Week-7 Challenge: DC Motor Control
1a a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
1b.Comment on the armature current shoot-up from the scope results.
‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Compare it with the H-bridge model.
Hbridge
Only Two quadrants of operation are possible. They are Forward motoring and reverse motoring,Higher armature current ripple,Less complex, Simulation is continuous and simulation time is more, Widely Used in Robotics
Four-Quadrant Chopper DC Drive
3.A brief about operation of BLDC motor.
A Brushless DC Electric Motor (BLDC) is an electric motor powered by a direct current voltage supply and commutated electronically instead of by brushes like in conventional DC motors. BLDC motors are more popular than the conventional DC motors nowadays, but the development of these type of motors has only been possible since the 1960s when semiconductor electronics were developed.
Working
When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.
Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction.
Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.
Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.
Advantages of BLDC Motors
Efficiency, as these motors can control continuously at maximum rotational force (torque). Brushed motors, in contrast, reach maximum torque at only certain points in the rotation. For a brushed motor to deliver the same torque as a brushless model, it would need to use larger magnets. This is why even small BLDC motors can deliver considerable power.
Controllability, BLDC motors can be controlled, using feedback mechanisms, to delivery precisely the desired torque and rotation speed. Precision control in turn reduces energy consumption and heat generation, and—in cases where motors are battery powered—lengthens the battery life.
high durability and low electric noise generation, thanks to the lack of brushes. With brushed motors, the brushes and commutator wear down as a result of continuous moving contact, and also produce sparks where contact is made. Electrical noise, in particular, is the result of the strong sparks that tend to occur at the areas where the brushes pass over the gaps in the commutator. This is why BLDC motors are often considered preferable in applications where it is important to avoid electrical noise.