September 2007

EMC filters cut costs and protect motors

Never again shielded cables

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The SineFormer™ from EPCOS is an output filter that is easy on the motor and makes shielded motor cables superfluous. It allows costs to be reduced while improving performance.

Asynchronous motors must often be operated at continuously variable speeds. Pulse-width modulated frequency converters with an unregulated input rectifier shape the DC voltage from the DC link circuit to an output voltage by using suitable driving of the semiconductors (usually IGBTs). The principle is shown in Fig. 1.

 FIGURE 1: BLOCK DIAGRAM OF A FREQUENCY CONVERTER
The AC converter forms a pulse-width modulated output voltage with steep edges from the DC link voltage
a: Sinusoidal input voltage
b: DC voltage in the DC link circuit
c: Pulse-width modulated output voltage (phase-to-phase voltage)

As Fig. 1 shows, the voltage at the converter output is very far from having an ideal sinusoidal shape. The square-wave pulses of typical converter output voltages have an edge steepness of between 5 and 10 kV/µs that causes high-frequency currents in the motor cable. The phase-to-ground voltage shows a similarly high edge steepness due to parasitic capacitances of the converter topology. Parasitic motor capacitances also lead to very high bearing currents that shorten the operating life of the motor.

Common mode interference necessitates shielded cables

Longer motor cables also have high parasitic capacitances to ground that are approximately proportional to the cable length and significantly influence the design of the power filter. In order to reduce the further electromagnetic propagation of the interferences on the motor cable by coupling or radiation, the cable must be shielded unless effective filter measures are taken on the motor side of the converter.

Shielding becomes more important the higher the frequency range is. It is already indispensable for the frequency range < 30 MHz. As illustrated in Figs. 2 and 3, even with an almost ideal positioning with a spacing of more than 50 cm between power and motor cables, the specified limits can rarely be met if the motor cables are unshielded. In practice, the installation specifications usually allow the motor and power cables at least to cross at right angles. Due to the stronger capacitive coupling, however, the interference spreads more extensively than if the cables are separated from each other, also reaching the power line and causing the limits to be exceeded to a much greater extent (Fig. 3). However, it is not only important to observe the EMC limits on the power line. If the high-frequency interference parts couple over to data and sensor lines, they produce signal errors and thus jeopardize the functionality of the system.

 FIGURE 2: CABLE CONFIGURATIONS AT CONVERTERS
Structure of a converter with a power line filter and an unshielded motor cable without an output filter.
Left: Power and motor cables are separated from each other by a significant distance
Right: Power line and motor cables are crossed at right-angles

 FIGURE 3: INTERFERENCE VOLTAGE WITH UNSHIELDED CABLES
Power supply side measurement of the interference voltage at a frequency converter with an EMC power line filter and a 100 m unshielded motor cable without an output filter: dependence of the measured results on the position of the motor cable, with reference to the limits specified in EN 55011 class A (group 1) and EN 61800-3 category C2

However, if the same test set-up consisting of power line filter, converter and motor is operated with a suitable shielded motor cable of equal length, then the specified industry limits are observed as expected (Fig. 4). Coupling of the interference from the motor cable to data and sensor lines can also be prevented or at least significantly reduced, so that operational failures can be avoided.

 FIGURE 4: INTERFERENCE VOLTAGE WITH SHIELDED CABLES
Measuring the interference voltage (power supply side) at a frequency converter with a power line filter and a 100 m shielded motor cable without an output filter. The specified limits are met.

Apart from these EMC problems, the user must also deal with the bearing currents in the motor. This is because the high edge steepness of the converter output voltage stimulates parasitic resonant circuits consisting of motor and cable capacitances as well as parasitic inductances. As a result, transient responses are superposed onto the converter output voltage, which produces voltage surges on the motor side, often with amplitude peaks that the motor is not designed to handle over the long term (Fig. 5). High bearing currents lead to premature wear of the bearings, but also stress the insulation of the motor winding, thus shortening the operating life of the motor.

 FIGURE 5: VOLTAGE SURGE DUE TO PARASITIC EFFECTS ON THE CABLE
Voltage surges lead to breakdowns of the motor insulation.

Essentially three different filter concepts may be used to alleviate this problem:

     • dv/dt filters

     • Sine wave filters

     • EMC sine wave filters (SineFormer)

At first sight, the least expensive solution would seem to be a dv/dt filter. Designed as an LC low-pass filter between the motor phases (differential mode), it reduces the steepness of the voltage pulses and the voltage peaks at the motor winding. However, as especially the higher-frequency parts of the common mode interference current to ground are not suppressed, shielding of the motor cable is necessary. Cable lengths of up to about 100 m are typically possible. However, greater lengths can lead to resonances and impermissible heating of the dv/dt filter, ultimately leading to destruction of the choke.

