TechArchive

Original article date: May 2000

PM-driven dynamometers to 300kWAndy Gardner of Meiden Europe explains why permanent magnets keep the heat down in dynamometers – and why it’s a good thing.

Permanent magnet motors are now being used by Meidensha Corporation for a new range of dynamometers with continuous ratings up to 300kW that eliminates gears, achieves ultra-low inertia and provides superior control capabilities.

The new transient dynamometers (PMDY) give the automotive engineer a low inertia equivalent to that of an automotive engine, permit rapid acceleration and deceleration, and make it possible to simulate cylinder firing patterns accurately. It reduces the space envelope requirement by more than 80% by eliminating gears and their cooling systems, while avoiding associated maintenance. Similarly, initial set-up before testing is easier and there are none of the safety hazards associated with the use of fuel, permitting 24-hour unmanned tests. Because the transient dynamometers are not so affected by temperature, humidity and atmospheric conditions, they ensure stable test conditions with high-accuracy repeatability.

Using a permanent magnet motor means that the PMDY unit’s inertia is around 14% of a conventional dynamometer, very much in-line with that of a car engine.

Inertia is closely related to the temperature rise in the motor. For an induction motor, copper loss of the rotor (secondary copper loss) is abnormally increased if the rotor size is reduced. In the case of a cage-rotor type 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, very little heat is generated in the rotor.

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

It’s well known that the rare earth permanent magnet (Nd-Fe-B) used has a far greater energy product than Alnico and ferrite ones. In addition, it experiences minimal changes in magnetic flux due to temperature changes and does not suffer irreversible magnetism loss, even at 200degC.

Unlike induction machines, the PM motor does not experience instability due to delay 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 superior to those of an induction unit.

As well as skewing stator slots to minimise the cogging effect experienced normally by PM motors, magnetic materials have been used for the stator wedges. This helps to reduce the harmonic losses by about 40% compared to non-magnetic wedges.

A major feature of PMDY is the control mechanism that ensures torque is maintained even if the permanent magnets lose any magnetism when the motor temperature rises. This is achieved by employing a flux observer to monitor the magnetic flux.

In operation, a variation in the magnetic flux is predicted by integrating the difference between voltage command of the current regulation system and the motor model output.

This field weakening control enables the PMDY to offer constant torque characteristics at the base speed, or below, and constant output characteristics from base speed up to maximum revolving speed. At the same time any increase in demand from power sources is avoided.

• Meiden Europe

May 2000