Power Circuit Design and Analysis of Controller for High-Power Axial Flux PMSM

░ ABSTRACT - Designing a high-power controller having high efficiency for permanent magnet motors is a challenge for developers in recent times and very few techniques are available. Design and analysis of power electronic drive for a high-power axial flux permanent magnet synchronous motor is presented in this paper. The motor under consideration here is having two outer stators and single permanent magnet rotor to drive the shaft. Control schemes and methodologies are the major concentration for research. Present paper explains a method to estimate the operational drive parameters and loss calculation according the selected power switches. In addition to this, thermal modelling is also important for designing a controller for high power machines. The hardware selection for high current switching operations is very critical for reliable and robust operation. Parts like IGBT module, gate driver, DC link capacitor, snubbers, current sensor sensitivities and estimation of losses are also critical along with control logic and processor. The importance of thermal analysis for high current switching drives is discussed and methodology for thermal analysis of power electronic devices is explained. The current work focuses on hardware level design of high-power drive for 150kW axial flux PMS motor.


░ 1. INTRODUCTION
In recent era, demand of high-power axial flux permanent magnet machine drives increased in view of industrial and scientific applications [1]. These machines have high power density, better magnetic and electrical loading capacity, long life and low noise [2].
For the power electronic drive, important requirement is component selection suitable for high current switching and minimizing losses. For the current application, SVPWM [3] control algorithms are implemented in the DSC and a master processor commands the DSC. ARM CORTEX M4 based architecture is adopted for the Master Processor. The space vector modulation and closed loop vector control algorithms are implemented in the DSC. IGBTs are selected for rated current and voltage requirements with necessary factor of safety. Numerous literatures [4] are available for control logics and techniques Axial Flux PMS motor [5]. In view of practical implementation care has to be taken for high current and high frequency switching.
The IGBTs will share the phase current by parallel operation. The parallel operation is opted for optimization of losses and description of heat. DC link capacitors, snubber capacitors, bus bars, DC-DC converters, sensors, processors, driver boards and power terminals are selected and accommodated in a best possible layout in order to optimize the space and weight of the overall power electronic drive system.  [6] are also critical along with control logic [7][8] and processor.
After an extensive study and search for the literature in this regard, it is observed there is a very minimal literature available for hardware level design. So, data from different developers and manufactures is collected and efforts put on presenting an elaborate methodology to estimate the specifications of the required hardware for the drive design.  Table 2 shows the important parameters that are derived from the motor to design the dive hardware. Few limitations related to thermal management, run time error and processing speed (Switching speed) have been eliminated with this new methodology.

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The following section explains clearly the design methodology, selection of power circuit hardware and simulations to estimate the functionality of the designed power electronic drive for 150kW axial flux PMS motor using simulations. Thermal analysis is also done using lumped parameter model to ensure safe operation at high current switching. Figure 1 shows the power circuit schematic for the 165kW power electronic drive system. The total controller has inverter to drive the 150kW axial flux PMS motor. As can be seen from the figure, the DC battery drives the inverter. The DC voltage is applied to the drive through a high-power contactor. The threephase inverter bridge of each motor are driven by respective adapter and gate driver boards. The vector control logic is implemented in the control board with one common digital signal controller. This processor on the control board acts like slave and are commanded by the master processor on the communication card. The control takes the necessary phase current and rotor position inputs from respective motors and with the help of vector control logic [9] generates the PWM signals for the gates of IGBT based inverters. The input DC link capacitors will help in maintaining the minimum current ripples on the DC bus of the system. The control card also implements the necessary protection logics for the motor drives.

░ 2. DESIGN METHADOLOGYTY
The 150kW axial flux motor is designed with two stator cores. The motor will be made of two 75kW stator cores operating in parallel and driving the same shaft. Both the stator cores are similar to each other. Table 3 shows the parameters required to design a drive for 150kW motor. The required input terminal AC voltage (phase peak) of the motor as per motor design parameters is 186 V.   Figure 2 shows the IGBT selected for the controller. As per the design, three IGBT modules will make total drive. Each of the IGBT is a HEX pack having six power semiconductor switches in it. Three IGBTs will make an inverter driving one motor. The top three switches and bottom three switches of the HEX pack IGBT operate in parallel. This means each of the HEX pack IGBT makes one leg of the inverter.  figure 2 is 300A and 1200V IGBT with HEX pack configuration [8].

░ 3. POWER ELECTRONIC SWITCHES AND SUITABILITY ANALYSIS
Current Rating of IGBT = 900 A Scaling factor for current overshoot capability = 1.4 Voltage rating of the IGBT = 1200 V Scaling factor for voltage capability = 2 Table 5 shows the important gate circuit parameters designed for the power electronic drive. Figure 3 shows the   Table 6 lists the losses of the inverter for the controller by using the different values related to switching and conduction losses [10]. The temperature dependent parameters are also considered.

