Why line chokes are used in VFD

Why line chokes are used in VFD:

Almost 100 % of the vfds are starting with series line choke. Line chokes are nothing but an inductance. I have used upto 280 mH line choke for my 400KVA ABB make vfd drive. Generally, more than 1 KVA rating vfds are preferred to operate with line choke both input side and output side. But line chokes are costlier, it takes amost 20% of the vfd orginal cost. let see Why line chokes are used in VFD.

Why line chokes are used in vfd:

  1. To protect thyristor form high transient current, because an inductance does not allow sudden change in current. All the semiconductors are having some tolerance to stand high transient current but beyond the limit which leads to semiconductor failure. At here come to the cost effect, a A445 model thyristor is cost around Rs 65,000 /- which is used in ABB VFD drives. To run a vfd with 100 % load we must need all thyristor.

Note: ABB drives comes with in build line choke but Siemens make VFD drives does not come within build choke, for this we need to install externally.


When transient current occurs practically:

  1. Grid voltage fluctuation. i.e grid suffers due to heavy earth fault or short circuit on high voltage side, it may happen frequently during rainy season. You can observe a sound “tharrr”, it is very mild at vfd side and heavy at generator side. Not for all vfd, it happened us in one of our 400 KVA vfd and 770 KVA vfd.
  2. If you have own generation station i.e home load plant. A small earth fault in the generator output terminal happened means, at that time you can observe that sound.
  3. High Voltage switching. (Switching on or off) , the high voltage circuit breaker operation.
  4. It works as reactive compensator.

Let see how line chokes are protecting

As we know the basic inductance formula:

V= L X dI/dt    ——————————–1

Here substitute t = 0 in number 1

Then V=∞

It is highly impossible, due to this reason, sudden change current is impossible in inductance. That why line chokes are used in VFD


A main purpose of these line choke is to protect thyristor form transient over current


  1. Mr.Mani,

    I am using an ABB ACS355 drive to power my 60 volts 3 phase induction motor of 1 HP. I am using a 2 HP 4.1 amp drive and running current is 1.2 amps in the drive, the output of the drive is connected to a three phase step down transformer which give phase to phase 10 volts. I am facing 2 problems. One is that the stating current being high the drive trips a number of times while starting or the motor does not attain full rpm. I am running drive at 100 Hz. Now i have reduced the ramp time to 3 seconds and it is working well. Can i use a line filter between drive and transformer to counter the initial load.
    Secondy i want to know if the voltage will drop with drop in frequency with the pot. The voltage rises when frequency is reduced from 100 to 50 and then after 50 Hz, it starts reducing. We use to pot to fine tune the voltage which can not be achived by transformer.

    • Hi, Harjit
      The exact answer for your second Problem is, the drive starts injecting more current when the frequency of the output voltage increases, it is only in variable torque VFD drives. the true concept is, whenever the output current of the motor reaches its threshold limit, the drive starts to reduce the output voltage as well output current to maintain steady operation (as per VFD concept v/f ratio should be constant). i think you are using Variable vfd drives. to archive full speed, you must reduce the motor loads.
      For your first problem, You can use scalar mode. or enable flux optimization mode in VFD.
      Line filters are only preferred to use of long-distance motor (>75mtr drive to motor distance)


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