# Electronic Circuits and Projects Forum

1. ## DC Motor Torque Calculation

How can I calculate the DC Motor Torque? Sorry that I only have very limit info.
DC Motor:
Voltage: 18 V
Current: 4 A
Factor in a 40 degree slope
The torque requires to carry 23 kg load.

2. The best I see is deriving a theoretical torque and that number would not even take motor inefficiency, drag and other factors into consideration. At that you would still need the rotational speed of the motor.

18 Volts * 4 Amps = 72 Watts
72 Watts / 746 Watts (1 HP) = .0965 HP
Divide the HP / RPM (Unknown)
Multiply by a constant of 63,025 (the answer will be expressed in LbFt of torque)

Even with the RPM known for the current and voltage the answer would be theoretical and likely not even close to the real world torque the motor was actually producing. Also, I am not even sure how correct what I stated was. I think it goes that way and am also unsure of the constant I used.

Maybe another member has a better solution?

Ron

3. Reloadron has given it the best shot from what's available. Unfortunately there isn't a way to tell for sure from this limited information.

If it's critical, the usual technique is to hook the DC motor up to a big heavy disc with a chopper wheel connected to a frequency datalogger and see how fast it can accelerate this purely inertial load. This will give you the torque vs. RPM curve.

The redneck approach is to clamp a ruler onto the shaft and keep adding weights until it can no longer move.

4. Thanks for you guys reply. Actually, I want to know the amount of torque before I purchase a DC Motor. I mean, how much the amount of torque that I need and so that I can allow my robot to carry 23 kg load?

5. The redneck approach is to clamp a ruler onto the shaft and keep adding weights until it can no longer move.
That being how I would have done it. I know I have some of those lead salt water fishing sinkers around here somewhere.

Thanks for you guys reply. Actually, I want to know the amount of torque before I purchase a DC Motor. I mean, how much the amount of torque that I need and so that I can allow my robot to carry 23 kg load?
That changes things. You need to know where the motor is and where the load is. There is quite a bit to it. From the motor shaft center for example 12 inches out if I have a 1 Lb weight I will need (on a horizontal plane) 1 LbFt of torque to lift the weight. This goes back to what Duffy mentioned as to the ruler and lifting a weight. In reality I would need more than that.

Ron

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