Week-7 Challenge: DC Motor Control

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

MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge is shown below,

The Simulink model consists of the following blocks:

DC Motor

• The motor is modeled using armature resistance (Ra), armature inductance (La), and back EMF (Eb).
• The equation governing the motor is: Va=Eb+IaRa+LadIadt
• where:
• Va ​ is the applied armature voltage,
• Eb=Kew is the back EMF,
• Ia is the armature current,
• Ra and La are armature resistance and inductance, respectively.

BJT H-Bridge

• The H-Bridge consists of four BJTs arranged in an H-configuration.
• By controlling the transistors, the polarity of voltage applied to the motor can be changed.
• The H-Bridge enables:
• Forward rotation
• Reverse rotation
• Braking
• Freewheeling operation

PWM Speed Controller

• The motor speed is controlled using Pulse Width Modulation (PWM).
• A PWM signal is applied to the base of the BJTs to regulate the average voltage supplied to the motor.
• A Proportional-Integral (PI) Controller is often used to adjust the duty cycle of the PWM signal.

Power Supply

• Provides the required DC voltage for the H-Bridge.
• Usually, a constant DC voltage is used (e.g., 12V or 24V).

Working Principle

1. PWM Control

o The input speed reference signal is compared with the actual motor speed.

o A PI controller generates the required PWM duty cycle.

The PWM signal drives the BJTs, switching them on and off.

Motor Operation Based on H-Bridge Switching

Forward Rotation: Two BJTs conduct to apply positive voltage to the motor.

Reverse Rotation: Opposite BJTs conduct, reversing the voltage polarity.

Braking Mode: Both upper or lower BJTs conduct together, short-circuiting the motor.

Freewheeling Mode: The motor keeps running due to inertia when switches are off.

Applications

• Small electric vehicles and robotics.
• Industrial automation for conveyor belt control.
• Home appliances using DC motors.
• Battery-powered motor control applications.

The IGBT Current

Initially (t < 0.5s): The IGBT current is zero, indicating that the transistor is off or that current is flowing through a different path (e.g., freewheeling diodes).

At t ≈ 0.5s: The IGBT starts conducting, and the current rapidly increases, reaching a peak. This suggests a sudden increase in power delivered to the motor.

After t ≈ 0.5s: The current gradually decreases, showing an oscillatory pattern. This could be due to:

- Motor reaching steady-state operation.

- PWM switching causing ripple.

- The motor’s back EMF opposing the current increase.

The Diode Current

Initially (0 < t < 0.2s): The diode current is high and decreases exponentially. This suggests that the freewheeling diodes are carrying current while the IGBT is off.

After t ≈ 0.2s: The diode current reduces to near zero, meaning that the power transistors are now controlling most of the current flow.

Interpretation & Possible Causes

The waveforms suggest the operation of a PWM-driven H-Bridge with motor freewheeling and switching transitions.

- Diode conduction at the start -- Indicates motor is regenerating energy.

- IGBT conduction rise -- Suggests a controlled startup or torque demand change.

- Oscillations in IGBT current -- Likely caused by PWM switching and motor dynamics.

----------------------------------------------------------------------------------------------------------------------------------------------

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

RPM, armature current and load torque

Motor Speed

• Initial rise (0s - 0.2s): The speed increases smoothly, indicating the motor accelerates as expected.
• Steady-state (0.2s - 0.6s): The speed remains constant at around 1000 rpm, showing that the motor reaches and maintains the desired operating point.
• Rapid decline (~0.6s - 0.8s): A sudden deceleration occurs, indicating that braking or a reversal is applied.
• Near zero speed after 0.8s: The motor stops completely, which suggests a shutdown, load removal, or braking event.

Armature Current

• High initial inrush current (~40A at t=0s): This is normal when starting the motor due to the low initial back EMF.
• Steady-state current (~5A after 0.2s): Once the motor reaches constant speed, the current stabilizes at a lower value, reflecting normal operation.
• Sharp negative current spike (~0.6s - 0.7s): This occurs during braking or speed reversal, where the motor acts as a generator and opposes the applied voltage.
• Final stabilization near zero (~0.8s onward): The motor is stopped, and no significant current flows.

Load Torque Gradual increase in torque (~0s - 0.2s):

The motor initially develops torque to overcome inertia and reach the required speed.

• Steady-state torque (~5 N·m between 0.2s - 0.6s): The motor maintains constant torque to drive the load.
• Torque drops (~0.6s - 0.8s): A sudden drop in torque occurs, likely due to braking or a command to stop the motor.
• Negative torque (~0.7s onward): The motor may be regenerating power (acting as a generator), or an external force is slowing it down.

The motor follows a controlled speed profile: acceleration, steady-state, and deceleration.

The armature current follows the expected pattern: high at startup, stable during operation, and negative during braking.

Torque behavior matches motor dynamics, with an increase at startup, stable running, and a drop when stopping

----------------------------------------------------------------------------------------------------------------------------------------------

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

• The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors.
• This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine.
• Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived.
• This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
• The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration).
• In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.
• The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field current is set to its steady-state value when a simulation is started.
• The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a constant DC voltage source. Armature current oscillations are reduced by a smoothing inductance connected in series with the armature circuit.

