Why the motor matters in a VSD compressor
We’re diving a little deeper this month into the different motors that can be found in variable speed drive (VSD) compressors. While VSD compressors are well known to be energy efficiency giants, did you realise that the efficiency gains and life expectancy of these compressors - as well as their service requirements - will differ depending for one, on the type of electric drive motor they have? And, this will ultimately impact your total cost of compressor ownership. In this blog post we therefore look briefly at the differences as well as the pros and cons of synchronous reluctance versus asynchronous motors.
If a variable speed drive (VSD) compressor is the right technology for your application and you’re now comparing products on the market place, you may have noticed that where some operate with an asynchronous motor (such as a permanent magnet synchronous motor or PMSM), others operate with a synchronous reluctance motor (SynRM). The question is - what difference does it make?
In an AC synchronous electric motor, the rotation of the shaft at steady state is synchronised with the frequency of the supply current. Here, the rotation period is precisely equal to an integral number of AC cycles. As the name applies - it is ‘in sync’ or synchronised.
Whereas with an AC asynchronous motor - such as a permanent magnet synchronous motor (PMSM) - permanent magnets are embedded into the steel rotor in order to create a constant magnetic field. Here, the stator carries windings which are connected to an AC supply in order to produce a rotating magnetic field.
But what does that all mean to you?
Materials, maintenance & service life
In a PMSM, the rotors will be made from iron or rare earth magnet materials (which are becoming increasingly difficult to source as a raw material and can be expensive or hard to dispose of when they’ve reached their useful life) and the removal of these rotors (e.g. for service) requires specialised tools. In addition, they are very sensitive to overheating which runs the risk of the magnets losing their magnetism. This results in a shorter life cycle or risk of compressor failure.
In comparison, with SynRM’s there is no use of aluminium, copper or rare earth magnet materials in the rotor, ensuring for one, ease and cost effective manufacture, as well as a trouble-free disposal at the end of their service life. In addition, no magnets in the rotor means no concerns when it comes to overheating, and the replacement of bearings and rotors when required is easy, as no special tools are required.
And, here’s another maintenance consideration to bear in mind. With a variable speed compressor which uses a PMSM, the motor and the compressor block (or airend) are one unit. When it comes to maintenance tasks, such as bearing replacement, this configuration makes the task rather elaborate. The result may therefore likely be a higher service fee and a longer period of downtime compared to maintaining a variable speed compressor that has a SynRM. Here, like three phase asynchronous motors, maintenance is far easier and therefore far more economical to undergo.
The energy efficiency advantage
Perhaps the greatest advantage of variable speed drive compressors which have a SynRM is their energy efficiency.
Let’s look first at specific power (kW/m3/min). A VSD compressor with a SynRM will generally have a higher specific power. To explain, let’s draw an analogy from the automotive industry. Understanding the specific power of a compressor, is like looking at the fuel efficiency or fuel economy of your car. So let’s say that's measured in litres/100 km (i.e. how many litres of fuel your car needs in order to travel 100 km). Here, the less litres required per 100 km - the more fuel efficient the car is.
With compressors we look at what is the required power input in kW of the compressor to generate e.g. xx m3/min of FAD Flow output. So, the less kW of energy required to produce the m3/min - the more energy efficient the compressor is hence the use of the “specific power” that equals kW/m3/min. This is a great tool which allows you to meaningfully compare different compressors at the same flow rate.
Coming back to our initial comment then that a VSD compressor with a SynRM will have a lower specific power this means that it will be more energy efficient, than VSD compressors without synchronous reluctance motors.
Performance in nominal point and partial load
A recent study demonstrated that a typical compressed air consumption profile is in the 30 to 70% range of the maximum. This is where a rotary screw compressor equipped with variable speed control and a synchronous reluctance motor can demonstrate its energy efficiency advantages in the partial load range to the full. Synchronous reluctance motors achieve significantly higher efficiency in the partial load range than asynchronous motors. This allows savings of up to 10% when compared with conventional variable-speed systems.
Synchronous reluctance motors boast significantly enhanced efficiency rates.
Choosing a VSD compressor which can deliver increased energy efficiency ultimately means lower operating costs and therefore lower life cycle costs.
When it comes to comparing VSD compressors it's also worth looking at the IE class. Frequency converters are graded by IE efficiency classes, while drive systems are placed in IES efficiency classes. Such uniform frameworks allow the comparison of drive systems (motor and FC) for every type of application.
Would you like to learn more about Kaeser’s Sigma Frequency Controlled (SFC) rotary screw compressor range? The variable-speed versions of Kaeser’s ASD to CSDX series rotary screw compressors from 18.5 to 90 kW feature a synchronous reluctance drive system from Siemens. The increased energy efficiency of these compressors is already benefiting numerous customers around the globe with lower operating costs.
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