EPE Association: EPE 2011 - DS3h: Topic 11: Adjustable Speed Drives

EPE 2011 - DS3h: Topic 11: Adjustable Speed Drives
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2011 ECCE Europe - Conference > EPE 2011 - Topic 11: Adjustable speed drives > EPE 2011 - DS3h: Topic 11: Adjustable Speed Drives

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   A Control system of the multistage active rectifier energizing power shovel multi-motor variable-frequency drive for mining applications
By Mikhail PRONIN, Oleg SHONIN, Gregory GOGOLEV, Alexey VORONTSOV [View]
[Download]

Abstract: The paper presents a multi-stage PWM-rectifier control system built up in abc-reference frame.The control algorithm takes into account the peculiarities of converter- machine/mains interactions during an operational cycle of the mining power shovel. The paper concludes by analyzing simulation results that have shown that proposed system ensures both bidirectional energy flow and stabilization of the DC-link voltage under condition that input voltage fluctuates because of weak mains characteristics.

   A FAULT TOLERANT CONTROL AND POWER ELECTRONIC FOR A PERMANENT MAGNET SYNCHRONOUS MOTOR DRIVE
By Rammohan Errabelli, Peter MUTSCHLER [View]
[Download]

Abstract: In this paper fault tolerant solutions for different components of the permanent magnet synchronous machine drive (PMSM) such as digital controller, inverter and feedback sensors are presented. In case of digital controller fault tolerance, three fixed point Digital Signal Processor (DSP) based controllers are used in parallel, running the same control algorithm. In case of fault tolerant inverter, a redundant leg is added, which replaces the faulted leg. Faulted leg isolation and redundant leg insertion is done by using independent back to back connected thyristors. Sensorless vector control algorithm is implemented in case of position sensor failure. A redundant current sensor is used in case of current sensor failure. The fault tolerance of the above system is verified using field oriented control (FOC) of a PMSM drive

   A Pumped Storage Power Plant with Double-Fed Induction Machine and Cascaded Frequency Converter
By Grigorii GOGOLEV, Alexey VORONTSOV, Mikhail PRONIN, Oleg SHONIN [View]
[Download]

Abstract: The paper presents a variable speed drive for pump-storage plant applications. A reversible turbine is energized by a double-fed induction machine (DFIM) exited on the rotor side with a cascaded frequency converter made up of M single-phase cells. The DFIM is started up as a synchronous machine by means of a CSI on the stator side and a VSI on the rotor side. An emphasis is placed on the control system aimed at the plant active and reactive power regulation. In order to balance out currents of the transformer secondary windings feeding single-phase active rectifiers, a phase disposition PWM strategy has been modified by introducing a consecutive cycling step-wise displacement of triangular wave offsets within a saw tooth period consisting of M triangle wave periods. The paper concludes by analyzing start-up responses and performance characteristics of the system in sub- and over-synchronous modes of operation.

   A Sensorless Control for Damping of Torsional Vibrations with Middle Voltage Induction Motor Drive for Compressor Application
By Koichiro NAGATA, Haruo NEMOTO, Toshio KATAYAMA, Yoshitoshi AKITA [View]
[Download]

Abstract: In this paper the converter control method against damping torsional vibration of megawatt class compressor system is proposed. This method is based on field-oriented vector control. The main block is very simple but the motor torque can be controlled to enlarge the electric dumping coefficient easily. And the proposed method has good robustness for motor parameters and can decrease torsional vibration without speed sensor. Simulations for several assumed conditions were done to check the stability and robustness for motor parameters. And measurement of shaft torque with 3.3kV/ 600kW induction motor and compressor system was done to check the effectiveness of the proposed method.

   Active du/dt filter dimensioning in variable speed AC drives
By Juha-Pekka STRÖM, Juho TYSTER, Juhamatti KORHONEN, Mikko PURHONEN, Pertti SILVENTOINEN [View]
[Download]

Abstract: Active du/dt is a new output filtering method to mitigate cable reflection based motor overvoltages. The target in active du/dt is to minimize the passive filter in terms of size and weight, and to provide better filtering performance than with conventional passive approaches. The active du/dt method uses a passive LC filter circuit at the output of the inverter with additional active control for controlling the filter output voltage. The active control is carried out using the same output stage as in the fundamental frequency modulation, therefore, no additional components are required. In addition, considerations of dimensioning the filter du/dt value according to the cable length and allowed voltage overshoot at the motor terminals resulting from cable reflection are presented.

