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PM motor eliminates dynamometer gears

01 June, 2000

PM motor eliminates dynamometer gears

Electric motors have for years helped automotive engineers to test vehicle transmission systems and drive trains by simulating engine behaviour accurately and cleanly in the laboratory. Traditionally, both AC and DC motors have been used in these dynamometers, but now the Japanese firm Meidensha is suggesting that permanent magnet (PM) motors have particular advantages in this application.

Conventional AC and DC dynamometers need gearboxes to achieve low inertia characteristics, but Meidensha says that its PM version - available in continuous ratings up to 300kW - can deliver extremely low inertia without needing gears.

It claims that PM-based transient dynamometers give the automotive engineer a low inertia equivalent to that of an automotive engine, and can simulate rapid acceleration and deceleration. They even make it possible to simulate different cylinder firing patterns.

Eliminating gears and their cooling systems reduces the space required for a dynamometer installation by more than 80%, and reduces the need for maintenance. Initial set-up before testing is said to be easier, and 24-hour unmanned tests are possible.

Meidensha suggests that the PM dynamometers produce an inertia very similar to that of a car engine - and around 14% that of a conventional dynamometer.

Inertia has a close relationship to the temperature rise in the motor. For an induction motor, copper losses in the rotor (secondary losses) are increased abnormally if the rotor size is reduced. In the case of a cage-rotor induction motor, this high temperature level can be sustained because no insulation material is used. However, the heat is transferred to the bearings causing different problems.

With a permanent magnet motor, the magnetic flux is generated by the magnets and torque is generated in conjunction with the stator current. Consequently, not much heat is generated in the rotor.

In addition, the PM motor does not experience excitation losses and its efficiency is high. For example, a 220kW PM motor achieves 96.7% efficiency, compared with 92.2% for a typical induction motor.

The rare-earth, neodymium-iron-boron magnets used in the Meidensha dynamometers have a greater energy product than Alnico and ferrite types. In addition, their magnetic flux is hardly affected by changes in temperature and they do not suffer irreversible losses of magnetism, even at temperatures as high as 200°C.

Unlike induction machines, PM motors do not suffer from instabilities caused by delays in the secondary magnetic flux. Consequently, because the stator current and torque have an almost proportional relationship, both torque current characteristics and transient response characteristics are better than those of an induction machine.

To minimise the cogging effect normally experienced by PM motors, the stator slots are skewed. Also, magnetic materials are used as stator wedges, helping to reduce harmonic losses by around 40% compared to non-magnetic wedges.

The PM dynamometers are not affected by atmospheric variables such as temperature and humidity, ensuring stable, repeatable test conditions.




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