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.