DC Motor Questions - Page 1
Forum section: electronics / beginners

author: Electro Fan
author_rank: Super Contributor
timestamp: October 20, 2015, 01:31:33 am
content: I have a small Radio Shack DC motor. I think it's spec'd for 3V but I'm not sure about the current.At 3V it draws about 135mA, but with my DC Power Supply set to 135mA or 200mA up to about 500mA the Constant Current light on the PS will flicker indicating that even though it is drawing 135mA the motor might like more current. When I turn the Power Supply up to allow 1 amp the motor continues to draw 135mA and the CC light no longer flickers; as I turn the current limit down toward 600mA the current light on the PS will start to flicker and significant flickering will return at 500mA. What is it about the nature of a DC motor that causes the intermittent draw of greater current (even though it supposedly only draws 135mA and it's being given 500mA)? (Maybe when the brushes are actually touching the current is different? I think this motor runs about 8-10k RPM so I would think the brushes are generally in contact. Just guessing).Thanks

author: DrGeoff
author_rank: Frequent Contributor
timestamp: October 20, 2015, 02:26:48 am
content: If you know someone with a scope, put a 1 ohm resistor in series with the motor and have a look at the current waveform.That might help you see that the current is not constant for motors. They consist of electromagnets (ie inductance) with magnetic fields being created and collapsed.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 20, 2015, 02:38:55 am
content: current requirement will increase with torque experienced by the motor. check stall motor and free running (no load) current draw spec, they are different...

author: rs20
author_rank: Super Contributor
timestamp: October 20, 2015, 03:03:50 am
content: Just to state it simply (even though others have already answered): it would appear that your motor is drawing anaverageof 135mA, but the current waveform haspeaksup to around 600mA. As Mechatrommer points out, these values will increase as you put the motor under load (right up to the extreme case where the motor is prevent from rotating entirely, i.e., "stalled").

author: Electro Fan
author_rank: Super Contributor
timestamp: October 20, 2015, 03:30:25 am
content: Quote from: DrGeoff on October 20, 2015, 02:26:48 amThanks, I'm using a Rigol DS2072. I experimented with various settings but not sure quite what waveform I am looking for - What I saw was something that varied over time with the largest peaks about 3x or so what might be the average peaks. I tried using averaging and various triggers.... any suggestions on the preferred scope settings? Thx

author: rs20
author_rank: Super Contributor
timestamp: October 20, 2015, 04:21:31 am
content: You shouldn't be expecting to see any kind of beautiful sine or square wave, itisgoing to be spiky and nasty. It's a mechanical motor after all with inductive speaks and all. All you're interested in is the peak value (the voltage at the peak of the highest peaks), and the average value. These can both be judged by eye by reading off the number of divisions on screen, or by using the vertical measurement functions on your scope. Since you're using a 1 ohm resistor, 1 volt on screen corresponds to 1 amp of current, so you can just use amps and volts (or milliamps and millivolts) interchangeably here.Waveform averaging is absolutely the wrong thing to do, you're not looking at a perfectly repetitive signal, so averaging will just destroy the very peaks you're interested in. Triggering isn't terribly important, since you're only interested in seeing the peaks, which you could do even with a completely untriggered signal. But if you want to see the highest peaks nice and stable on screen for some reason, turn the trigger level so high that it fails to trigger at all, and then back up a bit. That should allow you to see the highest peaks clearly.PS/ It would be more helpful to state actual readings (number of division, or even volts!) or include a screenshot, rather than just the vague "one thing was 3x the other thing".

author: Electro Fan
author_rank: Super Contributor
timestamp: October 20, 2015, 05:38:26 am
content: Quote from: rs20 on October 20, 2015, 04:21:31 amThanks - that's all helpful. Below is an image - 10x probe terminated at 50 Ohms, normal acquisition (not averaging). Seems like nearly a 30 Amp peak - is that possible?

author: rs20
author_rank: Super Contributor
timestamp: October 20, 2015, 05:46:42 am
content: A 10x probe shouldn't be terminated with 50 ohms at the scope... are you actually using a 10x probe with the probe switch (if relevant)set to 10x? Again, standard oscilloscope probes should be used with Hi-Z input impedance.Do you have the oscilloscope probe connectedacrossthe 1 ohm resistor? I.e., with the hook on one side of the resistor, and the ground clip on the other side? Be careful doing this (google the EEVBlog video "how not to blow up your oscilloscope" for more details):-- Either connect the resistor on the low side, on the negative side of the power supply, so that the ground clip on the side of the resistor is connected to ground anyway OR-- Make absolutely sure your entire circuit is floating (no earth reference, USB connections including arduinos etc)If you've got that all set up correctly, then yeah, that implies that you're seeing 30A peak, although the negative peaks are similarly huge, I can hardly make out any average current in there. I'm not sure this is correct. You should sanity check by replacing the motor with a resistor (leaving every other detail of the circuit intact), and verify that the figures you read off the oscilloscope match power supply readings and calculations and multimeter measurements.

author: Electro Fan
author_rank: Super Contributor
timestamp: October 20, 2015, 05:53:12 am
content: If I terminate at 1M Ohm the peak is close to 100 (Amps with the 1 Ohm resistor). I'll recheck per our instructions on the setup. Thanks

author: Simon
author_rank: Unknown
timestamp: October 20, 2015, 05:57:03 am
content: if you measure the resistance of the motor you will get the resistance that can cause the maximum current draw. Most motors draw 10x or more current to start, that will immediately trigger your supply.

author: Electro Fan
author_rank: Super Contributor
timestamp: October 20, 2015, 06:23:46 am
content: Quote from: Simon on October 20, 2015, 05:57:03 amI wouldn't have guessed this, but the DC motor sure seems to be reading 1 Ohm.

author: Electro Fan
author_rank: Super Contributor
timestamp: October 20, 2015, 06:34:24 am
content: Quote from: rs20 on October 20, 2015, 05:46:42 amOk, to confirm:Negative terminal on Power supply connected to 1 Ohm Resistor; other side of 1 Ohm Resistor connected to Negative terminal on DC Motor; Positive terminal on DC Motor connected to Positive terminal on Power Supply. Power Supply set for 3V, 1Amp max. (Now it reads about 120mA, but I think that's just an attempt to show the average on a slow display).Scope is set to 10x (with 10x probe), 1MOhm probe termination. DC coupling, Edge triggering, using your slightly backed off from the peak triggering technique.- Where do recommend placing the probe and the probe ground? Thanks- No Arduinos, the Ground (Green) on the DC Power Supply is connected to nothing; Scope and DC Power Supply both have Grounded AC plugs. I appreciate you keeping my gear and me safe

author: hayatepilot
author_rank: Frequent Contributor
timestamp: October 20, 2015, 07:54:37 am
content: Quote from: Electro Fan on October 20, 2015, 06:34:24 amYour Power Supply is floating, therefore you can connect the scope directly across the 1Ohm resistor.Greetings

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 20, 2015, 10:59:37 am
content: Quote from: Electro Fan on October 20, 2015, 06:23:46 amsmaller motor has larger ohm, bigger motor got lesser than that...Quote from: Electro Fan on October 20, 2015, 05:38:26 amnot likely, probably that just an inductive or common mode noise...Quote from: Electro Fan on October 20, 2015, 03:30:25 amno suggestion for preferred scope settings, but just buy a dc current probe, you can comfortably see whats going on... see attached example, 1st pic: blue trace is a small dc motor's current probe 1mV = 10mA during a high side pnp's collector switching (yellow trace), second pic is free running dc motor current at fully on high side pnp...