Although permanent magnet (PM) machines have been used in various small power applications for more than 100 years, they have become a top choice for industrial-scale motor and generator applications only during the last decade or two. They gained ground in megawatt-range applications after the invention of Neodymium magnets in the early 1980s. In addition to technological improvements, the improved reliability and reduced cost of power electronics have made PM machines more popular. Today, high-power PM machines are used, for example, in distributed power generators (wind, tidal and wave), traction applications (hybrid cars) and electric propulsion.

From the very beginning, The Switch has focused on developing megawatt-class variable-speed drives based on permanent magnet technology. This has been the right choice. Due to the favorable characteristics of PM machines over conventional technology, there have been many recent PM technology breakthroughs in a wide variety of applications.

PM technology offers higher efficiency, better reliability and straightforward simplicity. Fifteen years ago, not one of the most powerful wind turbines worldwide used PM generators. Today, 7 out of 10 do. The Switch has developed various megawatt-class PM systems for a number of global turbine suppliers over the last decade, and is currently supplying a 10 MW PM machine for the world’s most powerful wind turbines.

Permanent magnet shaft generators: Creating new savingsfor the maritime industry

PM technology offers higher efficiency,

better reliability and straightforward simplicity.

“Permanent magnet machines have become the top choice for industrial-scale motor and generator applications.”

Dr. Jussi Puranen,Product Manager,

Permanent Magnet Machines

PM shaft generators Talking point

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Why PM for maritime use?While the roots of the company are in the wind business, the recent global market slowdown motivated The Switch to develop PM technology for marine applications as well – including shaft generators, propulsion motors and auxiliary gensets. Fluctuating fuel prices and stricter emission regulations around the world are driving cleaner and more efficient power production in the maritime industry. PM technology enables a big step in this direction.

One of the most interesting and well-suited marine applications for permanent magnet machines is a direct-drive shaft generator system in which a ship’s main engine, often a two-stroke diesel, drives the shaft generator directly through the propeller shaft, without a gearbox.

Due to the low rotational speed of propellers in big ships, typically around 100 rpm, the only suitable generator type in the past has been the electrically excited synchronous generator (EESG).

Induction machines – in addition to being large and heavy – are inefficient at such low speeds. Additionally, they have a low power factor, which means a high current and consequently a larger frequency converter and heavier power cabling. Electrically excited synchronous machines, too, require substantial auxiliary equipment such as an automatic voltage regulator, exciter and rotating rectifier. In addition, they are also heavy and physically large.

Very early direct-drive shaft generator systems were designed with direct on-line (DOL) generators. These allowed only one fixed propeller speed during power production, due to the constant frequency required by the ship’s grid. Modern shaft generator systems now come equipped with frequency converters, allowing variable propeller speed while maintaining constant grid frequency.

The Switch’s approach has been to replace the EESG with a generator using permanent magnet excitation in the rotor. This is connected to the grid by a modern insulated-gate bipolar transistor (IGBT) frequency converter. By doing this, no external power needs to be taken from the grid for magnetization. Significantly increased power generation efficiency is the main benefit of a PM machine.

Complexity causes energy losses In the conventional EESG concept, a magnetic field is created by feeding current into the generator rotor winding, causing significant ohmic heat losses in the generator. This energy must be taken from the grid and fed through the automatic voltage regulator (AVR) that controls the field, which means energy losses in the AVR, as well.

To feed current into a rotating part (the rotor) with an EESG also requires a device called a brushless exciter – a small PM generator connected to the same shaft as the main generator. In addition, a rotating rectifier (a diode bridge) converts the AC current into DC.

All these extra components make an EESG system overly complex, resulting in significant additional energy losses, lower efficiency, increased fuel consumption and higher emissions.

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No gearbox is needed when the shaft generator is

connected directly to the propeller shaft.

By coupling the PMG with a frequency converter, no external power is needed

for magnetization.

Significantly increased power generation

efficiency is the main benefit of a PM machine.

The EESG system is complex, resulting in energy losses, lower efficiency, increased

fuel consumption and higher emissions.

PM shaft generators Talking point

© 2018 Yaskawa

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A PM machine produces a strong magnetic field

for its entire life cycle, which is needed for the

electromechanical power conversion process.

