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.
 Jan 06, 2013, 03:29 AM We want... Information! Hastings, New Zealand Joined Jan 2001 5,221 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.
Jan 06, 2013, 03:49 AM
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Quote:
 Originally Posted by Bruce Abbott 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 )
 Jan 06, 2013, 03:53 AM Registered User Antony (France) Joined Sep 2003 3,382 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
Jan 06, 2013, 04:05 AM
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Quote:
 Originally Posted by Fourdan 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?
 Jan 06, 2013, 04:11 AM Registered User Antony (France) Joined Sep 2003 3,382 Posts 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 07:51 AM.
 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).
 Jan 06, 2013, 06:12 AM Registered User Sweden, Gävleborg County Joined Jan 2004 853 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.
Jan 07, 2013, 03:05 AM
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Quote:
 Originally Posted by MagnusEl 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.