Permanent Magnet Synchronous Generators in marine applications – Interview with expert: Dr. Jussi Puranen

It was a while from hearing from us and it was definitely a while from latest drive technology article. That’s why I am happy to be back with new, hot material. This time we will look on Permanent Magnet (PM) Machines, particularly on Permanent Magnet Synchronous Generators (PMSGs) or Permanent Magnet Generators (PMGs) for short. Latest environmental IMO regulations are shifting the maritime industry into hybridization and energy efficient solutions, where PMSGs play a huge role. It is a topic in which I am involved for some time already, but this time it is not me who will do the talking. Let me present you our guest and expert in the field – Dr. Jussi Puranen from Lappeenranta in Finland.

Interview with expert: Dr. Jussi Puranen

Mateusz: Jussi, I am very happy, you agreed for an interview. Can you please tell us more about yourself?

Jussi: Jussi Puranen – working as a head of product line for electric machines at The Switch/Yaskawa. I have background on electrical engineering (MSc and DSc) and been working 16 years in the company in various positions including also engineering and R&D.

Permanent Magnet Synchronous Generators in general

Mateusz:  We will be discussing PMSGs today, as their trend is increasing lately in maritime industry. Let us starts form understanding, what they actually are. How does the Permanent Magnet Shaft Generator work?

Jussi: Basically it is really simple; there are strong permanent magnets fixed on the rotor (the rotating part), and as the rotor is rotated with prime mover (diesel engine), magnetic flux will induce voltage and current on stator (stationary part) according to Faraday’s law of induction.

PMSG rotor (source: The Switch/Yaskawa)

Advantages of Permanent Magnet Synchronous Generators

Mateusz: What are the advantages of PMSGs over conventional, Electrically Excited Synchronous Generators (EESGs)?

Jussi: In conventional machine, magnetic flux is created with big electromagnets where current flows in conductors. First of all; there are significant heat losses (Ohm’s law) which reduces the machine efficiency, thus increasing the main engine fuel consumption (all the mechanical power must come from that).

Secondly, it is not that straightforward to feed current into rotating part. Either brushless exciter (small generator) is needed, or alternatively slip-ring unit which needs constant maintenance and is prone to failures. This increases mechanical complexity of conventional machine, increasing its physical size and lowering reliability.

Electrically-excited machines need also AVR (automatic voltage regulator), which is the device (basically a small frequency converter) controlling the rotor current. It has some losses, costs money and takes space in a vessel. With PM machine this becomes obsolete.

Brushless EESG rotor (source: ABB)

Mateusz: What are the advantages of PMSG over less common, Induction Machine Generators (IMGs)?

Jussi: In geared applications (e.g. shaft generator with tunnel gear) where the generator rotates typically between 1000-1500RPM, induction machines are a good solution, as they are very simple and cheap machines with rather good efficiency.

However, in in-line type shaft generators, induction machines are far from optimal. This is because they don’t work well in slow-speed applications due to electromagnetic reason. Main drawbacks in slow-speed induction machines are low efficiency (~4-6 %-unit lower than with PM machine) and high current, which leads to bigger frequency converter needed (current can be ~25% higher than with PM machine with equal power and voltage). Slow-speed inductions machines are also physically very big and heavy.

Efficiency comparison between PM and EESG (constant speed, varying torque) (source: The Switch/Yaskawa)

Fields of applications of Permanent Magnet Shaft Generators

Mateusz: So in both cases we are achieving lower current consumption, saving the space on the vessel and on top of that EESGs have more complicated construction. Regarding maritime and offshore industry, where PMSGs prove their best?

Jussi: Typically PMGs are best in slow- and medium-speed applications. This includes for example in-line shaft generators and direct-drive or medium-speed (2 stage gear) wind turbines. For example, all of world’s ten most powerful wind turbine designs are done with PMG.

Mateusz: What kind of ships may benefit the most from this solutions?

Jussi: Basically all kinds of vessels, but especially those where slow-steaming is often applied. This is because PMGs have best efficiency at partial loads (not at rated speed). Savings come through higher efficiency, as less mechanical power needs to be taken from propulsion shaft. Also with shaft generator applications, savings come from lack of maintenance needed for gensets, as they can be shut-off most of the time.

Inline shaft generator arrangement (source: The Switch/Yaskawa)

Perfect couple: Permanent Magnet Generators and Variable Frequency Drives

Mateusz: I would like to ask you to give us some sneak-peak into technology. What constructions/technologies are dominating in PMSGs in maritime, offshore and renewables industry?

Jussi: All modern high power PM machines are done with Neodymium magnets, as they have the best performance/cost ratio. PM machines are mostly used in slow/medium-speed applications, let’s say up to 600RPM. With geared/high-speed applications, conventional induction machines are a good choice due to their cheapness and fairly good performance (although they still come behind PMG).

Mateusz: One of the differences between PMSG and conventional generators is that in PMSG the voltage on the stator terminals will appear as soon as the rotor starts rotating. Are the dynamics or operation principles of the PM machines any different as well?

