Internal permanent magnets

Paul Boughton

Electric motors are such an integral part of the world in which we live that it is not surprising that continual efforts are made to improve their performance. And with most nations in the world now agreeing that climate change is a reality and not just a theory, energy efficiency is more important than ever.

The evolution of the electric motor has come a long way since Michael Faraday’s early experiments, with today’s products being shining examples of how engineers can refine designs in different ways, whether the aim is to make motors small, powerful, dynamic or efficient.

Inspired by the quest to conserve energy, Yaskawa Electric has developed a new type of high-efficiency motor that also benefits from improved torque characteristics and a substantially smaller size than conventional ac induction machines. The new synchronous motors are designated IPM and use internal permanent magnets embedded within the rotor to create the superior flux density and distribution that gives the improvement in torque density (Fig.1).

Typical applications will be those where high torque is required, such as lifts/elevators, cranes/hoists and machine tool axes and spindles. However, the new IPM motors also offer advantages where large amounts of energy are consumed, such as in continuous-duty fans, pumps and compressors.

In high-torque applications, the IPM motors deliver a number of benefits. For example, in direct-drive spindles for machine tools, the heat-rise characteristic of the motors is one-third that of a conventional ac induction motor. The reason for this is the IPM motor suffers no secondary copper losses, as torque is generated by the permanent magnets rather than a secondary current flowing in the rotor. Minimising heat losses not only saves energy, but also helps maintain the precision of the machine tool.

In the lift industry, the trend is towards gearless direct drives where high efficiency and low audible noise are desirable. Increasingly, lifts do not have large motor rooms and, consequently, the IPM motor’s space saving means it can be sited within the lift shaft. Speed control is also enhanced over drives that use conventional ac induction motors and gearboxes – a feature that benefits crane and hoist applications as well.

Size considerations

New product development projects frequently lead to the latest model being more compact than its predecessor but, in the case of the IPM motor, the size reduction is considerable: compared with a standard ac induction motor, the physical dimensions are up to one-third smaller. But one of the advantages of the IPM technology is that it enables radical configurations to be designed. For example, Yaskawa Electric is offering a slimline, ‘flat’ motor for lifts/elevators and hoist applications where a very low profile is beneficial for installation in confined spaces (Fig.2).

Since about two-thirds of all the electricity used by developed countries is consumed in powering electric motors, it only takes a small improvement in efficiency to produce big savings – particularly in fan applications, where the net effect is that energy consumption increases as the cube of the frequency (speed).

Once again, IPM motors offer substantial benefits compared with traditional ac induction motors. These savings are further enhanced when the motor is coupled with Yaskawa’s F7S ac inverter with its integrated energy-saving characteristics.

The IPM synchronous ac motors are available with power ratings ranging from 0.4kW 200V up to 160kW in the 400V class.

But Yaskawa Electric is not alone in its work on internal permanent magnet motors. Omron has recently introduced its SGMBH servo motor series for high-power applications. In this case the company is emphasising the lower rotor inertia that results from the internal magnet configuration. This makes the motors particularly suitable for highly dynamic, high-power applications such as for the replacement of hydraulic systems.

SGMBH servo motors are available in power ratings from 22kW to 55kW with a rated speed of 1500rpm. Peak torque can be maintained for a maximum of five seconds, and customers can opt for either a high-resolution serial encoder or an absolute multi-turn encoder.

Motors in cars

The concept of a motor with internal permanent magnets extends beyond industrial applications. Honda has been at the forefront of research into hybrid petrol-electric vehicles and has even launched hybrid versions of its Civic and Accord models.

For at least two years, the company has been using an IMA (integrated motor assist) unit in such vehicles to boost the power and torque of the petrol engine. Data is not readily available for the latest version of the company’s hybrid powerplants, but the third-generation system, used in the 2004 Honda Accord, featured an electric motor that delivered up to up to 12kW (16.3PS) and 136Nm of additional power and torque to boost the output of the vehicle’s petrol engine.

For this third generation of IMA units, a new internal permanent magnet motor was designed to increase output density and improve efficiency. At startup, the Accord hybrid provides 26percent more starting torque than the Civic Hybrid. During driving, the motor delivers a 20percent increase in power and more than doubles the peak torque compared with previous incarnations of the hybrid system.

Moreover, the IPM motor is more efficient, converting 97.5percent of the available electricity into motive energy in assist mode, which compares with 94.6 per cent previously.

With such a diverse range of applications for which IPM motors are finding applications, the Yaskawa Electric engineers could be right when they predict that it is possible that one day all ac motors will constructed with internal permanent magnets.

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