   Effect of Machine Asymmetry on a Two-machine Direct Torque Controlled Induction Motor Drive
By Bhakti JOSHI, Mukul CHANDORKAR [View]
[Download]

Abstract: This paper is concerned with the controller-driven interactions between the induction motors of a single inverter, two-machine direct torque controlled drive system. The main control strategies used for such adrive system are the Master-slave (MS) control and the Mean control. The performance of these controlmethods can get affected by the inherent asymmetries in the induction machines. A commonly occurringexample of asymmetry is a stator winding turns short circuit fault. In a two-machine system, since bothmachines are connected to a common inverter, a fault in one machine affects the performance of the other machine through the drive control system. A detailed comparison of the machine interactions which take place in two-machine drive systems controlled by the MS and Mean control methods is done considering a situation where there is a stator inter-turn fault in one of the machines. An experimentally validated dynamic model of an induction machine with stator faults is used to analyse these interactions. The machine interactions are also studied for the case in which the load on the two machines is different. The paper presents a detailed analysis and experimental results of these machine interactions, in the steady state.

   Efficiency Measurements on Active du/dt Output Filtering
By Juho TYSTER, Juha-Pekka STRÖM, Juhamatti KORHONEN, Pertti SILVENTOINEN [View]
[Download]

Abstract: Active du/dt filtering method can reduce the size of filter component physical size, and achieve superior filtering results. This enables very low overvoltage with long motor cables. The losses and power dissipation associated with the method is addressed. A comparison with passive filters is introduced, and silicon carbide components evaluated. The use of silicon carbide free-wheeling diodes results in significant loss reduction at least in 5.5 kW–22 kW motor power levels. The power dissipation of the inverter is measured with a temperature rise/thermal resistance method, and the results compared with chamber calorimeter results.

   Energy Efficient Sensorless Scalar Control for Full Speed Operating Range IM Drives
By Giacomo SCELBA, Alfio CONSOLI, Giuseppe SCARCELLA, Mario CACCIATO [View]
[Download]

Abstract: This paper proposes a low-cost implementation of energy saving strategies in scalar control IM Drives including high speeds in a wide working range. This loss reduction control algorithm approximates and adapts to scalar control some optimized analytical relationships, proposed in vector control, for low cost implementation. Basically an “optimal” slip from the energetic point of view is calculated in every speed range condition and the IM drive is forced to operate at this suitable slip angular frequency, that assures loss reduction especially at light loads. Numerical simulations and experimental validation confirm the effectiveness of the proposed control strategy, highlighting pros and cons of such implementation.

   Hardware-In-the-Loop Simulation of a Complex AC-fed Motor Drive with Triple Active Front-End 3-level Rectifiers and Induction Motor Drive using an Intra-Step-Pa
By Weihua WANG, Loicq BAKAY, Christian DUFOUR [View]
[Download]

Abstract: This paper presents the Hardware-in-the-Loop (HIL) simulation results of a very complex AC-fed induction motor drive. The drive is composed of an AC-stage composed of 3 saturable zig-zag transformers in series, each connected to a 3-level NPC inverter and DC-link with RLC filter. The DC-link feeds another 3-level NPC-based induction motor drive and also comprises a precharge circuit. All switching devices of the circuit are controllable from the I/O points of the real-time simulator. Due to the high complexity of this drive and the requirement for full fault simulation capability, a new electric circuit solver called State-Space Nodal, based on state-space and nodal approach was used from within SimPowerSystems. The new algorithm is the first known example of an electric system solver algorithm that parallelizes the network equation on multi-core micro-processors without delays using computing threads. The use of parallel computing threads enables to gain up to 33\% on computational time when compared to standard sequential execution of the same algorithm.

