Week-7 Challenge: DC Motor Control

Q1.A)

The H-bridge connects to a power supply at the top end and is grounded at the bottom end. Each of the four switching elements can be toggled on or off independently, with certain limitations.

While the load can vary, H-bridges are most commonly used with brushed DC or bipolar stepper motors. In this discussion, we concentrate on their application as brushed DC motor drivers.

Static Operation:

The H-bridge's basic operating mode is straightforward: activating Q1 and Q4 connects the left lead of the motor to the power supply, initiating forward motion. Conversely, activating Q2 and Q3 initiates reverse motion. However, simultaneously closing both Q1 and Q2 (or Q3 and Q4) creates a short circuit, known as 'shoot-through,' which can damage the bridge.

The MATLAB demo model simulates the control of a DC motor using a BJT H-bridge. It incorporates a preset DC motor model (5 HP, 24V, 1750 rpm) representing a fan-type load. The armature voltage can be varied from 0 to 240V based on the duty cycle.

Simulation Results:

The simulation reveals that the motor starts in the positive direction with a 75% duty cycle, transitioning to negative direction motion at t=0.5 sec. The scope displays motor speed, armature current, and load torque, with 'Currents' indicating currents flowing in BJT Q3 and diode D3.

Q1.B)

l The armature current graph exhibits sudden spikes at 0 seconds and 0.5 seconds.

l These spikes correspond to the transition from positive to negative direction of rotation.

l The spikes are attributed to transient switching occurring in the circuit.

l As current is already flowing in the circuit, changing switches after 0.5 seconds adds to the current surge.

l Reducing the duty cycle from 75% to 50% significantly reduces the current surge.

l Hence, changing the duty cycle effectively mitigates the armature current spike.

Q1.C)

Four-Quadrant Chopper DC Drive (DC7) Block:

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.

l Represents a four-quadrant chopper drive for DC motors with closed-loop speed control.

l Utilizes a PI current controller to derive the chopper duty cycle for armature current control.

l Employs high switching frequency DC-DC converters, resulting in lower armature current ripple compared to thyristor-based drives.

l Requires four switching devices, increasing system complexity.

Working of Four Quadrant Chopper:

l In the above circuit diagram, the choppers are numbered CH1 to CH4. For first quadrant operation CH1 is made ON, for second quadrant operation CH2 is made ON and so on. To better understand the working of four quadrant chopper, we will discuss its operation separately for each quadrant.

First Quadrant Operation:

In this mode, CH4 is activated while CH3 is deactivated, and CH1 is operated. When both CH1 and CH4 are simultaneously on, the load connects directly to the source, resulting in the output voltage being equal to the source voltage (vo = vs). During CH1's off state, the load current flows through CH4 and D2. Consequently, both the output voltage (vs) and load current (io) remain positive. This operation represents a step-down chopper configuration.

Second Quadrant Operation:

Here, CH2 is activated while CH1, CH3, and CH4 remain off. When CH2 is on, the DC source drives current through CH2, D4, E, and L. Energy is stored in inductor L during CH2's on period. Upon CH2's deactivation, current is fed back to the source through D1 and D4. As the load voltage (Vo) is positive and the load current (Io) is negative, this operation constitutes the second quadrant. In this mode, power is transferred from the load to the source, resembling a step-up chopper configuration.

Third Quadrant Operation:

To achieve third quadrant operation, both the load voltage and current must be negative. CH1 is turned off, CH2 is activated, and CH3 is operated. When CH3 is on, the load connects to the source, causing the load voltage to match the source voltage. However, the polarity of the load voltage (vo) is opposite to the circuit diagram, resulting in a negative vo. The load current (io) flows in the opposite direction, also negative. Upon CH3's deactivation, the negative load current free wheels through CH2 and D4, maintaining negative vo and io. This operation mode involves power transfer from the source to the load.

Fourth Quadrant Operation:

In this mode, CH4 is activated while CH1, CH2, and CH3 remain off. Positive current flows through CH4, D2, L, and E when CH4 is on, with energy stored in inductor L. Upon CH4's deactivation, current is fed back to the source through D2 and D3. While the load voltage is negative, the load current remains positive, resulting in power transfer from the load to the source. This mode acts as a step-up chopper.

Each operation mode of the four-quadrant chopper or Class-E chopper offers distinct power transfer characteristics, facilitating versatile control over DC motors.

Comparison with BJT H Bridge:

l Four quadrant chopper operates in all four modes, while BJT H Bridge model operates in two modes: forward motoring and reverse motoring.

l Direction of rotation change in chopper achieved through current signal transition, whereas in H Bridge model, it's done by changing current polarity.

l Chopper has high switching frequency compared to H Bridge model.

l Armature current spike at switch transition is lower in chopper due to high switching frequency.

l Chopper commonly used in industrial applications, while H Bridge model is used in robotics.

