This study considers the design of surface-mounted permanent magnet electrical machines for high-speed applications and proposes a methodology to determine the maximum achievable power density. Power density is usually improved by increasing rotational speed. At high speed, a mechanical retaining system for the rotor magnets must be considered. As the speed increases, the thickness of the retaining sleeve becomes larger, reducing torque capability. There will be an optimal speed at which the output power will be maximised. Both structural and electromagnetic design must be considered simultaneously to properly address this design problem. To simplify the design procedure, static finite-element simulations are used for the electromagnetic analysis and analytical formulae are employed for retaining sleeve sizing. The procedure is aided by multi-objective optimisation algorithms. A case study based on the specification of an aeronautical actuator is presented. The performances that can be obtained using different iron cores, high-grade silicon steel, and cobalt iron steel are compared. Finally, results obtained from transient finite-element electromagnetic and structural analysis are presented to validate the feasibility of the proposed procedure.
Wide bandgap (WBG) power devices such as silicon carbide (SiC) can viably supply high speed electrical drives, due to their capability to increase efficiency and reduce the size of the power converters. On the other hand, high frequency operation of the SiC devices emphasizes the effect of parasitics, which generates reflected wave transient overvoltage on motor terminals, reducing the life time and the reliability of electric drives. In this paper, a SiC metal-oxide-semiconductor field-effect transistor (MOSFET) based two level (2L) inverter is systematically studied and compared to the performance of Si insulated-gate bipolar transistor (IGBT) based three level (3L) neutral point clamped (NPC) inverter topologies, for high speed AC motor loads, in terms of efficiency, overvoltages, heat sink design, and cost. A fair comparison was introduced for the first time, having the same output voltage capabilities, output current total harmonic distortion (THD), and overvoltages for the three systems. The analysis indicated the convenience of using the SiC MOSFET based 2L inverter for lower output power. In the case of the maximum output power, the heat sink volume was found to be 20% higher for the 2L SiC based inverter when compared to 3L NPC topologies. Simulations were carried out by realistic dynamic models of power switch modules obtained from the manufacturer's experimental tests and verified both in the LTspice and PLECS simulation packages.Even though the modern switching devices offer many benefits, they still experience many problems mainly connected to the fast switching such as high dv/dt-rates, also in combination with impedance mismatch (machine against cabling and surge), high di/dt, crosstalk [5], etc. Moreover, the fast switching leads to transient overvoltage which increases the strain for the machine's insulation and accelerated aging [4,6]. This, together with the fact that often, the power cable between the inverter and the motor is long, leads to significant voltage overshoot due to reflected wave phenomenon. All of these problems resulting from the high dv/dt spikes could degrade the reliability and efficiency of motor drive systems [7][8][9]. In some cases, motor transient peak voltages can be even up to 3-4-times the DC bus voltage [10]. Moreover, besides the effects of the overvoltages on a whole winding, also the stress on the inter-turn insulation of the motor windings must be considered, especially as the rise time becomes shorter. This can result in relevant voltage drops across one turn, going beyond the inter-turn insulation design limit considered under sinusoidal operation [6,7].Currently, insulated-gate bipolar transistor (IGBT) based two level (2L) VSI, and three level (3L) neutral point clamped (NPC) and T-type neutral point clamped (T-NPC) inverters are commercially available and widely used by industry. However, in some cases, the application of 2L inverters is limited due to several drawbacks. They are usually connected with increased losses (in the case of higher fundamental freq...
The use of fast wide-bandgap devices in high-speed electrical drives with steep pulse-width modulation voltage waveforms is the main cause of increased stress on the electric machine insulation system and, consequently, of reduced reliability of the whole system. This represents a major concern in safety-critical applications, such as in the aerospace and electric transportation fields. The novel contribution of this work is to assess the effects of electrical aging on complex insulation systems (i.e. a whole stator winding) by analyzing easily measurable macroscopic quantities. To this purpose, an electrical aging procedure is defined, and the effects produced on three tested motors are presented. Three different analysis methods are adopted with the aim of assessing the damage caused to an AC motor winding by such steep voltage waveforms and hence to evaluate the state of aging of the insulation system.
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