Measuring Brushless Motor Resistance (Two Digital Meter Method)

[kyleservicetech]

Originally Posted by dirtybird

Did you notice the Sears Craftsman #82369 meter is also made in China?

Would not be surprised, but I couldn't find a "Made in China" label or decal anywhere on my meter.

It's getting more and more difficult to find anything not made in China now days. I just found on the Internet that the trade deficit between the USA and China was some 200 billion dollars for last year. Lets see, a dollar is about 0.004 inches by 2.5 by 6 inches. So, 200 billion of them is about seven million cubic feet, or something like 2500 rail road cars full of dollar bills.

Kind of puts a different light on things.

[jjw]

Originally Posted by dirtybird

Did you notice the Sears Craftsman #82369 meter is also made in China?

Did anyone think it would have been made in the good old USA?... not likely. Virtually all consumer class electronic manufacturing (and even most "professional" level) has gone offshore. Its a simple matter of economics. US manufacturing priced itself out of what the majority of consumers are willing to pay quite a few years ago... you can pick which side you want to wag your finger at.

Been in the PC manufacturing biz for many years... where everything is made in Asia... spent a lot of time at the Asian manufacturing sites and am intimately familiar with labor rates, working conditions, tax advantages... the US and other 1st world economic countries simply could not compete. As an example... the PC you're using right now, that you probably spent a few hundred dollars for, would cost 5 to 10 times that amount if it and all its components were to be made domestically. If you had to pay 5 grand for the average PC, would you buy it?

Not trying to make a political point here... just stating fact. Just look here on this forum and watch how many people are now trying to get things as cheap as they can by buying stuff directly from China at places such as HobbyKing, OKHobby, RCTimer, etc., etc... no longer even willing to buy Asian made stuff sold at a domestic retailer...

As I've always said... as much as people complain about quality and customer service, they'll vote with their pocket book everytime...

-Jim.

[wingnutt]

Jjw – I am curious as to the value of measuring the DC resistance of a motors coils. Rm I can understand, since that can be used to help predict the performance capabilities of a motor in use. Other than checking for shorts or to make sure you didn’t miss a turn in rewinding, I cannot see where the value is useful, but would like to know if I am missing something.
A motor coils DC resistance is not the same as Rm and shouldn’t be used for performance prediction. I recently built a milliohm tester, using Lucien’s LM317 current source plans. On my most recent rewind, the DC resistance (Rdc) is 0.047 Ohm and the calculated Rm (at operation voltage) is 0.113 Ohm. Other motors I tested had the same discrepancy between Rdc and Rm.

Please understand that I really am curious and not trying to be contradictory at all. I just don’t want to not miss out on something.

[dirtybird]

Originally Posted by wingnutt

Jjw – I am curious as to the value of measuring the DC resistance of a motors coils. Rm I can understand, since that can be used to help predict the performance capabilities of a motor in use. Other than checking for shorts or to make sure you didn’t miss a turn in rewinding, I cannot see where the value is useful, but would like to know if I am missing something.
A motor coils DC resistance is not the same as Rm and shouldn’t be used for performance prediction. I recently built a milliohm tester, using Lucien’s LM317 current source plans. On my most recent rewind, the DC resistance (Rdc) is 0.047 Ohm and the calculated Rm (at operation voltage) is 0.113 Ohm. Other motors I tested had the same discrepancy between Rdc and Rm.

Please understand that I really am curious and not trying to be contradictory at all. I just don’t want to not miss out on something.

Whats Rm?
What does Motocalc use for their calculations?

[wingnutt]

Rm is the motors 'running' resistance or impedance. It is the combination of the coils resistance, reactance, inductance, etc. while being pulsed by the esc. Rm is what Motocalc and other performance prediction software use.