   Monitoring of Rotor Bar Faults in Induction Generators with Full-Size Inverter
By Goran STOJICIC, Matthias SAMONIG, Peter NUSSBAUMER, Gojko JOKSIMOVIC, Mario VAŠAK, Nedjeljko PERIC, Thomas WOLBANK [View]
[Download]

Abstract: Monitoring of generators is an essential part when operating a plant in remote areas. Nowadays demands on system reliability are very high and thus maintenance at regular intervals is necessary. Standard fault detection methods are usually based on several additional sensors. Such monitoring systems increase system costs and afford an additional source of fault. To avoid this disadvantage an alternative technique based only on the usage of sensors already available in the power inverter will be shown. The method is based on machine’s response due to voltage pulses generated by inverter switching. Measurement of the current response and subsequent signal processing leads to very detailed information about the machine state. Identification and separation of machine asymmetries obtained from two subsequent Fast Fourier Transformations offer a high sensitive fault indicator. The generation of the excitation pulses, current sampling and the signal processing chain as well will be described. Measurements for several levels of rotor bar fault severity are presented and the fault detection sensitivity and efficiency is proven.

   Motor terminal overvoltage suppression method for parallel inverters
By Juhamatti KORHONEN, Tommi LAAKKONEN, Toni ITKONEN, Juho TYSTER, Juha-Pekka STRÖM, Pertti SILVENTOINEN [View]
[Download]

Abstract: Inverter-fed motor drives can be subject to transient overvoltage problems caused by switching operations. An overvoltage suppression method for parallel inverters that uses no passive filtering components and a new modulation strategy are presented in this paper. The used modulation strategy allows pulse accumulation at the motor terminals which mitigates the overvoltage. The method is verified by prototype testing.

   Novel Switched Reluctance Motor Drive Circuit with Voltage Boost Function without Additional Reactor
By Yujiro KIDO, Nobukazu HOSHI, Akira CHIBA, Satoshi OGASAWARA, Masatsugu TAKEMOTO [View]
[Download]

Abstract: Switched reluctance motor (SRM) is attractive for the driving motor of electric vehicles (EVs) and hybrid electric vehicles (HEVs) from the view point which does not use rare-earth materials. However, special drive circuit, which is different from general three-phase inverter used for permanent magnet synchronous motor and induction motor, is required in order to drive SRM. By the way, in some of commercial HEVs, boost chopper combined with three-phase inverter is used in order to apply high voltage to motor windings at high-speed range. In this system, the size of the additional reactor, which is required in boost chopper, is a problem. Then, this paper proposes a novel drive circuit with voltage boost function without additional reactor for SRM. In addition, control scheme and controller configuration for the proposed circuit are described. Moreover, the effectiveness of the proposed circuit is verified by simulation and experimental results.

   Programmable Logic Device Based Brushless DC motor Control
By Alex VAN DEN BOSSCHE, Dimitar BOZALAKOV, Thomas VYNCKE, Vencislav VALCHEV [View]
[Download]

Abstract: In this article a three-phase BLDC motor controller for use in an Ultra-Light Electrical Vehicle is presented. The control is performed using a Programmable Logic Device (CPLD), which doesn’t require any additional processor. In this way a robust and low-complexity control is obtained. For extending the speed range of the BLDC, a phase advance circuit is implemented as well. The power consumption of the controller is very low which is an interesting feature in battery applications.

   Service Continuity of an IM Distributed Railway Traction with a Speed Sensor Fault
By Tahar ACHOUR, Maria PIETRZAK-DAVID [View]
[Download]

Abstract: This paper deals with a speed sensor fault in a Distributed Induction Motor (IM) Railway Traction. The advantages of multi-equipment modules in the Multi-Inverter/Multi-Machine topologies are exploited in order to guarantee the system service continuity in the presence of a speed sensor fault. Two solutions are proposed and compared. The first one is based on the analytical redundancy (speed observer introduction) and the second one applies the natural physical redundancy of speed sensor.

   Steady State Performance and Stability Analysis of Matrix Converter Fed Induction Motor Drive under Volt/Hertz Control
By Melaku MIHRET, Olorunfemi OJO [View]
[Download]

Abstract: A new generalized model for stability analysis based on the small signal analysis is proposed, for matrix converter fed induction motor drive system. This approach considers the analysis when the drive system operates at unity as well as non-unity input power factor at the source side. With the help of this model, different factors which affect the matrix converter stability are analyzed including stator and rotor resistance variation due to motor heating and the impact of non-zero reactive power at the source side. The paper also presents a complete and a general steady state analysis in the q-d synchronous reference frame when the whole drive system operates under a constant Volt/Hertz operation. Computer simulations are carried out to verify theoretical analysis on stability as well as on the steady state performance characteristics.