The structure of the sine wave filter is similar to that of the dv/dt filter. The sole but significant difference: the limit frequency of the LC low-pass filter lies between the maximum permissible rotating-field frequency and the lower permissible switching frequency of the converter. The advantage: the sine wave filter has a greater efficiency than the dv/dt filter, the switching-frequency parts of the (differential mode) phase-to-phase voltage disappear almost completely and the output voltage is sinusoidal. However, the LC circuit of the sine wave filter operates only symmetrically, so the (common mode) voltage of each phase with respect to ground still contains significant higher-frequency parts. Although this allows motor cable of much more than 100 m and greatly improves the motor protection, the insufficient common mode filter effect nevertheless means that shielding of the motor cable is still necessary.

SineFormer optimizes suppression and cuts system costs

To solve this problem, EPCOS has developed the SineFormer. It offers optimized motor protection and simultaneously reduces the system costs. The SineFormer technology not only produces a sinusoidal voltage between the phases, but also significantly reduces the common mode interferences between the phases and ground. The SineFormer consists of a symmetrical inductor and capacitors that form a sinusoidal voltage between the phases. These are supplemented by a current-compensated choke as well as capacitors to ground designed to significantly reduce the common mode interference parts on the motor cable. Fig. 6 shows the basic circuit diagram of a SineFormer.

 FIGURE 6: CIRCUIT OF THE SINEFORMER
The SineFormer attenuates both differential and common mode interference. This makes expensive shielded cables unnecessary and protects the motor.

The EMC concept with SineFormer is distinguished by numerous technical and cost benefits.

 SINEFORMER IN OVERVIEW

Technical benefits of the EMC concept with SineFormer:

Reduction of the dv/dt to <500 V/μs
Reduction of the acoustic noise produced by the motor
Significantly lower eddy current losses
Significant reduction of bearing currents
Avoidance of interference coupling from the motor cable to other power and signal lines
Improved EMC performance compared with shielded cables
Radiated interference within normative limits
Optimum reduction of interference (conducted and radiated) compared to other output filter solutions
No feedback loop to DC link of the frequency converter is necessary

Cost benefits of the EMC concept with SineFormer:

Unshielded motor cables can be used, thus reducing assembly expenditures, increasing the operating life and reducing cable costs
Motor size can be reduced
Motor operating life can be significantly increased
Longer motor cables possible (measured with up to 1000 m unshielded motor cable)
No maintenance costs, as the SineFormer is built without forced ventilation
Compact filter solution (not modular system) with lower volume and weight
Reduced demands on the power filter
Increased system availability
Also suitable as a retrofitting set

The special advantage of dispensing with shielded cables is that the SineFormer is simply less expensive, depending on the cross-section and length of the cables. Frequently, the costs of the filter are already compensated by the use of unshielded cables of around 100 m. If only the cost of the SineFormer and that of unshielded cables is compared with the cost of a sine wave filter and shielded cables, the break-even point can be achieved at cable lengths of less than 50 m – and this comparison does not even include the higher assembly expenses for shielded cables.

Fig. 7 shows the efficiency of operation of the SineFormer technology in an impressive way compared to Fig. 3. Even if the power line is crossed with an unshielded motor cable, the specified limits are met (here to EN 55011, class A – group 1 or EN 61800-3 category C2). The optimum performance of the new filter technology is clearly shown by the almost complete absence of coupling. Thanks to the use of SineFormer filters, the use of shielded cables can finally belong to the past. The system costs can be reduced and the system availability increased.

 FIGURE 7: MEASUIRING THE INTERFERENCE VOLTAGE AT THE SINEFORMER
Despite an unshielded cable, the permissible limits are observed.

SineFormer filters are also ideally suited for retrofitting, namely whenever EMC problems caused by the motor cable only occur during operation. It is naturally always important to select a suitable EMC filter on the power side, for example by using EPCOS’ new B84143D*R127 power line filter series for up to 300 m for class A and 200 m in class B (EN 55011).

The new SineFormer filters are distinguished from the sine wave filters currently available on the market by additional innovative features. They are available as a compact filter solution, thus obviating the extensive assembly costs typical of modular systems. There is also no need for a feedback to the DC link circuit. In the latter case, the relevant cable must be shielded on both sides. However, in many cases no such provision to contact a shield is made for the DC link circuit terminal at the converter, thus incurring a risk of high interference radiation.

Because the SineFormer needs no internal forced cooling, it is maintenance-free. In contrast, filters with fans have a major drawback, namely that the operating life of a commercially available fan depends on the ambient temperature: its operating life drops as the temperature rises. Worse still: because the temperature in the filter fluctuates as a function of the load current, the exact failure or maintenance time cannot be determined. As a rule, the fan can only be replaced by the filter manufacturer, and thus incurs further costs.

With the SineFormer, EPCOS has developed a pioneering filter technology that offers a convincingly superior EMC performance as well as impressive cost benefits.

 PRODUCT PROFILE: SINEFORMER

Rated voltage:

     • 520 V up to 180 A or 600 V from 320 A

Rated current:

     • 11 A up to 320 A (600 A and 1000 A in preparation)

Converter pulse frequency:

     • 4 to 8 kHz up to 180 A and 2.5 to 3 kHz at 320 A

Approvals:

     • UL/CSA up to 180 A

Authors:

Carsten Juergens, Product Marketing Manager, EMC Filters

Christian Paulwitz, Manager EMC Laboratory

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