Week-7 Challenge: DC Motor Control

Q1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Ans : H- Bridge Circuit 

In general H- Bridge is a rather simple circuit, containing four switchimg elements, with the load at the center, in an H-like configuration

The directions of current flow in H-bridge circuit are shown in following dia.

• During positive half cycle switches Q1 and Q4 are conducting and motor rotates in forward directions. The diode D1 & D4 are turn off conditions.
• During negative half cycle switches Q2 and Q3 are conducting and motor rotates in reverse directions. The diode D2 & D3 are turn off conditions.
• we cannot trigger the two switches of the same leg cause its creates low resistance path and short circuit occur.

Simulink model of Speed control of a DC motor using BJT H-bridge

Pulse Width Modulation 

Pulse Width Modulation is a technique for getting analog results with digital means. Digital control Creates a sqaure wave, a signal switched between on and off. This on-off pattern can simulate voltages in between full-on and off byy changing the portion of the time the signal spends on versus the time that the signal spends the duration of "on time" is called pulse width. To get varying analog values, we change or modulates, that pulse width. In this module 75% Pulse width taken and time period is 2e-3sec.

Output Result of current

The waveform shows that 0 to 0.5 sec IGBT current are zero and diode current is occure that means IGBT off and Diode is on conditions. After 0.5 to 1sec diode current are zero and IGBT current is occure that means diode off and IGBT is on conditions.

Output waveform of armature current, speed and load torque

• From the waveform speed is start from zero &  reaches uptp max. speed 1000RPM and after 0.5sec it will decreses to the -1000RPM
• The armature current start from zero to rise up to 40 to 45 A & decreses after immediately and reaches to near zero. After 0.5sec it will goes to negative direction nearer to zero value.
• The load torque follows the speed wavform in positive as well as neagtive direction

B. Comment on the armature current shoot-up from the scope results.

we notice that at the time of starting armature current a spike in the current flow and also in the negative directions of the current flow. if you want to reduce sudden rise in armature current waveform we have to reduce duty cycle 75% to 50% and see result as shown in following results

C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Comparisionof H-bridge and Four Qundrant chopper

Sr. No.	H-bridge Model	Four Qundrant chopper
1	The motor operates only two mode i.e. forward and reverse mode	The motor operates in all four quadrant i.e. forward motoring, reverse motoring, forward braking and reverse braking
2	Vechicle control is less as compare to Four qundrant chopper	Sophisticated control vehicle using Four qundrant chopper
3	Regenrative braking not possible	Regenrative braking possible
4	Signal are continuous	Signal are discrete

Q2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )

Ans : A 2-quadrant chopper has two types

• Type A Chopper (Class C chopper)
• Type B Chopper (Class D chopper)

Here only Type B Chopper (Class D chopper) has been explained

Simulink Model of 2-quadrant chopper

The inductor load value taken as L = 10e-3, DC Voltage Source = 12V

Pulse generator value are 

Amplitude = 10, Time Period = 0.02sec, Pulse width = 50%, Phase delay = 0

Run the model for 1Sec

Output Results

The output waveform of current is in positive direction and the voltage waveform in both positive as well negative directions occurs.

Reference Book : Power Electronics by K.B. Khanchandani

Q3. Explain in a brief about operation of BLDC motor.

Ans : 

What is Brushless DC motor?

Brushless DC electric motors also known as electronically commutated motors (ECMs, EC motors). 

Primary efficiency is a most importent feature for BLDC motors. Because the rotor is the sole bearer of the magnets and it doesn't require any power. i.e. no connections, no commutator and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ along with controllers, rotary encoders or a Hall sensor.

Construction of Brushless DC motor

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy. 

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit.  A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.

The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “Inrunner” style.

• Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.

• In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out runner style, but is capable of spinning at very high rpm.

Working Principle of Brushless DC motor

BLDC motor works on the principle similar to that of a Brushed DC motor. The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the BLDC motor, the current carrying conductor is stationary  and the permanent magnet is moving.

When the stator coils get a supply from 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.

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 which causes the motor to rotate.

Advantages of Brushless DC motor

• Less overall maintenance due to absence of brushes
• Reduced size with far superior thermal characteristics
• Higher speed range and lower electric noise generation.
• It has no mechanical commutator and associated problems
• High efficiency and high output power to size ratio due to the use of permanent magnet rotor
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
• Long life as no inspection and maintenance is required for commutator system
• Higher dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Low noise due to absence of brushes

Limitations of Brushless DC motor

• These motors are costly
• Electronic controller required control this motor is expensive
• Requires complex drive circuitry
• Need of additional sensors

Applications of Brushless DC motor 

Brushless DC motors (BLDC) use for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments etc. 

• Computer hard drives and DVD/CD players
• Electric vehicles, hybrid vehicles, and electric bicycles
• Industrial robots, CNC machine tools, and simple belt driven systems
• Washing machines, compressors and dryers
• Fans, pumps and blowers.

Reference Link : https://robu.in/brushless-dc-motor-working-principle-construction-applications/