EPE 2011 - DS3h: Topic 11: Adjustable Speed Drives
You are here: EPE Documents > 01 - EPE & EPE ECCE Conference Proceedings > EPE 2011 ECCE Europe - Conference > EPE 2011 - Topic 11: Adjustable speed drives > EPE 2011 - DS3h: Topic 11: Adjustable Speed Drives
	[return to parent folder]

	A Control system of the multistage active rectifier energizing power shovel multi-motor variable-frequency drive for mining applications By Mikhail PRONIN, Oleg SHONIN, Gregory GOGOLEV, Alexey VORONTSOV	[View] [Download]
Abstract: The paper presents a multi-stage PWM-rectifier control system built up in abc-reference frame.The control algorithm takes into account the peculiarities of converter- machine/mains interactions during an operational cycle of the mining power shovel. The paper concludes by analyzing simulation results that have shown that proposed system ensures both bidirectional energy flow and stabilization of the DC-link voltage under condition that input voltage fluctuates because of weak mains characteristics.

	A FAULT TOLERANT CONTROL AND POWER ELECTRONIC FOR A PERMANENT MAGNET SYNCHRONOUS MOTOR DRIVE By Rammohan Errabelli, Peter MUTSCHLER	[View] [Download]
Abstract: In this paper fault tolerant solutions for different components of the permanent magnet synchronous machine drive (PMSM) such as digital controller, inverter and feedback sensors are presented. In case of digital controller fault tolerance, three fixed point Digital Signal Processor (DSP) based controllers are used in parallel, running the same control algorithm. In case of fault tolerant inverter, a redundant leg is added, which replaces the faulted leg. Faulted leg isolation and redundant leg insertion is done by using independent back to back connected thyristors. Sensorless vector control algorithm is implemented in case of position sensor failure. A redundant current sensor is used in case of current sensor failure. The fault tolerance of the above system is verified using field oriented control (FOC) of a PMSM drive

	A Pumped Storage Power Plant with Double-Fed Induction Machine and Cascaded Frequency Converter By Grigorii GOGOLEV, Alexey VORONTSOV, Mikhail PRONIN, Oleg SHONIN	[View] [Download]
Abstract: The paper presents a variable speed drive for pump-storage plant applications. A reversible turbine is energized by a double-fed induction machine (DFIM) exited on the rotor side with a cascaded frequency converter made up of M single-phase cells. The DFIM is started up as a synchronous machine by means of a CSI on the stator side and a VSI on the rotor side. An emphasis is placed on the control system aimed at the plant active and reactive power regulation. In order to balance out currents of the transformer secondary windings feeding single-phase active rectifiers, a phase disposition PWM strategy has been modified by introducing a consecutive cycling step-wise displacement of triangular wave offsets within a saw tooth period consisting of M triangle wave periods. The paper concludes by analyzing start-up responses and performance characteristics of the system in sub- and over-synchronous modes of operation.

	A Sensorless Control for Damping of Torsional Vibrations with Middle Voltage Induction Motor Drive for Compressor Application By Koichiro NAGATA, Haruo NEMOTO, Toshio KATAYAMA, Yoshitoshi AKITA	[View] [Download]
Abstract: In this paper the converter control method against damping torsional vibration of megawatt class compressor system is proposed. This method is based on field-oriented vector control. The main block is very simple but the motor torque can be controlled to enlarge the electric dumping coefficient easily. And the proposed method has good robustness for motor parameters and can decrease torsional vibration without speed sensor. Simulations for several assumed conditions were done to check the stability and robustness for motor parameters. And measurement of shaft torque with 3.3kV/ 600kW induction motor and compressor system was done to check the effectiveness of the proposed method.

	Active du/dt filter dimensioning in variable speed AC drives By Juha-Pekka STRÖM, Juho TYSTER, Juhamatti KORHONEN, Mikko PURHONEN, Pertti SILVENTOINEN	[View] [Download]
Abstract: Active du/dt is a new output filtering method to mitigate cable reflection based motor overvoltages. The target in active du/dt is to minimize the passive filter in terms of size and weight, and to provide better filtering performance than with conventional passive approaches. The active du/dt method uses a passive LC filter circuit at the output of the inverter with additional active control for controlling the filter output voltage. The active control is carried out using the same output stage as in the fundamental frequency modulation, therefore, no additional components are required. In addition, considerations of dimensioning the filter du/dt value according to the cable length and allowed voltage overshoot at the motor terminals resulting from cable reflection are presented.

