High-speed drives: VFD perspective
In the introduction to high-speed drives [1] we have briefly described the design variants of semi high-speed and high-speed motors. The impact on construction of the motor is significant. High-speed motors can be quite different compared to the conventional machines. But how about the VFD? What changes need to be done when using the VFD to supply a high-speed motor? That is the topic of this post: High-speed drives from VFD perspective.
VFD design considerations for high-speed
First let’s look at some design aspects that likely need to be adapted for VFDs serving in high-speed applications.
Modulation technique and switching frequency
The modulation pattern at 250 Hz output frequency is not necessarily the same as the one used at 50/60 Hz output. Pulse width modulation (PWM) is the most widespread modulation of voltage source inverters (VSI). However, when it comes to high-speed application, other modulation schemes may be more suitable. A popular approach is to use a pulse pattern. Due to wide controllable speed range multiple pulse patterns are used. They provide good balance between power quality (harmonics) and device switching frequency.
Efficiency and losses
Losses of the VFD in high-speed configuration shall remain at reasonable level. Ideally, the losses would be similar to those of a conventional motor drive. Overall, the efficiency improvement by eliminating the gear shall not be eaten up by loss increase of the inverter. Moreover, higher losses would require larger re-cooler which is just another reason to keep them low.
Semiconductor current capability
Current capability of the switching device (such as IGBT or IGCT) is not constant but depends on the effective switching frequency. The reasons behind are the switching losses and the design maximum junction temperature.
Typical semiconductor current capability as depicted in Figure 1. At switching frequencies just above the “rated” (design) switching frequency is derating is moderate. When going further to higher frequencies the slope of the capability curve gets steeper representing more significant derating.
Motor friendliness
As just explained, the ratio between switching frequency and fundamental output frequency is lower for high-speed drives compared to “50/60 Hz” VFD fed motors. At the same time, the current and voltage waveform supplied to the motor shall be of a good quality. High harmonic content would cause additional losses in the motor. This is a challenge as the heat cannot be easily evacuated. In fact, in case of the high-speed motor compressor, the electric motor is cooled by the explosive process gas.
Slow roll mode
So called slow roll may be applied to cool down the shaft string before it is brought to standstill. It shall avoid bending of the shaft when stopped in “hot state”. Bending must be avoided as it is especially critical in high-speed operation (the higher the speed the larger the centrifugal forces for given imbalance).
Experience with VFDs for high-speed drives
Although some people may consider high-speed medium voltage drives as something “new”, they are available since several decades and their combined operational experience has reached multi-million operating hours.
A) Load-commutated inverters (LCI)
LCI is very mature and reliable technology that is available on the market since 1970s. Due to easy power and voltage scalability a very high-power drive can be realized without too much complexity and with low parts count. Although majority of the LCI drive applications have rated output frequency in the range 40 to 60 Hz the drive can reach output frequency up to 100…120 Hz with minimum modifications. In combination with 2-pole turbo-motor a direct drive with up to 7’200 rpm can be realized.
First projects semi high-speed LCI driven motors had already been realized in early 1980s and some installations therefore reached over 35 years operational experience demonstrating high reliability and availability.
Selected references:
- Year 1984: 9’600 kW @ 6’000 rpm (pump)
- Year 1986: 13’000 kW @ 6’400 rpm (compressor)
⇒ First references with LCI in combination with motor at 6’000 rpm from early 1980s ⇒ about 40 years of operational experience
B) Voltage source inverters (VSI)
Nowadays VSI drives represent vast majority of all new electric drive installations. There are several topologies on the market. In medium voltage range the most common are 3-level neutral point clamped (3-L NPC) inverter, 5-level neutral point clamped (5-L NPC) inverter, multi-winding multicell or modular multilevel converter (MMC, M2C). Advantage of the multilevel inverters is that they can reach high overall switching frequency without a significant power derating (switching frequency of individual semiconductor remains at medium level).
In high-speed configuration more than 250 Hz fundamental output frequency can easily be reached (field reference up to 260 Hz, technical feasibility up to at least 400 Hz).
VSI type of converter can supply both induction and synchronous machine and the true high-speed applications typically consist of cage induction machine and active magnetic bearings (AMB). Nevertheless, there are high-speed motors running on conventional fluid film bearings.
Selected references:
- Year 2007: 11’500 kW @ 11’200 rpm (compressor)
- Year 2011: 14’800 kW @ 9’175 rpm (compressor)
Installed base
The figure below shows selected high-speed drive systems in operation. Note that the figure reflects only one VFD manufacturer (ABB) and only selected references up to the year 2019/2020.
In line with the previous description, the LCI drives dominate semi high-speed range up to approx. 6’400 rpm. VSI based motor drives allow true high-speed up to 15’000 rpm with potential for further increase. Another thing that we can observe is the power-speed capability. The higher the speed the lower the shaft power. This relationship is dictated by the mechanical integrity of the rotor of electric machine.
Conclusion
As illustrated in this post, the VFD design for a high-speed application does not significantly differ from a conventional VFD. It is either a standardized product with small adaptations or even completely free of any modifications. The electric motor, on the other hand, is the component with higher complexity. One step from a conventional motor is motor with increased speed and ‘standard’ bearings. If the speed range requires magnetic bearings the complexity goes again one step higher.
References
[1] High-speed drives: Introduction, https://mb-drive-services.com/high-speed-drives-introduction/
[2] To gear or Not to gear? https://mb-drive-services.com/to-gear-or-not-to-gear/
[3] Challenges of high-speed motor design, https://mb-drive-services.com/challenges-of-high-speed-machine-design/
2 Comments
Tomi Ämmälä · May 23, 2023 at 5:21 pm
This topic has special intrest for me; it was maybe 2018 when we needed to do control upgrade on Troll C -platform high speed crude oil export pump -drives (Megastar W). Its original modulator (from 80’s) was able to reach 166Hz freq with 300Hz switching freg. Pwm/vector control
Originally the new design (carrier wave) was able to reach just 69Hz, when it ran out of gears.
The solution to reach the desired 100Hz was to introduce one more gear, adjust parameters and at higher speed change to MP3C.
Now you are talking about 260Hz output, but IGCT is not that fast of component, what would be the required swfreq and how does voltage waveform look like at top speed?
Br, Tomi
admin · May 29, 2023 at 8:23 pm
Hi Tomi,
thank you for leaving such interesting comment and sharing your experience. As you say, PWM may not be the most efficient way as high switching frequency leads to increased losses and power (current) derating of the semiconductor switch such as IGCT. Thus, pulse pattern may be a good approach to get reasonable quality of output voltage while keeping moderate switching frequency. Due to wide speed range, multiple pulse patterns are used (the higher the fundamental frequency the lower the pulse pattern). Another point might be certain modification of the inverter output filter.
Best regards,
Martin
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