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Old Jan 06, 2013, 04:11 AM
Nereth is offline
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Measuring winding resistance


Hi,

I've decided I want to measure the winding resistance on some motors since I don't really trust the hobbyking spec (I love HK, but not for their integrity).

I don't have a multimeter that can read such low numbers but I thought of an alternative idea and I want you guys to tell me what could go wrong, since I don't understand what I'm doing that well.

I have access to a voltage controlled power supply, I figure I will set it to about .5 volts, put it across two of the winding terminations of a motor, use a multimeter to measure the actual voltage (.5xxxx) and another multimeter to measure the current draw, and get the winding resistance as such:

R=V/I.

Now, what I am afraid of is a multitude of effects that I don't know about making this experiment end badly or uselessly. Here are some I can think of, and I invite anyone to tell me more stuff to watch out for:

1) First, would this value of resistance be the actual useful value that is normally quoted for a motor? Or does that value normally include some other effects based on the fact that there are 3 windings etc.

2) If the motor is rated for 50 amps, and I pull 50 amps like this, I am assuming that the winding would fry, since normally it only pulls 50 amps momentarily before another winding takes over? In other words, would a safe target current actually be less than the motors normal rated current, since we are putting all the current through a single winding? What current should I aim for (too little volts or amps and I might run into measurement precision inaccuracies)? How quickly should I do the test to ensure the winding doesn't heat up too fast?

3) Are there better methods for this?

Thanks for your time and please let me know if I am doing something stupid - these motors are still good and have homes in planes! I don't wanna kill them.
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Old Jan 06, 2013, 04:29 AM
Bruce Abbott is offline
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You have the right idea, but you need something to limit the current. The simplest way is to just put a resistor in series, and wind the voltage up until you get the current you want.

For example, if you use a 4.7 Ohm 10W resistor and set your power supply to 4.7V, then the current will be 1A (slightly less if the winding resistance is high). 1A is a good value to set because then the voltage across the winding is equal to its resistance, eg. 1mV = 0.001 Ohm, 100mV = 0.1 Ohms.
Old Jan 06, 2013, 04:49 AM
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Quote:
Originally Posted by Bruce Abbott View Post
You have the right idea, but you need something to limit the current. The simplest way is to just put a resistor in series, and wind the voltage up until you get the current you want.

For example, if you use a 4.7 Ohm 10W resistor and set your power supply to 4.7V, then the current will be 1A (slightly less if the winding resistance is high). 1A is a good value to set because then the voltage across the winding is equal to its resistance, eg. 1mV = 0.001 Ohm, 100mV = 0.1 Ohms.
Is this a safety thing? The power supply can be current limited incase of unexpected mistakes.

Or is this the solution to not having enough time to make the measurement before things overheat - artificially bump the resistance so the current is lower? Would a lower voltage from the power supply be an equally valid solution?

(Not trying to avoid doing what you are saying, just trying to figure out why I am doing it )
Old Jan 06, 2013, 04:53 AM
Fourdan is offline
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Hi
Personnally I am using a Constant Current PS (CCCV) Mastech
Generally I first adjust to 5A (and around 3V limit) with a short.

I connect just one or two seconds (to avoid temperature rising)
I lock the stator in a support
I note also the ambient temperature C on my table

The Rm value could be "corrected" to 20C (0.0062 /C)
It is useful to compare several motors with similar Kv and weight

Louis
Old Jan 06, 2013, 05:05 AM
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Quote:
Originally Posted by Fourdan View Post
Hi
Personnally I am using a Constant Current PS (CCCV) Mastech
Generally I first adjust to 5A (and around 3V limit) with a short.

I connect just one or two seconds (to avoid temperature rising)
I lock the stator in a support
I note also the ambient temperature C on my table

The Rm value could be "corrected" to 20C (0.0062 /C)
It is useful to compare several motors with similar Kv and weight

Louis
Is locking the stator nescessary? Won't it just jump to a position and then stay there?
Old Jan 06, 2013, 05:11 AM
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Hi
Locking the stator is not mandatory ..
But I prefer to do so ... to avoid the "jump"
Louis
Last edited by Fourdan; Jan 07, 2013 at 08:51 AM.
Old Jan 06, 2013, 05:21 AM
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I think I figured out why we need a resistor in series, Bruce - The power supply is limited to 3a =D, not sure how low the voltage can go (it isn't with me atm).
Old Jan 06, 2013, 07:12 AM
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I just limit the current to 1A on my PS, and then measure the voltage dropp over the windings with a digital multimeter, milliVolts =milliOhms with 1A.
Old Jan 07, 2013, 04:05 AM
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Originally Posted by MagnusEl View Post
I just limit the current to 1A on my PS, and then measure the voltage drop over the windings with a digital multimeter, milliVolts =milliOhms with 1A.
That's fine if your power supply can maintain regulation when driving into an almost dead short with significant inductance. Mine won't (the meter reads 1A, but my scope shows output voltage oscillating at a high frequency).

Quote:
Originally Posted by Nereth
Is locking the stator necessary?
With a brushless motor, no. You only need to lock the rotor when measuring a brushed motor.
Old Jan 07, 2013, 07:03 PM
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https://www.rcgroups.com/forums/showthread.php?t=580151 Here is a link to a lot of discussion about how to do this I too have been interested in obtaining this ability so I can measure what I wind. Frustratingly I haven't achieved it yet. The mostly due to the lack of money to buy the proper equipment, but in an effort to find another route I was planning on building one of the power supplies laid out in the linked thread. My Dad has quite a supply of electronic parts, lab equipment and the engineering ability to create any type of electronic circuit/ device one might want. That became my second obstacle is when I showed him the circuit, explained the goal he couldn't understand it's relevance to accurately measuring a motor performance etc(?). My main explanation was that its a required measurement for software like Motocalc. Next Q: dose the soft ware need the R of one phase or all 3, A: I don't know. Then the black magic explanation of the inner workings of the universe and the relative ways to measure it ensued and I'm no closer getting accurate measurements like resistance or Kv for my pea brained world . In other words there is more than one way to skin a cat which is covered in the thread. The soft ware issue comes up in the thread because sometimes the software prediction is way off from the real world result. ie. I made several attempts with a drill press to measure Kv and the results obtained with my Fluke 88 after using a formula I found in one of the free calculators/ motor winding guide to correct for the meter reading RMS, didn't come very close to what was measured when run through an ESC and measured with the Emeter suite and verified with a hand held tach , DMM and watt meter. I have also measured these tools against Eagle Tree and they are all in agreement within a percent. I have Motocalc and have found it to be hit and miss and I figure garbage in garbage out. I have found that using various free BM prediction tools when averaged over several motor ESC combo test is in agreement with the real world measurements but sometimes the bench measurements and the prediction tool disagree by 20% other times there dead on.http://www.allaboutcircuits.com/vol_1/chpt_8/9.html with all that being said here is my next plan http://www.keithley.com/products/acc...robes/?mn=5808


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