[dirtybird]

How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motors

[MustangMan]

Originally Posted by dirtybird

How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motors

In the simplest sense Rm is the DC resistance of the windings between any two phase leads. Because it is substantially less than 1 ohm in any of our model motors it's a little difficult to measure with a standard multi-meter. The trick to getting a good measurements is to push a significant DC current, like an amp or two, through the winding and measure the voltage drop with a meter capable of millivolt resolution. The current must be known accurately as well. Knowing both the current and the voltage drop you can compute the Rm with Ohm's law ( R = V / I ). Note that you must measure the voltage drop at the point where the motor wires exit the motor, or very close to it to get an accurate measurement. Otherwise you'll be including the resistance of your test harness wiring in the measurement, which you don't want to do. Commercial instruments designed for this purpose use "4-wire" measurements rather than 2 wires like a standard DMM uses to measure resistance in order to eliminate the resistance of the measurement leads from the problem.

[jjw]

Originally Posted by wingnutt

Jjw – I am curious as to the value of measuring the DC resistance of a motors coils. Rm I can understand, since that can be used to help predict the performance capabilities of a motor in use. Other than checking for shorts or to make sure you didn’t miss a turn in rewinding, I cannot see where the value is useful, but would like to know if I am missing something.
A motor coils DC resistance is not the same as Rm and shouldn’t be used for performance prediction. I recently built a milliohm tester, using Lucien’s LM317 current source plans. On my most recent rewind, the DC resistance (Rdc) is 0.047 Ohm and the calculated Rm (at operation voltage) is 0.113 Ohm. Other motors I tested had the same discrepancy between Rdc and Rm.

Please understand that I really am curious and not trying to be contradictory at all. I just don’t want to not miss out on something.

Hi... For the purposes of estimating DC motor performance, Rm (or terminal resistance) is the motor coil DC resistance... I typically measure this using a constant current source and feeding this current through the coil, measuring the voltage across the coil, then using ohms law to calculate resistance.

When you say you calculate Rm, I can only guess that you are running the motor at some known volltage, current and load conditions and then back calculating the coil resitance... all completely valid.

The fact that you find different numbers than a measured DC coil resistance is probably due to other losses seen in real world motors... magnetic (or iron) losses (eddy current and hysteresis) and mechanical losses (friction and windage)...

-Jim.

[Larry3215]

There has been quite a bit of debate on the value or validity of using the measured dc resistance vrs the calculated resistance.

There are a number of very smart people who's opinions I value who disagree on this point

Its my understanding the using the calculated 'Rm' ends up including the esc resistance and wiring, connectors etc. That might be fine for over all predictions for a specific "system" but doesnt necessarily tell you about the motor itself.

Changing the esc or its settings or the rpm and/or the power range your operating in all will effect the calculated values.

So if your calculated value was done using brand x esc at timing Y, but you end p runing the motor on brand z esc at tming Q, then you predictions may be meaningless.

At the same time, taking only the motors dc resistance ignores the esc and connectors, timing settings etc etc. But that may give you a more honest look at the motor itself if thats what your trying to evaluate or compare.

Both have value I suppose depending on the situation and what your looking for.

[kyleservicetech]

Originally Posted by dirtybird

How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motors

Something I've done over the past few years, is set up a particular motor/prop/esc/battery pack. Then run the motor and prop, checking its current/voltage/watts with a meter such as an Astroflight whattmeter.

Then, assuming the battery is putting out the voltage under load as advertised, I check the actual motor RPM against the motocalc predicted RPM. If they vary substantially, I just save the motor under a different name, then "tweak" the motocalc motor characteristic "KV" number so the motocalc RPM matches the actual motor RPM.

If the battery has a different voltage under load, you can also "tweak" the battery constants to match the real world.

That way, as you change props in motocalc, the predicted RPM's should be a little closer.

Not very scientific, but it gets you a little closer.

Just a note, I've found that sometimes motocalc has the WRONG motor constants, as compared to the motors web pages. This was noted on the Hacker motors several times, as well as Eflites.

