New roles for permanent magnet technology

February 20th, 2012, Published in Articles: Vector

by Jouni Ikäheimo, ABB

For many decades, permanent magnet technology has been used in smaller scale applications where its favorable weight-to-performance ratio is an advantage. Its use in bigger electrical machines, however, has not been that common.

This, however, is changing. The increasing demand for new solutions for wind power generators and special motors for industries such as pulp and paper, marine, and traction has created new markets for electrical machines. This in turn has focused attention on the high power density and high efficiency which can be achieved using permanent magnet motors and generators.

ABB has introduced this type of motor in larger sizes, an example of which is in marine propulsion. The company has also developed the permanent magnet motor to provide high accuracy and reliability for industrial applications requiring high torque at low speed. Permanent magnet technology has opened the door for new solutions in wind power generators. Today, the largest permanent magnet motor weighs seven tons.

The actual motor design is a radial flux construction, air or water-cooled, with a permanent magnet rotor. The temperature of the permanent magnet rotor remains naturally low, allowing higher power densities.

The company’s permanent magnet motor is a synchronous motor which, as there is no rotor slip (see box), provides better accuracy than standard, asynchronous, motors. In an asynchronous motor, the slip varies according to speed and load. With a synchronous motor, it is simpler to optimise the speed, while the elimination of slip compensation improves the dynamic motor control.

Like all synchronous motors, the speed of this motor can only be controlled with a variable-speed drive. Furthermore, the synchronous motor control must be specifically developed for permanent magnet flux control. ABB’s direct torque control (DTC) method has been enhanced to achieve this.

Water-cooling gives higher power density and compact drive cabinets, while the higher protection class enclosures allow more freedom for drive placement by reducing the exposure of the drive components.


Rotor slip

The main difference between a synchronous motor and an asynchronous induction motor is that the rotor of the former is magnetised and turns at the same speed as the rotating magnetic field.

The synchronous speed of a motor is that speed it would achieve theoretically if speed were only a function of the network frequency and the number of poles in the motor. Ideally, the rotor should follow the rotating magnetic field in the stator exactly. In an induction AC motor, however, the load will cause the rotor to slip in relation to the magnetic field and friction in the motor will add to this slippage. The slip can be in the region of 5%.

Some frequency converters feature slip compensation to reduce this. The speed drop can then be reduced to about 10% of the nominal slip. If very high control accuracy is required, a speed controller with pulse encoder is used.

The synchronous motor has electromagnets or permanent magnets built into its rotor. These lock the rotor into a certain position when confronted with another magnetic field. The speed of a synchronous motor can therefore be controlled with a high degree of accuracy over a large speed range by supplying it via a frequency converter, without the use of a feedback device.


Synchronous motor construction

The construction of the traditional synchronous motor is more complicated than that of the asynchronous motor, so it requires more maintenance. However, the permanent magnets in the ABB motor simplify its construction by creating a constant flux in the air gap, thereby eliminating the need for the rotor windings and the brushes normally used for excitation in synchronous motors.

This has resulted in a motor that combines the high-quality performance of the synchronous motor with the robust design of the asynchronous induction motor. The motor is energised directly on the stator by the variable-speed drive.

The synchronous motor can deliver more power from a smaller unit. To power the in-drives, for example, of a paper machine directly at 220 to 600 rpm with a conventional asynchronous motor would require a motor frame substantially larger than that of a 1500 rpm motor. The new motor type has, in most cases, the same size or is even smaller than the existing induction motor.

The permanent magnets are made from neodymium iron boron (NdFeB). NdFeB is the most powerful magnetic material available at room temperature, with high values of flux density at very high values of magnetisation. It is also extremely resistant to demagnetisation. Furthermore, NdFeB is less costly and brittle than samarium cobalt, another rare earth material that was used widely in the 1980s.

 

Fig. 1: Drive configuration with a) conventional induction
motor drive, gear and jack shift, and b) the Direct Drive.

High efficiency, less maintenance, no gearbox

Standard induction motors, normally designed to run at 750 – 3000 rpm, are not particularly suited for low-speed operation as their efficiency drops with the reduction in speed. They may also be unable to deliver sufficiently smooth torque across the lower speed range.

This is normally overcome by using a gearbox which, however, is a complicated piece of machinery that takes up space and needs maintenance as well as considerable quantities of oil. High performance at low speed is sometimes achieved by using a DC drive. Using a permanent magnet motor, the company has found a solution that eliminates the gearbox completely. This solution will save on motor maintenance as the permanent magnet motor is very robust and the maintenance it requires is similar to that for standard AC induction motors.

