Comments: The isolation by so-called phase shifting transformer (or rectifier transformer) disrupts a direct galvanic path for the common mode current, which tends to exists with passive harmonic filers, e.g. Lineator (Matrix Passive Harmonic Filter) by Mirrus International
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Consider a "drive" as all of the components that start with the rectifier and end with an inverter. This drive will generate harmonics that will enter the power system supplying the drive. The characteristic harmonics will be at 5, 7, 11, 13, 17, 19, 23, 25... . A drive isolation transformer, if of the multi-pulse type, will create cancellation of these harmonics within the transformer. As an example, a transformer with 2 secondary windings phase displaced by the appropriate amount can result in the reduction of the 5, 7, 17, 19... harmonics. As far as the power system is concerned, it now sees basically the 11, 13, 23, 25... harmonics with lower levels of the 5, 7, 17, 19....This requires a special transformer design and more components in the VFD. So initial investment may be a consideration.
A passive harmonic filter consists of R, L, C components tuned in a specific manner to shunt away from the power system certain harmonic frequencies generated by the drive.
The problem with passive filtering is it is sensitive to the power system electrical quantities. Changes in things like tie breaker positions, adding / removing loads can change the filter tuning frequencies and therby impact filtering effectiveness.
Passive filtering usually incorporates capacitors that typically have a shorter life than inductive components such as transformer coils.
Comment on the previous posting marked ///\\A passive harmonic filter consists of R, L, C components tuned in a specific manner to shunt away from the power system certain harmonic frequencies generated by the drive.
The problem with passive filtering is it is sensitive to the power system electrical quantities. Changes in things like tie breaker positions, adding / removing loads can change the filter tuning frequencies and therby impact filtering effectiveness.
///The matrix (inline) filters (e.g. Lineator) are also passive harmonic filters; however, they have some advantages over the shunt passive harmonics filters, e.g. do not produce inadvertent resonance, have high input impedance to higher frequencies, etc. Visit
The isolation transformer provides opportunity for voltage transformation as well as capability for phase-shifting. If you have multiple drives, use of delta-wye transformers for half of the drives to provide 30 degree phase shift can provide substantial reduction in THD due to cancellation effect. The isolation transformer can also provide isolation of ground faults if delta-wye unit is used.
But if harmonic reduction is the issue, transformer generally is not required - an input line reactor can provide equivalent harmonic reduction.
Comment: The phase shifting transformers can be applied way up to a 72 pulse input AC-DC conversion (rectifier), e.g. at arc furnaces, to obtain very low current and voltage THDs on the rectifier input.
Hi WheelerTI,
here is additional info for your comparison Drive Isolation phase-shifting XFMR vs Passive Harmonic Filters:
1. Shorts on filters:
a)they have a leading power factor when they unloaded,
b)the presence of the Cap Bank boosts the input voltage of the Drive when the Drive in stand-by mode (bad for the drine input suppressors),
c)some filters have a significient increase of their impedance that helps with the harmonic mitigation but creates more the voltage drop under full rated load; this voltage drop can affect on inverter bus supply and you can loose you output power to the motor.
2. Shorts on Isolation phase-shifting XFMRs;
a)a larger size than filters definately,
b)a higher cost due to the copper utilization for a two windings, a copper cost has increased lately on market.
Solutions: check with the Drive manufacturer, they have more options for you:
phase-shifting AutoXFRMs, active Harmonic Filters and so on.
Thanks, NNG.
Comment on the previous posting: The passive harmonic filters are more diversified nowadays. It is necessary to ramify the statements to describe applicable characteristics to applicable category of passive harmonic filters. E.g. visit
