Unknown motor - measure power from resistance?

I found a motor laying around and all I can come up with is a part #, that goes to a special order for a machine. It’s a brushed DC motor, kind of heavy and I believe it was used as a servo motor for the CNC machine. The encoder is not attached, it’s just a double shaft motor with 2 leads, and 1 additional lead to the metal shell that is GREEN. It’s kind of bulky about the width of a 12-ounce soda can, and just short of it’s length.

Can I determine or calculate anything by measuring the resistance of the leads on this unknown (or any other) brushed DC motor?

Yes, you can calculate the stall current if you know the DC resistance at rest.


Use an Ohm meter to measure the resistance between the motor leads, turn the shaft a few degrees and do it again until you go a full turn, watch for higher (or lower) than normal R values that might signal an open (or shorted) armature winding or worn out brushes.
Don't spin the motor fast with your meter attached (it will act as a generator & may damage your meter), only take a reading while the motor is stationary, please report your findings.

Also you should repeat the test with one Ohmmeter lead connected to the green ground wire and the other to each motor lead in turn, you are looking for an open circuit (at least 100k anyway), anything lower may indicate a grounded armature (or field, if it's a series wound motor) winding.

You cannot calculate anything from measuring the armature resistance since two parameters are required to calculate a third.

If it is a DC motor with permanent magnet pole pieces its speed will be proportional to the applied voltage.

As has already been said, it will generate a voltage if you spin its shaft.
By using an electric drill connected to its shaft and spinning it at the drill’s speed you can measure its output voltage. This will then give you a reasonable indication of its expected drive voltage to produce a similar speed. However, it will not indicate its designed operating parameters with respect to voltage, current, speed or output shaft power (Ps)

The motor power and torque can be estimated by weighing it! The properties of iron and copper are
pretty constant between motors, so similar sized motors will have similar thermal behaviour (which
limits continuous power dissipated in the winding, which is related to output power - assuming a
reasonable motor efficiency).

Rotor volume is roughly related to torque, hence overall motor size is as well.

The motor power and torque can be estimated by weighing it! The properties of iron and copper are
pretty constant between motors, so similar sized motors will have similar thermal behaviour (which
limits continuous power dissipated in the winding, which is related to output power - assuming a
reasonable motor efficiency).

Rotor volume is roughly related to torque, hence overall motor size is as well.

But not if you have no prior experience or a collection of similar motors to use as reference points.

If the OP posts a photograph of the motor alongside a measuring scale, a rough dimension of the output shaft, an estimate of the brush cross section dimension and all text printed on the motor casing he might get a sensible suggestion as to its capability

Similar size motors with datasheets are available!

Pictures of motor

I found this link for google search “electrocraft 0642 09 030” (supplier is asking $500.00 for this replacement part)

Replacement motor for HAAS cnc

To me, there doesn’t appear to be $500.00 of hardware here - but I’m cheap. If they have a contract with HAAS, then can probably charge $5,000 for a fan motor since their customers are at the mercy of parts repair to get their machines back run again. It’s not a spindle motor, that thing is about 12" diameter and needs a hoist to lift. Probably a servo to drive cross slide, material feed, turret, or otherwise.

I searched around electrocraft website after an hour, and got lost in all the PDF documents and crapola about how great electrocraft is and how wonderful the company is to do business with. I just want to look up a part number in their database, and not sign up to have a sales-person call me about price quotes for a 10,000 pallet shipment of custom motors.

There are 4 spots to unscrew and replace the brushes. I did not open them to inspect the brush size - but I will do that if anyone is actually interested. The plastic screw is 16mm or .615" (battery in my calipers going dead).

Double output shafts, 1/2" (12.71 mm) about 3/7" (19mm) long.

Test 2-leads at idle no motion:

5.5 ohm resistance at idle no motion. 1/4 turn, returns same 5.5 ohm resistance value. Same for 1/2 turn. 3/4 turn meter settles down at 5.3 ohms. I turned it one more time about 1/4 way and now resistance is 6.4 ohm. turn a little more 6.0 ohm, then settle down to 5.2 ohm. I am using a cheap handy portable digital multimeter.

Test one lead to Ground wire connected to shell:

54.2 ohm. turn 1/8: 53.8 ohm. turn 1/4: 53.9 ohm turn 1/4: 54.0 ohm. 1/8 turn: 43.8 ohm. 1/8 turn: 54.1 ohm.

I hook up a 6-cell lithium ion battery pack, 24 volts. The motor PACKS A WOLLOP. The motor spins strong - and quiet. I tried using my fingers clamp to slow it down, and definitely has no intention to do that (yes I know, it’s not safe - I’ve got it clamped down, and there’s no sharp edges). I tried using some paper towel and slow it down with my hands, and it keeps on trucking. I did not test the RPM yet, I have to make a wheel or something for my sensor to pickup in order to get the rpm. Feels low rpm I guess 300 maybe ?? Not sure.

Picture 2

p.s. I guess the motor to weigh between 8-10 pounds. My triple beam balance can not measure above 500 grams - so I can’t give a precise weight.

