Difference between Soft Starter and VFD

We use direct online starter and star delta starter for starting of induction motor. An induction motor takes about 6 times current of its full load current (FLC) when we start the motor on DOL.

Even the motor takes 3-4 times current when we start the motor on star delta starter.

Soft Starter

The electrical system experiences a heavy stress when we start a large rating motor on DOL or star delta. The driven equipment also experience a heavy mechanical stress.

To mitigate the above situation, soft starter is used for jerk less starting and stopping of an induction motor.

Advantages of Soft Starter

Advantages of use of soft starter are as follows.

  1. Reduced mechanical stress on motor and driven equipment
  2. Reduced electro-mechanical stress on the power system
  3. Increased life of the motor

Limitation of the soft starter

The soft starter can be used for starting and stopping of motor at reduced voltage in order to reduce the starting current for jerk less starting.

We can not use soft starter for speed regulation purpose.

Working Principle of Soft Starter

The soft starter has anti-parallel SCR is each phase. The output voltage of the each phase is controlled using controlling of firing angle. As the firing angle is reduced the output AC voltage increase.

At starting, the SCRs are fired at maximum firing angle in order to get low output.

More importantly, the output of the soft starter at the reduced voltage has the same frequency. The soft starter does not change the frequency. The flux in the motor is proportional to the ratio of voltage and frequency.

The V/f ratio is low when the motor is started at low voltage. The starting torque of the motor is low when it is started with soft starter.

Therefore, the soft starter is suitable for those applications where starting torque requirement is low. If the starting torque requirement is high, the soft starter will draw a large current and may trip with overcurrent fault.

When motor achieve its rated speed the bypass contactor is made on to bypass the SCRs in order to supply a perfect sinusoidal voltage to induction motor.

Variable Frequency Drive (VFD)

VFD does two functions.

  1. Soft starting and stopping of an induction motor
  2. Speed control of the motor as per process requirement

The speed of an induction motor is proportional to the frequency and inversely proportional to the number of poles.

Ns = 120f/p

The VFD has following sections.

  1. Converter section
  2. DC Bus
  3. Inverter section

Three phase AC voltage is first converted into AC voltage. The output of the rectifier is further filtered to remove the AC components in the DC. The DC voltage is again converted into AC by PWM technology.

The PWM inverter control the width of the pulse and as a result the output voltage gets varies with an increased frequency. Thus the V/f ratio of the inverter output is maintained constant.

The motor is capable to deliver the constant torque from minimum speed to base speed of the motor.

The VFD is popularly used in industries for fan and pump application. The power drawn by the fan and pump is proportional to the cubic of speed. Thus, at reduced speed substantial power saving can be achieved.

Comparison between Soft Starter and VFD

The below comparison table shows the brief differences between the soft starter and VFD drives.

Difference between Soft Starter and VFD

We have to use the VFD drives if the process application requires speed control of motor as soft starter do not have this capability.

Author: Satyadeo Vyas

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