What are the benefits of using EMC filters in frequency converters?
Although most manufacturers offer inverters with internal interference suppression, external EMC filters are still required in some applications. The two EMC filters of EPCOS are ideal for external applications of frequency converters.
With the continuous expansion of the inverter market, fierce competition, cost pressures and technological innovations have led to a continuous decline in product prices and a shrinking volume, while at the same time expanding the field of application of frequency converters. Today's inverter manufacturers generally offer products with built-in interference suppression devices, but this only guarantees EMC compliance under precisely defined operating conditions. If the limit is exceeded, an external filter needs to be configured and additional interference tests are performed, which means more cost and ultimately higher system cost.
According to the inverter product description, the maximum operating range that meets the EMC requirements is usually related to the inverter switching frequency and the maximum cable length between the inverter and the motor. In general, a higher switching frequency means stronger interference. Longer cables mean that a larger common mode current will flow through the common mode choke to suppress interference. If the actual applied cable exceeds the maximum cable length as defined in the drive's product manual, then a non-compliance with the limit will occur. In addition, the built-in noise suppression coil of the inverter may even saturate, causing the entire filter to completely fail.
1 EPCOS B84143A and B84143B Series Filters
The EPCOS B84143A and B84143B series EMC filters have the following features:
1) is an optimized solution for long motor cables and full load operation;
2) high insertion loss performance;
3) Easy to install, light weight (0.58~13.5 kg), small volume (51.4 mm I 63 mm I 165 mm ~ 110 mm I 220 mm I 440 mm);
4) Suitable for all industrial power supplies, maximum voltage 520 V, 50/60 Hz;
5) The product has UL and CSA certification, and the maximum certified current is 200 A;
6) Custom solutions that provide current up to 2 500 A;
7) High overload capacity, which can withstand an overload of up to 2.5 times the rated current.
Its outline drawing is shown in Figure 1.
Now we test the performance of commercial inverters with built-in interference suppression devices under different application conditions to illustrate the effect of these two EMC filters in inverter applications.
2 long cable drive problem
In general, the maximum operating cable length of a frequency converter with a built-in interference suppression device is 5 m. The test curve with internal interference suppression device is shown in Figure 2. The frequency converter under test has an internal interference suppression device with a motor cable length of 5 m or 50 m and a motor power of 11 kW. In the case where the motor cable length is 50 m, the limit is exceeded. Tests have shown that the maximum conducted emissions under these conditions meet the requirements of EN55011/Class A. However, if the drive uses a longer cable, the conducted interference on the power line will increase and the built-in filter of the drive will not guarantee adequate noise rejection. Since the common mode current generated by the high parasitic capacitance of the shielded cable itself will also increase with the cable length, these currents will cause the common mode choke coil of the internal filter to saturate, so EMC requirements will be difficult to meet. It is necessary to configure an external filter.
In this example, the frequency converter is equipped with a 50 m cable. The test record clearly shows that the coils of the internal filter are saturated, causing awkward irritating noise, so an external filter must be configured to ensure that the EMC limits are met again. The corresponding EMI test curve for an inverter with an external filter and a motor power of 11 kW with a shielded cable length of 50 m is shown in Figure 3. The EPCOS ultra-compact, low-cost filter, model number B84143-A25-R105, is used in this application.
3 effects of saturation
The installation of an external filter with a comparison test curve with and without the internal common-mode coil is shown in Figure 3, showing that the interference is reduced, but still not below the limit. Prove that the external filter is not suitable for this topology, because the coil inside the inverter is saturated and becomes an additional source of interference. The interference generated by the saturation coil is directly superimposed on the interference signal of the inverter itself. This is because when the common mode AC current flows through the common mode choke and is constantly saturated, due to its nonlinear characteristics, the choke behaves like an additional broadband interference source. Therefore, the characteristics of coil saturation can be reflected by the test peak (PK) and quasi-peak (QP), and it can be seen that the peak and quasi-peak have exceeded the limit. In contrast, the effect of coil saturation on the mean (AV) is very limited. The wideband interference due to coil saturation is correspondingly more clearly captured by the wideband PK and QP detectors, rather than the narrowband detector AV.
At first glance, the test curve with built-in coils is surprising in the range of frequencies from a few hundred kHz to 3 MHz. This is because of the common mode choke, whose inductance is determined by its ferrite material, which not only suppresses common mode interference, but also has a very low leakage inductance (the leakage inductance is due to the partial magnetic field passing through the air instead of the ferrite medium). produced). This part of the leakage inductance is not affected by the saturation of the coil, but it is almost no help for other frequency bands that exceed the limit due to coil saturation. If the same frequency converter uses the EPCOS filter of the same type as B84143-A25-R105, but removes the internal choke of the inverter, the result is shown in Figure 3, which makes it easier to meet the limit.
For this reason, the interference suppression coil inside the inverter is removed. This is a very good unfiltered effect of the frequency converter. The inverter without internal filter still contains some basic components that suppress interference, such as well-connected capacitors. Internal capacitance is necessary so that high frequency interference on the power line connecting the frequency converter to the filter is effectively reduced to prevent noise from coupling to the filter power line. Otherwise, the connection cable must be shielded.
However, the reality is that if the user uses an internal interference suppression component with a common mode choke coil, it is inevitable to use a higher performance external filter at the same time, for example, the model number B84143 shown in Figure 4. EPCOS double-section filter for B25-R110. The frequency converter under test has an internal coil with a motor cable length of 50 m and a motor power of 11 kW. The external filter is B84143-B25-R110 as a supplement to the internal interference suppression device.
This ensures that at least the interference is below the corresponding limit and meets EMC requirements. The peak PK in the test curve fluctuates drastically at frequencies up to 0.5 MHz because the internal choke is still running at saturation. However, the quasi-peak QP curve will be smooth, significantly below the limit.
4 parallel problems
When the interference at the power input is increased with the length of the motor cable, this problem will be more serious if the inverter drives multiple parallel motors at the same time. We can think of the shielded cable as a simplified circuit with multiple series inductors and multiple capacitors connected in parallel and grounded (multiple types of series circuits). These capacitors are connected in parallel, and the total capacitance to ground is parallel. The number of cables increases. However, on the other hand, the equivalent circuit of the shielded cable is very complicated, because the presence of the in-line inductance, the parallel connection of the two cables is not equivalent to the parallel connection of the two parasitic capacitors.
However, the frequency converter drives two parallel motors (in this case a motor with 7.5 kW and 11 kW), each with a 25 m cable connection. This configuration is more than a single motor, 50 m long. The configuration of the cable is more demanding, and the test curve of the parallel connection of the two motors is shown in Figure 5. Install the external B84143-A25-R105 filter, which is a comparison test curve with and without the built-in interference suppression device. Compare Figure 3 with Figure 5. It can be seen from the two motors with a cable length of 25 m. The interference is significantly higher than a motor with a cable length of 50 m. This phenomenon can also be reproduced in the comparison test after removing the internal coil of the inverter. If the low-cost filter B84143-A25-R105 is used, the target below the limit can be achieved, even in the case where the internal coil is removed, but the margin is significantly smaller than in the case of a single motor 50 m cable.