Electric power produced by a shaft generator system can be much cheaper than power

produced by auxiliary gensets.

PMG has a 2–4% higher efficiency throughout the

entire operating range.

The PM shaft generator system – how does it help?Using a shaft generator system for onboard electricity makes sense. A ship’s main engine – typically a two-stroke engine for large ships – operates at significantly higher efficiency than gensets powered by four-stroke engines. Auxiliary gensets normally have specific fuel oil consumption (SFOC) well above 200 g/kWh, while the SFOC for two-stroke engines is around 160–170 g/kWh.

Furthermore, two-stroke engines use cheaper heavy fuel oil, while four-stroke engines generally use more expensive marine diesel oil. All this means that, depending on a ship’s operating profile, the electric power produced by a shaft generator system can be significantly cheaper than power produced by auxiliary gensets.

Design specifics: PM vs EESGWith today’s volatile fuel prices, a shaft generator makes more economic sense than ever. Even further savings are possible when a PM machine is used as the generator instead of a conventional electrically excited one.

All electrical machines need a strong magnetic linking of the stator and rotor for the electromechanical power conversion process. In conventional machine types such as induction machines or electrically excited synchronous machines, this magnetic field is created with dedicated winding – in essence, a large electromagnet. In PM machines, however, the magnetic field is created with permanent magnets instead of electromagnets. Once permanently magnetized, a product produces a strong magnetic field for its entire life cycle, typically spanning over 20 years or more. Nearly all modern high-power PM machines utilize so-called Neodymium-Iron-Boron magnets or so-called “Neo magnets” made by sintering and magnetizing them to their full magnetic field strength.

When these Neo magnets are mounted onto a generator rotor, a magnetic field equal to that of an EESG can be created much more simply, resulting in significantly better efficiency due to the lack of rotor field winding losses.

The rather low speed of direct-drive EESG generators means weak electromagnetic induction. Many coil turns are needed in the rotor field winding of conventional EESG units, resulting in relatively high ohmic heat losses in the rotor field winding due to high resistance. Consequently, this lowers the efficiency throughout the operating range.

In low-speed generators, rotor field winding losses as a share of total generator loss are typically as high as 20% to 30%, causing a significant increase in main engine fuel consumption per produced kilowatt hour.

In contrast, there is no current flow or ohmic heat loss related to magnetization in PM machines. Therefore, they operate at significantly higher efficiency, especially in low-speed applications.

Although the PM machine efficiency premium is typically less than 1% unit when compared to conventional machines in high-speed applications rotating between 1,000–2,000 rpm, the difference can be several percentage units in low-speed applications, such as direct-drive shaft generators.

Talking point PM shaft generators Talking point

In addition to increased efficiency, advantages of

PM generators include smaller size and weight and also lower inertia.

The following figure compares the efficiency of a 1.5 MW shaft generator system using an EESG machine with one using PM technology.

While the most obvious benefit of a high-efficiency PM machine is its fuel savings, there are a number of other advantages.

Another major advantage of a PM generator is its significantly smaller physical size compared to other generators. Due to the low electrical losses of PM machines, their power density can be designed to be substantially higher than that of EESG machines without any risk of overheating. This high power density results in a physically compact size.

The advantage over EESG machines is further increased because a PM machine needs no brushless exciter. This makes the machine much shorter axially, saving space.

The rotating weight and inertia of a PM machine is also significantly reduced. A conventional EESG generator has massive field poles mechanically connected to the rotor core, which increase rotor weight and inertia.

Modern Neodymium magnets are so powerful that it is possible to replace field poles with a radial height of ~200 mm by using only a ~20 mm thick magnet.

The following photos show a comparison of the rotor mechanical construction of these two machine types.

Figure 1: An efficiency comparison between a PM and EESG machine in the 1.5 MW range, measured at constant speed with varying torque. Note that the PM machine operates at a 2–4% higher efficiency throughout the operating range.

Efficiency comparison

Output power (% of rated)

90

91

92

93

94

95

96

97

98

99

0 20 40 60 80 100 120

PM generator

More than 2%

Conventionalsynchronousgenerator

Figure 2: In the rotor of an EESG on the left (source: www.gepowerconversion.com), the exciter on the left and massive field poles result in large weight and physical dimensions. In the rotor of a direct-drive PM shaft generator on the right, the rotor is mainly hollow, resulting in small weight and physical dimensions in the axial direction.

