Project:
Permanent Magnet Drives in the More-Electric Aircraft
From January 1997 to August 2000
Project Leader(s): Dr D.J. Atkinson
Contact: Dave.Atkinson@ncl.ac.uk
Sponsors: EPSRC
Partners: Lucas Aerospace, Rolls Royce International R&D, Welwyn Electronics
This project has researched fault tolerant electric drives for safety critical systems, with particular emphasis on avionic applications. The following items of research have been undertaken:-
- (i) A search for power electronic converter and electrical machine topologies which offer high functionality in the event of component failures, with the minimum use of redundant power devices and components.
- (ii) Research into fault detection techniques that determine which components have failed and the manner of failure before fault propagation occurs.
- (iii) Development of control methods which reconfigure power device switching strategies in the faulted drive in order to maintain drive integrity and performance.
- (iv) Use of hybridisation techniques to produce high power density, reliable hardware.
The above topics are predominantly generic in nature, but are demonstrated with a single converter and drive. The demonstrator application of a 16kW, 13000 revs/min. aircraft fuel pump drive has been chosen as a suitable mid power range drive which requires all of the above research items to be addressed. Researchers in the U.S. have developed switched reluctance drives for the aerospace market because of their inherent fault tolerance. However, this research has demonstrated that with careful design, a similar degree of fault tolerance can be achieved with a permanent magnet machine, with a substantial saving in drive mass. This is particularly important for aerospace applications.
The aim has been to develop a permanent magnet machine drive which can continue to operate with either power device or winding faults. It has become clear that the most successful design approach uses a multiple phase drive in which each phase may be regarded as a single module. The operation of any one module has minimal impact upon the others, so that in the event of that module failing the others can continue to operate unaffected.
A novel machine design has been developed, built and tested, in which there is effective magnetic, electric, thermal and physical isolation between phases. This machine is driven by a modular converter which contains new fault detection and post fault control methods. The figure below shows the demonstrator stator
Power hybrid circuits have also been designed and constructed as part of this project. The figure below shows a hybrid for one phase with embedded fault detection current sensor.
A fault tolerant permanent magnet demonstrator drive has been constructed. Tests on this drive have shown that it is possible to detect all the machine and power electronic faults considered. It has been shown that if appropriate action is taken the drive can continue to operate indefinitely in the presence of any of these faults. In particular the research has demonstrated that:
- The drive can continue to operate with an open circuit winding.
- A permanent magnet machine can be designed with 1 p.u. reactance to limit the current in a shorted phase to rated current.
- The remaining healthy phases of the drive are unaffected by a shorted phase and the drive can continue to operate.
- Undetected turn to turn faults will result in currents many times larger than rated current flowing in the faulted winding. If no action is taken the fault is likely to propagate rapidly.
- A single shorted turn can be detected by monitoring the sampled current each PWM cycle. Shorting the phase reduces the fault current to rated value.
- The drive can continue to operate with a shorted or open-circuit power device.
- Shoot through current can be detected within the short circuit withstand time of the power devices and the healthy device turned off again.
- In the event of each fault, the healthy phases are capable of compensating for the torque loss resulting from the failure.
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Dr Dave Atkinson
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Professor Alan Jack
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Professor Barrie Mecrow
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