What bothers me is the meaning of no load current (free current) which is the source of my problem. I want my circuit's MCU to warn me if a motor exceeds 30% load. But how to calculate load?
There is a simple brushed DC motor characterized as below (made up characteristics).
Stall current: 1.5A
Free current (no load): 0.15A
Some load applied: 0.45A
Nominal voltage: 3V
Let's say I have a circuit consisting of nominal voltage source, a motor, some form of load and an MCU along with other necessary components allowing me to measure current flow. What is the load if I measure 0.45A current?
I can think about two solutions:
1st solution related to stall current only:
Motor parts create some load which results in free current and in fact no load means 10% load.
free current load: 0.15A / 1.5A = 0.1 -> 10% load
applied load: 0.45A / 1.5A = 0.3 -> 30% load
therefore 100% load: 1.5A / 1.5A = 1 -> 100% load
2nd solution related to stall current and freerun current:
If it is described as no load then I subtract this current when calculating load and no load means 0% load.
free load: (0.15A - Ifr) / (1.5A - Ifr) = 0A / 1.35A = 0 -> 0% load
applied load: (0.45A - Ifr) / (1.5A - Ifr) = 0.3A / 1.35A = 0.22 -> 22% load
therefore 100% load: (1.5A - Ifr) / (1.5A - Ifr) = 1.35A / 1.35A = 1 -> 100% load
where Ifr = 0.15A (freerun current)
When I use 1st solution rhen my firmware will generate warning at 0.45A current draw. 2nd solution will fire warning at 0.405A current draw
I have two questions:
1) Which solutoin is correct (if any)?
2) How to calculate load if I drive motor at 2V? (In relation to stall current equal 2V/3V * 1.5A = 1A?)