Project 1
AIM: To model a fan type load in Simulink and to control its speed by using the PWM method.
Solution :- This model will give an idea about electro-mechanical system where electrical control of Dc motor will result in speed variation of fan by applying PWM technique for varying voltage and current through the motor we can implement the variation of speed and torque at output .By controlling the electrical parameter of motor result in change of speed of Fan. Fan connected on shaft of DC motor.
Pulse Width Modulation:
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 in between full-on and off by changing the portion of the time the signal spends on versus the time that the signal spends the duration of "on time" is called the pulse width.
Permanent Magnet Motor
A DC motor consist basically of two parts, the stationary body of the motor called the “Stator” and the inner part which rotates producing the movement called the “Rotor”. For D.C. machines the rotor is commonly termed the “Armature”.
Generally, in small light duty DC motors the stator consists of a pair of fixed permanent magnets producing a uniform and stationary magnetic flux inside the motor giving these types of motors their name of “permanent-magnet direct-current” (PMDC) motors.
Generally, in small light duty DC motors the stator consists of a pair of fixed permanent magnets producing a uniform and stationary magnetic flux inside the motor giving these types of motors their name of “permanent-magnet direct-current” (PMDC) motors.
The motors armature consists of individual electrical coils connected together in a circular configuration around its metallic body producing a North-Pole then a South-Pole then a North-Pole etc, type of field system configuration.
As the armature rotates electrical current is passed from the motors terminals to the next set of armature windings via carbon brushes located around the commutator producing another magnetic field and each time the armature rotates a new set of armature windings are energised forcing the armature to rotate more and more and so on.
So the rotational speed of a DC motor depends upon the interaction between two magnetic fields, one set up by the stator’s stationary permanent magnets and the other by the armatures rotating electromagnets and by controlling this interaction we can control the speed of rotation.
The magnetic field produced by the stator’s permanent magnets is fixed and therefore can not be changed but if we change the strength of the armatures electromagnetic field by controlling the current flowing through the windings more or less magnetic flux will be produced resulting in a stronger or weaker interaction and therefore a faster or slower speed.
Then the rotational speed of a DC motor (N) is proportional to the back emf (Vb) of the motor divided by the magnetic flux (which for a permanent magnet is a constant) times an electromechanical constant depending upon the nature of the armatures windings (Ke) giving us the equation of: N ∝ V/KeΦ.
PWM Method
As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.
The power applied to the motor can be controlled by varying the width of these applied pulses and thereby varying the average DC voltage applied to the motors terminals. By changing or modulating the timing of these pulses the speed of the motor can be controlled, ie, the longer the pulse is “ON”, the faster the motor will rotate and likewise, the shorter the pulse is “ON” the slower the motor will rotate.
In other words, the wider the pulse width, the more average voltage applied to the motor terminals, the stronger the magnetic flux inside the armature windings and the faster the motor will rotate
Model Diagram in Simulink
Model Explanation:
Torque exerted by fan represented by T
Load torque i.e. T proportional to square of speed i.e. w^2
T= K . w^2
By varying the electrical parameter of motor i.e. voltage and current we are controlling load torque or load speed.
For fan load we don’t have any block to represent a fan so we are representing by a subsystem
to the right side there is w and it is multiplying by product block then it is become w^2
Then gain block is used that is the constant of proportionality 8.8e-8 Unit of this gain block is N*m/(rad/s)^2
Output of this subsystem is fed to the Aerodynamic block. Actual name of this block is Ideal torque source block. This ideal torque source block is going to apply torque on the shaft of DC motor.
For getting the speed we used Motion sensor.Value of the motion sensor w is given to the subsystem w and the value of the torque is given to the aerodynamic block and therefore completing the loop.
Controlled voltage source is connected to the controlled PWM voltage and H-bridge is connected through PWM voltage.
Pulse generator block used as input to the controlled voltage source.
Parameters of H- bridge:
Parameters of PWM controlled :
DC motor configuration :
Output Speed - Torque characteristics :
Output in Scope :
Conclusion :
From the above model and from the assumed parameters we found that the speed, Torque & angular displacement of a fan is shown in scope waveform matches with desired response.
A PWM-controlled DC motor using an H bridge circuit was modelled in Simulink and results were analysed. The output plots justified the theory discussed. The load torque characteristics of a fan is obtained.