I can manually read the armature resistance of a 3 phase brushless electric RC motor by connecting each phase in turn in series with a calibrated low value resistance, and reading the low voltage across the calibration resistor and motor phase, and calculating the resistance of the phase. I do this three times for each phase, making 9 readings in total, so it is quite time consuming. I have many motors to test.
I would like to automate this using an Arduino or possibly a Teensy 3.2 (for its improved A2D). However I struggle with the electronic circuit required to allow me to read the voltage across the calibration resistance and then the phase, and then change the phase.
Note the motors have three wires, lets call them A, B and C. I need to read A-B, A-C, and B-C.
Assuming you want to know how to do it ......Might it be sufficient to connect one phase to 0v and , via two series resistors, power the other two phases alternatively and together . You may be able to get enough data that way to “ check” your motors . You need to do the sums, but you can calculate and measure the average current and the two phase currents , this might give you enough information ( eg if compared to a good motor) to satisfy .
If resistance of each phase is A,B,C , then
with one phase connected I=V/(A+B)
other phase I =V/(A+C)
both powered Itotal= V/( A+ (CB/C+B))
In each you should be able to predict the current and maybe get your answer .
Other wise you are looking at some clever switching arrangement, probably with relays- three relays with changeover contacts switching its phase connection to either Ov or the supply in series with measuring resistor .Measure phase voltage and power supply voltage with relays appropriately set .
To me it looks like measuring A - B will have the parallell circuit of A-C-B. That would call for a number of equations. High time for circuitry theory work!
@hammy
If there is one resistance between A and B, one resistance between B and C and one resistance between C and A they are connected in a triangle inside the motor if I'm right.
Could be ... in which case the equations are different ( let u sort that out ) -I had assumed the windings were In “star” arrangement.
While I’m here , because of commoning issues measuring the drop across series resistors means you need to measure voltage one side, then the other of the series resistor both to 0v . This will give poor resolution - better to use a current source and just measure the phase voltage
@hammy
You're perfectly right! It could be a star connection! Still, use Ohms law and set up the equations. Anybody that has cut up brushless motors and know?
Check logic MOSFETs. What magnitude of resistance would it be in the motor? Look for Ron for the transistor regarding that the gate has the voltage You can give it.
But…. the kind of relays You should use would be more like signal switching relays, not clumsy power relays. Look into old telefon station stuff....
@OP
More interesting info. That indicates that the central point in the Y connection is available. That cancels the need for a djungle of equations. Apply +voltage to the centre, some resistors and use 3 analog input to read....
hammy:
While I’m here , because of commoning issues measuring the drop across series resistors means you need to measure voltage one side, then the other of the series resistor both to 0v . This will give poor resolution - better to use a current source and just measure the phase voltage
Yes, indeed. When I measure the motors manually I use two voltmeters, one across the calibration resistor, and one across the motor phase. None of the voltmeter wires are current carrying. I try to read both voltmeters simultaneously. Then the motor phase resistance is just the voltage ratio * calibration resistance.
@railroader - no the central connection is hidden within the motor and not accessible.
BTW the motor's armature resistances vary between about 0.200R and 0.010R
@OP
Thanks for the eduction! Brushless motors didn't exist during the 1970:ies when I collected my master degree.
Then it remains to set up the equations for the 3 measurements and figure out how to design the measurement hardware.
Normal measuring arrangement is low ohm tester which induces a constant current of 1 amp through two inner probes and the voltage is read via two outer probes.
Voltage reading is resistance value.
Any shorts are usually detected with a growler instrument.
bluejets:
Normal measuring arrangement is low ohm tester which induces a constant current of 1 amp through two inner probes and the voltage is read via two outer probes.
Voltage reading is resistance value.
Any shorts are usually detected with a growler instrument.
Yes, I understand this. That is what I do manually now.
I am looking for help in designing a circuit to allow me to measure the resistance of the three phases quickly and easily, just plugging the motor in and pressing a Start button.