Which is better - VFD or soft starter?

There can be a lot of reasons for using a variable frequency drive (VFD) for such a motor, but in that voltage/power range, there are LOTS of other starting methods that are nearly as good, and a lot cheaper.

Just depend on the characteristics of your supply system, and what your concerns are during start, and I am sure quite a number of electrical engineers will be able to advise which starting mode will be cheaper, easier to run, and easier to maintain, then choose a VFD.

The main application to select the variable frequency drive is when the process driven by the motor requires special consideration, mainly in terms of a variation in motor speed, and there can be a lot of reasons for that, too.

For Medium Voltage motor, like 11kV, and in the most of case we don't use variable speed drive, because it's too expensive. The best way is to run it DOL with one condition that your network has the necessary capacity for the inrush current.

By the way, if this motor will be, after running, working all time at normal speed, you don't need to use variable frequency drive at all, and if you network doesn't have the necessary capacity for inrush current, you can choose a "Medium Voltage Soft Start" that cheap than VFD.

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.

The issues that need to be considered before select a motor starter, include:
• Type of load the motor is driving: Does it require a high starting torque?
• The time required for the motor to reach speed
• The best control method of a VFD for the load: Is voltage boost required in order to obtain the required starting torque? What is the minimum operating frequency of the variable speed drive? Will it cause high starting current?
Also, we must not get confused and think very simply that soft starter and inverter drive have the same role. VFD drives are more expensive and soft start are not able to regulate frequency! There are many other considerations before deciding if a variable frequency drive or soft starter is needed for your load.