INTERNATIONAL JOURNAL FOR RESEARCH & DEVELOPMENT IN TECHNOLOGY

High Speed Brushless DC Motors (BLDC) with power ratings of a few tens of watts to a few kilowatts and rotational speeds in the range of 1000 to 35000 revolutions per minute are employed in blowers and compressors. The trend toward higher rotational speeds is mainly driven by the need of a higher power density in emerging applications. The main challenges in these drives are to ensure low losses and high efficiencies while operating in wide speed ranges. In this paper, a new BLDC motor drive with a hybrid switching mechanism employing pulse-width modulation (PWM) control at low-speed region and pulse-amplitude modulation (PAM) control at high-speed region is introduced with an improved voltage gain technique at the front-end side for high speed applications. The simulation results are also discussed and analyzed in order to verify the effectiveness of the proposed design. INTRODUCTION Recently, there has been an increasing trend in requests for Brushless DC motors for high speed applications of BLDC motors especially in compressors and blowers. But these applications demands low losses, low cost and maintenance, and compactness. The trend toward higher rotational speeds is mainly driven by the need of a higher power density in emerging applications. Thus to assure the above mentioned characteristics, an appropriate driving method need to be designed while operating for wide speed control ranges. For BLDC motor drives, the inverter can be controlled through either PWM with a fixed DC-link voltage or PAM with controlled DC-link voltage amplitude. The pulse width modulation PWM-controlled BLDC motor drive system is the most commonly applied technique for driving a motor inverter bridge. Unfortunately, the PWM techniques can induce serious current and torque ripples for the ironless stator motor. In addition, the high-frequency and large-range current ripple will inevitably increase the copper and iron losses at high speed. The comparative results using conventional PWM techniques have been fully explored here to highlight the advantages of PAM techniques at high speed operations [1]. At low speed regions, say upto 700 rpm, BLDC motors are driven with pulse-width modulation (PWM) inverters and by ideally applying sinusoidal three-phase currents. Due to the high fundamental frequency of the currents, the PWM frequency has to be increased, or an ac filter is employed in between the inverter and the motor. At speeds above 700 rpm, pulse-amplitude modulation (PAM) inverters are suggested. In those converters, the DC-DC buck-boost converter can be used to vary the dc link voltage (voltage at the inverter input) and to control the dc link current, whereas the inverter switches at the fundamental frequency of the machine, limiting the switching losses and limiting the need for a highbandwidth current control. This paper organized as follows. The current section presents an introduction and gives the objective of the work. Section II explains the literature review on the performance comparison between PWM & PAM techniques and the need of a hybrid switching technique. Section III is focused on the circuit description and block diagram explanation of the existing and the proposed motor drive system. Section IV is based on the control theory of the PAM and PWM control modes and proposes a hysteresis comparator to switch between the two modes. The final section deals with the simulation model and results of the proposed design. II. OPERATIONAL COMPARISON BETWEEN PWM AND PAM TECHNIQUES In most of the BLDC drive systems, the three phase inverter is controlled by the PWM techniques by keeping the DC-link Paper Title:Simulation of Hybrid Switching Technique for driving High-Speed BLDC Motor ISSN:-2349-3585 |www.ijrdt. org 11 voltage constant. Furthermore, the PWM control is extensively employed in low and high power BLDC motor drives. The motor performance is these drives are decided by the commutation control techniques. Commutation control of inverter provides smooth rotation of the stator magnetic field. The interaction between stator magnetic field and the permanent magnet gives the torque. The typical PWM control contains six different PWM techniques namely, H-PWM_L_ON, H_ON_L_PWM, PWM_ON, H_PWM_L_PWM, and PWM_ON_PWM respectively [2]. For instance, when H_PWM_L_ON is employed, the high-side power device is controlled by the PWM chopper signal every consecutive 120° in a fundamental period. Meanwhile, the low side of the same leg control signal is shifted by 180° without the PWM chopper signal, as compared to its high-side one, to clamp the related inverter output to the negative dc-link rail. The control signals for the other two legs are shifted by 120°and 240°, respectively. But ineffectively, these PWM techniques have many drawbacks while operating at high-speed regions. It includes: a) High frequency harmonic waves with the frequency of (kωs ± nωs) are introduced because of the PWM