Jussi: Well, yes and no. Magnetic field is a magnetic field whether it is produced by electromagnets or permanent magnets. Biggest difference however comes from the rotor design; PM machine does not have so-called damper winding, which is needed for direct on-line start to accelerate the machine up to rated speed, and also to adapt to fast load changes. PM machines do not have this, which is the reason they always require frequency converter, which on the other hand allows variable speed operation, which is often the most important thing.

PMSG with VFD topology

Mateusz: Is Variable Frequency Drive (VFD) necessary to operate PMSG?

Jussi: Yes, VFD is always needed with PMG, they cannot be operated directly on-line.

Mateusz: I think advantages of VFDs in motor control are already known to our readers, but what are advantages of VFDs in power generation system?

Jussi: It allows fully variable speed for the system, and full torque starting from zero speed. For example in shaft generator application, it allows fully variable speed for propulsion line, allowing vessel to operate at lower than rated speed while shaft generator is connected to grid. Without VFD, it would be able to operate only at rated speed, because generator frequency (which depends on speed) must match the grid frequency (50/60 Hz).

Power can flow in both directions!

Mateusz: So it is basically matching the generator and grid frequency. Does VFD allows power to flow in both directions? What are restrictions in that matter?

Jussi: There are both one-directional, and bi-directional frequency converter. One-directional VFDs can only operate in one mode (either motor or generator, not both). With bi-directional VFD (active rectifiers both on the grid side and machine side), both modes can be used. This allows for example PTI function in shaft generator applications, making it possible to operate shaft generator as a propulsion motor in special cases (boost mode or take-me-home function). Bi-directional VFDs are bit more expensive and have bit lower efficiency, but gives much more freedom/flexibility for vessel operation.

Mateusz: When generators onboard are feeding the ships grid, we refer to it as PTO (Power take out). Can you please tell us more about PTI, take-me-home and boost functions? What are they and why would we like to use them?

Jussi: PTI (Power take in) i.e. boost mode can be used for example when the vessel is moving in heavy head-wind or going through ice, requiring very high propulsion power. Then electric power is made by genset and fed into shaft generator operating as a motor. It means that it is not necessary to dimension the main engine to meet this max power which can happen maybe only once a year, and the remaining 10…15% of max power can be done by shaft generator/motor. It means that bit smaller (and cheaper) main engine can be selected.

Take-me-home mode can be applied, if there is a fault in main engine, and the shaft generator can be used to rotate the propeller so that the vessel can safely move into nearest port for repair. It however needs a clutch between shaft generator and main engine, to decouple the diesel from propulsion.

PTO mode (source: The Switch/Yaskawa)
Boost mode (source: The Switch/Yaskawa)
Take me home mode (source: The Switch/Yaskawa)

Sensor or sensorless control?

Mateusz: Some PMSM applications / control algorithms require the use of encoder installed on the machine’s shaft, while others prove well in sensorless control. Is encoder required for PMSG or can we implement sensorless control of the generator?

Jussi: Operation of PMSG in sensorless mode is fully normal. Because it is a synchronous machine, speed is always defined exactly by frequency, which can be accurately controlled by VFD. In induction machines there is a slip, making this more complex, but with synchronous machines it is not a problem. It is often common to apply encoder for other purposes (e.g. to give speed signal for external automation/power management system).

Mateusz:  Can you please tell us more about PMSG control algorithms? How is maximum power output maintained by VFDs?

Jussi: VFD can control speed and torque independently, and the power is a function of them both. So any power at any speed is possible, of course power cannot exceed the rated values of SG and VFD.

Other technological considerations

Mateusz: The Switch PMSGs are often equipped with two separate stator windings. Is it a standard PMSG solution on the market and how is it affecting the operation of the PMSG system?

Jussi: Actually whether it is a single winding or dual winding machine, it is the same from the stator/machine operation point of view. Stator is the same in both and it is only a question how the cables are connected into frequency converters. Often it is practical to use two smaller VFDs in parallel instead of one big (due to redundancy reasons), and this is called dual-winding machine. But from the machine performance point of view it will make no difference.

Mateusz: Overall, can PMSG be considered as a safer solution than other technologies and why?

Jussi: I would say yes, because they are simpler mechanically, meaning less points of failure.

PMSG without shaft installed (source: The Switch/Yaskawa)

Troubles with Permanent Magnets?

Mateusz: So onto the troubles section! If the PMSG rotor is suspended on the shaft, there is always some chance for eccentricity between rotor and stator. What could be an outcome of that and how are we dealing with it?

Jussi: That is true, rotor is never exactly in the center due to different manufacturing tolerances. This causes so-called unbalanced magnetic pull on the rotor which needs to be taken into account in mechanical calculations. Of course the same applies for conventional machines, but with PM machines, this force is also present during alignment of the generator into shaft, which must be taken into account. Conventional machines do not have magnetic field during the installation/alignment.

Types of rotors in PMSGs.

