What efficiency can you expect from your drive system?
In general, the efficiency of a drive system utilizing VFD is high. The differences come from the VFD topology, dimensioning and technical solution. As discussed in our previous post, the user shall always compare total efficiency of a complete drive system. The comparison of individual components (e.g. two VFDs or two motors) might be misleading and might impose a wrong conclusion.
What components shall be considered in the system efficiency:
– Input isolation transformer
– Input harmonic filter (if applicable)
– Variable frequency drive (VFD)
– Output harmonic filter (if applicable and not already included in VFD)
– Output reactor (if applicable and not already included in VFD)
– Output transformer (if applicable)
– Electric motor
– Gearbox (particularly when comparing geared drive with a gearless solution)
What efficiency values can you expect from drive components? The listed values are indicative. For comparison the total system efficiency is the right quantity for benchmarking purpose.
xfmr … Transformer, η … Efficiency, VSI = Voltage Source Inverter, LCI = Load-commutated Inverter, DFE = Diode Front End, AFE = Active Front End, VAR = Reactive power (also VAr)
The integrated transformers are usually optimized for smallest possible footprint allowing high temperature rise and therefore higher losses and lower efficiency. There are always exceptions, but generally the integrated transformers are designed for insulation temperature class 180°C or even 220°C with forced cooling allowing more losses and higher temperature rise. External transformer is easier to be optimized for higher efficiency, especially when located outdoors with less footprint constrains.
Above figures do not show the energy consumption required for re-cooling. Air cooled VFD typically requires air conditioning system (HVAC) to maintain the indoor temperature. Liquid cooled VFD, in case of no water on site, requires a dry cooler. The liquid cooling is more efficient and liquid cooled VFD has lower auxiliary power consumption for re-cooling.
In the next section we compare six drive system configurations on a model example of 16 MW drive. Let’s assume that the load is a compressor. The motor drives the load through a speed-increasing gear except for case 3 that is gearless high-speed.
Case 1 is a standard voltage source inverter type of VFD with diode rectifier and induction motor. Inverter has three or more levels (phase to ground).
Case 2 is very similar to case 1; it just has additional output sine filter as integral part of the VFD for superior output waveform.
Case 3 is a gearless high-speed drive system. VFD is same as in case 1, just slightly adapted for higher output frequency and equipped with an output reactor. Motor has magnetic bearings; it is either stand-alone machine or integrated moto-compressor.
Case 4 is a voltage source inverter with active front end (AFE) rectifier that allows regenerative braking. Inverter is same as in case 1. Input power factor is inherently controlled to 1.0.
Case 5 is again voltage source inverter with AFE. However, this one is in transformerless configuration, also called direct-to-line. It also allows regenerative braking. There is an input filter to reduce higher order harmonics, decouple the rectifier from the grid and eventually also to limit short circuit currents.
Case 6 is a classic load-commutated inverter (LCI) with input filter for harmonic reduction as well as power factor compensation. The losses of this filter are rather small as it is a shunt (i.e. the power from the grid to the motor does not flow through this filter). Motor type is synchronous (induction machine is not possible with LCI).
What can we observe?
The first two cases (conventional systems) have almost identical system efficiency. The extra losses in the sine filter are approximately offset by lower motor losses (sine filter eliminates harmonics). If synchronous motor would be used instead of induction machine, the system efficiency would boost by approx. 0.6%-1.0%. The figure would get close to the efficiency of LCI drive system.
The high-speed gearless drive system has somewhat lower VFD and motor efficiency, but there is a significant gain due to elimination of gearbox. Overall, the system efficiency is expected to be quite similar like previous cases 1 and 2.
The active front end VSI shows the lowest system efficiency due to higher rectifier losses. However, that picture would change in case of frequent regenerative braking. If braking capability is not required then a diode rectifier type is usually preferred (for seldom braking a diode front end with braking chopper might be attractive).
The transformerless solution eliminates the transformer losses. However, this is offset by additional losses in the rectifier (again AFE) and losses in the input filter. The system efficiency is therefore comparable to the diode front end type with transformer. This configuration would gain on energy efficiency in case of regenerative braking as already discussed.
LCI solution shows overall superior efficiency due to high efficiency of both VFD and motor. As already said, large VSI with diode rectifier comes close to this figure in case synchronous motor is utilized instead of induction type.
The goal of this post is to explain some considerations on drive system efficiency and demonstrate them on simple examples. The main take-away is that the user shall compare the system efficiency and not to benchmark the efficiency of individual components (great example is the high-speed motor in above table). Don’t forget to check if the figure provided by manufacturer is typical or guarantee value, if it includes harmonic losses and if auxiliary power was considered or not.
Let’s drive for more energy savings and short payback time. Cheers from MB Drive Services.
References
[1] IEC 61800-4: Adjustable speed electrical power drive systems – Part 4 (–> Chapter 11: Efficiency determination)
[2] World record in energy efficiency of synchonous motor, https://new.abb.com/news/detail/1789/ABB-motor-sets-world-record-in-energy-efficiency-saves-half-a-million-dollars
[3] Saving energy with variable speed drives, http://www.abb.com/cawp/seitp202/c253ae5e6abf5817c1256feb0053baf7.aspx
[4] Energy savings with variable frequency drives, https://www.youtube.com/watch?v=LYDSbOyUV4I
