PWM CONTROL TECHNIQUE
A vehicle is supposed to continuous change of speed
especially in urban areas. Therefore the controller technique
should be as simple as possible as well as quick response. On
the other hand drive train controller should be robust due to
safety point of view. It should be able to handle some sort
of common electrical faults or mechanical shocks till fault
tolerant system takes an appropriate decision to handle the
fault at least up to the point for repairmen.
PWM technique is one of the most popular speed control
techniques for BLDC motor. In this technique a high frequency
chopper signal with specific duty cycle is multiplied by switching
signals of VSI. Therefore it is possible to adjust output
voltage of inverter by controlling duty cycle of switching
pulses of inverter. A three phase VSI with MOSFET switches
is modeled in MATLAB/SIMULINK to supply BLDC motor.
Duty cycle of chopping signal can be determine by a PI
controller [12] or toggle between two predefined duty cycles
(high and low duty cycle) [5]. In this paper a PI controller is
designed to control duty cycle of chopping signal according
to speed error. An embedded MATLAB function is written to
generate high frequency (10 KHz) according to the defined
duty cycle value. There are three different methods to apply
chopping signal to the VSI. These three modes are: apply
chopping signal 1- to upper side switches of each leg of
inverter, or 2- to lower side switches of inverter, or 3- to all six
switches at the same time. In each mode applied voltage to the
motor is different, thus output characteristics of BLDC motor
is also different. From now on numbers are used to specify any
of switching modes. In this paper behavior of BLDC motor
for various switching modes in normal and critical condition
are compared and discussed.