COMPARISON WITH H BRIDGE

	H BRIDGE	4 QUADRANT CHOPPER
1	Works in Open Loop system(Non- feedback mechanism).	Works in closed-loop system(feedback mechanism). The armature voltage is controlled by PI regulators
2	The motor is not separately excited	The motor is separately excited with a constant 150 V DC field voltage source
3	It cannot work in regenerative mode.	It can work on all 4 quadrants.
4	We need to change the polarity in order to make the motor move forward or backward	We need to interrupt the signals.
5	Simulation is continuous	Simulation is discrete
6	The simulation takes more time and is less efficient	The simulation takes less time and its more efficient

=======================================================================================================================

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

The above model is a 2nd -quadrant chopper modeled using Simulink software. The main components are :

• A pulse generator
• Current measurement
• Voltage measurement
• Diode
• MOSFET
• Vcope
• NOT gate
• Battery

The working of the same is given below:

• Type C chopper is obtained by connecting type –A and type –B choppers in parallel.
• We will always get a positive output voltage V0 as the freewheeling diode FD is present across the load.
• When the chopper is on the freewheeling diode starts conducting and the output voltage v0 will be equal to Vs.
• The direction of the load current i0 will be reversed. The current i0 will be flowing towards the source and it will be positive regardless of the chopper is on or the FD conducts.
• The load current will be negative if the chopper is or the diode D2 conducts.
• We can say the chopper and FD operate together as a type-A chopper in the first quadrant.
• In the second quadrant, the chopper and D2 will operate together as type –B chopper.

Simulation model:-

Output:-

Results:-

Motor Speed

• 0s - 0.2s: Speed increases as the motor accelerates.
• 0.2s - 0.6s: Speed remains stable (~1000 rpm).
• 0.6s - 0.8s: Rapid deceleration due to braking or reversal.
• After 0.8s: Motor stops completely.

Armature Current

• High initial current (~40A) at startup due to low back EMF.
• Stable current (~5A) during constant speed operation.
• Negative spike (~0.6s - 0.7s) during braking, showing regenerative action.
• Near zero after 0.8s, as the motor stops.

Load Torque

• Torque rises at startup (~0s - 0.2s) to drive the load.
• Stable torque (~5 N·m, 0.2s - 0.6s) during normal operation.
• Torque drops (~0.6s - 0.7s), then turns negative, indicating braking or regenerative mode.

=======================================================================================================================

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

• Brushless DC motors are common in industrial applications across the world.
• At the most basic level, there are brushed and brushless motors and there are DC and AC motors.
• Brushless DC motors,do not contain brushes and use a DC current.
• These types of motors are highly efficient in producing a large amount of torque over a vast speed range. In brushless motors, permanent magnets rotate around a fixed armature and overcome the problem of connecting current to the armature. Commutation with electronics has a large scope of capabilities and flexibility. They are known for smooth operation and holding torque when stationary.
• A Brushless DC motor has only two basic parts: the rotor and the stator. The rotor is the rotating part and has rotor magnets whereas the stator is the stationary part and contains stator windings.
• In BLDC permanent magnets are attached to the rotor and move the electromagnets to the stator. The high-power transistors are used to activate electromagnets for the shaft turns. The controller performs power distribution by using a solid-state circuit.

Working Principle and Operation of BLDC Motor

1. BLDC motor works on a principle similar to that of a conventional DC motor, i.e., the Faraday's law which states that whenever a current-carrying conductor placed in a magnetic field it experiences a force.
2. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the case BLDC motor, the current-carrying conductor is stationary while the permanent magnet moves.
3. When the stator coils are electrically switched by a supply source, it becomes an 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 a trapezoidal shape.
4. Due to the force of interaction between the electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
5. Consider the figure below in which the 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.
6. The permanent magnet rotor with North and South poles align with stator poles causing the motor to rotate.
7. Observe that the 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). In this way motor moves in a clockwise direction.

Advantages of Brushless DC Motor

The advantages of a BLDC motor are:

1. Brushless motors are more efficient as their velocity is determined by the frequency at which current is supplied, not the voltage.
2. As brushes are absent, the mechanical energy loss due to friction is less which enhanced efficiency.
3. BLDC motor can operate at high speed under any condition.
4. There is no sparking and much less noise during operation.
5. More electromagnets could be used on the stator for more precise control.
6. BLDC motors accelerate and decelerate easily as they are having low rotor inertia.
7. It is a high-performance motor that provides large torque per cubic inch over a vast sped range.
8. BLDC motors do not have brushes which make them more reliable, high life expectancies, and maintenance-free operation.
9. There are no ionizing sparks from the commutator, and electromagnetic interference is also get reduced.
10. Such motors cooled by conduction and no airflow are required for inside cooling.

Disadvantages of Brushless DC Motors

1. BLDC motor cost more than a brushed DC motor.
2. The limited high power could be supplied to the BLDC motor, otherwise, too much heat weakens the magnets and the insulation of winding may get damaged.

==================================================================================================================