Is this a valid method to measure the inductance of a brushed DC motor? - Page 1
Forum section: electronics / beginners

author: fishandchips
author_rank: Regular Contributor
timestamp: April 14, 2017, 09:40:46 pm
content: Hello, I came across the following simple method to measure the inductance of a DC motor. I need some expert advice. Could forum users please check if the method is valid? How accurate will the inductance measurement be using this method?http://www.dos4ever.com/inductor/inductor.htmlIn Step 4, it reads "Connect the unknown inductor parallel to the oscilloscope (Fig. 2.1)." I cannot see an inductor parallel to the oscilloscope. In the case of trying to find the inductance of a DC motor, am I supposed to connect a wire from the DC motor to the ground of the scope while connecting the other wire from the motor to the input of the scope? Usually to feed the input from the functional generator to the oscilloscope and see the signal on the screen, I just use one cable.I cannot see clearly what is in the white circle in Fig 2.1B. Any idea?Thanks

author: Benta
author_rank: Super Contributor
timestamp: April 14, 2017, 10:26:40 pm
content: Seems overly complex to me.When I have an unknown inductor, I feed it with a square wave voltage and measure the resulting triangular current through the inductor.Why do you need to know the inductance of your motor? It will likely vary quite a lot with armature angle.

author: fishandchips
author_rank: Regular Contributor
timestamp: April 14, 2017, 11:35:25 pm
content: I need to model the motor in computer simulation.

author: neko efecktz
author_rank: Regular Contributor
timestamp: April 14, 2017, 11:50:59 pm
content: It looks like the inductor is just above the scope terminals.looks like a square base with a round coil, all encapsulated.There is one in the picture at the top of the page.

author: fishandchips
author_rank: Regular Contributor
timestamp: April 15, 2017, 01:11:47 am
content: I see. So, if I want to measure the inductance of the DC motor, how do I connect the two wires from the motor to the scope/function generator?

author: rstofer
author_rank: Super Contributor
timestamp: April 15, 2017, 06:26:38 am
content: Yes, the two wires going from the signal generator to the scope are extended to connect to the 2 wires on the motor. That's what it means but I suspect it is intended for small inductors, not some motor.There are other methods:https://electronics.stackexchange.com/questions/182116/whats-the-easy-way-to-measure-a-dc-hobby-motors-inductanceGoogle has more...

author: mtdoc
author_rank: Super Contributor
timestamp: April 15, 2017, 06:39:20 am
content: I like that. Seems pretty simple to me.I'm going to have to try that and check it againt my DEE-5000.

author: fishandchips
author_rank: Regular Contributor
timestamp: April 15, 2017, 12:07:02 pm
content: Quote from: rstofer on April 15, 2017, 06:26:38 amYes, I came across this but I am not sure the following:"in series with a resistor, and put the scope across the resistor. Adjust the resistor value"First the author wrote in series with "a" resistor. What value?Then, the author wrote "adjust the resistor value". Isn't the resistor value fixed?Does that mean a potentiometer that one could adjust the resistor value? What value should I get?

author: rstofer
author_rank: Super Contributor
timestamp: April 15, 2017, 01:37:04 pm
content: Quote from: fishandchips on April 15, 2017, 12:07:02 pmThe author is talking about testing small inductors, not motors. Why would anybody care about the inductance at 20 kHz in a motor? 1 kHz, maybe but 20 kHz seems over the top.Second, is the inductance constant whether the motor is stalled or running full speed? I doubt it. We haven't discussed how big the motor is but if it is very large, the signal generator may not be very happy with the current draw. So, maybe you should use something like 50 Ohms, assuming that the motor looks like 0 Ohms and the generator should be able to drive 50 Ohms.You're only looking for phase shift and by measuring across the resistor, you are getting the current waveform. To me, that means that one side of the resistor is ground and that's where both scope channel grounds connect. Channel 1 probe on the top of the resistor (current waveform) and Channel 2 probe on the ungrounded signal generator output at the motor (voltage waveform). Measure the phase angle.

author: rstofer
author_rank: Super Contributor
timestamp: April 15, 2017, 01:46:12 pm
content: Here are some better SPICE models for a motor. Google is full of examples of motor simulationhttps://www.precisionmicrodrives.com/application-notes/ab-025-using-spice-to-model-dc-motors

author: fishandchips
author_rank: Regular Contributor
timestamp: April 15, 2017, 01:56:41 pm
content: Thanks. In this case, do you think the methods presented in:http://www.dos4ever.com/inductor/inductor.htmlandhttps://electronics.stackexchange.com/questions/182116/whats-the-easy-way-to-measure-a-dc-hobby-motors-inductancemay not be suitable to measure the inductance of a 12V DC motor? I am trying to get the inductance for modelling in simulink. Is it better to get a LCR meter instead? I am trying to save some money but if it is more accurate and easier to get the inductance using a LCR meter, maybe I have to buy it.

author: qibeike
author_rank: Newbie
timestamp: January 28, 2019, 10:17:07 pm
content: Hey, did you find a way to measure the inductance of the motor? I'm trying to model in simulink an electric vehicle too and I need the internal inductance of the motor.Thanks in advance.

author: rstofer
author_rank: Super Contributor
timestamp: January 29, 2019, 12:26:27 am
content: The method is valid even if the frequency seems over the top.Measure the DC resistance of the winding. Then add a series resistor that will make the motor + resistor value match the generator output - probably 50 Ohms.Apply a signal, perhaps a couple of volts and measure the current by reading the voltage across the external resistor. From the voltage and current you get total impedance. You know the two resistances (motor + external resistor in series) so all you need to do is get the inductive reactance and then solve for L which is a function of frequency.http://support.ctc-control.com/customer/elearning/younkin/motorParameters.pdfPage 4 talks about it...I'm not sure if the numbers will make any sense. Try a few frequencies and see how it turns out. When I was playing with a DC motor simulation, I just picked 80 uH and called it good. It's far more important that your model tracks the physical constants of the motor. These may need to be measured with a tachometer, voltmeter and ammeter.https://web.stanford.edu/class/me161/labs/Lab06MotorConstants.pdf