300kW dynamometers offer big advantages

Edited by the Engineeringtalk editorial team Mar 22, 2000

Permanent magnet motors in a new range of dynamometers with continuous ratings up to 300kW help eliminate gears, achieves ultra-low inertia and provide superior control capabilities.

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 low inertia equivalent to that of an automotive engine, permit rapid acceleration and deceleration, and make it possible to simulate accurately cylinder firing patterns.

Plus, it adds the benefits of reducing the space envelope requirement by more than 80% by eliminating gears and their cooling systems, whilst also avoiding associated maintenance.

Similarly, initial set-up before testing is easier and there is none of the safety hazards associated with the use of fuel, permitting 24-hour unmanned tests.

Further, because the transient dynamometers are not so affected by such variables as temperature, humidity, atmospheric conditions they ensure stable test conditions with high-accuracy repeatability.

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

Inertia has a close relationship 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.

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 200oC.

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 and induction unit.

As well as the technique of skewing stator slots to minimise the cogging effect experienced normally by PM motors, magnetic materials have been used for stator wedges.

This helps to reduce harmonic losses to the tune of around 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 loose 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.