The combination of fewer components and simpler configuration reduces plant engineering hours, facilitates installation, allows more efficient use of floor space, and reduces spare part inventories.

Simpler configuration also improves the availability of the production machinery. Less maintenance means fewer production interruptions and start-ups, decreased raw material waste, increased end product quality and reduced wear in the production machinery. Maintenance and repair work can also be carried out faster.

Pulp and paper industry

Paper machines require large numbers of high-accuracy, low-speed drives. As permanent magnet technology is helping to eliminate gearboxes across a wide range of industries, this particular solution has a ready market in the paper industry.

This solution provides a high torque drive directly coupled to the in-drive of the paper machine section. Eliminating the gearbox saves space and installation costs, as the user only has to prepare the foundations for one piece of driving machinery. This also allows more freedom in the design of the mill layout. Getting rid of the gearbox and brushes not only reduces maintenance requirements, it also saves energy.

The permanent magnet motor is the heart of a direct drive solution. This consists of the motor, controlled by a low-voltage AC drive based on the ACS600 frequency converter, connected directly to the paper machine, without gearboxes or pulse encoders.

Direct drive solution technology improves drive controllability, enabling the paper machine drive to run without a pulse encoder, as synchronised motors allow very exact control without feedback. The accuracy is as good as that of an induction motor in variable speed operation with a feedback device. This means the pulse encoder can be eliminated, further reducing the need for maintenance. This is particularly beneficial in the paper industry where poor reliability of feedback devices contributes to production stoppages. It can also reduce design complexity, as the feedback devices sometimes can be difficult to integrate in the system or have to be positioned in places that are difficult to reach .

The better electrical efficiency of the new drive has a direct impact on power consumption. Savings increase considerably with further reduced speed.

Following two successful pilot projects, the first Direct Drive system was installed in August 2002 at the Finnish paper company M-Real, on the line manufacturing packaging materials for the medical and cosmetics industries.

Fig. 2: Frequency control of a permanent magnet
excited gearless wind turbine generator.

Propulsion

A new market for electrical machines has been created by the introduction of the podded propulsion concept for ships. The electrical motor is mounted in a bulb, which is attached to the hull of the ship, and together these form the main propulsion system. The speed of the motor is controlled, as is the direction of the propulsion force in relation to the ship.

The typical power range of these motors is 400 kW to 20 MW. One ship is normally equipped with between one and three propulsion units, and rigs equipped with dynamic position systems may use up to 10 units. Known as “Azipod”, this propulsion system was originally developed for ice breakers and ice-going vessels. Compared with conventional mechanical propulsion, Azipod has increased the popularity of the system in other types of vessels such as cable layers, dredgers, shuttle tankers, chemical and product tankers, support vessels, motor yachts, drill-ships and semi-submersible rigs. This propulsion system is especially appreciated by owners of big cruise vessels where the total propulsion power is in the region 40 to 60 MW. Azipod allows excellent ship maneuverability, low vibration and noise levels, high efficiency, low emission and passenger comfort.

Permanent magnet motor technology has been used in the development of a highly standardised modular concept known as “Compact Azipod” which has been designed for a propulsion power range of between 400 to 5000 kW. Permanent magnets and DTC have been the main factors for improving the performance and extending the applicability of Compact Azipod.

The motor module is cooled by the surrounding seawater, allowing high power density for the motor and simple construction with a low number of parts. The pod housing diameter can be kept small, giving improved propeller hydrodynamic efficiency. This, together with the high motor efficiency, means high overall efficiency and low fuel consumption.

Wind power

Wind power represents another growing application area for electrical machines. Increased demand presents new challenges for the wind power plant concept in terms of higher output, higher availability, lower noise level and cost-effective solutions.

ABB has three different permanent magnet technologies available for high, low and medium speed wind turbine generators. In the low speed (Direct Drive application) version, the turbine and the generator are combined to form a compact and structurally integrated unit. The medium speed unit is a very compact unit with the turbine main bearing and the permanent magnet generator integrated into a single stage gearbox. The high-speed permanent magnet generator is a compact solution with a maximum output of 3,6 MW from a 500 mm frame.

As permanent magnet motors are used more widely, the price of the magnetic materials, which today are comparatively high, is expected to drop. When this happens, it will be possible to use permanent magnet motors in normal industrial drives where they will save energy through better efficiency, as losses are reduced considerably.

Contact Mark Sheldon, ABB, Tel 010 202-5868, mark.sheldon@za.abb.com

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