What is the Difference between Soft Starter and VFD?

The main difference between VFD and Soft Starter is that a VFD can increase or decrease the speed of the motor, while a soft starter only controls the smooth start and stop of that motor. Soft starter is an electronic device  used for  starting and stopping  the  induction motor smoothly. The soft starter is kind of reduced voltage starter. The soft starter starts and stops an induction motor without any jerk. An induction Motor takes high current about 5-7 times current  of its full load current when we start the motor through  direct online starter(DOL). 

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At start, the motor generates heavy torque. As a result, the motor creates electro-mechanical stress on the power system. In addition to this, it  creates mechanical stress on the motor and driven equipment as well. Even if we feed 58.7 % of line voltage through star delta starter, the motor takes high current. Though current on star delta start is less than the current on DOL start, but still the current is high.

Disadvantages  of DOL or star delta starter Compared to Soft       starter 

  1. High starting current cause electrical and mechanical stress on the power system.
  2. High starting current cause mechanical stress on the motor and driven mechanical equipment

Contrary to DOL and Star Delta starter, the soft starter increase the output voltage gradually at start.  The gradual increase of voltage limits the high starting current and torque. Thus,the driven equipment  does not experience mechanical stress.

Similarly, during stopping of the motor, the soft starter decrease the voltage  gradually in a ramp down manner. This result into, the driven equipment gets soft stopped.

Advantages of soft starter Compared to DOL Starter

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

The soft starter is most suitable for jerk-less starting and stopping of the induction motor. However, we can not use the soft starter for speed regulation of induction motor. On the other hand, VFD is used to control the motor speed from zero to its base speed. This is the major difference between soft starter and VFD.

Disadvantages of soft starter Compared to VFD

The soft starter is suitable for jerk-less  starting and stopping of an induction motor, however we can not use it for speed control of the motor.

Let us know understand the working principle of soft starter and VFD for understanding the difference between soft starter and VFD.

Working Principle of Soft Starter

We will  first discuss  about working of the soft starter to have better understanding. This will ease the understanding about what is the difference between soft starter and VFD.  The power diagram of soft starter is as given below. The soft starter has the anti-parallel SCR in each phase.

power diagram of soft starter
                 Power Diagram of Soft Starter

We can control the output voltage of each phase by varying the firing angle of  silicon controlled rectifiers (SCRs). The output voltage increase with decrease of firing angle.

relationship between firing angle and output of soft starter
                                  Output Voltage Vs Firing angle

When full line voltage reaches, the soft starter controller switches on the bypass contactor. As a result, the  motor gets  pure sinusoidal voltage. Again at the time of stopping , the soft starter controller shifts the system  from run contactor (bypass contactor) to soft starter mode for smooth stopping of the motor.

Soft starter controls the voltage only, the frequency does not change with change in voltage.

Disadvantages of Soft starter

The soft starter is used where speed control of an induction motor is not required. The soft starter does not maintain the constant v/f ratio, and hence the flux is less than the rated flux capacity of the motor. The torque delivering capacity of the motor depends on the flux. Thus  soft starter is not suitable for those applications which demands a higher starting torque.

Soft starters are used for the motors which almost start on no load- ( coupled equipment demands very low torque at start) Example – Air Compressor. In other words, it is not possible to drive the loads which demands higher starting torque.

General Misconception about Soft Starter

Someone believe that if we run the motor through soft starter, the motor draw less starting current  and driven equipment runs smoothly and jerk-less. However, the starting current depends on the type of the load driven by soft starter. The starting torque delivery of the motor with soft starter is very low than its rated capacity. The motor draw a large current for starting of high inertial loads. Also, the motor produce very low starting torque. Under this condition, the motor may stall. 

Therefore, before selecting the soft starter for motor, we must check the speed-torque characteristics of driven equipment.

Variable Frequency Drive (VFD)

We use VFD for:

  • Speed control of induction motor
  • Jerk-less  starting and stopping of induction motor

The speed of an induction motor is proportional to the frequency and numbers of poles. We can calculate the speed of an induction motor using the following formula.

induction motor speed formula

The flux of the motor is proportional to the ratio of voltage and frequency. Therefore, for maintaining the constant flux in the motor along with speed control, the VFD maintain the   V/f ratio constant.  The PWM inverter maintains the V/f ratio constant  by simultaneously increasing or decreasing the frequency and voltage. Whereas, the soft starter does not maintain the constant V/f ratio and flux in the motor is not constant. This is one of the major difference between soft starter and VFD.

Block Diagram of VFD 

Block diagram of VFD
                  Block Diagram of VFD

The VFD has three major sections.

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

Converter section

Converter or rectifier section has six diodes. The diodes rectify the AC voltage into DC voltage. The magnitude of the DC voltage is;

Vdc = 1.35 X Vrms

DC Bus

The rectified voltage contains ripples in it. Capacitor bank filters the AC components and produce ripple free DC voltage.

Inverter Section

The inverter converts DC voltage into AC through PWM process. The inverter controls both frequency and voltage simultaneously to maintains the v/f ratio constant.

   Summary- Major difference between Soft Starter and VFD             

  • The soft starter controls the voltage only, the frequency always remains unchanged. VFD changes the  voltage and frequency simultaneously to maintain the constant flux in the stator air gap length.  This is the major technical difference between soft starter and VFD.
  • We use soft starter for soft starting and stopping the motor. The VFD also soft starts and soft stops the motor.The main purpose of the VFD is  to control the speed the motor coupled with energy saving and smooth process control. We can achieve the tremendous power saving in fan and pump applications  with use of VFD because power drawn by pumps and fans is proportional to the cubic of the speed.
  • Soft starter is cheaper than VFD.
  • Soft starter is suitable for low starting torque demanding equipment. Contrary to this, the motor driven by VFD can deliver the  rated starting torque, and even more than the rated torque for definite time through enabling torque boosting feature of VFD.
  • Soft starter injects harmonics in the system at the time of acceleration of the motor only. After reaching base speed, soft starter switches on the bypass contractor and thus, motor gets cleaner sinusoidal supply. VFD continuously remains in the circuit, and as a result injects harmonics in the system. Moreover, the motor gets PWM wave which is not the pure sinusoidal supply.  The harmonics impacts the motor performance in many ways.

Related Posts:

  1. Why V/F Ratio Is Kept Constant in VFD?
  2. Direct Online Starter (DOL Motor Starter) : Circuit Diagram and Working Principle
  3. Star Delta Starter | Working Principle, Power and Control Circuit Diagram
  4. Why Induction Motor Takes High Starting Current?
  5. Harmonics and Harmonic Frequency in AC Circuits
  6. Working Principle of VFD
  7. Thyristor Protections-notesforengineering


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