How to calculate NM for a motor without specifications

When we are working with our first DIY in some cases we bought some wiper motors in our local city/country to make the initial prototypes. Those motors in some cases don't have the information to make the necessaries calculations, and after our initial prototypes, we need to buy motors with more or less force depends on our actual wiper motors.

After finding in some forums we made the next:

1. Connect our motor (with gearbox) to a power supply (with almost 25 A, 12 or 24 V depends on our motor) and connect the multimeter to obtain the Amperage. https://www.wikihow.com/Measure-Amperage. Our motor going to consume more or less amperage proportionally to the effort that needs to work with the load.

For our case, our motor works with 12 v DC and we use a power supply with 40A.

2. Incrementally try to put more load to the motor until the rpm decrease. In this point, you can see the multimeter and obtain the Amperage over load.

With the maxload until decrease the speed, the multimeter shows 9 Amp.

3. Now we need to calculate the rpm of our gearbox, for that connect the motor without load and count how many times does the engine gearbox turn in one minute.

Our RPM: 40

4. Make the next:

Motor Voltage * Amperage (obtained in the point 2) = watts

In our example -> 12 * 9 = 108 watts

5. Using the next calculator online put watts and rpm and obtain the NM.

Divide your watts between 1000 to obtain kW.

In our example -> 108/1000 = 0.108 kW

http://wentec.com/unipower/calculators/power_torque.asp




I hope this post help anybody that needs motors information without specifications.

Note that electrical input power is not equal to axle output power. For example, if there is a gearbox with worm gear, efficiency could be only 75%. Which would mean under 20Nm of torque instead of nearly 26Nm in your example.