Technical Articles
A Variable Frequency Drive is an electro-mechanical drive used to control the speed & torque of AC Motors by varying the input voltage & frequency of the motor. VFDs are widely used in conveyor systems, pumps, blowers, machine tools & other such applications which require variable speed along with variable torque. But a VFD, being a non-linear load, is a source of harmonics in the line.
Active Harmonic Filters are the ideal solution for harmonic mitigation. They compensate the harmonics generated by VFD by generating the same harmonic components in the opposite phase. As a result, Power Factor can be maintained close to unity & the supply current will be a pure sinusoidal wave with no harmonic contents.
However, AHFs have a very high cost per ampere if they are used to compensate for the harmonics generated by the VFDs. The sizing of the AHF can be reduced by reducing the harmonic content at the output of the VFD.
Hence input line chokes are used at the output of the VFD. This is an inductor coil which opposes rapid change in current because of its impedance.
Input Line Chokes will:
As can be seen from the above waveforms, Line choke lowers the harmonics distortion present in the current waveform & increases the impedance against unusual voltage spikes.
Line chokes are classified by their percentage impedance, which is the voltage drop due to the impedance at the rated current expressed as a percent of rated voltage. The most commonly used line chokes have 3% or 5% impedance. Thus the line choke will apply this impedance when rated current is flowing through the vfd. 3% impedance line chokes absorb voltage spikes & current surges & also prevent nuisance tripping of switchgear. 5% impedance line chokes are used for reducing the harmonics generated due to the vfd.
Let us see an example of the effect of input choke on a 6 pulse drive.
| 6-Pulse Drive Input Choke Analysis | Percentage Impedance |
|---|---|
| Transformer kVA Rating (kVA) | 2500 |
| Secondary Voltage (V) | 415 |
| % Impedance | 6 |
| Total Transformer Load (%) | 75% |
| % Non Linear Load | 55% |
| 6 Pulse Drive with no choke (kW) | 1000 |
When 6 pulse drive is used without a choke, the overall Current THD is 30%
Now, 440 A 3 Ph 3 Wire AHF is sufficient to reduce the %TDD to required limits.
Hence by using 5% Input Line choke alone, requirement of AHF current has decreased from 720 A to 440 A. Neutral Compensation is no longer needed. Thus, using an input choke results in direct monetary saving by reducing the capital expenditure. Also, the losses due to higher harmonic currents are reduced.
Variable Frequency Drives are widely used for controlling the speed of AC Motors, but these are also the sources of harmonics in the system. If AHF is used alone for harmonics mitigation, the cost per ampere of the solution will be very high. This cost can be reduced by using either 3% or 5% Input Line Chokes. A combination of 5% Line Chokes & Active Harmonic Filter has been shown to reduce the current & voltage harmonics within IEEE 519 Limits in the case study mentioned.
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
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