What has a transformer and a gear in common?
Transformer and gearbox are two quite different devices. Nevertheless, they have something in common. The analogy described in this article may help to understand how a drive system is dimensioned and configured, what limitations the designer faces and finally how do transformers and gears help to standardize certain system components while adjusting the entire system to the project specific conditions and requirements.
Role of transformer and gear in a drive system
For the illustration we will consider a typical VFD based system that is used to control the speed of a centrifugal compressor. We can assume a voltage source inverter (VSI) but the same is applicable also for other VFD topologies.
There is an incoming circuit breaker which is dedicated for the specific drive system. Next component is an input isolation transformer. It adjusts the voltage level and supplies the grid side of the VFD. The voltage or current is then rectified in the intermediate dc link before the inverter generates an output voltage or current of a variable frequency that is supplied to the electric motor. The motor converts the electric power into mechanical power in terms of shaft torque and speed. In case of turbomachinery as the driven load, there is mostly a
speed increasing gear. The driven load, such as compressor, is connected to the high speed shaft.
Role of the transformer
An input transformer provides several functions to the drive system. However, the most important functionality is the adaptation (transformation) of the grid voltage to a voltage level that the drive expects at its input terminals. For this reason, almost every medium voltage VFD requires an input isolation transformer. The only exception is so called “transformerless” (direct-to-drive/direct-to-line) VFD variant that eliminates the input transformer and replaces it with an input reactor. Such solution is restricted to specific grid voltages matching the VFD input voltage.
By using an input transformer (mostly a step-down transformer) the VFD can be designed for a narrow range of nominal input voltages. It allows economically optimal configuration and supports standardization of the VFD design.
How would the situation look like without the transformer? It would be very challenging for many reasons. Certain VFD topologies cannot easily scale the input voltage level. Also, the nominal rectifier input voltage often does not correspond to a standard grid voltage. Other topologies can eventually support the voltage scaling but the consequence is a non-economical solution.
Role of the gear
The purpose of the gear is to adjust the shaft speed. There are speed-increasing and speed-reducing gears. In case of a variable-speed turbomachinery driven by electric motor drive, a speed-increasing gear is used.
The gear ratio is defined by the number of teeth of the tooth wheels. Use of a gearbox allows an optimal motor design. The motor nominal speed can be selected independent on
the nominal speed of the driven load. Such degree of freedom allows to use more “standard” motor with a better performance. For example, a conventional 4-pole motor with a nominal speed of 1’500 rpm or 1’800 rpm can be used. Such motor mostly does not have any critical speed within the operating speed range which is a significant advantage in case of a compressor drive. Moreover, standard fluid film bearing technology (e.g. sleeve bearings) can be used.
Similar like the transformerless VFDs, there are also gearless compressor drives. However, they are (still) quite rare and require special technology. The VFD for a high-speed gearless drive is almost identical to a conventional VFD. The difference is a higher output frequency associated with higher switching frequency and potential modifications of the output filter circuitry. Motor is of a special design. It is a 2-pole machine, mechanically optimized for high speed.
How do transformer and gear help standardize the motor-drive design?
After understanding the role of transformer and gear in a drive system, we look at the resulting benefits for the VFD and motor design. The transformer allows to standardize the nominal input voltage of a VFD. This is essential thing since the grid voltage in each project/site can generally be different. Typical levels…
Purchase premium and get access to the full article.
Summary
The blog post describes typical modularization of variable speed drive systems and illustrates the concept by making an analogy between an input transformer and a gearbox.
In a simplistic way, a transformer is an electric adapter on the grid side while the gear is a mechanical adapter on the motor/load side.
Want to know more?
Purchase our premium subscription and learn more about this and many more topics from the world of variable frequency drives (VFD) and drive systems.
References
[1] VFD transformers: Introduction, MB Drive Services, November 2019, available online, https://mb-drive-services.com/vfd-transformers-introduction/
[2] Challenges of high-speed motor design, April 2022, available online, https://mb-drive-services.com/challenges-of-high-speed-machine-design/