control; these harmonics will increase both the copper loss. b) PWM control decreases the motor power factor (large lag angle (δ); when the active power is constant, the small power factor will increase the reactive power, apparent power, and motor loss. c) The large lag angle (δ) will result in a large commutation delay it has a significant influence on the phase current and drive performance at high speed. In order to counteract these effects, another popular control method for the BLDC motor is the PAM control. For PAM control, 120°commutation control, i.e., the so-called six-step mode is generally used and the dc-link voltage can be adjusted according to the error between the speed and its reference. The variable dc voltage can be adjusted by a DC-DC converter, and the inverter legs participate only in commutation instead of involved in modulation. The PAM control is superior to the PWM control for providing good performance at high speed. Under PAM control, following are the merits: a) Power factor is far higher. b) Less harmonic content. c) Less torque ripples. d) Higher efficiency. e) Low motor losses. Although the PAM control for the BLDC motor is predominant to the PWM control as discussed above, still the PAM control is not employed in the whole speed range for the following two reasons: a) Wide-ranging dc-link voltage fluctuation can limit its application at low and medium speed. b) At low speed, it is unable to produce a small enough dc voltage to achieve the PAM control due to the small phase-winding resistance and phase-winding inductance. Combining the merits and demerits of both PAM and PWM control, it is being inferred that both the methods are imperative for driving a high-speed BLDC motor [3]. PAM control is the main drive method, since the high speed region is the most common working range. Thus the aim of the PWM control is merely to guarantee that the high-speed motor can be accelerated to a high speed. It is just an auxiliary drive method. The operation time of the PWM control is far shorter than that of the PAM control, thus mitigating the disadvantage of the PWM control. The transition process between PWM and PAM can be achieved by a hysteresis comparator [4]. III. PROPOSED MOTOR DRIVE SYSTEM The conventional BLDC drive circuit structure block diagram and topology are shown in Fig.1 and Fig.2. It consists of a single phase AC supply source, a single-phase uncontrolled rectifier along with an output filter, a DC link and a three-phase inverter on the motor supply side. Existing motor drive system works in PWM control mode for both low and high speeds. Thus the actual speed is sensed and it is compared with the reference speed. The error in speed is processed through a PI (Proportional Integral) controller. PI controller along with the saturation block limits the error within the prescribed limits. It is then compared with a carrier wave of suitable high switching frequency to generate PWM gate pulses for the inverter switches. Thus, by varying the inverter switches duty cycle, the PWM controlled BLDC motor drive can be implemented. Paper Title:Simulation of Hybrid Switching Technique for driving High-Speed BLDC Motor ISSN:-2349-3585 |www.ijrdt. org 12 Fig.1: Conventional BLDC drive structure But the problem with such a drive circuit structure is that, the output voltage obtained from the single-phase uncontrolled rectifier, is more than 30 V. Though the three-phase inverter is controlled by the PWM scheme for reducing the voltage, the start-up current is still far larger than the rated current. Moreover, the inverter cannot work anymore, as the protection circuit blocks the inverter due to the large current. So this common circuit structure is not employed. Fig.2: Topology of existing motor drive system Also PWM controlled BLDC motor drives suffers from various drawbacks as discussed earlier. Due to the PWM period interruption caused by the commutation and limitation of the resolution of PWM generator, more torque ripple can be produced in the performance of PWM-controlled BLDC motor drives. This phenomenon will become more serious when the motors work at high speed. Moreover, it will lead to high motor loss. In order to satisfy the commutation control of high-speed BLDC motor in a widespread range and improve the motor efficiency, a hybrid drive method combining PWM and PAM is proposed. When the motor runs at low speed, the PWM control is adopted with a fixed dc-link voltage. When the motor speed reaches a threshold value (the enough backEMF can counteract the dc-link voltage); the PAM control works with an adjustable dc-link voltage. Fig.3 shows block diagram of proposed motor drive system. When the motor starts, the initial DC voltage of the three-phase inverter should be smaller enough to limit the startup current, as the backEMF is too small to counteract a large dc voltage at the very low speed. Therefore, a suitable drive circuit should be first selected to reduce the initial dc voltage and guarantee the safe moto