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Talking point PM shaft generators Talking point

Traditional technologies require substantial

auxiliary equipment and are heavy and large.

Increased efficiency results in lower emissions.

With fewer components, reliability increases.

Practice has proven the efficiency of PMGs.

When comparing the total weight of a PM direct-drive shaft generator with an electrically excited one, the PM machine is often 30–40% lighter in total weight, as well as taking significantly less space in the engine room.

Low inertia is another critical factor when considering the torsional vibration characteristics of an entire propeller line. A PM machine typically has inertia that is only 15–25% of an EESG’s due to the hollow-rotor construction that improves torsional vibration characteristics.

In some cases, the high inertia of an EESG results in an additional need for a torsional vibration damper, a very expensive component.

What are the gains?While the basic operating principles of a PM machine are similar to those of a conventional EESG – with permanent magnets replacing electromagnets – a PM machine has significant advantages.

The primary benefit is increased generator efficiency, resulting in reduced fuel consumption and lower emissions.

A second benefit is that no separate exciters or AVR units are needed to supply the magnetization current into the rotating part because a PM machine is brushless by nature. This makes the whole system much simpler.

With fewer components, reliability increases. The smaller physical size of the PM machine means more effective use of engine room space. The very small inertia makes handling propeller line vibrations easier, without the need for torsional dampers. Plus, recovery after blackouts is straightforward and immediate, since no external energy is required for magnetization.

Experience with direct-drive shaft generators: from theory to practiceIn early 2014, The Switch received its first orders for four PM shaft generators from Finland-based WE Tech Solutions Oy. The generators were rated at 1,500 kW PTO (power take out) power when used as a generator and 1,000 kW PTI (power take in) power when used as a motor.

Since the core technology had already been proven in harsh wind environments, making the needed modifications for marine use was quite straightforward. Engineering work focused mainly on making the design comply with certification requirements and also ensuring the machine could withstand the heavy vibrations of diesel engines.

The first machines were successfully tested at the end of 2015 and received approval by Lloyds Register.

Figure 3 shows the full-load back-to-back tests of these machines at The Switch Vaasa factory in Finland. In this test, two identical machines were coupled through a common shaft, one acting as a motor and the second generating full power for the network.

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PM shaft generators Talking point

Generator performance was well in line with calculations: the measured full-load efficiency was 96.4%, a ~3% unit increase over that of a conventional EESG.

The first installation took place in early 2016 at the AVIC shipyard in Shanghai for the Ternsund vessel commissioned by Terntank Rederi. The main engine was Wärtsilä’s V31 dual-fuel which, combined with a PM shaft generator, provides a highly efficient and environmentally friendly propulsion system.

Since August 2016, M/T Ternsund and its sister ships have been operating without any technical problems. As of today, more than 30 shaft generator systems have been ordered from The Switch, most of them already successfully tested, certified and delivered.

ConclusionsAs the interest towards PM shaft generator systems is increasing, so are the required output power levels.

For applications such as floating storage regasification units (FSRUs), discussions are currently going on for the use of direct-drive PM shaft generator systems up to 10 MW and more.

To be prepared for this challenge, The Switch has scaled up its shaft generator offering with two new frame sizes. Currently, the product portfolio covers a torque range of up to around 2,000 kNm.

We are also investing in a new testing center in Lappeenranta, Finland, giving us the capacity to test electrical drive trains up to 15 MW and becoming one of the few in the world with this capability. This testing facility will support our vision of delivering the most high-powered, efficient technology to our global customer base.

The Switch will set up a new testing center with a capacity to

test electrical drive trains up to 15 MW to serve its

global customer base.

About the author: Jussi Puranen currently works as Product Manager for Permanent Magnet Machines at The Switch, Finland. His specialties are the electromagnetic design and R&D of permanent magnet generators and motors. Puranen holds M.Sc. and D.Sc. (Technology) degrees in Electrical Engineering from the Lappeenranta University of Technology (LUT), Finland.

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PM shaft generators Talking point

Figure 3: PMM 1000D shaft generator with a rated power of 1,500 kW during full-load tests at The Switch Vaasa factory in Finland.