	Effect of Machine Asymmetry on a Two-machine Direct Torque Controlled Induction Motor Drive By Bhakti JOSHI, Mukul CHANDORKAR	[View] [Download]
Abstract: This paper is concerned with the controller-driven interactions between the induction motors of a single inverter, two-machine direct torque controlled drive system. The main control strategies used for such adrive system are the Master-slave (MS) control and the Mean control. The performance of these controlmethods can get affected by the inherent asymmetries in the induction machines. A commonly occurringexample of asymmetry is a stator winding turns short circuit fault. In a two-machine system, since bothmachines are connected to a common inverter, a fault in one machine affects the performance of the other machine through the drive control system. A detailed comparison of the machine interactions which take place in two-machine drive systems controlled by the MS and Mean control methods is done considering a situation where there is a stator inter-turn fault in one of the machines. An experimentally validated dynamic model of an induction machine with stator faults is used to analyse these interactions. The machine interactions are also studied for the case in which the load on the two machines is different. The paper presents a detailed analysis and experimental results of these machine interactions, in the steady state.

	Efficiency Measurements on Active du/dt Output Filtering By Juho TYSTER, Juha-Pekka STRÖM, Juhamatti KORHONEN, Pertti SILVENTOINEN	[View] [Download]
Abstract: Active du/dt filtering method can reduce the size of filter component physical size, and achieve superior filtering results. This enables very low overvoltage with long motor cables. The losses and power dissipation associated with the method is addressed. A comparison with passive filters is introduced, and silicon carbide components evaluated. The use of silicon carbide free-wheeling diodes results in significant loss reduction at least in 5.5 kW–22 kW motor power levels. The power dissipation of the inverter is measured with a temperature rise/thermal resistance method, and the results compared with chamber calorimeter results.

	Energy Efficient Sensorless Scalar Control for Full Speed Operating Range IM Drives By Giacomo SCELBA, Alfio CONSOLI, Giuseppe SCARCELLA, Mario CACCIATO	[View] [Download]
Abstract: This paper proposes a low-cost implementation of energy saving strategies in scalar control IM Drives including high speeds in a wide working range. This loss reduction control algorithm approximates and adapts to scalar control some optimized analytical relationships, proposed in vector control, for low cost implementation. Basically an “optimal” slip from the energetic point of view is calculated in every speed range condition and the IM drive is forced to operate at this suitable slip angular frequency, that assures loss reduction especially at light loads. Numerical simulations and experimental validation confirm the effectiveness of the proposed control strategy, highlighting pros and cons of such implementation.

	Hardware-In-the-Loop Simulation of a Complex AC-fed Motor Drive with Triple Active Front-End 3-level Rectifiers and Induction Motor Drive using an Intra-Step-Pa By Weihua WANG, Loicq BAKAY, Christian DUFOUR	[View] [Download]
Abstract: This paper presents the Hardware-in-the-Loop (HIL) simulation results of a very complex AC-fed induction motor drive. The drive is composed of an AC-stage composed of 3 saturable zig-zag transformers in series, each connected to a 3-level NPC inverter and DC-link with RLC filter. The DC-link feeds another 3-level NPC-based induction motor drive and also comprises a precharge circuit. All switching devices of the circuit are controllable from the I/O points of the real-time simulator. Due to the high complexity of this drive and the requirement for full fault simulation capability, a new electric circuit solver called State-Space Nodal, based on state-space and nodal approach was used from within SimPowerSystems. The new algorithm is the first known example of an electric system solver algorithm that parallelizes the network equation on multi-core micro-processors without delays using computing threads. The use of parallel computing threads enables to gain up to 33\% on computational time when compared to standard sequential execution of the same algorithm.

