How many times the continuous current is the motor's start-up (inrush) current?
In brushed DC motors the start-up current is typically 5-8 times the rated current and lasts 0.5-3 seconds depending on the load and the inertia. When a soft start (ramp) is used this surge typically drops to 2-4 times the rated current; on the KS250 the ramp is built in, on the PT500 it is set with the RMP trimmer.
At the moment of start-up the rotor is not yet turning, so no back EMF is produced. At that moment the motor behaves like an almost short-circuited load limited only by the winding resistance; the current therefore goes far above the rated value. As the rotor accelerates the back EMF rises and the current falls to its steady value.
Typical magnitudes:
- Start-up (locked rotor / inrush) current: 5-8 times the rated current.
- Surge duration: 0.5-3 seconds depending on the load and the inertia.
- Surge with a ramped start: 2-4 times the rated current.
This surge causes trouble in three places: the driver's peak current window, the fuse's time-current curve and the instantaneous voltage drop in the cable. The solution is usually a ramp. The KS250 has soft start and stop functions; on the PT500 the ramp time is set with the RMP trimmer. The longer you make the ramp, the smaller the surge and the less the mechanical stress, but the slower the response at the start of motion.
Additional note: because DC chopper drivers apply the armature voltage gradually, by their nature they provide a degree of soft start; a separate soft-starter device is generally not needed.
The full topic: DC Motor Driver Selection Guide