Feb 19, 2007, 08:21 PM
Registered User
Data

# Mega 16/25/4 Motor Constants

I have been using Mega 16/15/xx and 16/25/xx for the last few years, and have noticed that the 16/25/xx motors never matched my MotoCalc predictions. I read several posts on RCgroups stating that the Mega armature resistance figure for the 16/25 series is incorrect. In the past I spent a long time measuring motor constants, but for the last few years have been putting most of my effort into flying e-gliders, rather than poking around inside motors. However I decided to put together a good quality and accurate-as-i-can-make-it no load test facility to measure my stock of motors, and here is the first result.

The motor resistance Rm is measured by comparing it to a 0.050R 0.1% calibrated shunt resistor. I also have a 0.010R 0.1% shunt and I used the setup to measure this (replacing the motor in the circuit), and got exactly the right answer, so I am confident the tester is accurate to better than 1%. The motor resistance is read on all three phases at a current of about 1A to 4A, so the heating is minimal. In operation the motor resistance will be considerably higher due to heating.

The no load current Io is measured using a Power Analyser Pro, with a resolution of 50mA.

Kv is measured using the drill press method (using the 95% of the peak voltage), and by free running the motor at several voltages, and calculating the Kv taking the motor resistance into account. In the free running tests the RPM is measured with an optical tachometer and by measuring the controller frequency using a 4 digit multimeter. I had hoped to use a frequency counter but haven't yet had time to interface it correctly. All three Kv measurements are very slightly different, but this is positive, as it shows each method is reliable and reasonably accurate. The controller used is a Hyperion 80A unit, set to lowest advance. This should give the lowest (and best) Kv and Io figures.

Enough of the methodology, here are the results:

Mega 16/25/4 Motor Test 20/2/07
Motor resistance = 0.059 R
Kv drill press = 1330 RPM/V
Kv opto tach = 1325 RPM/V
Kv Hz meter = 1332 RPM/V
Io = 0.83 A @10V

The attached files show the results in more detail.

Unfortunately the conclusion is that the Mega 16/25/4 has 2.6X the resistance and only 95% of the torque of the official (but erroneous) Mega figures. IMO it is high time they corrected this misinformation.

I will be posting more test results as I measure my motors.

Regards,

Neil.

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Last edited by Neil Stainton; Feb 19, 2007 at 08:26 PM.
 Feb 20, 2007, 03:48 AM Tony Rogers Neil, Does Motocalc match you observed performance when using the new measured motor parameters?
 Feb 20, 2007, 04:29 AM Registered User Thread OP I haven't yet checked but I have been using a guesstimated 0.050R Rm figure recently to more closely match MotoCalc's predictions with my meter readings. The actual 0.059R Rm number is even worse than my guesstimate, but I am confident it is correct, and will result in a good match. Regards, Neil.
 Feb 20, 2007, 05:25 PM Registered User The Rm values megamotor posts on their website for the 16/15/xx are for 2-phases. However, all the other motors on their website are for a single phase resistance, so they need to be doubled to get the correct Rm number. If you go to the czech website, you will see the resitance numbers for all their motors are for a single phase and not two. I dont know why they do this. Programs such as motocalc and e-calc need 2-phase resitance numbers to get an accurate prediction. So, for the 16/25/4, mega posts an Rm of 23 mohm but the number you really need 46 mohm, which is a little closer to your measured resitance of 59 mohm.
Feb 20, 2007, 05:40 PM
Registered User
Quote:
 Originally Posted by tho98027 but the number you really need 46 mohm, which is a little closer to your measured resitance of 59 mohm.
I agree with everything except the above. I have measured both my Mega 16/25/4 motors several times, to high accuracy, and their resistance is 59 and 60 mOhm +/- 1mOhm. So even the Mega official one phase resistance is 28% too low.

Regards,

Neil.
Feb 20, 2007, 05:54 PM
Registered User
Quote:
 Originally Posted by Neil Stainton I agree with everything except the above. I have measured both my Mega 16/25/4 motors several times, to high accuracy, and their resistance is 59 and 60 mOhm +/- 1mOhm. So even the Mega official one phase resistance is 28% too low. Regards, Neil.
Hi Neil

My experience on a Mega ACn 16/7/17
Rm announced by Mega 0.31 ohm
I measured (3 years ago) 0.618 0.620 0.621 ohm (no heating/room temp)
Close to double
I informed immediately Dan Mathyas (Mega)
It seems their convention to give one section Rm (star connecting)

Louis
 Feb 21, 2007, 03:41 AM Registered User Thread OP It is possible the self heating of the 3A test current affected the Rm a bit, but I doubt it is significant as: a) I only applied the test current for a few seconds for each phase. b) The 9 Rm values measured are not increasing with time. I haven't yet measured by other Mega motors but my Neu is spot on the official figures, so I am confident of the accuracy. Neil.
Feb 21, 2007, 06:05 AM
Tony Rogers
Quote:
 Originally Posted by tho98027 The Rm values megamotor posts on their website for the 16/15/xx are for 2-phases. However, all the other motors on their website are for a single phase resistance, so they need to be doubled to get the correct Rm number...
That would be true if the motor was wye (star) wound but I thought all Mega motors were delta wound in which case, the effective resistance is 2/3 of a single phase.
 Feb 21, 2007, 12:16 PM Registered User Hi Neil- Are you using a 4-wire measurement for the resistance measurement? If not, the resistance of connections and leads could be significant sources of error. -Tracy
 Feb 21, 2007, 12:47 PM Registered User Thread OP Yes I am. The 0.1% shunt has 4 pins, and for the motor I use crock clips onto the motor connectors. Neil.