Your Friendly Electrical Salesman?

Essentially, they are simplified versions of variable-frequency-drives (VFD) or variable-speed-drives (VSD).

While you can vary the speed or frequency of a VFD driven motor, a soft-start merely injects a voltage and frequency that ramps up from zero (or a starting value if you configure it). Once it reaches maximum frequency and voltage, it stay there until you stop it.

There are usually two ways to stop: coast to stop (just remove the power from the motor) or soft-stop (ramp the voltage and frequency down to zero).

What do they use them for? In some applications, it keeps the mechanical parts from falling apart due to the sudden start of the motor (if you weren't using a soft-start). In pumps, the soft-start prevents the hammering of the pipes when the fluid is suddenly forced to move.

There's just one thing that I can't answer and that is the reverse soft-start. I don't know what that is. 'Never heard of it.

The main factor in soft starting is so as to do two things Prevent the sudden high speed form casusind damage as you say but the other is to prevent a sudden power surge from causing fuses to blow or contactors/trips from being damaged/tripped out by the surge current for a large motor it can be infinate until it starts to rotate. Reverse soft start is like an electrical break it controls the motors run down to a stop.

the other is to prevent a sudden power surge from causing fuses to blow or contactors/trips from being damaged/tripped out by the surge current

Thanks, BrainWave. Missed that one.

Reverse soft start is like an electrical break it controls the motors run down to a stop

So, it's the same as the soft-stop that I mentioned. Personally, I wouldn't use such a term since "reverse" usually means (in the case of motors) is to spin the other way. If I were a customer, I would think that you're starting the motor in reverse in a soft way (whatever that means). 'Not something I'd want, definitely.

Yes it is a way of applying either regenerative breaking or bring the motor slowly to a controlled stop. Ref other posts.

In areas where there are domestic p.c.s and such running off the same mains they also prevent surges and voltage drops that may affect the appliances as the starting current can be limited . Rex Devantier.

I use soft stop on a conveyor with top heavy parts. We slow the conveyor down at stop to keep parts from falling over.