SENSORLESS STARTING METHOD OF 9-SLOT 8-POLE PERMANENT MAGNET BLDC MOTOR FOR ELECTRIC VEHICLE

• Main Author: Satria, Alfi
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/33449
• Institution: Institut Teknologi Bandung

Electric vehicles are starting to develop again nowadays in line with the increasing awareness to improve environmental quality. Internal combustion engine vehicles which use fossil fuels and produce air pollution, in the next few years will be replaced by electric vehicles in several countries, including in Indonesia. The research related to electric vehicles that is interesting now is research on electric motors and energy storage media (batteries). In the field of electric motors, the use of permanent magnet motors begins to dominate and shift the use of induction motors because they are more efficient, more reliable and low maintenance. Among permanent magnet motors, brushless DC motors or BLDC have the potential to be used more in the future because of the ease of control. To control the DC motor properly, the motor control voltage must be applied at the right time to produce the maximum torque. This process is called commutation. In a conventional DC motor, this commutation is done mechanically by split rings and brushes, which acts as position sensors as well as ignition switchs. Whereas in the BLDC motor, the commutation process is carried out electronically using power electronics devices. The switching of power electronic devices must be carried out in the proper rotor position. For this reason, position sensors such as Hall effect sensors are used. The use of this position sensor is now being removed due to reducing price, volume, cabling, and sensor failures anticipation. This control method is called the sensorless control method. The popular sensorless method used is based on the detection of back electromotive forces (back EMF) signals. But this method can only be applied to speed above a certain value because the back EMF is proportional to speed. The method cannot be used for low speed or starting from a standstill. The popular starting technique used is to rotate the rotor in the direction of a certain position, then accelerate to a certain speed where back EMF can be detected. The disadvantage of this technique is that the motor can rotate backward, or the generated torque is small, or the sequence of the switch ignition can cause the motor to jerk. This is not desirable in some applications such as electric vehicles. One method that can be used to detect the position of the rotor in standstill or low speed conditions is based on saliency. Rotor position changes can cause winding inductance variations and by analyzing the detected inductance value in the motor winding, the rotor position can be estimated. After that the motor is excited and accelerated according to the commutation sequence based on the position of the rotor. One of the BLDC motor configurations developed by ITB in the national electric car research project is the interior permanent magnet BLDC motor with 9 slots and 8 poles. The BLDC motor with fractional slot configuration and concentrated winding has a low cogging torque advantage and the interior permanent magnet make the motor safe to operate on high speed, but on the other hand it has a shortage of unbalanced magnetic pull due to the asymmetrical stator winding. This unbalanced magnetic force is caused by a unique magnetic field distribution on the stator windings so that for one inductance value on the stator winding due to the permanent rotor magnetic flux only associated with one particular rotor position. On BLDC motors with integer slot configurations, for one inductance value perceived by the stator winding due to permanent magnet rotor flux, there are two possible rotor position in opposite direction, so an additional algorithm must be conducted to determine the real position. The first stage in the sensorless starting method for BLDC motor with 9 slots and 8 poles is the learning process towards the winding inductance characteristics due to the rotor position, by mapping the stator winding inductance value in the form of winding current response amplitude, to the rotor position. To map the inductance in the stator winding of the BLDC motor with 9 slots and 8 poles, the motor is injected with high frequency low voltage current, so that the rotor does not move. The current response value due to the voltage injection is then stored in the lookup table. When starting the motor, the same high frequency voltage is injected to the motor, then the amplitude of current response is compared to the data in the lookup table so that the rotor position estimation is obtained. After the sector of rotor position is known, the motor is excited using the right commutation sequence which based on the position of the rotor. The BLDC motor with 9 slots 8 poles has unique magnetic field distributions in the stator due to the rotor position, so the current response generated using this method is easier to analyze and estimate the rotor sector position with just one observation so that the application of this method can use a simple and inexpensive microcontroller.