A magnetic stirrer is an indispensable instrument in any laboratory setting. Commercially available stirrers are costly and have a defined life-span. The construction of a low-cost magnetic stirrer based on pulse width modulation is described. The design reported uses mainly recycled parts which results in a robust and efficient device at a very reasonable cost of ∼US$17.50, which will be particularly attractive in developing countries. The project requires a multidisciplinary approach and can be utilized as a teaching exercise.

Robust, Efficient, and Economical Magnetic Stirrer: A Device Based on Pulsed Width Modulation, Built Using Mainly Recycled Parts

The prototype of the rice drying machine is a device used for rice dryers that are not weather dependent. The driver of this prototype rice drying machine uses ½ HP induction motor as its main mover. To be able to adjust the rotation of the induction motor using Variable Frequency Drive (VFD) brand LS iC5 with 1 HP power. The rotation needed to rotate the load can be done by changing the frequency and voltage so that the rotation speed of the induction motor can be adjusted as needed. From the testing of a no-load induction motor with a frequency adjustment range from 5 Hz to 13 Hz, the induction motor speed is 142.8 rpm (rotation per minute) to 375.3 rpm. The measured voltage is 76.5 V to 116.7 V and the measured current is 2.5 A to 1.6 A. From the results of the test of a no-load induction motor it can be concluded that the ratio of frequency to voltage is directly proportional while the ratio of frequency to current is inversely proportional. In testing induction motors with a frequency adjustment range from 5 Hz to 9 Hz, they produce the same conclusions as to the induction motor load testing. VFD is certainly appropriate to be used as a speed regulator as needed.

Analysis of the implementation of three-phase Variable Frequency Drive on the prototype of a rice drying machine

In this paper investigates the influence of high frequency switching of power transistors of an autonomous voltage inverter on the distortion of the output quasi-sinusoidal signal. To solve the discovered problem, a modified autonomous voltage control algorithm is proposed, based on the use of different pulse-width modulation frequencies in one half cycle. The standard two-level sinusoidal pulse-width modulation is formed as a result of the intersection of the modulating sinusoid with the reference sawtooth signal. In classical pulse-width modulation, the ratio of the frequency of the reference signal to the modulated frequency is always constant and equal to an integer, i.e., this pulse-width modulation is synchronous. An algorithmic solution is proposed for the formation of a modified sinusoidal pulse-width modulation, when the frequency of the reference sawtooth signal during one period of the modulated sinusoid changes twice from a smaller value to a larger one and vice versa.

Microprocessor Control System for a Three-Phase Voltage Inverter with a Modified Algorithm

We consider the calculation of modulation power losses in a system "induction motor–inverter" for various pulse width modulation (PWM) methods of the supply voltage. Presented values of modulation power losses are the result of modeling a system "DC link - two-level three-phase voltage inverter - induction motor - load". In this study the power losses in a system "induction motor - inverter" are computed, as well as losses caused by higher harmonics of PWM supply voltage, followed by definition of active power consumed by the DC link for a specified value mechanical power on the induction motor shaft. Mechanical power was determined by the rotation speed and the torque on the motor shaft in various quasi-sinusoidal supply voltage PWM modes. These calculations reveal the best coefficient of performance (COP) in a system of a variable frequency drive (VFD) with independent voltage inverter controlled by induction motor PWM.

https://iopscience.iop.org/article/10.1088/1755-1315/90/1/012042/pdf

Evgenii Ten

Papers

Effect of different methods of pulse width modulation on power losses in an induction motor

Microprocessor Control System for a Three-Phase Voltage Inverter with a Modified Algorithm