TDK Electronics · TDK Europe

EMC Output Filter Solutions

High-performance output solutions for frequency converters


Frequency converters are used for regulating the speed of AC motors. In applications, in which the converter and motor are connected by a longer cable, parasitic capacitances occur between the conductors and to ground. In addition, the rise time of the square-wave pulses of the converter output voltage is in the range from 5 to 10 kV/µs, which causes high-frequency currents in the cable on every switching operation. These have a whole range of negative impacts, which can be nearly eliminated by using EPCOS products such as motor chokes (dv/dt filters), sine-wave filters and SineFormer. Such adverse effects include:

  • The superimposition of high-frequency currents on the cable reduces the amount of current available for operating the motor. To compensate for this, the converter must therefore be dimensioned for a higher power rating.
  • High-frequency currents, with their high switching frequency content, cause losses both in the cable and in the motor.
  • As a proportion of the high-frequency currents are conducted to ground, they cause asymmetric interference. If unshielded motor cables are used, this would consequently result in inadmissibly high interference fields. For this reason, expensive shielded motor cables are generally used.
  • Moreover, the steep rising edge of the converter voltage excites parasitic oscillating circuits that consist of cable and motor capacitances as well as line inductances. Their transient characteristics overlay the converter voltage. On the motor side, this primarily results in momentary voltage overshoots which can far exceed the rated motor voltage and exert a load on the motor insulation due to partial discharges, which in turn can cause a motor to fail.

Overall, this creates the following problems at the converter output:

  • Very high-frequency reactive currents in the motor cable
  • EMC problems
  • Overvoltage at the motor caused by steep voltage gradient and long motor cable
  • Damage to the motor insulation
  • Bearing damage due to leakage currents through the motor bearings
  • Motor noises

TDK offers a wide range of EPCOS filters and reactors:

  • Motor chokes (dv/dt filters)
  • Sine-wave filters
  • SineFormer

All reactors are manufactured with an UL-approved insulation system.

Output Filter Concepts

Compared to the various concepts for output filters, the new SineFormer is both technically and economically the superior solution.

Concept with dv/dt choke

 SF Konzepte A circuit

Advantages and disadvantages
  • + Reduces dv/dt peaks significantly
  • + Low cost solution
  • - Motor cable is limited to approx. 100m
  • - No reduction of noise
  • - Shielded cable necessary

Concept with sine-wave filter

 SF Konzepte B circuit en

Advantages and disadvantages
  • + Reduces dv/dt peaks significantly
  • + Forms a sinusoidal wave between the phases
  • + Reduces noise
  • - Reduces eddy currents
  • - Relatively expensive solution because shielded cables still have to be used

Concept with SineFormer

 SF Konzepte C circuit en

  • + Motor size can be reduced
  • + Motor operating life can be significantly increased
  • + Unshielded motor cables can be used, thus reducing assembly expenditures, increasing the operating life and reducing cable costs
  • + Longer motor cables possible (ub to 1000 m unshielded measured)
  • + No maintenance costs, as the Sine Former is built without forced ventilation
  • + Compact filter solution (not modular system) with lower volume and weight
  • + Reduced requirements for the power line filter
  • + Increased equipment availability
  • + Also suitable as a retrofitting set
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Advantages of a SineFormer solution compared with other output filter solutions using examplary measurements

Frequency converter without output filter

 SF Measurements A circuit


High-frequency interference requires a shielded motor cable.

 SF Measurements A measurement1
Figure 1:

Phase-to-phase voltage

 SF Measurements A measurement2
Figure 2:


Frequency converter with dv/dt choke

 SF Measurements B circuit


Steep current and voltage rises at the output of the frequency converter are evened out by the inductance, while the parasitic capacitances of the motor cable are charged and discharged less powerfully. Motor chokes are used mainly for protecting motor windings against voltage spikes.

 SF Measurements B measurement1
Figure 3:


Frequency converter with sine-wave filter

 SF Measurements C circuit


Sine-wave filters are designed as LC filters. Unlike motor chokes, however, their limit frequency, lies between the output frequency and the converter clock speed. As the sine-wave filter mainly suppresses the symmetrical interference between the lines, the interference acting on the phase-to-ground voltage is hardly reduced at all. Therefore, the motor leads still require shielding. Sine-wave filters reduce the motor noise and the eddy current losses and permit the use of motor leads much longer than 100 m.

 SF Measurements C measurement1
Figure 4:

Phase-to-phase voltage

 SF Measurements C measurement2
Figure 5:

Phase-to-ground voltage

Frequency converter with SineFormer

 SF Measurements D circuit

 SF Measurements D measurement1
Figure 6:

Phase-to-phase voltage

 SF Measurements D measurement2
Figure 7:

Phase-to-ground voltage

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Bearing Currents

Comparison of bearing currents in the various output filter solutions

High frequency common-mode interference is generated by the switching behavior of IGBTs. This interference propagates over the motor cable and is conducted to ground via the parasitic capacitance of the cable and the motor. Currents flow through the bearings in the motor, which can lead to increased wear and tear and premature damage of the motor bearings.

Bearing currents without output filters

 SF Currents A circuit

 SF Currents A measurements

Bearing currents with sine-wave filter

 SF Currents B circuit


The series inductor of the sine-wave filter mainly has a symmetrical effect and only to a small degree asymmetrical. For this reason the high-frequency interference continues to flow through the motor cable, meaning that it is still necessary to use a shielded motor cable. Moreover, bearing currents are only slightly reduced.

 SF Currents B measurements

Bearing currents with SineFormer

 SF Currents C circuit

 SF Currents C measurements

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Motor chokes for drives B86301U*R000/S000

 Pic Portfolio Chokes

Motor chokes reduce the voltage stress at the motor and the dv/dt increase at the frequency converter output.


  • 4 to 1500 A / 520 V
  • 1% impedance reactor
  • Easy to install
  • Low weight
  • Compact design
  • Design complies to IEC 60076-6
  • UL approved isolation system class F (155 °C)

Sine-wave output filter series B84143V*R227/R229/R230

 Pic Portfolio SineWave filter


  • Complete design from 4 A up to 720 A / 520 V (R227/R230)
  • 690 V version: R230 up to 204 A
  • Designed for motor cables up to 1000 m
  • Slim design unique in the market
  • UL approved isolation system

SineFormer®: Best output filter solution

 Pic Portfolio SineFormer

  • Complete design
    from 6 A up to 180 A / 520 V (-R127 series)
    and 95 A up to 320 A / 760 V (-R290 series)
  • Commercial advantages
    System-cost savings due to the use of unshielded cables
    • Automatic cost savings from a motor-cable length of approx. 100 m
  • Technical benefits
    Longer life cycle of the motor, motor noise reduction, substantial compensation of bearing currents and eddy current losses, no forced ventilation necessary
    • Maintenance-free (fan would have a life cycle of 2 to 4 years only), no feedback to the DC link needed
    • Reduction of all kinds of radiation sources by easy installation
  • Installation advantage
    Unshielded cables are lighter and more flexible
    • Cost savings during installation
  • Logistics advantage
    Unshielded cables are standard products