Project 1

Objective: To model a fan-type load in Simulink and control its speed by using the PWM method.

Abstract: This model shows how to use the Controlled PWM Voltage and H-Bridge blocks to control a fan load using a DC motor. The DC Motor block uses manufacturer datasheet parameters, which specify the motor as delivering mechanical power at a rated and no-load speed when run from a DC supply. Hence if the PWM reference voltage is set to its maximum value, then the motor should run at the no-load speed. If it is set to a minimum value, then it should run at the rated speed. The Simulation model parameter is set to Averaged for both the Controlled PWM Voltage and H-Bridge blocks, resulting in fast simulation. To validate the averaged behavior, we can change the Simulation mode parameter to PWM in both blocks.

Keywords: PWM, H bridge, DC Motor

Introduction:

This paper presents the work for the design of the fan speed control system using Pulse Width Modulation by establishing a connection between an H bridge circuit and a Dc motor. A fan-type load equation has been used to analyze the output results instead of a physical model of a fan in Simulink. The speed of the fan is varied according to the input voltage using the PWM technique.

By changing or modulating the timing of these pulses the speed of the motor can be controlled. A stepwise change in the speed variation is given and the full load speed is observed. In this model, all input ports of the H-Bridge block except the PWM port are connected to the ground. As a result, the H-Bridge block behaves as follows:

• When the motor is on, the H-Bridge block connects the motor terminals to the power supply.
• When the motor is off, the H-Bridge block acts as a freewheeling diode to maintain the motor current.

Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. Digital control creates a square wave, a signal switched between ON and OFF. This ON-OFF pattern can simulate voltages between full-ON and OFF by changing the portion of the time the signal spends versus the time that the signal spends the duration of "ON time" which is called the pulse width. To get varying analog values, we change or modulate, that pulse width.

The torque-speed curve of a load is a plot showing its torque requirement on the y-axis versus the speed on the x-axis. It’s the torque requirement of the load from zero speed to full speed. It is application/load specific and varies with different load types. Loads like centrifugal pumps and fans follow a quadratic speed torque curve, where torque varies square times the speed. These are also called constant torque loads or applications.

Model Diagram in Simulink:

Model Description:

The DC Motor block uses manufacturer datasheet parameters, which specify the motor as delivering 10W mechanical power at 2500 rpm and no-load speed as 4000 rpm when run from a 12V DC supply. H-bridge block represents a motor drive. The block can be driven by the Controlled PWM Voltage block in Averaged mode.

PWM Input:

Result:

Speed-Torque characteristics (XY Graph):

Conclusion:

A PWM-controlled DC motor using an H bridge circuit was modeled in Simulink and results were analyzed. The output plots justified the theory discussed. The load torque characteristics of a fan are obtained. The full speed was observed for a full voltage of 12V from the DC motor.