Variable frequency drive

If there is no need for a variation of speed, the Soft Starter is the ideal solution. Soft Starting solutions will enable a reduction in equipment size, reduced demand on the electrical network, zero harmonics (after starting), reduced maintenance on equipment, user friendly configuration and set up.... and a great commercial offer. Although the starting conditions can be very harsh (High Inertia), from units in new projects, through replacement of DOL starting and gears to replacement of slip ring motor starting with Soft Starters.

When Starting a Motor across the line, you can draw up to 8 times the rating of the full load amps of the motor. A variable frequency drive or a soft starter limits the starting current due to a pre-charge circuit built into the drive or starter. VFD Drives are better because you can program the ramp time in the firmware.

A soft starter is in the circuit for so short a time, it is not likely that the fluting is coming from the variable frequency drive. My logic is that fluting is a low current long time event. Bearing damage that could occur from the very short and very infrequent duration of starting would have to be a very high energy (for that short time), and would more likely be pitting.

In order to understand better the starting requirements using an AC drive and the selection of a VFD for your motors, a number of motor starting simulation studies (ETAP or other software) can be conducted for the load types and variable frequency drive control mechanism of the motor, in order to determine the drives size, control method and settings. These studies can verify that the drive and motor system meet the operational requirements. There are usually a set of speed-torques curves for each application. During the acceleration, frequency and voltage are generally controlled such that full voltage is reached at the design rated RPM of the motor following a ramp. Motor voltage and current are regulated to provide speed and torque control to the load. The voltage to frequency ratio is held roughly proportional. In practice torque developed at very low frequencies falls off and extra voltage above the value of the volts / HZ ratio (or other ctl) is automatically applied to regain the lost torque. The variable frequency AC drive modify the speed but also the input voltage in direct proportion.

Soft starters and variable frequency drives are two completely different electric devices. Variable frequency drive is used for AC motor speed control, it can change output voltage and frequency at the same time; actually, soft starter is a regulator for motor starting, only changes output voltage but not the frequency. Variable frequency drive has all the features of soft starter, but its price is much more expensive than the soft starter, and the structure is much more complex.