What are ramps (soft start and soft stop) good for, and how much should I set?
A ramp raises the voltage applied to the motor gradually instead of instantly, suppressing the starting current. Starting current is typically 5–8 times the rated current, but with a soft start it drops to 2–4 times. Soft start and soft stop are built into the KS250; on the PT500 they are set with the RMP pot.
At the moment of starting the rotor is stationary, so there is no back-EMF and the motor behaves almost like a shorted load. That is why locked-rotor current is far above the rated value, and depending on the load it can last 0.5–3 seconds.
What a ramp buys you:
- A smaller current pulse, so the fuse does not blow needlessly.
- Less voltage sag on the supply line, and less stress on the battery and cabling.
- Lower mechanical shock torque, so gears, chains, ropes and couplings last longer.
- The load moves without swinging; critical in lifting and platform applications.
How to set it: start with a short ramp time and lengthen it step by step until the current pulse is acceptable and the motion is free of jolts. Too long a ramp also causes problems; the motor stays at low voltage and high current for a long time, and with heavy loads it may stall before the start is completed. In lifting applications the stopping ramp matters especially: an abrupt stop swings the load and shocks the rope.
Note: chopper-type DC drivers apply the armature voltage gradually anyway, so by their nature they provide a certain amount of soft starting; a separate soft-starter device is usually not needed.
The full topic: DC Motor Driver Selection Guide