Judith Apsley

Analysis of multiphase induction machines with winding faults

The paper shows how the techniques of generalized harmonic analysis may be used to simulate the steady-state behavior of a multiphase cage induction motor with any form of open-circuit or short-circuit fault in the stator winding. The analytical model is verified using a four-pole machine with a 48-slot stator. Each coil of the stator winding of this machine is brought out to a patchboard that enables the stator to be configured for single-phase, two-phase, three-phase, four-phase, six-phase, or 12-phase excitation. Experimental results are compared with computer predictions for a six-phase machine with both open-circuit and short-circuit faults.

Propulsion Drive Models for Full Electric Marine Propulsion Systems

Integrated full electric propulsion systems are being introduced across both civil and military marine sectors. Standard power systems analysis packages cover electrical and electromagnetic components, but have limited models of mechanical subsystems and their controllers. Hence electromechanical system interactions between the prime movers, power network and driven loads are poorly understood. This paper reviews available models of the propulsion drive system components: the power converter, motor, propeller and ship. Due to the wide range of time-constants in the system, reduced order models of the power converter are required. A new model using state-averaged models of the inverter and a hybrid model of the rectifier is developed to give an effective solution combining accuracy with speed of simulation and an appropriate interface to the electrical network model. Simulation results for a typical ship manoeuvre are presented.

Analysis of multi-phase induction machines with winding faults

The paper shows how the techniques of generalised harmonic analysis may be used to simulate the steady-state behaviour of a multi-phase cage induction motor with any form of open-circuit or short-circuit fault in the stator winding. The analytical model is verified using a 4-pole machine with a 48-slot stator. Each coil of the stator winding of this machine is brought out to a patch-board that enables the stator to be configured for single-phase, two-phase, three-phase, four-phase, six-phase or twelve-phase excitation. Experimental results are compared with computer predictions for a six-phase machine with both open-circuit and short-circuit faults

Derating of Multiphase Induction Machines Due to Supply Imbalance

Open-loop variable-voltage variable-frequency control should offer an effective control strategy in many applications of multiphase machines, but increasing the number of phases can make the machine more sensitive to imbalance in the supply voltage, causing unequal current sharing between phases. This paper uses simple expressions based on an extended symmetric-component approach to quantify the degree of current imbalance and provide a way of calculating the derating required. This paper shows that the additional stator copper losses are concentrated in the phase with the highest terminal voltage. However, the pulsating torques, rotor losses, and total stator losses due to supply imbalance in a multiphase machine are less than for the three-phase case. Experimental results for the same machine when connected with four and six phases show good agreement with the simple analysis method proposed. A more detailed generalized harmonic analysis algorithm is used to validate the torque and loss calculations.

Modelling and Analysis of Electro-Mechanical Interactions between Prime-Mover and Load in a Marine IFEP System

This paper reports on the simulation of a marine Integrated Electric Full Electric Propulsion (IFEP) system to assess its ability to absorb variations in propulsion or auxiliary load without excessive degradation of the electrical supply quality or imposing excessive demands on the prime movers. IFEP systems are expected to yield economic benefits to ship operators by permitting the capacity of ship engines in use to be more closely tailored to the electrical demand of auxiliary and propulsion systems. However, the extent to which these savings can be realised at times of low demand is dependent on the ability of the shipboard electrical system to absorb disturbances. In this paper, simulations are conducted for a variety of frequencies of load variation, and the results assessed. Measures which might be taken to reduce the observed effects are suggested.

The Doubly Fed Induction Machine as an Aero Generator

Modern aircraft require a robust and reliable supply of electrical power to drive a growing number of high-power electrical loads. Generators are driven by a mechanical offtake from the variable speed gas turbine (GT), while a constant frequency ac network is preferred. Here, doubly fed induction machines are advantageous since they can be controlled, through a fractionally rated converter, to decouple electrical frequency from the mechanical drive speed, making control of the network frequency possible. However, the converter must be suitably rated, according to drive speed range, electrical voltage and frequency regulation, and power requirements. This paper develops and validates a simulation model of the doubly fed induction generator (DFIG) system, which is applied to find the power flow through the machines stator and rotor connections over a wide mechanical speed range in order to size the converter. A field-orientated control scheme is implemented, to provide stand-alone voltage and frequency regulation across a drive range of ±40% synchronous speed, on a purpose-built 6.6-kW hardware test platform. Based on the mechanical speed range of an aero GT and the identified converter sizing, the suitability of a DFIG for aero applications is appraised. It is shown that a converter rated at 18% of full system rating can be used to meet the aircraft electrical specifications and offers a potential improvement in aircraft performance, with no additional mechanical components.

Winding Configurations for Five-Phase Synchronous Generators With Diode Rectifiers

Wound-field synchronous generators are widely used in aircraft and marine electrical systems. As the electric power requirements increase, there is renewed interest in dc power networks, but the electrical source remains a synchronous generator. The combination of a diode rectifier with a wound-field multiphase generator reduces the voltage ripple on the dc network and increases fault tolerance, compared with an equivalent three-phase system. It also increases the options in terms of the winding design and configuration. This paper uses circuit modeling, including harmonics, informed by static finite-element results, to understand the wound-field generator performance for star and polygon connections of both short- and fully-pitched coils. Experimental results are used to validate the models. A polygon connection of short-pitched coils is shown to give good generator utilization for a healthy machine. However, careful design is required to prevent circulating harmonic currents. Under winding open-circuit faults, the polygon connection requires significant derating, making the star connection the preferred option.

Diode rectification of multiphase synchronous generators for aircraft applications

Increased electrical loading on aircraft power systems has led to interest in DC electrical networks for aircraft applications. This paper investigates a wound-field synchronous generator and uncontrolled diode rectifier as the DC source and analyses the power quality of the AC and DC sides of the rectifier. The paper gives expressions for voltage and current distortion for a multiphase generator as a function of the number of phases. For a star-connected machine, the power quality on the AC side is shown to deteriorate rapidly as the phase number increases, resulting in increased losses and a higher rating requirement. A delta connection provides better utilisation of the generator; however, at the expense of the fault tolerance. Mathematical models for the multiphase system are developed and the analysis is supported by simulation results; verified through laboratory testing.

Reduced losses in die-cast machines with insulated rotors

This paper reports an investigation into the influence of interbar currents on the performance of die-cast cage induction machines. This work is based on an extensive set of experimental measurements using a series of specially made die-cast cage induction motors with bars insulated using alternate processes. Experimental measurements were undertaken separately by the industrial manufacturer and at the university laboratories, and these clearly demonstrate that increasing the insulation of the rotor cage inhibits the flow of interbar current and reduces the total motor losses and increases the motor efficiency as a consequence. The experimental measurements were further validated using an analytical interbar model. Further experimental measurements also demonstrate that insulating the rotor cage also modifies the stator torque in a counterintuitive fashion.

De-rating of multiphase induction machines due to supply unbalance

Open-loop variable voltage, variable frequency control should offer an effective control strategy in many applications of multiphase machines, but increasing the number of phases can make the machine more sensitive to unbalance in the supply voltage, causing unequal current sharing between phases. This paper uses simple expressions based on an extended symmetric components approach to quantify the degree of current unbalance and provide a way of calculating the de-rating required. The paper shows that the additional stator copper losses are concentrated in the phase with the highest terminal voltage. However, the pulsating torques, rotor losses and total stator losses due to supply unbalance in a multiphase machine are less than for the three-phase case. Experimental results for the same machine when connected with four and six phases show good agreement with the simple analysis method proposed. A more detailed, generalised harmonic analysis algorithm is used to validate the torque and loss calculations.