[MustangMan]

Originally Posted by dirtybird

How do you calculate Rm?
If you can't or don't want to answer this, can you refer me to a book or an article that will tell me ?
When I went thru EE college(60 years ago) they did not have such a device.
They had a 3 phase synchronous AC motor but this appears to be a different motor.
In the description of the motor characteristics Motocalc lists resistance, KV, and no load current. When I changed the resistance from what the HC spec stated to what I measured, the predicted performance characteristics changed significantly.
I am not trying to argue, I just want to get to the bottom of this so I can get accurate performance predictions of these cheap motors

Commercial/Industrial 3-phase motors are indeed a bit different in detail from our little motors but operate using the same general principles. Smaller 3-phase motors use squirrel cage rotors just like the smaller 1-phase motors use. In these motors the "field" magnetization of the rotor is "induced" by the slip between the theoretical rotation of the stator fields and the mechanical rotation of the rotor at a slightly slower speed. That's why induction motors never achieve true "synchronous" speeds. Larger motors, which operate synchronously, have field windings on the rotor which creates the field magnetization. These must be fed DC current through clip rings whereas our motors have permanent field magnets.

[dirtybird]

Originally Posted by MustangMan

In the simplest sense Rm is the DC resistance of the windings between any two phase leads. Because it is substantially less than 1 ohm in any of our model motors it's a little difficult to measure with a standard multi-meter. The trick to getting a good measurements is to push a significant DC current, like an amp or two, through the winding and measure the voltage drop with a meter capable of millivolt resolution. The current must be known accurately as well. Knowing both the current and the voltage drop you can compute the Rm with Ohm's law ( R = V / I ). Note that you must measure the voltage drop at the point where the motor wires exit the motor, or very close to it to get an accurate measurement. Otherwise you'll be including the resistance of your test harness wiring in the measurement, which you don't want to do. Commercial instruments designed for this purpose use "4-wire" measurements rather than 2 wires like a standard DMM uses to measure resistance in order to eliminate the resistance of the measurement leads from the problem.

I have an adjustable lab power supply(Mastec 5020) that I can connect to two motor leads in series with a 10 ohm resistor and adjust to 1 amp within +/- 0.1V. I need the resistor or the PS thinks its output is shorted. I then verify the current with the clamp on meter. I then read the voltage at the two leads with a millivolt meter. The MV reading then converts directly to milliohms.
The Sears meter has a 200MV range. The Harbor Freight $3 meter also has a 200MV range. Both meters agree.
The resulting reading is not the coil resistance. The coil resistance is 3/2xRmeasured if the motor is delta connected. It is 1/2x Rmeasured if the motor is Y connected.
The measured resistance I get agrees with the MFG spec for the Scorpion and Eflight motors but not for the Turnigy.
The Turnigy runs with a lot of vibration in the no load test. I suspect its bearing wont last long.
The Eflight is very smooth running but it is an inrunner.

[kyleservicetech]

Originally Posted by MustangMan

Commercial/Industrial 3-phase motors are indeed a bit different in detail from our little motors but operate using the same general principles. Smaller 3-phase motors use squirrel cage rotors just like the smaller 1-phase motors use. In these motors the "field" magnetization of the rotor is "induced" by the slip between the theoretical rotation of the stator fields and the mechanical rotation of the rotor at a slightly slower speed. That's why induction motors never achieve true "synchronous" speeds. Larger motors, which operate synchronously, have field windings on the rotor which creates the field magnetization. These must be fed DC current through clip rings whereas our motors have permanent field magnets.

If you want a demonstration of that "slip" between the three phase rotating magnetic field, and the squirrel cage rotor, try to locate an especially powerful magnet, samarium cobalt or similar.

I've got some that are 3/8 by 1 1/2 by 3 inches (They pull 400 pounds on a one inch thick steel block) Take that magnet, and swipe it very rapidly near an aluminum sheet, at least 1/4 inch thick. (Be danged careful with magnets like this. I got a finger smashed between one of those magnets and a steel magnetic structure. That finger didn't stop bleeding for 8 hours.)

The magnetic drag caused by the magnet inducing short circuit currents into the aluminum sheet is very noticeable. In fact, I was able to measure the voltage generated across a 1/2 inch sheet I've used on my oscilloscope. The scope showed a few millivolts across that sheet, indicating the short circuit current generated in that aluminum sheet was something around 1000 amperes.

