Electromagnetic compatibility (EMC) in frequency converters can be very problematic if not addressed properly in the initial design. EMC ensures the proper operation of devices to avoid negative electromagnetic interference (EMI) effects. Good design takes into account the control, design, and function of each device to prevent such interference.
EMC occurs at the input and output of frequency converters. To ensure the elimination of negative interference at the output, design engineers should evaluate EMC filters and reactors based on the desired results.
Frequency converters are used for regulating the speed of asynchronous motors. Often the motors and converters are in different parts of the equipment and need to be connected by a longer cable. In such applications, parasitic capacitances occur between the ground and the conductors. Furthermore, there are high-frequency currents in the cable during each switching operation that are caused by the rise time of the square wave pulses on the converter output voltage. This is typically in the range of 5-10 kV/μs.
These high-frequency currents/high-switching frequency content can bring about losses both in the motor and in the cable connecting it to the converter. When high-frequency currents make their way onto the cable, it reduces the amount of current that is available to operate the motor. To deal with this, a converter with a higher power rating needs to be used in the design. If not, the interference from the longer cables will cause the overcurrent protection switch of the converter to be tripped, stopping the system from operating properly.
This problem is worsened if unshielded motor cables are used, due to increased interference from high-frequency currents that are conducted to ground causing asymmetric interference. This often results in failure in the design, which is why shielded motor cables are typically used even though they add significant cost to the design.
Finally, there is a steep rising edge that occurs within the converter. The resulting voltage excites parasitic oscillating circuits that are comprised of motor and cable capacitances and inductances. These temporary features add themselves to the converter voltage. This causes momentary voltage spikes that surpass the motor voltage rating and add unnecessary load on the motor insulation, causing the motor to fail over time.
These issues cause noticeable EMC problems, motor noises, damage to motor insulation, and bearing damage due to current leakage in motor bearings. Less noticeable issues that engineers can use to diagnose these problems include high-frequency reactive currents in the long motor cable, and overvoltage in the motor from the steep rising edge (Figure 1).
The use of output filters can help suppress interference. While the degree of design changes that are needed to suppress interference at the converter outputs changes from design to design, there are several things that may be done to reduce interference. Popular alternatives include changing cable length to reduce interference, evaluating and changing the spectrum of frequency causing the interference, and changing the motor type or choosing one with a higher power rating. Motor chokes, sine-wave filters, and EMC sine-wave filters also help suppress interference.
In long motor cables, motor chokes — or more properly dv/dt chokes — are common components that can help to suppress interference because all the motor currents move through them (Figure 2). They are often placed serially to protect motor windings from voltage spikes. As the steep rising edge of the current and voltage begins to happen at the frequency converter output, the chokes help even them out with inductance, resulting in less parasitic capacitances and less powerful discharges in the motor cables.
Motor chokes that are designed for a rated voltage of 520 VAC and current capabilities of between 8 and 1500 A are best used in motors with frequencies between 0 and 400 Hz, and with longer motor cables that have a length of up to about 100 m. There are also smaller chokes that are designed for lower clock speeds and current capabilities. Regardless of what choke is specified, it should be manufactured with the UL-approved T-EIS-CF1 insulation system and comply with IEC 60076-6 standards.
Sine Wave Filters
Sine wave filters should be used when there is a more significant demand for interference suppression. These are most often designed as LC filters (an inductor-capacitor filter). These filters act as an electrical resonator, storing energy oscillating at the filter’s resonant frequency. They differ largely from motor chokes in that their limit frequency lies between the output frequency and the frequency converter clock speed.
Sine wave filters primarily suppress symmetrical interference between lines. Sine wave filters reduce the motor noise and the eddy current losses that permit the use of motor leads much longer than 100 m. However, there is little suppression of the interference that acts on phase-to-ground voltage. As a result, shielding is still needed on motor leads.
Typical sine wave filters are designed for continuous currents of between 4 and 320 A at rated voltages from 520 to 690 V. Depending on the type of the filter, the limited clock speeds of the converters should be between 1.8 and 16 kHz.
EMC Sine Wave Filters
EMC sine wave filters are augmented with a current-compensated choke and capacitors to ground. While standard motor chokes and sine wave filters reduce voltage spikes on the motor cables, they have minimal impact on the interference acting on the phase-to-ground voltage. This is why shielded motor cables are required with standard sine wave filters. In addition, there is little suppression of motor bearing currents with sine wave filters and motor chokes.
With EMC sine wave filters, the design can use a motor cable without any shielding due to its circuit design. The design reduces asymmetric interference on the motor cable. As a result, using this type of filter in combination with unshielded cables can often reduce costs of the design (Figure 3). However, cross section and cable length may cause different dynamics in the design. EMC sine wave filters operate even when unshielded motor cables and power cables cross (and even touch each other), and prevent coupling of interference from the motor cable with other network and signal cables.
When cable lengths are over 100 m, the use of an EMC sine wave filter is much more economical than the use of shielded cables. However, the cost effectiveness of using an EMC sine wave filter and unshielded cable is reduced significantly at less than 50 m, where shielded cable costs are typically higher and the cost of an EMC sine wave filter start to break even.
EMC sine wave filters reduce the dv/dt to less than 500 V/μs, and significantly reduce the eddy current losses and motor bearing currents. This provides the best possible reduction of the interference (conducted and radiated) in comparison with other output filter solutions.
Regardless of the design, electromagnetic interference in the converter output can be addressed using motor chokes, sine wave filters, and EMC sine wave filters. The result of less overall interference will be better performance, fewer spikes, reduced costs, and a longer service life of the motor.
This article was written by Carsten Juergens, Director of Product Marketing for Power EMC Filters at EPCOS Inc., a TDK Group Company, Iselin, NJ. For more information, Click Here.