What can the ohms of a DC motor tell me about itself?

I have a lot of motors and all type of size but mainly they are all small to mid size adult hand size or so , I took out my multimeter and switch to ohms settings and grabbed the ground and power wires of each motor to see what type of reading I would get, what confused me is that, I found that most bigger ones had high ohms of 1.5 to 2.5 ohms while the smaller ones had higher ohms of 4.5 to 14 ohms.

Can someone break it down for me?

• The resistance will tell you the stall current (LRA = Locked Rotor Amperes) but only if you know the operating voltage. For a given motor size and construction (and therefore horsepower), a higher resistance probably indicates a higher operating voltage. – Spehro Pefhany Jul 1 '15 at 18:16
• This is not your exact question, but if the resistance deviates from what it is supposed to be, you can be fairly sure that the motor is bad. Of course, you have to have some idea what the resistance is supposed to be in the first place. From a specification, or by comparing with a known good sample. – mkeith Jul 1 '15 at 19:58
• well for the low resistance "1.5ohms" i have 4-5 and they are all the same so im going to say that this is what it supposed to be but thanks on that note very useful (resistance deviates from what it is supposed to be, you can be fairly sure that the motor is bad) – Frank Jul 2 '15 at 0:48

In general terms, the motors with the lower resistance will draw more current (ohm's law), and therefore use more power, and (hopefully), generate more mechanical power.

• when you say resistance you mean "ohms" the reading from the meter in setting ohm is resistance? – Frank Jul 1 '15 at 18:05
• @Frank'sDesig'Nature, yes, an Ohm meter measures DC resistance. The unit of resistance is the Ohm. – Solomon Slow Jul 1 '15 at 18:17
• A stalled DC motor with lower DC resistance will draw more current and hopefully, create more torque. Things get complicated—at least, more so than I understand—when the motor shaft is allowed to spin. Back EMF and all that... – Solomon Slow Jul 1 '15 at 18:19
• i must admit that i was actually looking to see which motor will have more torque, i guess i was not trying todo the whole wire splitting between power and multimeter to that power and multimeter ground to battery power terminal and reading amps on the meter i thought the ohm setting will tell me this without doing all that – Frank Jul 1 '15 at 18:22
• tho i have to add that the motor who game my the highest reading 15ohms belong to a mini car vacuum that i took apart and that motor def is powerful so im not sure why im reading that the lower resistance will power more then this one? not to mention that the small ones will work at 3/5v with only 1A while the 15ohm one will not unless i crank up the amps/current and voltage since amp is propositional to the voltage and when i took this vacuum apart there where like 15ohms small battery linked up to each other "so i kept those batteries" - – Frank Jul 1 '15 at 18:30

For a given motor, torque is proportional to current, back emf to rpm. This means that at some rpm, the back emf is sufficiently large that the difference between supply voltage and back emf is so small it cannot generate a current larger than the friction. This is the maximum no-load rpm (for DC motors, but AC work similar).

• i keep hearing what your saying just about every where but is like ppl just repeating what they hear "they dont break it down beyond that" so to my understanding when you say "supply voltage and back emf is so small it cannot generate a current larger than the friction." – Frank Jul 3 '15 at 3:08

I have a brushed DC motor with 13.5 0hms resistance. I use that to calculate the STALL current at maximum torque (More current = More torque). That is pretty much accurate with the datasheet spec.
At 3V, when motor is stalled current shoots to about 3/13.5 ~=170 mA

• ok so basically if my motor is 3-12v and i want to run it at 12v that motor has a resistence of 1.5ohms should i do the math for that ? cuz it comes up to 8 soooo 9 what ? 8 amps ? at 12v that motor will suck up 8A cuz of its resistance? – Frank Jul 3 '15 at 3:07
• Yes, Motor I have will consume maximum power (increasing torque) until the current increases (limited by V/Rmotor). If stall current is not limited by other means. later, it will overheat and burns – Umar Jul 3 '15 at 3:11
• i keep hearing what your saying just about every where but is like ppl just repeating what they hear "they dont break it down beyond that"so to my understanding when you say "supply voltage and back emf is so small it cannot generate a current larger than the friction."- – Frank Jul 3 '15 at 6:43
• - Are you saying that the back emf is large enough that its just about equal to that of voltage currently getting? Meaning, -if i was using the motor as a generator and spun the shaft to the point of RPM's equivalent to that of if i was giving it 12v, it will generate 12v of off that back emf? so therefor it reach its maximum- – Frank Jul 3 '15 at 6:43
• Yes. Back emf increases as the motor is tried to stop (increased load) so that the motor can sustain the same speed. When stalled, the EMF will increase and the drop will be supply/DC off state resistance. While running with typical load, the emf will be [supply] minus [current * DC off state resistance] – Umar Jul 3 '15 at 7:03