Variable frequency drive Ramping

VFDs offer many of the same advantages as reduced-voltage & soft start starters. The timed speed ramp-up feature found in variable frequency drives is similar to the soft start function of starters. However, the variable frequency drive timed speed ramp-up generally has a much smoother acceleration than soft starting, which is usually done in steps. Soft starting with a variable frequency drive reduces the frequency initially supplied to the motor & steps up the frequency over a preset period of time. Variable frequency drives with soft start capabilities have replaced many of the older types of reduced-voltage starters. While variable frequency drives offer soft start capabilities, true soft start starters are not considered VFDs.

Ramping is the ability of a variable frequency drive to increase or decrease the voltage & frequency to an AC motor gradually. This accelerates & decelerates the motor smoothly, with less stress on both the motor & connected load. Ramp-up is generally a smoother acceleration than the stepped increases used in soft starters. The length of time pre set for the speed ramp-up can be varied from a few seconds to 120 seconds or more, depending on the VFD capabilities.

Timed ramp-down is a function of variable frequency drives that provides smooth deceleration, bringing the motor to a full stop in a preset time. Acceleration & deceleration are separately programmable. Depending on VFD parameters, ramp down times can vary from fractions of a second (when used with dynamic braking) to more than 120 seconds.

The ramp-down function is applied in processes that require smooth stops, but also require the process to stop within a given period of time.