Let's look at the question of how you start the Induction motor

Soft Starter

Direct-on-line and star/delta motors are standard. However, at levels over 10 kW, star/delta has traditionally been more common. At levels over 10 kW, installations and mechanical parts are rarely strong enough to withstand the associated inrush current. 

Now more than ever, soft starters are used instead of star/delta for levels over 10 kW. Soft starters increase voltage gradually to generate a smooth, steady acceleration, and are perfectly compatible with direct-on-line and star/delta motors. Although a soft starter does smoothly and gently start a motor, and bring it up to full speed in a controlled manner, it does not provide substantial energy savings or variable speed control.

Variable Frequency Drive

The variable speed drive is also known as a frequency converter and is known for having very accurate speed and torque regulation. It has a rectifier bridge followed by an inverter. Both allow voltage amplitude and frequency to be varied.

A variable speed drive is more efficient, but there are a number of other benefits to consider. For instance, with variable speed drives, starting and stopping operations are made smooth and perfectly controlled. Slow starting also minimizes inrush currents that often accompany a motor suddenly starting. Inrush currents cause voltage sags that can have damaging effects on the motor and other affected equipment. Variable speed drives are sometimes called variable frequency drives. This is because the drive receives AC power at a constant frequency, such as 50 or 60 Hz, and converts it into a variable frequency for supply to the motor.

They are found in many applications such as elevators, HVAC systems, pumps, fans, cranes and conveyors.  

Direct starting methods are inexpensive, but they do not perform as well. Direct-on-line and star/delta starting methodologies can be stressful on mechanical parts and gear trains. They also draw high inrush currents. The soft starter preserves the efficiency of gear trains and mechanical parts, but does nothing to master speed or torque, which are the main factors to control in a process to make significant energy savings.

Here we see a table comparing different starting methods. As we can see, variable speed drives not only yield better performance, but have lower operating costs as well.