Philip Mellor

Rotor loss in permanent-magnet brushless AC machines

The eddy-current loss in the permanent magnets of brushless AC machines is usually neglected, since the fundamental air-gap field usually rotates in synchronism with the rotor, and time harmonics in the current waveform and space harmonics in the winding distribution are generally small. However, an important category of brushless AC machine design is emerging in which the fundamental component of the stator MMF has fewer poles than the rotor, the torque being developed by a higher order MMF harmonic. The fundamental and lower order MMF harmonics can then give rise to significant rotor eddy currents. An analytical model is developed to predict rotor-induced eddy currents in such machines, and to quantify the effectiveness of circumferentially segmenting the permanent magnets in reducing the rotor loss.

Rotor loss in permanent magnet brushless AC machines

The eddy current loss in the permanent magnets of brushless AC machines is usually neglected, since the fundamental airgap field rotates in synchronism with the rotor and time harmonics in the current waveform and space harmonics in the winding distribution are generally small. However, machine designs are emerging for which the fundamental component of the stator MMF has fewer poles than the rotor, the torque being developed by a higher order MMF harmonic. The fundamental and lower order MMF harmonics can then give rise to significant rotor eddy currents. An analytical model is developed to predict rotor induced eddy currents in such machines, and to quantify the effectiveness of circumferentially segmenting the permanent magnets in reducing the rotor loss.

Thermal Modeling of a Segmented Stator Winding Design

This paper presents a thermal analysis of a segmented stator winding design. As the thermal performance is one of the main factors limiting a machines output capability, a thermal test on a complete prototype machine is an essential part of the design process. However, for the segmented stator winding design, a test-informed thermal analysis on a single stator tooth can be performed prior to the manufacture of the full machine. This approach allows for a rapid and inexpensive assessment of the thermal performance of the complete machine and early identification of design modifications needed. The research has been applied to the design of a highly efficient and compact permanent-magnet traction motor. A thermal model for a single tooth was developed and supported by tests to identify key heat transfer coefficients. A number of winding assemblies were compared, and the most promising was selected for the final motor prototype. The results from the approach are compared with thermal test results from the complete machine.

A computationally efficient iron loss model for brushless AC machines that caters for rated flux and field weakened operation

This paper presents a simple and computationally efficient approach for predicting iron loss within a field orientated controlled brushless AC permanent magnet machine which can cater for both rated flux and field weakened operation. The proposed method is readily incorporated as part of the design process and is based on two discrete time step 2D magnetostatic finite element field solutions describing the open circuit and short circuit operation of the machine. Parameters obtained from these analyses are used alongside the standard d-q equivalent circuit to generate a map for the iron loss across the entire machine working envelope. Test results taken from a concentrated wound brushless AC traction motor are used to validate the technique.

Performance of a Prototype Traveling-Wave Actuator Made From a Dielectric Elastomer

The primary aim of the research is to demonstrate the fabrication and operation of a traveling wave actuator made from a silicone dielectric elastomer. Multiple folded stack configurations of a silicone are assembled to create individually controllable regions in a single device, allowing a traveling-wave pattern of electrical stimuli to be applied to each active region. The prototype actuator is sandwiched between two friction surfaces allowing motion in response to the traveling wave. A number of issues related to the research and development of the prototype actuator are considered, including traveling-wave principle, folded stack design, actuator fabrication, and electrical control. A prototype is tested with a bespoke multiple-channel high-voltage converter to assess the performance characteristics of stroke, force, and frequency. Practical velocities and forces are achieved; however, a number of challenges are discussed in order to increase performance to comparable levels exhibited by commercial actuators with high-force long-stroke capabilities.

Low-cost high-bandwidth current transducer for automotive applications

Electric actuators in automotive systems may require a current transducer for control purposes. A low-cost non-invasive transducer design based around a current transformer (CT) is described in this letter. The problem of droop in the output signal is addressed by controlling the CTs secondary terminal voltage in response to the core flux change. This is detected using a tertiary flux-change sense winding. The circuit is suited for use in high-temperature automotive environments. The bandwidth of the circuit is high allowing it to be used in peak current mode or hysteresis control schemes. Operation when sensing a 100-A current pulse of 10-ms duration is demonstrated

Thermal analysis of a segmented stator winding design

This paper presents a thermal analysis of a segmented stator winding design. As the thermal performance is one of the main factors limiting a machines output capability, a thermal test on a complete prototype machine is an essential part of the design process. However, for the segmented stator winding design a test-informed thermal analysis on a single stator tooth can be performed prior to the manufacture of the full machine. This approach allows for a rapid and inexpensive assessment of the thermal performance of the complete machine and early identification of design modifications needed. The research has been applied to the design of a highly efficient and compact permanent magnet (PM) traction motor. A thermal model for a single tooth was developed and supported by tests to identify key heat transfer coefficients. A number of winding assemblies were compared and the most promising was selected for the final motor prototype. The results from the approach are compared with thermal test results from the complete machine.

High-Fidelity Low-Cost Electronic Current Sensor for Utility Power Metering

The design of a high-fidelity electronic current sensor for utility power-metering applications is described. The sensor is based around a current transformer with a low-permeability core material in order to yield a high dc tolerance and improved immunity to extraneous dc magnetic fields. The transformer is configured with a flux-change sense winding and feedforward of the voltage developed across the secondary winding and burden resistances. This minimizes the error due to magnetizing current which would otherwise be high with a low-permeability core material. Experimental results are given for a 60-A sensor designed for single-phase 50- and 60-Hz systems. Measured phase error is less than 0.6° at 50 Hz with 60-A dc current superimposed onto the current under measurement. No Hall-effect sensors or core-gapping operations are required. Combined with simple analog electronic circuitry, this provides a low-cost solution.

Power Device Gate Driver Circuit With Reduced Number of Isolation Transformers for Switched Reluctance Machine Drive

The combined asymmetric half-bridge power converter topology is often used to drive switched reluctance machines (SRMs). This letter describes a transformer-isolated power semiconductor gate driver circuit that uses only two transformers to supply all three power devices in a combined asymmetric half bridge with local power supplies and drive signals. Therefore, the entire gate driver circuitry for a six-switch converter driving a four-phase SRM requires only four isolation transformers, thereby reducing cost. Operation of a prototype circuit is demonstrated.

EMI reduction with a soft-switched auxiliary commutated pole inverter

PWM-controlled power converters are increasingly used in the More Electric Aircraft (MEA). However, Electromagnetic Interference (EMI) is introduced due to the high dv/dt and di/dt slew rates of their fast-switching power devices. Though EMI is conventionally mitigated by adding filters that can be often heavy and bulky, a better solution would be to address it at source, in order to achieve higher power density systems. This paper investigates how soft-switching topologies can attenuate EMI by addressing it at source. Specifically, the auxiliary commutated pole inverter (ACPI) is employed to actively control the dv/dt of the output voltage. The effect this control has on the output voltage frequency spectrum is examined with analytical models. Subsequently, a control scheme is developed for regulating the output voltage to the desired waveform and simulation results are presented. A 1.4-kW ACPI phase-leg prototype is utilized for proof of concept. Reductions in the harmonic content of the output voltage are observed in the range of 1 to 20 MHz. It is expected that the ACPI can benefit from better EMI performance and can contribute to the overall effort of achieving high power density systems for MEA applications.