Dealing with critical speeds
This post explains some basic considerations related to variable speed motors and their critical speeds. The fact that motor operates at variable speed makes it more challenging from rotordynamic perspective in order to avoid critical speeds within the operation speed range. At the same time, VFDs allow higher flexibility in selecting the rated electric frequency and rotor pole number to optimize the performance.
Variable speed motors and critical speeds
Every electric motor, like other rotating equipment, has one or more critical speeds. At such speed the resonance (either lateral or torsional) is excited and the motor experiences high vibration. Magnitude of these oscillations depends on the damping, resp. amplification in resonance. In general, vibration has negative impact on lifetime of mechanical components. Therefore, the desire is to avoid operation at a critical speed.
Put it simple, there are two types of rotors with regard to critical speed:
— Subcritical rotors
— Supercritical rotors
Subcritical rotor is such that operates below the critical speed. It means that the maximum operation speed of the rotor is below the critical speed with a separation margin defined by the design standard (usually 10-15% separation margin).
Supercritical rotor has its operation speed range above the first critical speed, i.e. the minimum operation speed is higher than the critical speed with appropriate separation margin.
Now looking at typical designs of industrial motors we realize that:
– 2-pole machines usually have supercritical rotor design
– machines with 4 and more poles mostly feature subcritical rotors
2-pole variable speed motors and critical speeds
The rotor of a 2-pole machine is supercritical with regards to critical speed. It means that during the start-up the critical speed needs to be passed. With increasing power it becomes increasingly challenging to keep the critical speed out of the operating speed range.
4-pole variable speed motors and critical speeds
4-pole motors (and generators) run below their first lateral critical speed. This allows them to operate across their entire speed range without forbidden speed windows – a particularly important feature when the motor is supplied from a VFD.
When considering a fix speed motor operating as direct on-line, the critical speed is less of an issue. Either the motor is subcritical and no special measures are needed or it is supercritical and the machine needs to pass through the critical range fast enough. With variable speed the topic becomes more complex. The wider the speed range the more challenging it becomes to keep the critical speeds outside of the operating range.
Sometimes it is unavoidable to have a critical speed inside the operation speed range. VFDs support this by allowing to define several narrow speed ranges – so called “skip bands” or “restricted speed ranges”. Operation within such range is restricted by the VFD control software.
While skip bands are a common solution, they obviously limit the speed range and therefore also the performance of the system.
In fact, VFDs do not necessarily make the situation around critical speeds worse. Drive solution also allow more flexibility how to combine number of pole pairs and stator frequency to reach desired speed.
4-pole high-speed machines
One example of potential that VFDs offer: 4-pole high-speed approach [5, 6].
As mentioned before, 2-pole rotors are typically supercritical and wider operation speed range might be cumbersome. Using a 4-pole machine and doubling the electric frequency one can achieve the same shaft speed and at the same time better mechanical performance of a 4-pole rotor. It paves the way to 4-pole gearless variable high-speed machines.
Very high-speed machines and active magnetic bearings
In true high-speed range (typically above 8’000 rpm) a 4-pole solution is theoretically still possible. However, at some point the stator frequency gets too high making the VFD design less cost effective and less meaningful. Special 2-pole machines are used. They often feature solid rotor with superior mechanical characteristic. Moreover, such high-speed applications utilize magnetic bearings hence mitigating issues with critical speeds [7]. In fact, active magnetic bearings can introduce additional (active) damping for superior vibration performance.
Conclusion
Rotor design is crucial for good performance and particularly for low vibration of the electric machine. Variable speed operation imposes an additional challenge for the rotordynamics. Subcritical rotors are generally more suitable in combination with VFDs as they do not restrict the speed range. Supercritical rotors can principally be used as well, but require more attention and eventual use of “restricted speed ranges”. Last but not least VFDs support alternative concepts, such as 4-pole machine with double supply frequency. Hence, it is possible to keep the same rotation speed (e.g. avoid gearbox) without compromising the rotor performance.
References
[1] Power, speed and torque, https://mb-drive-services.com/power-speed-and-torque/
[2] Motor start with VFD, https://mb-drive-services.com/vfd-motor-start/
[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[4] ABB Motors and Generators, https://new.abb.com/motors-generators
[5] Timo P. Holopainen, Pieder Jörg, Olli Liukkonen, “Comparison of two- and four-pole VSD motors up to 4000 rpm”, Tutorial, 45th Turbomachinery & 32nd Pump Symposia, Houston, Texas, Sept. 12-15, 2016
[6] Alberto Tessarolo, Gianfranco Zocco and Carlo Tonello, “Design and testing of a 45-MW 100-Hz quadruple-star synchronous motor for a liquefied natural gas turbo-compressor drive”, IEEE Trans. on Industry Applications, Vol. 47, No. 3, May/June 2011
[7] MAN Energy Solutions: High-speed oil-free integrated motor-compressors, https://www.man-es.com/oil-gas/products/compressors/sealed-compressor
