Current problems with DC motor interacting with a H-Bridge IC

Question

How do DC motors work in stall I have been told (from my engineering project seminars at uni) that if your motor has a stall current of 5.6A, you should use a 5.8A fuse to cut it off after it begins to stall (but not cut out EVERY time, as it might just get bumped or some other temporary interruption occurs, so we don't blow a fuse every time). Does the motor begin to stall and goes beyond the stall current after a brief period of time, blowing the fuse to cut it off (this doesn't seem right to me so I'm asking)?

The other thing I'm curious about it controlling the max current my motor can draw. So I have an H-Bridge IC with a maximum rated current (to power the motor) of 5A. The motor has a stall current of 5.6A so can I use a series resistor to reduce the maximum current to 4.8A with a 5A fuse (using similar reasoning as in the previous paragraph)?

Answer 1

No, the stall current can be calculated from the supply voltage and the motor`s winding resistance, so it won't draw more than the stall current.

Therefore a fuse rated for more than the stall current is useless as protection against permanent stall. (It can still protect against other faults - wiring shorts for example).

what you can do is employ a "motor-rated" breaker or "class C" breaker somewhere between rated current and stall current - perhaps around 3A in your example (rated current is probably about 2A). Or a "slow blow" fuse.

Either of these devices will allow a short term overload, to permit the stall current for long enough to start the motor or overcome a small obstacle. But eventually the bi-metallic strip in the breaker heats up, and if the overcurrent persists for a second or so, the breaker will trip. Or the fuse wire melts. (NB if you restart the motor before the current sensor has cooled. it will trip faster next time)

Exact breaker rating requires information we don't have here, such as the spin-up time of the motor, the inertia of its load, and the details of the breaker. Find and study the datasheets for a few motor-rated breakers - they plot the trip time against current.

Description of the different classes of breakers here. (Note these breakers are for AC mains, but similar issues apply to DC protection)