Espritmodel.com Telemetry Radio
Reply
Thread Tools
Old Jan 06, 2013, 03:11 AM
Registered User
Joined Jun 2012
811 Posts
Discussion
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.
Nereth is offline Find More Posts by Nereth
RCG Plus Member
Sign up now
to remove ads between posts
Old Jan 06, 2013, 03:29 AM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,169 Posts
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.
Bruce Abbott is online now Find More Posts by Bruce Abbott
Reply With Quote
Old Jan 06, 2013, 03:49 AM
Registered User
Joined Jun 2012
811 Posts
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 )
Nereth is offline Find More Posts by Nereth
RCG Plus Member
Old Jan 06, 2013, 03:53 AM
Registered User
Fourdan's Avatar
Antony (France)
Joined Sep 2003
2,863 Posts
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 20°C (0.0062 /°C)
It is useful to compare several motors with similar Kv and weight

Louis
Fourdan is offline Find More Posts by Fourdan
Reply With Quote
Old Jan 06, 2013, 04:05 AM
Registered User
Joined Jun 2012
811 Posts
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 20°C (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?
Nereth is offline Find More Posts by Nereth
RCG Plus Member
Old Jan 06, 2013, 04:11 AM
Registered User
Fourdan's Avatar
Antony (France)
Joined Sep 2003
2,863 Posts
Hi
Locking the stator is not mandatory ..
But I prefer to do so ... to avoid the "jump"
Louis
Fourdan is offline Find More Posts by Fourdan
Last edited by Fourdan; Jan 07, 2013 at 07:51 AM.
Reply With Quote
Old Jan 06, 2013, 04:21 AM
Registered User
Joined Jun 2012
811 Posts
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).
Nereth is offline Find More Posts by Nereth
RCG Plus Member
Old Jan 06, 2013, 06:12 AM
Registered User
MagnusEl's Avatar
Sweden, Gävleborg County
Joined Jan 2004
851 Posts
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.
MagnusEl is offline Find More Posts by MagnusEl
Reply With Quote
Old Jan 07, 2013, 03:05 AM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,169 Posts
Quote:
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.
Bruce Abbott is online now Find More Posts by Bruce Abbott
Reply With Quote
Old Jan 07, 2013, 06:03 PM
Suspended Account
United States, FL, Pompano Beach
Joined Oct 2011
1,425 Posts
http://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
zeroback is offline Find More Posts by zeroback
Reply With Quote  (Disabled)
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion best wind-resistant multirotor samwei1950 Multirotor Talk 20 Dec 24, 2013 10:08 PM
Mini-HowTo Measuring IR (Internal Resistance) with a CB II lquick Batteries and Chargers 12 Sep 03, 2012 04:45 PM
Discussion cheap balancer capable of resistance measure eagle75 Batteries and Chargers 44 Jul 25, 2012 08:02 AM
Gallery Design to measure winding resistance? vintage1 DIY Electronics 11 Jan 15, 2006 07:02 PM
Design to measure winding resistances? vintage1 Batteries and Chargers 0 Dec 16, 2005 06:32 AM