Week-7 Challenge: DC Motor Control

AIM -

1. To explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

2. To develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

3. To explain the operation of BLDC motor.

OBJECTIVE 1 (A) - To explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Simulink Model -

Block Explanation -

• In this model 4 switches (Q1,Q2,Q3,Q4) are used , and they are arranged in H shape so it is called H Bridge .
• DC motor block is used which is connected to 300 V battery supply .
• Form block is used to recieve signals from Goto block with the specified tag .
• Pulse generator block is used for generating the pulse .
• Scope (Current) is used to give the output of IGBT Current and Diode Current .
• Another Scope is used to give the output of Speed , Armature current and Load Torque.

Working -

• When DC motor operate the Armature voltage will vary that is if Duty cycle will decrease the armature current will decrease.
• Switch Q1 and Q4 are Forward Motoring and Forward Braking , when both the switch will close than current will start to flow from source .
• Switch Q2 and Q3 are Reverse motoring and Reverse Braking , when both the switch will close than current will start to flow from source .
• All switch contains Emittor , Collector and Gate .
• When switch becomes close than current will be able to flow and voltage will not appear across the switch .
• All 4 switch (Q1 ,Q2,Q3,Q4) will not operate at same time otherwise short circuit will appear .

(B) Armature Current Shoot up from scope result .

Conclusion -

Armature current changes when we change the duty cycle. If we reduce the duty cycle than the armature current will reduce.

(C) - The Four-Quadrant Chopper DC Drive (DC7) block.

Comparision Of DC7 Block and H- Bridge Block -

DC7 Chopper Block -

• Simulation takes less time than H-bridge block.
• Regenerative braking is possible .
• Higher efficiency .
• Used in automobile to control DC motor speed and direction .
• To change the direction of motor control current signals are interrupted one another.

H- Bridge Block -

• Time requires for simulation will be more.
• Regenerative braking is not possible , there will be frictional breaking .
• To change the direction of motor polarity of current is changed.
• Used in Robotics technology .

OBJECTIVE 2 - To develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

Blocks Used -

• Mofset Switch (Metal Oxide Semiconductor Field Effect Transistor) are higher speed switching device which operates on higher frequency.
• Diode Block - A diode is a semiconductor device that essentially acts as a one-way switch for current. It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction.
• Current Measurement Block - The Current Measurement block is used to measure the instantaneous current flowing in any electrical block or connection line.
• Voltage Measurement Block - The Voltage Measurement block measures the instantaneous voltage between two electric nodes.
• Power gui Block -

  The powergui block allows us to choose one of these methods to solve your circuit:

  • Continuous, which uses a variable-step solver from Simulink

  • Discretization of the electrical system for a solution at fixed time steps

  • Continuous or discrete phasor solution

  The powergui block also opens tools for steady-state and simulation results analysis and for advanced parameter design.

WORKING -

• In this kind of chopper MOFSET switch is used in which average voltage will always be positive .
• When the Mofset is ON the current will from source to load ,also gate terminal is connected to the pulse generator which generates square wave pulses at regular intervals.
• From Diode the current will flow and inductor will stores the energy .
• When Mofset will OFF the Diode will start conducting and output volt will become negative.
• When MOFSET 1 will turn ON the current will completes its path through Diode 1 .
• The result will stored in the scope .
• For Regenerative Braking and Motoring this type of chopper configuration is used .

RESULT -

OBJECTIVE 3 - To explain the operation of BLDC motor.

What Is Brushless DC Motor -

• A Brushless DC Electric Motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply.
• It uses an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows.
• The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor.

Construction Of BLDC Motor -

• In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature winding 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.
• Inrunner - 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 -

Lorentz Force Law - 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.

Working -

• When the stator coils get a supply from source, it becomes electromagnet and starts producing the uniform field in the air gap.
• 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.