more efficient at lower currents of long flights. The higher Kv motor will be less efficient at lower current levels but MORE efficient at high burst currents involved in severe aerobatics.

>>And one more question for now - why is it that running less than, say, 70% on the ESC governor causes higher ESC temperatures (do these motors pull higher current when the voltage is low to maintain horsepower)? I know there's a lot here, but these are the missing links for me right now! Thanks to all who can help.<<

Two things - the current flow during the shorter time the ESC is feeding power is higher hence more motor heating. Plus, when you feed in lots of collective one of two things will happen. If your motor isn't big enough it will just fall off the torque curve bogging the headspeed and draw a fair amount of current. If your motor is burly enough the current will go WAY up and possibly pop your ESC. Best to run higher ESC percentages and go to a smaller pinion to get the headspeed where you want it.
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Old Dec 17, 2002, 09:36 AM
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New England
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Chris, thanks for the info! Based on your response it sounds like these motors normally operate in the 80 - 100% speed range, with current draw proportional to power required for the type of flying, is that right? Also, do these motors have a standard shaft length/diameter/thread (e.g. 1/4 - 20 for nitro engines) where the pinion attaches?

Dave
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Old Dec 17, 2002, 10:36 AM
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St. Mary, Maryland, United States
Joined Dec 1996
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Quote:
Originally posted by Tazcat
Chris, thanks for the info! Based on your response it sounds like these motors normally operate in the 80 - 100% speed range, with current draw proportional to power required for the type of flying, is that right? Also, do these motors have a standard shaft length/diameter/thread (e.g. 1/4 - 20 for nitro engines) where the pinion attaches?

Dave
Pretty much, if you you ran a decent motor @ 100% with no load and measured the RPM to added more and more load until the RPM sagged to 80% the current would be very high. The brushless heli ESC's have a governor mode, with brush motors you need to set a throttle curve which looks something like 100-90-80-90-100 for a symetrical -10 to + 10 pitch curve and constant headspeed.

There are 3 standard shafts sizes depending on power. 2mm for the little indoor FP machines with some hotter motors using 1/8 " shafts. The 7-10 cell heli's will either use 1/8" or 5mm and the larger helis are all 5mm. 1/8" is the same as 3.2mm BTW
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Old Dec 17, 2002, 08:44 PM
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Dave, welcome to the EZone!!!!!!

I am moving this to PS.
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