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Variable Frequency Drive versus Soft Starter in Harmonics

Variable Frequency Drives (VFD) that do not have either an AC line reactor or a DC bus choke, will typically exhibit a THDi greater than 85% at all loading conditions. The THDi is dependent on the conduction angles of the rectifier. Any inductive reactance in the supply, plus the overall impedance will reduce the peak charging current to the capacitors and increase the conduction angle and reduce the THDi.

So VFDs with standard 6 pulse rectifiers will produce harmonic currents in the supply ranging from around 38% to 100% depending on the load and the reactance in the circuit. VFD with active front end can have a reduced harmonic current down to around 5% provided that the active front end is optimised for harmonics rather than regeneration. These harmonic currents are drawn from the supply the whole time the variable frequency drive is operating.

If we compare this with a soft starter, the majority of soft start applications are "three wire" connections and as such, the minimum conduction angle is 120 degrees which will yield a THDi of around 40%. In a real application, the start current would be a minimum of 300% and commonly 400 - 450%. At this start current, the conduction angle will be in the order of 160 degrees and so the THDi will be less than 40%.

The soft starter does draw a high harmonic current during the duration of the start current only, and the THDi is typically small while compare to a VFD without a harmonic filter. The same argument can be applied when soft start is selected.

Once the ac motor is up to speed, the SCR conduction is 180 degrees per SCR, so there are no additional harmonic currents introduced by the soft starter, and commonly, modern soft starters include a bypass contactor shorting out the SCRs.

The start current drawn by the soft starter is higher than the input current to the variable frequency drive and so the absolute amplitude of the harmonics may be higher relative to the ac motor rating than with some VFDs, but only for a short time. I would expect that more typically the actual harmonic current amplitudes during start are not too different from a VFD with a 3% reactor.

The cumulative affect of multiple VFDs running on a supply can result in a very high level of harmonic current and when this is imposed on the supply impedance, is reflected back as a high harmonic voltage. We have large areas here where the THDv is well in excess of 5% and commonly up to 15%. This level of harmonic voltage causes significant supply losses and motor heating. Any capacitance on the supply acts to short out the harmonic voltages resulting in very high currents flowing through the capacitors. Where there is a combination of capacitance and inductance, there is a resonant frequency and if that resonant frequency is close to the frequency of one of the harmonics, then the voltage amplitude at that frequency will be greatly increased. Detuning the LC combination, will reduce the harmonic voltage gain, but will not eliminate the harmonics.

Harmonic mitigation is becoming increasingly prominent and in this region, it is mandatory to mitigate the harmonics to better than 8% THDi for all VFDs in the rural area. This rule applies to new installations and also to existing installations. The cost of mitigation is very high, commonly doubling the cost of the VFD.

One of the disadvantages of choosing a VFD when compare to soft starter is that they have a continuous significant harmonic current draw, minimum 38-40% for standard variable frequency drives.

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