░ 4. INVERTER LOSS CALCULATION
As the selected gate external resistance is 2 ohms, the parameters dependent on this are reconsidered from the IGBT datasheet respective graphs.

░ 5. CONTROLLER DESIGN AND PERFORMANCE ANALYSIS
The 150kW axial flux motor with 02 stators (each 75kW) and two central rotors has to be controlled by a single inverter. Inverter controls the parallel of two stators along with one central permanent magnet rotor. The controller uses PID control technique [11] to drive the motor. In order to achieve a smooth and stable motor control performance, gains of various loops play crucial role. These gains are designed using Ziegler & Nichole's [12] method and are fine-tuned by simulation. Also, a ramp command is applied for the speed reference at the rate of 1800RPM for 8 seconds. Table 7 shows the selected control parameters. The designed motor control and its performance for various speeds and loads has been validated using simulation studies in SIMULINK.   Figure 3 shows the simulation block for the purpose of analysis. Figure 4 is the speed and current controller subsystems and figure 5 shows the equivalent PMSM motor model.       figure 12 shows the various important electromechanical parameters viz. back EMF, speed response, AC winding currents and voltages, D-Q currents, Active and reactive power and load torque of the for 150kW motor with given reference speed as a ramp of zero to rated i.e., 1800rpm in 8 seconds.

░ 6. THERMAL ANALYSIS
The IGBTs are mounted on the controller box (al alloy). The drive heat sink has cooling channels on the outer periphery which circulates the water at 20 litres/minute in order to take away the heat generated in the IGBTs.
The heat losses generated at the IGBT junctions will transfer to its sink and to the drive shell and to the cooling liquid. Also, some amount of heat will be dissipated through convection and radiation process.
Thermal analysis [10] of 165kW power electronic drive is presented in this section considering these heat transfer aspects. Radiation heat transfer coefficient [11] has been derived.
t1 -Temperatures of the surface t2 -Temperature of ambiance σ -5.669x10 -8 W/m 2 K 4 ε -Emissivity (0.7 to 1, 0.8 in present case) F12 -View factor (1 for dissipating surface, 2 for absorbing surface) Hr -Radiation heat transfer coefficient Rr -Radiation thermal resistance A -Radiating surface area = 1 * Following are a few important correlations for natural convection for a horizontal cylinder. In the lumped parameter thermal circuit parameters are calculated by the resistance given in the datasheet [8]. The thermal resistance for one IGBT junction to the casing is given. But in the present case we have 3 IGBT modules for controlling single 150(2*75) KW motor. So, coming to the number of junctions we have 6 switches in a single IGBT module and 3 modules which results in total 3*6=18 junctions for a controller. Hence the resultant thermal resistance network is 18 resistances connected in parallel between the sink and controller.  Hence, we know when N symmetric resistances are connected in parallel resultant resistance is 1/N times and the current source (losses in transistor) is multiplied by N. As given in the data sheet thermal resistance from junction of single IGBT to sink (Rth(j-s) tr) = 0.116. In the lumped parameter circuit, the resistance represented is the total resistance due to 18 resistances in the controller is 0.116/18=0.006444C/W. And the current source is the total losses due to the 18 IGBT (switching and conduction losses) is 18* 155.141=2782.51W. similarly, In the anti-parallel diode as given in the data sheet thermal resistance from junction of single diode to sink (Rth(j-s) d) = 0.218C/W. In the lumped parameter circuit, the resistance represented is the total resistance due to 18 resistances in the controller is 0.218/18=0.01211/W. And the current source for the total losses due to the 18 diodes (switching and conduction losses) is 18*65.545=1179.81W. Table 8 lists the thermal resistance parameters corresponding this lumped parameter thermal network which was shown in figure 13. Figure 14 presents the Simulink simulation of the external cooling thermal analysis.

░ 7. CONCLUSION & FUTURE SCOPE
Power circuit for a high-power drive with high efficiency for a 150kW permanent magnet synchronous motor is designed and methodology is clearly explained. The motor considered here is a state-of-the-art motor configuration having two outer stators and single permanent magnet rotor to drive the shaft. Operational drive parameters like DC bus voltage, winding currents, switching frequency and also taking current ripple Website: www.ijeer.forexjournal.co.in Power circuit design and analysis of controller for High-power Axial Flux PMSM currents are calculated and suitable hardware specifications are derived. Critical parts selection such as IGBT module, gate drive and loss calculation procedure are done. Inverter and PWM control were also implemented in the above simulation studies and the simulation based fine-tuned control parameters are used in the practical implementation of the motor. Also, the closed loop control performance of the motor is satisfactory. For a very high-power dense drive the thermal analysis is very critical. It is observed that the junction temperature is within the limits i.e. less than 85 degrees Celsius as per the datasheet. This ensures a robust and faultless operation of the drive while switching high currents. This work can be implemented to design higher power drives validating the performance in switching high currents and to design a thermally robust controller.