Conclusion:

In conclusion, the MATLAB demo model illustrates the speed control of a DC motor using a BJT H-bridge, showcasing its operation, simulation results, and the impact of changing duty cycles on armature current spikes. Additionally, comparing the model with the Four-Quadrant Chopper DC Drive (DC7) block highlights differences in operation modes, switching frequencies, and applications.

Q2) The objective is to develop a Simulink model for controlling a DC motor using a two-quadrant type C model DC chopper.

Circuit Diagram for Two Quadrant Chopper:

The two-quadrant chopper, also known as a type C chopper, is constructed by connecting type A and type B choppers in parallel. This configuration ensures a positive output voltage (V0) due to the presence of the freewheeling diode (FD) across the load. When the chopper is activated, the FD conducts, and V0 equals the source voltage (Vs). The load current (i0) reverses direction, flowing towards the source, and remains positive regardless of the chopper or FD operation. However, if the chopper or diode D2 conducts, the load current becomes negative. In the first quadrant, the chopper and FD function together as a type A chopper, while in the second quadrant, the chopper and D2 act as a type B chopper.

The average voltage is always positive, but the average load current may be positive or negative. Power flow can occur from the source to the load (first quadrant) or from the load to the source (second quadrant). Simultaneous activation of both choppers should be avoided to prevent short circuits in the supply lines. This chopper configuration is commonly used for regenerative braking and motoring.

Two Quadrant Chopper Model Using Simulink:

The Simulink model consists of two MOSFET blocks, diode blocks, and a DC voltage source connected as depicted in the circuit diagram. The DC voltage source has a voltage of 24 volts. Additionally, an RLC branch block with an inductor (L) serves as the load. The motor load (LE) is simulated by connecting a 12-volt DC voltage source in series with the inductor.

A pulse generator block generates gate pulses, and a NOT gate ensures that the MOSFETs do not close simultaneously to prevent short circuits. The pulse generator has an amplitude of 10 to clearly visualize the pulses on the scope. The period is set to 0.02 seconds, and the pulse width is 50%.

Scope Result:

The voltage waveform remains non-negative, with a minimum value of zero and a maximum value always in the positive direction. However, the current waveform fluctuates, displaying both positive and negative values. The pulse amplitude on the waveform is 10, as indicated by the pulse waveform.

Conclusion:

The Simulink model effectively simulates the operation of the two-quadrant chopper system, showcasing forward motoring and regenerative braking actions. Understanding and analyzing these results are essential for optimizing the control of DC motors in practical applications.

Q3) BLDC Motor Operations

Introduction:

The Brushless DC (BLDC) motor, also known as a synchronous motor, stands out for its high efficiency compared to other motor types. Unlike conventional motors with brushes, BLDC motors feature a rotor equipped with permanent magnets, eliminating the need for brushes.

Construction:

In a BLDC motor, the stationary stator coil generates variable DC voltages, creating a rotating magnetic field to interact with the permanent magnet rotor. By controlling the magnitude and direction of current flowing through the stator windings, the motor's operation is regulated.

Operation of BLDC Motor:

BLDC motors typically feature three winding terminals labeled U, V, and W, each connected to a single point. The angle shift between two coils is typically 120 degrees. The rotation principle involves the magnetizing of coils, which attract the rotor's permanent magnet, causing it to align with the resultant arrow. Continuous switching of the flux ensures the rotor magnet continually chases the rotating magnetic field induced by the coils, resulting in rotor movement.

Advantages of BLDC Motor:

BLDC motors offer several advantages:

l High efficiency, capable of continuous control at maximum torque.

l Even small BLDC motors deliver significant power.

l Precise torque and rotation speed control can be achieved using feedback mechanisms.

l Reduced energy consumption and heat generation.

l High durability and low electric noise generation.

l Elimination of sparks typically seen in brushed motors due to brush and commutator interaction.

Control of BLDC Motor:

Control of BLDC motors involves adjusting the voltage into each coil using inverter circuitry, typically employing pulse width modulation (PWM). PWM adjusts voltage by varying the pulse ON time, known as the duty cycle. Increasing the duty cycle raises the voltage, while reducing it lowers the current. While 120-degree conduction control requires two-phase voltage control, sinusoidal control utilizes three-phase voltage control, necessitating appropriate inverter circuitry for driving BLDC motors.

Conclusion:

BLDC motors offer superior efficiency and control compared to traditional motors, making them ideal for various applications. Understanding their construction, operation, advantages, and control mechanisms is crucial for leveraging their benefits effectively in practical systems.