	Monitoring of Rotor Bar Faults in Induction Generators with Full-Size Inverter By Goran STOJICIC, Matthias SAMONIG, Peter NUSSBAUMER, Gojko JOKSIMOVIC, Mario VAŠAK, Nedjeljko PERIC, Thomas WOLBANK	[View] [Download]
Abstract: Monitoring of generators is an essential part when operating a plant in remote areas. Nowadays demands on system reliability are very high and thus maintenance at regular intervals is necessary. Standard fault detection methods are usually based on several additional sensors. Such monitoring systems increase system costs and afford an additional source of fault. To avoid this disadvantage an alternative technique based only on the usage of sensors already available in the power inverter will be shown. The method is based on machine’s response due to voltage pulses generated by inverter switching. Measurement of the current response and subsequent signal processing leads to very detailed information about the machine state. Identification and separation of machine asymmetries obtained from two subsequent Fast Fourier Transformations offer a high sensitive fault indicator. The generation of the excitation pulses, current sampling and the signal processing chain as well will be described. Measurements for several levels of rotor bar fault severity are presented and the fault detection sensitivity and efficiency is proven.

	Motor terminal overvoltage suppression method for parallel inverters By Juhamatti KORHONEN, Tommi LAAKKONEN, Toni ITKONEN, Juho TYSTER, Juha-Pekka STRÖM, Pertti SILVENTOINEN	[View] [Download]
Abstract: Inverter-fed motor drives can be subject to transient overvoltage problems caused by switching operations. An overvoltage suppression method for parallel inverters that uses no passive filtering components and a new modulation strategy are presented in this paper. The used modulation strategy allows pulse accumulation at the motor terminals which mitigates the overvoltage. The method is verified by prototype testing.

	Novel Switched Reluctance Motor Drive Circuit with Voltage Boost Function without Additional Reactor By Yujiro KIDO, Nobukazu HOSHI, Akira CHIBA, Satoshi OGASAWARA, Masatsugu TAKEMOTO	[View] [Download]
Abstract: Switched reluctance motor (SRM) is attractive for the driving motor of electric vehicles (EVs) and hybrid electric vehicles (HEVs) from the view point which does not use rare-earth materials. However, special drive circuit, which is different from general three-phase inverter used for permanent magnet synchronous motor and induction motor, is required in order to drive SRM. By the way, in some of commercial HEVs, boost chopper combined with three-phase inverter is used in order to apply high voltage to motor windings at high-speed range. In this system, the size of the additional reactor, which is required in boost chopper, is a problem. Then, this paper proposes a novel drive circuit with voltage boost function without additional reactor for SRM. In addition, control scheme and controller configuration for the proposed circuit are described. Moreover, the effectiveness of the proposed circuit is verified by simulation and experimental results.

	Programmable Logic Device Based Brushless DC motor Control By Alex VAN DEN BOSSCHE, Dimitar BOZALAKOV, Thomas VYNCKE, Vencislav VALCHEV	[View] [Download]
Abstract: In this article a three-phase BLDC motor controller for use in an Ultra-Light Electrical Vehicle is presented. The control is performed using a Programmable Logic Device (CPLD), which doesn’t require any additional processor. In this way a robust and low-complexity control is obtained. For extending the speed range of the BLDC, a phase advance circuit is implemented as well. The power consumption of the controller is very low which is an interesting feature in battery applications.

	Service Continuity of an IM Distributed Railway Traction with a Speed Sensor Fault By Tahar ACHOUR, Maria PIETRZAK-DAVID	[View] [Download]
Abstract: This paper deals with a speed sensor fault in a Distributed Induction Motor (IM) Railway Traction. The advantages of multi-equipment modules in the Multi-Inverter/Multi-Machine topologies are exploited in order to guarantee the system service continuity in the presence of a speed sensor fault. Two solutions are proposed and compared. The first one is based on the analytical redundancy (speed observer introduction) and the second one applies the natural physical redundancy of speed sensor.

	Steady State Performance and Stability Analysis of Matrix Converter Fed Induction Motor Drive under Volt/Hertz Control By Melaku MIHRET, Olorunfemi OJO	[View] [Download]
Abstract: A new generalized model for stability analysis based on the small signal analysis is proposed, for matrix converter fed induction motor drive system. This approach considers the analysis when the drive system operates at unity as well as non-unity input power factor at the source side. With the help of this model, different factors which affect the matrix converter stability are analyzed including stator and rotor resistance variation due to motor heating and the impact of non-zero reactive power at the source side. The paper also presents a complete and a general steady state analysis in the q-d synchronous reference frame when the whole drive system operates under a constant Volt/Hertz operation. Computer simulations are carried out to verify theoretical analysis on stability as well as on the steady state performance characteristics.