testing motor coil resistance
Matt Battle
Posts: 172
Hello all-
I am reading the Building robot drive trains book and its telling me how to test for the stall current. I don't know if I am doing this right. I attached 9V to the motor and I believe I have my Multi-Meter set to OHMs and I place the probes on the motor terminals and I am getting no readings from them. What do you think I am doing wrong?
-Matt
I am reading the Building robot drive trains book and its telling me how to test for the stall current. I don't know if I am doing this right. I attached 9V to the motor and I believe I have my Multi-Meter set to OHMs and I place the probes on the motor terminals and I am getting no readings from them. What do you think I am doing wrong?
-Matt
Comments
One does not use an ohmmeter in/on a·live circuit as·doing so will turn it into a "nohmmeter."
** Edit **
Stall current or Locked Rotor current
Many manufacturers quote a locked armature, or stall current, rating. This is the peak current that a switch or relay will have to handle if it quickly applies full battery voltage to the motor and is quoted so this can be properly rated.
When a speed controller is used, the picture gets more complicated. However if the speed controller has an internal current limit (as do all controllers sold by 4QD) then the stall current is not a useful figure as the controller will limit its output current and will never give more current than it is designed to give. If the motor ever stalls, the current limiting will protect the controller.
You should never allow the stall current to flow for more than a fraction of a second: it will damage most motors extremely quickly! So a controller with a current limit will also be helping to protect the motor if it is chosen properly.
Post Edited (PJ Allen) : 3/8/2006 9:20:05 PM GMT
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·1+1=10
Hmm, I guess I misread it. I thought it was weird when it told me to do that????? Well, I disconnect the power like Paul said and I used the lowest setting on my tester to measure the coil resistance and it gave me a reading of 7.9 which I rounded to 8 ohms if that is okay to do? So when the book ask for the supply voltage, that is the voltage I supply it with or is that what the motor is rated at?
I am also about to test for the no-load current. The book is telling me to use a low-value, high wattage type for this test. What if I don't have one of those? What can I use instead? Thanks guys.
-Matt
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·1+1=10
So, OK, your motor (coil) has a resistance of 8 ohms.· There isn't anything that you can infer from that, other than if you apply 8V then it'll draw 1A (1/2 that voltage gets 1/2 that current; Ohm's Law.)·
Is that OK?· Who can know?· There's no telling.
And, for the no-load current test, you will be measuring the voltage drop across the resistor (set your meter for volts, not resistance), so you will, in this case, ·hook up the motor to the intended power source (as shown in the diagram).
PAR
·
PJ Allen-
Well, I am trying to figure out how much total amps that the motor will draw but I guess not. Need help with this one. I can't trust the source that I got the motor info from.
-Matt
PAR
Thank you PAR. I have to motors I am going to use, they are the ones that came with the roomba. What I am trying to do is find the max AMP that these motors will draw so I know if I got the right power tranistors for it. I don't want to cause a fire or damage to anything.
-Matt
Post Edited (Matt Battle) : 3/9/2006 12:49:32 PM GMT
Your request has yielded some comedy I am unaccustomed to in these forums, so I would like to thank you for that.
For what you are trying to do, hook up your multimeter as per Paul's suggestion, apply your chosen voltage and read the current on the meter. You will find the current will change as you apply load by slowing the spinning motor in your fingers, unless it is a really big motor, then you will probably want to use somebody else's fingers.· So, this way you can simulate a heavy load - all the way to stall.
You seem to have the right idea for transistor choice, allow enough for a conservitive rating, you may need to look into heatsinking, but that is another topic, and little dc motors are electrically noisy which can result in other problems.· To fix that, fit a ceramic capacitor across the motor terminals.· the best size is good, but in the absence of knowing that, try 0.1uF· Fit·across the motor terminals, not at the end of the supply lead.
hope this helps,· Bongo
Bongo-
Its going to be hard for me to test the current change on the motors because its pretty much housed in a casing and its connected with to a wheel with a rubber type band(the name for this has left my mind). When I grab the wheel even slightly the motor stalls. So testing it is going to be a problem. Also what am I more worried about more, voltage or amps? I am going to play around with some h-bridges tonight using some cheap motors I got out of some xmods cars. I want to figure out which one I like the most.
-Matt
There is a relay driver that you may come across in your research, the UNL2003.· This has the diodes inbuilt with 7 or 8 darlington arrays on the one IC.· Each one is rated at 600mA output, so if you use 2 of these in parallel, there is a 1.2A capibility.
Know nothing about H bridges, but I suspect that they are a bit of an overkill for your motor.· But don't let that stop you from checking them out, it is the way to learn.
bongo
He must have an H-bridge if he wants to reverse direction. It just won't happen without it.· Relays can work at an H-bridge too.