Housing is 3.25" diameter. Housing length is 4.25". I guess this is a NEMA-34 size mounting pattern, since it’s definately larger than NEMA 23 motors, which are my largest steppers I own (here) to compare to.

Picture 3

5 ohms armature resistance seems rather high for a motor which "packs a whack" unless it is a series wound unit, or your measurement is in error.

The fact that you estimate it's running at around 300RPM seems strange, but again, if it is a series wound motor then motor speed is inversely proportional to field flux. A high series field flux produces slow speed.

What resistance does your meter read with just the probe leads connected together.

Assuming (dangerous game) the stall current at 24 volts was around 5 amps (based on your 5 ohm measurement) then running current might well be less than half that value so let's say 2 amps. So wattage would probably be no more than the 50 watts at 24volts

You shouldn't have any resistance between the supply leads and the casing.

By "pack a wollop", is a subjective opinion from feeling the power with my fingers.

I round guessed at 300 rpm for 24 volts, and do not have any confidence with that number. So I do more tests:

My digital portable meter reads 0.00 when I touch the two leads together. When I take my tests, I take at least 3 readings for each measurement - by remove leads, re-attach (3+ times for each reading).

Red lead to case: 48.2 ohm
Red lead to SHAFT: 53.0 ohm

black lead to case: 48.8 ohm
black lead to SHAFT: 58.3 ohm

I hooked up motor again, to a 3s lipo rectangular battery 12.31 volts. The speed is probably what my cordless drill speed is if I press trigger 1/2 way. Gator clip test wires for this test were only 20 gauge, I am embarrassed to admit. 16 gauge wires from battery to watt meter.

I hooked up "watt meter" commonly used for R/C hobby cheap ebay sale. At idle, the meter reads:

.54 amps. 12.25v 6.6 watts

If I clamp both shafts with both hands, 3 fingers very tightly the motor will slow down but still continue to spin. "Watt meter" reads:

1.89 - 2.12 amps. 26.1 watts (peak). voltage drop to 12.14

A general point on measuring very low resistances.

It's called the Kelvin 4-WIRE system .

Supply a known current to the resistor with two wires.

Measure across the resistor with an isolated voltmeter.

then R = V/I


A couple of E642 servo motors are described here. They appear to be rated at about 275 Watts, which is consistent with the weight and heavy construction.

Many companies rip off their customers by charging outrageous prices for replacement parts, so charging $500 for a $25-$50 motor would not be very unusual.

One reason industrial servo motors are expensive is because they are designed to last say 30 to 40 years in the
field and thus maintainance and spare parts and servicing become a large cost to the manufacturer... They
are not consumer grade or even commodity items given the complexity of large motion-control setups. :slight_smile:

Fortunately there are more and more cheap and cheerful servomotors becoming available for hobby CNC use
which don't have those costs.

Ok, so now i think an important parameter is the volts per rpm. And the only other test, I can think of is a load test. For a dynamic load, I think I need to build an electronic variable brake.

I thought if I had a large enough motor, I hook up to a powered motor to act as a generator. This way I can use a pwm signal on the generator side to vary the load the generator uses, while the powered test motor was in motion.

There's a motor at the junk yard, is 12" x 24" unmarked labeled as "DC MOTOR". It was too heavy for me to move without having to move 10 other motors that were just as heavy.

What kind of horsepower could I expect to get from that - if the bearings were still good or I replaced them - and the windings were not shorted? What type of resistance could I expect to measure from that - would I need a different meter?

What kind of horsepower could I expect to get from that - if the bearings were still good or I replaced them - and the windings were not shorted? Who knows ?

What type of resistance could I expect to measure Electrical

would I need a different meter? Who knows ?

OK those are dumb answers but so were the questions

Your previous post on using a motor as a generator acting as a dynamic brake (dynamometer) was good and I’ve used that technique numerous times to measure motor performance. Rather than using PWM though might I suggest you use simple linear resistors. First you need to establish that the motor will generate and here again I suggest you use an electric drill as a test drive motor. Measure the ‘generator’ output and then start to load it with resistors. Vehicle lamps form quite good high wattage/low voltage resistors though they are non-linear since resistance rises with voltage due to heating effect.

You won’t need a ‘large’ motor - even one the same size as the unit under test will be adequate since it will permit you to establish numerous operating parameters.

Work on the basis of say 80% energy transfer from input power to dynamometer load power

I was going to use a car alternator for the load tester. There's piles of them at scrap yard, I know a guy who will let me scavenge for lunch money. I wanted to grab another one try and convert it to a motor to see what happens. But I'm guessing the current needed to make it useful is probably too large to offset the size of the batteries to make a electric motorcycle that wasn't lame.

For the 12" x 24" motor - I will jumper a 12v car battery and see what kind of noise or smoke it makes - if the windigns were bad then hopefully it would show then. I have a magnet-clamp style multi-meter I would use to measure the current draw from that. I would use this motor to bolt directly to the transmission for the pickup truck with blown v8.