Should I use a fast-acting or a slow-blow fuse?
For motor loads a slow-blow (T/TT) characteristic should be preferred. The starting current rises to typically 5-8 times the nominal value and lasts 0.5-3 seconds; a fast-acting fuse takes this normal pulse for a fault and blows. ATO/blade type automotive fuses generally have a fast-acting characteristic.
What makes the difference is the time-current curve. A fast-acting fuse reacts in a very short time to current above its rating; a slow-blow fuse tolerates short overshoots for a certain period and blows on a sustained overshoot.
Motor loads are exactly the loads that create this need for tolerance. At start the rotor is stationary, there is no back-EMF, the current rises to typically 5-8 times the nominal value and stays high for 0.5-3 seconds. A fast-acting fuse interprets this entirely normal pulse as a fault and blows at every start.
Selection guide:
- Slow-blow (T/TT): Loads with a starting pulse such as motors, actuators and compressors.
- Fast-acting (F/FF): Electronic boards, low-power loads with constant current.
- ATO/blade: Common and cheap, but mostly of fast-acting characteristic; if you are going to use them, check the time-current curve.
Reducing the pulse is a second solution: a ramped start brings the current pulse down to typically 2-4 times the nominal value. Soft start and stop are included in the KS250; on the PT500 the ramp is set with the RMP pot. Using the ramp relieves both the fuse and the mechanics.
The full topic: DC Motor Driver Selection Guide