8 ohms at 5 volts is = 1300· 625 ma. So you have destroyed that 500ma transistor.·[noparse][[/noparse]8 ohms at 9 volts is still too much.]
It would be better to use 5amp rated FETs and have the headroom for stalls, abuses [noparse][[/noparse]such as short circuits], and miscalculations. The is essentially called 'derating the 5amp FET' and since the additional cost is near to nothing, it eliminates the need to ponder the actual stall current.
If one has a test bench in a big wonderful labratory or if all suppliers would provide all required data, it would be just a matter of fitting formulas. But design in the real world is about making things work with what you have and what you know.
Testing comes to two flavors -- Non-destructive and Destructive.
If I have a motor that burns up during my stall test, what have I learned and what have I achieved? If it doesn't burn up, I have some useful information, but still not the actual limit.
In other words, if you like the motor and want to use it, the testing must be designed to get useful information without damaging it. If you have a spare or several, you can load it up and then determine what voltage will set it aflame.
In comes the Rule of Thumb -- just provide a 5amp curcuit at a operating reasonable voltage for you 1.3amp motor. Maybe you are over-built by 400% or maybe only 150%. No one knows for sure, but you know you are not designed for immediate failure.
If you really want to test for voltage, heat causes most of the damage. Just run the motor at a nominal voltage and see if it gets too hot to touch or stays reasonably cool. A lot of 6volt motors will run fine at 12volts and at times I think the manufactures just change the labels. But keep in mind that some motors are meant for intermittent use, some are meant for light loads only, and some are meant to run 7/24 under heavy loads.
So look at it , does it seem like a throw away hobby motor or an industrial design. If it is an expensive, durible motor, the manufacture wants to present it as looking the part. Otherwise, you might choose a prettier one.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)
······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
Post Edited (Kramer) : 3/12/2006 11:02:24 AM GMT
You can't directly measure the internal resistance of a permanent magnet DC motor using an Ohm meter.· Internal resistance·has to be calculated.
Lock the shaft, and measure the current draw (the stall current).· Choose a battery that will not drop its voltage when the motor is stalled.· For most motors, I like to use a D-Cell battery.· Then calculate the internal resistance R = V/I
Locking the shaft is not dangerous if you use a low voltage source.· 9V batteries for this test, in my opinion, are worthless because they have very high internal resistance (10 ohms is not uncommon), they can't source very much current, and their voltage drops when you make the tests, which skews the results.
Pete
Something I have noticed is Parallax's strong emphasis on interest and education. This has some meaning to me, as a person who never finished high school.
When I bought my first micro development board, there was no user groups. It was an SX 18 with a proto board Rev A. The board was full of holes that I did not know what to do with. So I made my own (for the 28) with PCB terminal strips attached. I had that (and still do) and the SX manual. The parallax proto board had AC or DC written on it. As there was a bridge rectifier, filter cap and 7805 reg on the board I used an A/C plug pack. It did not work and somebody from parallax emailed with advice to use a bigger plug pack. I swapped to a DC supply and it worked. I emailed the Parallax guy with my opinion that the filter cap was too small. I guess he did not believe me, but the same thing came up on the sx site not all that long ago.
Now there is more information freely available than you can poke a stick at. So why the questions?
·A) Is it that it is very difficult to find exactly what you are looking for.
C) Or is the question a brand new one – never before asked?
[list=100][/list]I like A, and think that pointing somebody in the direction of the information they are looking for is of great assistance. I am of the opinion that B is a little limiting. And I read these forums searching for C.
If you can make a motor run in one direction with a transistor, figuring out how to run it in reverse is not that difficult. The question becomes which way is the best for what I am trying to achieve and with resources there are readily available.
But one thing at a time here, else we are turning a very simple exercise that anyone can do, into something very confusing. There is a huge professional education system out there for that.
bongo
Here is an H-bridge that can handle 2 amps if you wire them together: www.hobbyengineering.com/H1048.html. The wiring setup is the same as the L293D (see www.solarbotics.com).
Say Kramer.... when I went to school, 5 volts onto an 8 ohm load produced a current of .6 Amps. Has the "new math" influenced us that much?
Cheers,
Peter (pjv)
I am going try some 2N4401 and 2N4403 and see if they can handle whatever the motor can throw at them. If not then I will try some TIP41 and TIP42's. Now if those don't work time for some Heavy duty power transistors. I am looking at the power transistors that is on the roomba PCB and they are smaller then the TIP41 and TIP42's so I if thats a factor then I should be fine with using them but who knows.
-Matt
The 2N4401/03 are a great pair, but not suitable for 0.5 Amps. Eventhough their "maximum" rating will just permit it, their gain falls off rapidly with increasing currents, and they just won't saturate well enough, and that means heat. Their domain is in the 100 mA range.
The TIP's are a much better choice.
Cheers,
Peter (pjv)