VFD switching frequency
The heart of every variable frequency drive (VFD) is the power semiconductor. By switching on and off in a proper modulation scheme the frequency conversion functionality is achieved. Let’s look at various aspects of VFD switching frequency.
Switching frequency of medium voltage VFDs
Having medium voltage VFD topology in mind it is important to clarify one aspect right at the beginning:
Switching frequency of the semiconductor device is generally not the same as the overall effective swiching frequency of the VFD. Especially for multilevel topologies there is obviously a large difference between both of them. That is in contrast to low voltage VFDs that often have just a two-level inverter stage.
Keep it in mind before making a too quick judgement or a wrong conclusion. It is important to compare apples with apples.
Line- and load-commutated converters
There is a group of converters that are “externally commutated”. In a diode rectifier the diodes conduct when a positive voltage is applied and block in case of negative polarity of voltage. Diode converter is passive and the diodes commutate with the frequency of the main supply. In a thyristor converter the semiconductors are switched on by a gate signal. They are switched off through an external voltage. On the grid side the external voltage is the grid voltage. On the machine side the external voltage is the voltage of the synchronous machine. Therefore, the commutation is again happening with the frequency of that external voltage (usually 50 Hz or 60 Hz on the grid side and variable on the machine side) and the switching frequency is relatively low.
Self-commutated converters
Majority of modern power converters is based on self-commutated switching elements (IGBT, IGCT, IEGT). Those semiconductors can be switched on and off through a gate signal. It means that their switching frequency is completely independent on the frequency of the power supply or fundamental frequency of the machine. The self-commutated converters open many possibilities with respect to switching frequency and modulation scheme. One example is the notoriously known pulse-width modulation (PWM).
The switching frequency can be tuned based on power range, machine characteristic or project-specific requirements. The experience shows that some flexibility for fine tuning is not always needed, but definitely something nice to have.
Considerations about VFD switching frequency
Now the question is how to select the most appropriate switching frequency? There are several considerations to be made. To name at least few of them:
One limitation factor is the semiconductor device itself. There is normally a range of switching frequency stated in the datasheet. The upper limit may come from the power part or eventually from the gate unit. Normal operation is usually quite far from the upper limit.
Switching frequency also affects the cooling. The higher the switching frequency the more efficient cooling is required so that the junction temperature is maintained within the design limits of the semiconductor.
Reaching very high switching frequency is often possible with certain power derating. If the VFD has such power margin available then it might be an option. When the power derating leads to a next bigger size of the VFD then it obviously affects the cost, footprint etc.
Selection of VFD switching frequency also affects the quality of the waveform. However, it is not only the switching frequency that plays a role. Equally important is the modulation scheme. With smart modulation strategy you can achieve lower harmonic distortion while maintaining same of lower switching frequency than a conventional PWM.
As expected, switching frequency directly impacts the switching losses and thus overall VFD losses. Higher losses put more demand on the cooling system, as already mentioned. Besides that, the capitalized value of losses influence the life cycle cost of the VFD or drive system.
What is a typical switching frequency?
A general statement is hardly possible. It depends on specific product and its topology, type of semiconductor, power range, application and its requirements etc.
The switching frequencies of low voltage drives are generally significantly higher than the switching frequency of medium voltage VFDs. For medium voltage drives it depends whether the power semiconductors are of low voltage type (multi-cell and multi-level topology) or if medium voltage devices are built in. Multi-level topologies can work with relatively low switching frequency per device while having decent output waveform. As the power increases, the importance of optimally selected switching frequency becomes quite relevant.
On a device level the switching frequency is in the range of several hundred Hz, on the converter level we can expect switching frequency of few kHz.
The higher the better – true for switching frequency?
Selection of the most suitable switching frequency is a multi-objective optimization. It is not a race Who can switch the fastest? Certain minimum switching frequency is required for stable operation during steady state condition and transients. Switching frequency can be used to adjust the quality of a waveform (e.g. stator current). Nevertheless, it shall be balanced with other criteria incl. the economical aspect. Moreover, there might be other means how to improve specific performance indicator without boosting the switching frequency. They allow better utilization of the equipment and reduced life cycle cost.
Going deeper in the subject
The VFD switching frequency is not just one value, at least not on the inverter side with variable output frequency. How does the switching frequency scale across the speed range of the machine? What are the most suitable modulation methods? When to use which one of them? What are the pros and cons of carrier based methods versus hysteresis based strategies? Where does the minimum and maximum limit come from? How is the switching frequency related to the inverter-motor compatibility? How much is the impact on overall losses and efficiency? How does the switching frequency influence the harmonic spectrum? What is the relation between switching frequency and audible noise? Is there any influence on motor vibration?
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References
[1] How to choose a medium voltage VFD: Cooling type, https://mb-drive-services.com/choose-mv-vfd-cooling/
[2] VFD dimensioning: Derating and uprating, https://mb-drive-services.com/vfd_dim_derating_uprating/
[3] How to choose a medium voltage VFD: Motor compatibility, https://mb-drive-services.com/choosing-mv-vfd-motor-compatibility/
[4] What efficiency can you expect from your drive system? https://mb-drive-services.com/energy-efficiency-part-5/
[5] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
1 Comment
Power semiconductors - Comparison of IGBT and IGCT - MB Drive Services · February 12, 2022 at 6:38 pm
[…] is a relationship between rated voltage, rated current and effective switching frequency [4]. Hence the semiconductor capability shall be presented as “3-D figure” linking […]
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