To Gear or not to Gear?
“To Gear or not to Gear: That is the question!”
Dear readers, it is April 1, the Fool Day. However, we are not going to fool you. Instead, we have just allowed ourselves to paraphrase the famous quote by Shakespeare from Hamlet, Act 3 Scene 1. Have you ever asked that question yourself? Of course, we refer to electric drives and to the point whether to use a direct drive or a geared solution.
Direct drive versus geared drive
When designing an electric motor for a specific application there are two options:
(a) Directly match the load speed
(b) Use a gear to increase or decrease the speed
Therefore the question: To gear or not to gear? The variety of drive applications is so wide that no general statement is possible. Obviously, both solutions have their pros and cons. In some cases, one solution is clearly preferred against the other. In other cases, both solutions are viable alternatives.
Geared drives
Geared solutions have a long tradition. The gear is used to adapt the machine speed (and also torque) to the required load speed (and load torque).
Applications such as mills in mining industry or marine propulsion drives typically use speed-decreasing gear. The load speed is low and motor directly matching that speed would be large and heavy. Therefore, it is usually more practical and economical to optimize the electric motor and then select the gear ratio accordingly.
On the other side of the spectrum we have the turbomachinery. Centrifugal and axial compressors may operate at speeds from approx. 4’500 rpm to over 10’000 rpm. Here a conventional solution consists of a four pole electric motor (e.g. 1’500 rpm) and a speed-increasing gear. One advantage is a relatively standard motor design. Moreover, no special considerations about the bearings are required. As we have already written in [2], four pole machine design is more convenient from critical speeds point of view. The rotor is usually sub-critical which is a better fit for variable speed operation. And of course, these motors are well proven with many references installed across the industry.
Hint:
For those not so familiar with mechanical engineering you can imagine the electric equivalent: Presence of a gear is like having an output transformer [3] between inverter and motor. In this way the inverter nominal output voltage becomes independent on motor voltage.
Gearless (direct) drives
The other group of electric drives is gearless, i.e. the motor speed corresponds to the load speed (neglecting fancy things like torsional twist etc). Therefore, these drives are also called direct drives.
What is the motivation to eliminate the gearbox? One reason is to simply eliminate that one component in the drive train. When a speed-increasing gear is eliminated we talk about “high-speed drives”. That is actually what this short series will be about. Going gearless in the turbomachinery applications might have few advantages. Each gear has certain losses (typically 1.5-2.5%) so elimination of the gear may slightly boost the efficiency of the system. Although, there might be some additional losses in the VFD and motor. We will address this point more in detail in a separate post. High-speed motor is also much more compact than his conventional counterpart. Remember that motor mass is proportional to torque. And torque is proportional to the ratio of mechanical power and angular speed. The higher the speed for certain mechanical output the lower the torque.
Combining a compact high-speed motor together with the absence of a gear results in attractive reduction of overall footprint compared to the conventional solution. In specific cases, for instance replacement of gas or steam turbines with an electric drive, high-speed solution might be the only possibility.
Of course, there are also some challenges. High-speed motors are almost exclusively two pole machines. Their design often leads to a super-critical rotor that poses certain challenges to the operating speed range and mechanical stability. However, there are rotor designs with rigid behavior available addressing this issue.
Another point is the bearing design. Generally, such bearings need more care and fine-tuning. There are high-speed motors using fairly standard oil film sleeve bearings. The other option is to use magnetic bearings. This option brings some extra benefits such as no wear and absence of lubrication oil. The stiffness of magnetic bearings can be controlled electronically by the control system.
High-speed: Where does it start?
You may wonder what speed is actually high speed. That is a fair question and I have not seen a general definition. In our posts we consider conventional speed up to 3’600 rpm (two pole motor with 60 Hz supply). Afterwards is a semi high-speed range that goes up to roughly 6’400 – 7’500 rpm depending on manufacturer, machine frame size etc. And finally there is the “true high-speed” above 7’500 rpm that often features active magnetic bearings.
Note that above ranges are our own classification for better understanding. The ranges apply for medium voltage multi-megawatt drive systems. Obviously, for smaller machines it is much easier to achieve higher speeds.
Technology trend
Both geared and direct drives co-exist and each technology has specific strengths. However, in general we observe a slightly increasing trend for direct high-speed drives. Reason for that is the readiness of the technology. First high-speed motors were manufactured more than 20 years ago. So there is a decent field experience in that area. VFD technology evolved meanwhile so that higher output frequencies – up to 300 Hz or more – can be achieved without sacrificing other characteristics. The VFD technology is so mature that even the concept of 4-pole semi high-speed motor design [5] becomes viable and draws more attention.
High-speed experience of one renowned VFD manufacturer is shown in Figure 1. Some of these installations meanwhile have over 35 years of field experience. As we can see above, even LCI technology enables high-speed or at least semi high-speed realization. LCI can be designed for output frequency up to 100…120 Hz. That corresponds to 6’000…7’200 rpm of a two pole turbomotor. Higher speeds are realized almost exclusively with VSI technology. There is no hard limit for the maximum output frequency. Instead, it is more an optimization between switching frequency, losses, output quality and potential power derating.
Summary
Both geared and gearless drives are essential part of industrial applications. Each solution has specific advantages and potentially also few drawbacks. Progress in technology – in motor as well as VFD design – opens more opportunities for high-speed applications. Therefore, we can expect more high-speed solutions in the future.
If you are interested in this series on high-speed, just stay tuned to our blog.
References
[1] Power, speed and torque, https://mb-drive-services.com/power-speed-and-torque/
[2] Variable speed motors and critical speeds, https://mb-drive-services.com/variable-speed-motors-and-critical-speeds/
[3] VFD output transformers – Where do we need them? https://mb-drive-services.com/vfd-output-transformers-where-do-we-need-them/
[4] Torsional series, https://mb-drive-services.com/category/torsional/
[5] T. Holopainen, P. Jörg, O. Liukonnen, “Comparison of two- and four-pole VSD motors
up to 4000 RPM”, 45th Turbomachinery and 32nd Pump Symposia, Houston, Texas, 2016
[6] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[7] ABB motor and generators, https://new.abb.com/motors-generators
[8] Birr Machines, https://www.birr-machines.com/
[9] MAN Energy Solutions, https://www.man-es.com/
[10] WIKOV – Premium gear manufacturer, https://www.wikov.com/en/
