The three-phase synchronous reluctance motor (SynRM) is presented as a possible alternative in all-electric ship applications. The basic features of this motor with regard to the other types of motors are shown. The structure of the motor and specifically the structure of its rotor are analyzed, while the basic operating principles are presented and references on commonly used control strategies are made. In this paper, a demonstration of a reluctance motor fed by a voltage source inverter (VSI) takes place. To demonstrate the operation of the motor fed by a VSI, an example using a scalar control method is implemented, where harmonic injection PWM (HIPWM) is used to drive the VSI. Experimental results on a commercially available motor are shown, focusing on the harmonic content of the current.
Sea transportation plays a crucial role in the development of human civilization for more than 5,000 years [
In modern times, the constant progress in electric and electronic systems had also an important effect on ships and led to the first commercial adoption of electricity-driven technology by the cruise ship industry in the late 1980s [
Propulsion systems consume the major part of the total energy in conventional ships [
From the above, it is obvious that the need for more efficient electric machines in the future ships will not be restricted to those used for the propulsion systems. In every ship, there is a great need for middle- and low-power motors. These are used in a large variety of applications, such as winches, hoists, and pumps for water or fuels. It is also a fact that the navy is considering the replacement of hydraulic systems and other traditionally mechanical systems by power-electronics-driven electric motors. Also, electric motors are used in combustion air fans, refrigeration plants and ventilation systems. While electric propulsion motors should have a very small weight-to-power ratio, for the large amount of supplementary drive systems needed it is required that an optimum solution concerning efficiency, cost, and performance should be made. It is possible that this optimum solution can be given in the near future by the reluctance motor.
Reluctance motors are considered as a subcategory of synchronous motors with a salient-pole rotor. They differ from other types of motors, since their rotor does not have windings and is constructed from inexpensive materials like iron (instead of expensive magnet, like permanent magnet synchronous motors); thus, there is no concern with demagnetization [
Although the reluctance motor has a long history [
Early evolution of reluctance motors led to two different types: switched and synchronous reluctance motors. Although a significant progress has taken place concerning design and control of switched reluctance machines [ their stator is identical to that of an induction motor, so the stator can be constructed in the already available assembly line. They can achieve high power densities in regard to their size. They can function without a starting cage, since they can start in synchronous mode. Compared to the induction motor, the SynRMs achieve high torque output in regard to iron losses and higher efficiency [
The exploitation of these characteristics of SynRM in applications is still on the way, with many of these focusing on hybrid electric vehicles [
Moreover, the most appropriate converter topologies should be examined, which will lead to adequate performance of the reluctance machine. As far as converter topologies for use in ship applications are concerned, Controlled Rectifier, Load Commutated Inverter, Cycloconverter, and Voltage Source Inverter are the most popular ones [
In this paper, the performance of SynRM will be studied through the utilization of a simple PWM control method. This control scheme will be applied on a mass-production motor, and attention to the harmonic content of the current which is the main cause for torque ripples will be paid.
As it has been already mentioned, the stator of a SynRM is identical to the stator of an induction machine. The construction of the rotor, however, is more complex. In general, the main concept for the construction of the SynRM is the optimization of the rotor in order to maximize the produced (reluctance) torque. The evolution of the SynRM rotors has led to diverse structures, as it is shown in the Figure
Two different rotor designs (transverse and axially laminated rotor).
The individuality of the SynRM’s rotor has led to limited adoptions from the industry. This is more characteristic for the axially laminated rotor, although the high
As it is well known, the stator windings generate a spatial sinusoidally distributed magnetomotive force (MMF) in the air-gap between stator and rotor. The rotor of SynRM is constructed by ferromagnetic steel and nonmagnetic laminations. Despite the absence of field winding in the rotor, the rotor due to the presence of flux paths (Figure
In order to have a comprehensive representation of SynRM, it is significant to present the equivalent circuits (Figure
Equivalent circuits of SynRM in rotor reference
For this reason, the equations are expressed in
The corresponding equations in the rotor reference frame are the following [
The developed torque is given by the following equation:
From the previous analysis of SynRM, we can make some useful conclusions about the role of the inductances
The main characteristics that differentiate this type of motor from other types, such as synchronous and induction motors, are the absence of field winding on the rotor and the fact that there is no rotation slip. Many control methods (more or less complicated) have been proposed for this motor along the years [
In order to have a simple block diagram of a scalar control method for the SynRM, the open-loop
Open-loop control of a SynRM.
