# Calculate supply voltage based on motor specs

I'm a software engineer with no experience in building electronic circuits. I'm trying to operate 2 motors from a Raspberry Pi via a H-bridge motor driver. Given the motor specs like torque, RPM and current draw, I'm trying to understand how I can calculate the supply voltage and current requirements for driving the motors.

If you have the rated torque and RATED RPM (not no-load RPM) you can calculated the rated output power, assuming they were given at the same voltage, then you can calculate the output power. You can then "guess" an efficiency to get the input power.

With input power and the current draw, you can calculate the voltage.

However, I find it difficult to believe that you were provided the rated torque of the motor but were not provided more basic information like voltage.

No-load RPM and, no-load current, and stall current are kind of useless here.

If you're trying to figure out how much current the motor will draw based on a given torque, then all you have to do for a DC motor is to draw a linear graph for the speed-torque curve. RPM on the Y-axis, torque on the X-axis. At the top left at zero torque is the no-load RPM. At the bottom right of the graph at the zero torque is the no-load RPM. Connect those two points with a line.

The point of maximum power output will be at 50% stall and torque 50% no-load RPM. The point of maximum efficiency is 1/7th the stall torque and 6/7ths the the no-load RPM. It is better to overload the motor a little bit (i.e. load torque a bit higher) around the point of maximum efficiency since efficiency rapidly decreases as you underload the torque below the point of maximum efficiency. The efficiency drops off much more slowly as you increase the torque above the point of maximum efficiency.

On this graph, you can also add a second Y-axis for current to also have a current vs RPM curve. At the bottom left at zero torque is the no-load current. At the top right at the stall torque RPM is the stall current. Connect these two points with a straight line.