What is the difference between dv/dt filter and sine filter?
Almost every voltage source inverter (VSI) has some kind of inverter output filter. Design of the output filter has major impact on the drive characteristic and performance. For this post we have picked two key representatives. They are widely used in drive products and perfectly suitable for the explanation. Let’s talk a bit about the difference between dv/dt filter and sine filter.
Purpose of an output filter
Principle of a voltage source inverter is to convert the quasi-constant voltage in the dc bus into an alternating voltage of variable (adjustable) frequency. VSI does this DC/AC conversion by switching specific voltage vectors. The AC voltage waveform consists of voltage steps. Modern power semiconductors allow very rapid rise of the voltage (necessary to keep low switching losses). This high rate of change of voltage, commonly called just dv/dt, poses certain stress on the insulation system (power cable, stator etc). Moreover, it defines the bandwidth of the inverter and thus the frequency range that the VFD can excite at its output.
An output filter shall improve the compatibility within the drive system. It also improves the “motor friendliness” to some extent. Output filter generally reduces the maximum dv/dt and the maximum frequency that the inverter can excite (adding damping at higher frequencies). The filter allows to use certain length of motor cables without exciting cable resonance. It shall reduce the overshoot at motor terminals. Some filters also reduce the harmonic content of the motor current, minimize the common mode voltage seen by the motor or even provide power factor compensation and voltage boosting. The available functionality depends on the type of output filter and its design.
Types of output filters
There are several types of output filters. The filter design has major impact on VFD performance. They can vary from small compact units up to relatively bulky systems. In VFD applications those output filters must be suitable for operation at variable frequency. Practically all of them have a low-pass characteristic. Lower frequencies pass through the filter without any attenuation while high frequencies are damped (suppressed). The frequency where the suppression starts is called cut-off frequency. And the selection of the cut-off frequency is the major differentiator of the filter designs.
dv/dt filter
First output filter in our description today is a dv/dt filter. As the name says, the primary purpose is to limit the rate of change of voltage steps, i.e. reducing the dv/dt value and preventing undamped oscillations in the inverter-cable-motor system. Alternative names are slope filter or EMC filter. Cut-off frequency of such filter is typically several tens of kHz. Thus, the filter is small and compact and losses of dv/dt filter are mostly negligible. As the cut-off frequency is well above the switching frequency, the dv/dt filter has practically no impact on the harmonics in the output waveform.
f_fund < f_sw < f_cut-off
Advantage of a dv/dt filter is the compact size, negligible losses and the fact that it does not negatively impact the dynamics of the drive system (e.g. quick change of motor torque). The design is relatively straight forward. Most dv/dt filters are L-R-C type with small inductor in the main path and R-C combination as a shunt. Values of L and C determine the cut-off frequency while the resistance provides the desired damping.
The effect of dv/dt filter can be reinforced by adding an output reactor (choke). Reasons for the output reactor can be various: improved motor friendliness, synchronous bypass feature, paralleling of multiple VFDs to one motor etc. See blog post [1] for more details.
Sine filter
Sine filter is on the other side of the spectrum. The filtering effect is very strong so that the output waveform after the filter is almost sinusoidal. It means that the cut-off frequency is quite low, just in the range of few hundred Hz and basically below the switching frequency. It means that the size of the filter components is significantly larger compared to the dv/dt filter. Therefore, sine filter impacts the overall footprint of the VFD. It also generates heat losses that need to be evacuated.
f_fund < f_cut-off < f_sw
Sine filter is superior with respect to the quality of the output waveform. Harmonic content of motor current is typically negligible (< 2%). Earthed sine filter practically eliminates the common mode voltage on the motor side. Thus, sine filter supports high motor friendliness and retrofitting of DOL motors. However, without appropriate measures, the losses of a sine filter can be considerable. Moreover, the filter contains bulky components that increase the overall footprint of the VFD. Because of these reasons, sine filter is used in low and medium power range but becomes rare in high power domain.
Comparison
Although the dv/dt filter and sine filter have similar principle of blocking or suppressing higher frequencies from the drive output, the effect on the VFD performance is very different.
Best example is a 3-level NPC converter (see e.g. here). Equipped with a dv/dt filter this drive supports motor cables with a length of several hundreds of meters. At the same time, the topology allows highest dynamic performance (platform of choice for rolling mill applications or test stands with demanding dynamics). Motors for this application shall be specifically designed for the inverter duty.
The same topology, 3-level NPC, but with an output sine filter, is the preferred choice for clean output waveform (e.g. supporting retrofit projects with existing DOL motors) or very long motor cables. The price for superior motor friendliness is the limited dynamic performance.
A more detailed comparison of both mainstream output filter solutions is available for our premium subscribers:
- Footprint
- Losses
- Cable length
- Dynamic response
- Motor friendliness
- Power factor compensation
- Impact on control
- Impact on protection
References
[1] Reactors in drive systems: Where do we find them? https://mb-drive-services.com/reactors-in-drive-systems-where-do-we-find-them/
[2] What is the definition of low voltage and medium voltage VFD, https://mb-drive-services.com/what-is-the-definition-of-low-voltage-and-medium-voltage-vfd/
