How to estimate motor current under load

Question

How do you estimate the average current under a typical load of a brushed DC motor?

I'm trying to size the right motor, motor driver and battery to propel a small two-wheeled platform using this guide, and that's allowed me to calculate the minimum unloaded RPMs and minimum stall torque I need, but when I go to find motors that match these specs, they only list unloaded current and stall current, neither of which will be my typical use case. The platform will never be weightless but neither will it have so much that it'll be stalled.

Since my specs require a motor that's both fairly fast and high-torque, the only motors I'm finding that satisfy that are usually high-current, some with a stall current of 6A or more. Now I'm trying to pick an h-bridge motor-driver and battery, and there's a pretty big cost difference between those that can power 500mA motors vs 6A motors. Even batteries rated at 10Ah or more usually can't source that for more than a few seconds, and batteries that can will typically cost hundreds of dollars, which is over my budget.

Since I don't need the motor to source the stall current, I'm hoping to use a small driver and battery and halt the motors if I detect a current spike that lasts for more than a couple seconds, indicating a stall.

Is there a formula, or at least a "rule-of-thumb" for estimating typical motor current under load given the free-run and stall current? e.g. If unloaded motor current is X mA and stalled motor current is Y mA is it fair to approximate that the typical motor current under load will be (X + Y)/2 mA?

Answer 1

It's not unusual to find that under near-full-power-out conditions the motor will be about 50% efficient. This is a big motor but it still applies: -

Next, you estimate how much mechanical power you need (2 pi n T) where n is revs per second and T is torque in newton.metres. If your dc motor's data sheet doesn't provide this information (like the graph above) then get one that does.

If you don't know how to calculate your mechanical power requirements you need to learn and start with: -

Force = mass x acceleration courtesy of Isaac Newton

Mass is what you want to move and acceleration is final speed divided by time to get to that final speed (i.e. what you want it to do).

Force x distance = work done and work done per second = power.

Don't forget friction as a loss and if it moves fast then there will be other losses due to moving air around.