The user can set the desired speed and this command is translated to the corresponding voltage signals through a function generator, for example, a microcontroller. These signals are driven to a 3-phase PWM inverter, which is presented in the following chapter. The 3-phase inverter is fed by DC voltage, which is produced through a rectifier topology [
The three-phase VSI topology consists of six power switches (e.g., IGBTs), which are controlled by six corresponding signals (Figure
VSI topology.
As it has been already mentioned, PWM techniques are used for the production of the output voltages in a VSI. The main purpose of the VSI is to produce a fundamental voltage harmonic with controlled rms value and frequency.
A properly designed PWM VSI offers the following characteristics: high switching frequency, which leads to smaller passive harmonic filters; precise regulation of the fundamental harmonic frequency and rms value; reduced high-order harmonic content.
Many PWM techniques have been proposed along the years, which in general aimed at the optimization of the harmonic content, better exploitation of the DC voltage source, and reduction of switching losses [
Among them, one of the simplest but also the most popular one is the Sinusoidal PWM (SPWM) technique, where three-phase sinusoidar reference signals are compared with a triangular waveform. The corresponding output for each phase is positive when the value of the sine wave is greater than the value of the triangle, and zero in the opposite case. If we denote by
Harmonic injection PWM (HIPWM) is a modification of SPWM, where the 3 sine waves contain additionally a small percentage of 3rd and 9th harmonics of the fundamental frequency. This modification leads to better exploitation of the input DC voltage, resulting in higher maximum line, as shown in Figure
(a) SPWM and (b) HIPWM voltage signal and FFT analysis.
To evaluate the operation of the selected HIPWM technique, a three-phase inverter has been simulated using appropriate software and a comparison of the classic SPWM and the HIPWM technique was done. In Figure
An inverter with the same basic characteristics and operating with the same HIPWM method as in the simulation has been designed and constructed the main components of the experimental device are shown in Figure
Main components of inverter topology.
The inverter uses IGBTs as switching elements, since they offer low on-state impedance and adequate switching characteristics [
Flow chart of the developed open-loop control technique.
For the experiments, a 4-pole SynRM constructed by
Sketch of the SynRM under investigation.
For evaluating the performance of the system, experimental results have been carried out at no-load condition. This way, a “worst-case scenario” is examined, as oscillations are greater at no-load condition.
In Figure
HIPWM gate signal.
As it is mentioned, in the experiments an investigation of the current harmonics has been performed. In Figure
Current harmonics when the motor is fed by the utility grid.
When the topology shown in Figure
With the reference speed set at 50 Hz, the resulting harmonic content of the current is shown in Figure
Current harmonics at 50 Hz (fed by a VSI).
Current Waveform at 25 Hz (actual speed 746 rpm).
Harmonic content of current at 25 Hz (fed by the VSI).
A study on the synchronous reluctance motor has been made in this paper. The experimental results have verified the smooth and precise performance of the motor at 50 Hz. Furthermore, it has been observed that the energy of the higher-frequency current harmonics at low speeds, such as 25 Hz, becomes noticeable. To improve performance, a more sophisticated control method for mass-production SynRMs has to be sought. To this direction, the adoption of control methods already available for other synchronous motors has to be considered. In any case, the possibility of using SynRM in all-electric ship applications is subject to further study, since the characteristics of this type of motor show a great promise for the future.
Current
Voltage
Rotational speed of the rotor
Inductance
Electromagnetic torque
Flux
Pole pairs
Stator resistance
MMF force.
Components of a space phasor in
Stationary and rotating frame, respectively.