Mateusz: What about loss of the magnetization in permanent magnets over the years and due to improper handling? Is there something to be worried about?

 Jussi: With proper machine design, the reduction in magnetic flux during the lifetime is in-significant, less than 1%. One way to accelerate the reduction in magnetic flux would be poor cooling design. If the magnets would operate at too high temperature vs magnet grade most of the time, then the aging would accelerate. But with proper design, this is not an issue.

Regarding handling, yes, permanent magnets are very fragile and mechanical shocks can easily crack them. So this is something to take into account during design. Magnets can be embedded inside rotor core, or then placed inside stainless steel containers, making it very unlikely for them to experience any shocks.

Mateusz: How difficult is maintenance of PMSG? Would crew be able to maintain PMSG system with their own experience and expertise or will services be necessary to do the work?

Jussi: Maintenance of PMGs is very minimal and crew can easily handle this. It mainly consist of some visual checkings every now and then, and replacing some sealings once in few years.

PMSG without shaft installed (source: The Switch/Yaskawa)

Economics of Permanent Magnet Synchronous Generators.

Mateusz: That sounds like really reliable systems, but let’s talk about future and its economics. What is the costs of complete system in comparison to EESM and IM solutions?

Jussi: I would say that there is no major difference, perhaps PM based system is bit more expensive (maybe 10-20%), but in the long run it becomes cheaper due to savings in fuel consumption.

Mateusz: Can you provide any numbers?

Jussi: Typically we don’t do these calculations, as it is something that the shipowner needs to do, we focus only on generator. Also we don’t have access to vessel operating profiles, running hours and so on.

Here is a comparison between PM shaft generator and induction shaft generator in LNG vessel. Here the savings are 70000 USD annually, with our assumptions of main parameters. Compared to EESG, savings are not that high, perhaps half since it has better efficiency than induction machine. This vessel had also 2 shaft generators per vessel, so for smaller vessels with single screw, savings are obviously smaller. On the other hand fuel is now more expensive than last summer when this was written.

Mateusz: Installing PMSG together with VFD on newbuilding ship, can be profitable. Is this true for retrofits as well?

Jussi: Yes, same laws of physics apply for retrofits as well.

Mateusz: But can they still return the investment?

Jussi: This is the calculation vessel operator/owner needs to make, and it depends on many parameters like vessel operation profile, annual running hours, fuel price and so on. It is not just about the generator type. But one big reason for PM shaft generator retrofits is that without shaft generator, older vessels cannot meet new emission codes and they are forced to install shaft generator system to cut the emissions.

Future of Permanent Magnet Synchronous Generators and their market

Mateusz: What is the future of PMSG?

Jussi: In wind turbines, they have completely replaced the conventional technologies in most powerful turbines. Now they are becoming a standard solution also in shaft generators and large propulsion motors. So I would say that the future is bright. Also IMO new emission regulations (EEXI and CII) are forcing the ship owners to reduce emissions, and PM technology is one way in helping to meet this goal.

Mateusz: So what will be the future of maritime and offshore industry sectors? What technologies will dominate them?

Jussi: As mentioned, offshore wind industry has already completely switched into PM machines, and now this same is happening in marine. My feeling is that direct-driven PM machines without gear will become a standard choice in marine Megawatt class applications.

Mateusz: Will PM machines replace EESGs on diesel auxiliary generators in future as well? 

Jussi: Difficult to say, and it mainly depends on the vessel electric system topology. If there is a DC grid and electric propulsion, then the frequency converter can be split so, that there is half converter on the genset side and half converter on the propulsion motor side. Then it would make sense to also apply PM genset (which we have also done).

But with vessels having AC grid, it is much cheaper to use directly on-line gensets without frequency converter. Then PM genset obviously cannot be used. With AC grid on the vessel, there would have to be full converter on gensets and full converter on propulsion motors, which would be very costly solution.

Mateusz: Jussi, Thank You very much for your time and expertise. I think the interview will give our readers good foundations on PMSGs and related aspects.

Conclusions

Together with Dr. Jussi Puranen from The Switch / Yaskawa we discussed Permanent Magnet Synchronous Generators, their principles, advantages, technology, operation, economics and future – bright future.

As IMO emissions regulations get stricter, vessel owners and operators are forced to looking for greener alternatives and PMSGs and hybridization, in which PM machines take a part, as one of the answer for the problem.

All of world’s ten most powerful wind turbine designs are done with PMG and maritime industry is also switching to PM machines. PMSG are particularly desirable for low-speed in-line shaft generators attached to main engine intermediate shaft.

In comparison to EESGs and IMs, PMSGs have few advantages: Smaller dimensions, lower weight, higher efficiency and more reliable construction. All of this factors translates to lower fuel consumption and then lower emissions and higher cost savings.

Complete installation in fact may be 10-20% more expensive than conventional system, but savings will payback with interest.

Let us know what you think. If you have any comments or questions, please leave them in the comment section. If you are also attracted by hybridization, drives and innovations within maritime industry, stay tuned for the next posts!

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