Simulation of Pulse Amplitude Modulation based Voltage Control Technique for the Speed Control of BLDC Motor using Class-E Load Resonant Converter

Efficient power conversion systems are the major concerns of any industry. Thus BLDC motors gained higher priority in wide range of applications. Speed control of BLDC motors are very crucial in this attire .Many of the power electronic circuits used for this are subjected to very high switching losses ending up with reduced overall system efficiency. In this paper speed control of BLDC motor is achieved using a Class-E load resonant converter as the front end converter. Simulation of the proposed model was done in MATLAB/SIMULINK. Keywords—BLDC motor,Class E load resonant converter, Converter topologies.


INTRODUCTION
Permanent magnet brushless DC motors gained its full fledged acceptance in many research fields such as aerospace, traction, instrumentation etc due to key advantages like high torque, powerfactor, efficiency, simple control technique, low cost of maintenance. The elimination of brushes as well as the connection of windings to the control circuits makes it more suitable for high reliable, robust operations. Unlike a brushed DC motor, the commutation is controlled electronically. Hall effect sensors embedded into the stator provides exact rotor positions at every instant. The hall effect [3] signals provide switching pulses to the inverter providing ac signal to drive the motor.
DC-DC converters are used in a wide range as the front end converter to obtain a regulated dc output voltage. Even though the high rate of switching losses and the increased device stress urged the need of a high power density high efficient converter which can only be achieved using soft switching techniques. A resonant converter is the most suited choice. Phase shift modulated and load resonant types are the major categories of conventional resonant converters. Load resonant converters can be further divided into series, parallel, series-parallel resonant converters. Among which the seriesparallel converter, LLC resonant converter [1]can be used for constant output voltage power supplies and is a good choice for high switching frequency applications without the effect of generated electromagnetic interferences.
In this paper, the speed control of BLDC motor fed by a LLC resonant converter is and its mode of operation is explained and simulated using MATLAB/SIMULINK.

II. SPEED CONTROL OF BLDC MOTOR
There are three types of speed drives for a BLDC motor. Constant speed drive where the driving motor runs more or less the fixed speed. Discrete speed settings can be obtained using Multi speed drives.
Step less change in speed and multi speed ranges can be together performed with Variable speed drives depending on the application. The motor speed can be varied up to 150 % of rated speed with a penalty on torque value. The motor can deliver a high torque maximum upto peak torque as long as the speed torque curve is followed, mainly during the applications that require frequent starts and stops and frequent reversals with load on the motor. Hall sensors are used to provide the exact switching pulses to the inverter thereby controlling the motor speed.

A. LLC resonant converter
Among the classifications of resonant converters LLC resonant converter has the most potential since it can achieve zero voltage switching in operating load range and zero current switching for rectifier diodes under some desired conditions. The LLC resonant tank is excited using the square wave generated from the switching circuit. Transformer and rectifier circuit scales and rectifies the resonant sinusoidal current which is filtered by the output capacitors and dc voltage is obtained.
Depending on the load current conditions and the applied voltage, LLC resonant converters can operate in three modes Power delivery operation, occurring twice in a switching cycle when the resonant tank is excited by positive and negative voltages during the first and second half of the switching cycle respectively. The difference in the resonant and magnetizing currents flows through transformer rectifier assembly delivering power to the load.
Freewheeling operation, occurs only when the resonant current reaches magnetizing transformer current, following a power delivery operation. The transformer secondary current reaches zero thereby disconnecting the secondary side rectifier hence the magnetizing inductor is made free to enter the resonance with the resonant inductor and capacitor.

B. BLDC motor drive
A voltage source inverter is used to fed a BLDC motor. The exact position of rotor at every instant is obtained with the help of hall sensors mounted on stator. From the hall signals the switching logic is determined to provide triggering pulses to the inverter switches.
The actual and reference is compared to produce an error signal, which is then fed to the PI controller, generated control signal will then modify the speed accordingly. The switching pulses for the LLC resonant converter are obtained using pulse amplitude modulation method. Figure 3 depicts the simulation circuit for the LLC resonant converter fed BLDC motor drive .The total simulation time was 1.2 seconds and the input supply was a dc source of 24 V.  The speed control of a BLDC motor fed by a LLC resonant converter was modeled using MATLAB/SIMULINK. It can be concluded that use of LLC resonant converter as the front end dc-dc converter is the most suited choice since it reduces the switching losses and device stress to a great extent. High power density and high efficiency can hence be achieved without fearing the effect of electromagnetic interference/ noices. Thus LLC resonant converters are widely used in battery chargers, renewable energy and transportation systems. Since the back emf of the motor is directly proportional to rotor speed and field strength of the motor, any change in one of the parameter cause the other to modify accordingly.