But, 1000 amperes and a few millivolts still is only a few watts. :o

[kyleservicetech]

Originally Posted by jjw

Been in the PC manufacturing biz for many years... where everything is made in Asia... spent a lot of time at the Asian manufacturing sites and am intimately familiar with labor rates, working conditions, tax advantages... the US and other 1st world economic countries simply could not compete. As an example... the PC you're using right now, that you probably spent a few hundred dollars for, would cost 5 to 10 times that amount if it and all its components were to be made domestically. If you had to pay 5 grand for the average PC, would you buy it?

As I've always said... as much as people complain about quality and customer service, they'll vote with their pocket book everytime...

-Jim.

Yeah, my first PC was purchased in the early 1980's for $1500. A lot of money then, still is. The processor ran at a blazing 4 megahertz, it had one 8 inch floppy drive, and NO HARD DRIVE. Memory was something like 16,000 bytes. And that was made in the USA.

What I had before that was a cobbled up computer system built up with some 60 integrated circuits, from scratch with some 1000 feet of wire wrapping. Got it to work, and used it for simple word processing.

Learned a lot from that project, one of which was to damned certainly NEVER do that again.

[Gohmer]

Originally Posted by MustangMan

Well, we've managed to drag this thread WAY OFF TOPIC! Interesting stuff though.

Depends on what you find interesting. Puts me to sleep

[kyleservicetech]

Originally Posted by kyleservicetech

Good grief, and one wonders why we have no jobs in the USA, and many other countries.

I just purchased a new Hacker A50-12S motor. The package says "Designed in Germany". Then it says, "Made in China". At any rate, its performance is identical to my other Hacker A50-12S motor.

[hillbillynamedpossum]

Doesnt a megaohmmeter measure "loaded" resistance?

(FYI..... I work on stationary generator sets 7-60 KW as well as HVAC stuff)

I use a Freed megger that you plug in, let the tubes warm up and then after about 5 min you can check the insulation resistance and the loaded winding resistance. This is real handy to find a rotor or stator that has a winding that is opening under a load. The point is would this work to find the true resistance?


Possum

[kyleservicetech]

Originally Posted by hillbillynamedpossum

Doesnt a megaohmmeter measure "loaded" resistance?

(FYI..... I work on stationary generator sets 7-60 KW as well as HVAC stuff)

I use a Freed megger that you plug in, let the tubes warm up and then after about 5 min you can check the insulation resistance and the loaded winding resistance. This is real handy to find a rotor or stator that has a winding that is opening under a load. The point is would this work to find the true resistance?


Possum

I'm not certain which kind of meter you have. The Megohmmeters we used before I retired were of the high voltage kind. They would apply up to about 5000 Volts DC, while measuring the DC current through the insulation under test.

None of the Megohmeters we had in the shop were capable of checking winding resistance. For that, we used Micro-ohmmeters, that could measure resistance down to one micro-ohm. (If you gotta ask how much these cost, you can't afford it!)

Per industry standards, we tested out equipment under hypot conditions, where we would apply up to 80 KV (Depending on unit type) 60 Hertz for insulation and contact high voltage withstand capabilities. They also have to withstand BIL (Basic Insulation Level) tests where some 200 KV is applied to the circuit breakers for microsecond periods of time.

It was real strange to watch a vacuum interrupter be tested at hypot until the vacuum arced over the OUTSIDE of the interrupter, a distance of some 10 inches. That knowing the vacuum contacts were only separated by about 1/2 inch inside the bottle.

These tests are conducted behind lead shields. That's how you generate X-rays.

[hillbillynamedpossum]

My Freed meggers test at around 500 VDC. Ive got two, an old one I have never even tried to use and a fairly new one. They are pretty cool, like a cross between a hi pot and the "fish shocker" hand crank meggars that im sure you knocked the p*ss out of new guys with.

Any way, I was just curious. Id even be willing to peddle the old one to the right person.

Possum