Week-7 Challenge: DC Motor Control

1)

A)

When we talk about Speed control of DC motors using H-Bridge.

As we can see the circuit and terminal explanation of the DC motor. As seen in the circuit there are 4 switches SW1, SW2, SW3, and SW4. When the switches are turned on in a synchronized pattern, such as SW1 and SW4 are closed at the same time would make terminal A positive and B as Ground so the direction of the motor would follow the counter-clockwise direction as shown in the 2nd DC motor in Figure 1 (i). Also when SW2 and SW3 are closed and the rest 2 are open B becomes positive and A is ground. The direction of the motor is now the third motor figure of Figure 1(i) i.e. in a clockwise direction. So in this way the direction of the motor can be controlled and regulated according to the requirement.

The name being H-Bridge is due to shape of the circuit s shown in figure 1(ii).

       Figure 1            (i)                                                                              (ii)

The generalized circuit as shown in Figure 1(ii) can be made using actual electronic switches i.e. BJT.

Figure 2: H-Bridge circuit for DC motor speed controller using BJT.

So in the circuit shown above, we can see 4 NPN transistors/ BJT Q1, Q2, Q3, and Q4. So as we know for such fast switching speed, general manual switches can't be used and hence for this transistors/BJT are used as they can switch at higher speeds and higher rates. The physics of BJT is that it can be used as a switch, amplifier, filter, rectifier, and oscillator. So in this we use it as a switch. 

If we provide enough positive input to the base of an NPN transistor, it will turn ON (Saturation area), and if we provide 0 input, it will turn OFF (Cut off region). In this circuit, the motor will move forward if Q1 and Q4 are turned ON simultaneously, and it will revolve in the opposite direction if Q2 and Q3 are turned ON simultaneously.

The controlling of the DC motor through BJT switching can be done by changing the duty cycle of the BJT.

Figure 3: Different Duty cycles

Just like in figure 3, we can assign different duty cycles to change the duration of the DC motor power supply.

The drive cycle if we have to explain can be said that the time period in which the supply is ON for a particular BJT for the operation of the H-Bridge.

, where $\delta$ is the duty cycle.

When utilized in power switching applications, the bipolar junction transistor (BJT) functions as an IGBT. Between the collector and the emitter(in the range of 1 V), a forward voltage Vf develops when it is conducting (BJT functioning in the saturated area). Consequently, the BJT device can be modeled using the IGBT block.

The triggering of the IGBT block doesn't work on the gate signal controlling, but actually, it works with the help of the Simulink signal generator, which is nothing but binary in nature i.e. 0 for 'OFF' and 1 for 'ON'. 

The  DC motor used here as we can see is a separately excited motor and has the parameters as 5 HP 24V 1750 rpm.

And the H-Bridge works on the same principle mentioned above.

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1)

B)

Simulation Output:

Case (i) Pulse Generator Parameters:

Period: 2 milliseconds

Pulse Width (% of period) : 75%

As we can see in the Speed graph in which the graph starts attains the rated speed in about 0.13 seconds and as soon as the duty cycle is over at 0.5 seconds the plot starts to decelerate i.e now the speed is reducing with the passage of time because the supply to the DC motor has been stopped for the remaining part of the cycle. And now at about 0.65 seconds, the motor attains the negative speed i.e. -1000 rpm which means the motor has now entered into generator mode.

When it comes to armature current versus time, the Ia(armature current) is having a spike in it at the time of starting the DC Motor. This is because, at the time of starting, the motor requires a high amount of torque to come out of standstill or regenerative condition, which can be done by intaking a high amount of current as T ⍺ IΦ. Then again as soon as the duty cycle finishes, Ia starts to reduce and as the DC motor enters regenerative mode hence there is a sudden dip in the graph of Ia, which again rises to a certain higher value at about 0.65 sec.

For the torque versus time plots, the torque and speed plots can be seen as similar at the time of starting. At the time of regenerative mode, there is observed a small plateau at the time when the current plot touches the lowest mark, which can be concluded that at the particular time the motor tries to come back to its original nature of motoring instead of generating and tries to oppose the motion of EMI.

Case (ii) Pulse Generator Parameters:

Period: 2 milliseconds

Pulse Width (% of period) : 50%

As we can see in the Speed graph in which the graph starts doesn't attain the rated speed till 0.5 seconds, as the duty cycle is over at 0.5 seconds the plot starts to decelerate i.e now the speed is reducing with the passage of time because the supply to the DC motor has been stopped for the remaining part of the cycle. And now unlike Case (i), the DC motor doesn't attain a stable negative speed i.e. -1000 rpm.

When it comes to armature current versus time, the Ia(armature current) is not having a spike in it as in Case (i). This is because the pulse generated or supply is less as compared to Case (i). Then again as soon as the duty cycle finishes, Ia starts to reduce and as the DC motor enters regenerative mode hence there is a sudden dip in the graph of Ia, which again rises to a certain higher value at about 0.57 sec.

For the torque versus time plot, the torque and speed plots can be seen as similar at the time of starting. At the time of regenerative mode, we observed there is a plateau or the graph flattens for some time, and then the plot further reduces to a negative slope. 

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1)

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

A four-quadrant DC-supplied chopper drive for DC motors, often known as a DC-DC PWM converter, is represented by the Four-Quadrant Chopper DC Drive (DC7) block. This drive has four-quadrant operation and closed-loop speed regulation. The machine's reference armature current is output by the speed control loop. The chopper duty cycle matching to the commanded armature current is determined using a PI current controller. To get the necessary PWM signals for the chopper, this duty cycle is then contrasted with a sawtooth carrier signal. 

There are numerous applications of the Four-quadrant chopper

• Speed Controller (DC)
• Current Controller (DC)
• Regulation Switch

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2)

Type C chopper or Two Quadrant class A chopper:

Type-C or Class-C Choppers are within the group of choppers that operates in both the first and second quadrants. In essence, this means that in this chopper, electricity can either flow from source to load or load to source. Two Quadrant Class-A chopper is another name for this type of chopper.

How does Type-C chopper works?

A Class A and Class B chopper, as is well known, operate in the first and second quadrants, respectively. Therefore, it is conceivable to have chopper activity in both the first and second quadrants if we connect both of these types of choppers in tandem. In actuality, Class-A and Class-B choppers are connected in parallel to create Class-C or Type-C choppers. The circuit diagram for this kind of chopper is shown in the figure above.

We can see that the Class-A Chopper is formed by the chopper CH1, the Free-Wheeling Diode (FD), and the load, whereas the Class-B Chopper is formed by the chopper CH2, the D2, and the load. These two choppers are attached in parallel. chopper CH1 should be turned on to get first quadrant operation, and chopper CH2 should be turned on to get second quadrant operation.

Let us discuss the possible cases that can happen with CH1 and CH2 for the application of the chopper

Case 1: When CH1 is switched ON/OFF

When chopper CH1 is turned ON, source Vs is connected directly to the load, making source voltage and load voltage the same. According to the circuit schematic, which is assuming a positive voltage, the direction of load current is from source to load. 

The free-wheeling diode (FD) enters the circuit when CH1 is switched OFF because it becomes forward biassed and shorts the load. The output voltage Vo, therefore, drops to zero. However, as indicated in the circuit diagram, the Io continues to decline through the FD and L in the same manner. The action of the chopper is therefore in the first quadrant as a result of the average output voltage Vo and current Io being positive.

Case 2: When CH2 is switched ON/OFF

Current flows through chopper CH2 and the load when chopper CH2 is turned ON by load DC source E. This current is considered to be negative because its direction will be the opposite of what is indicated in the circuit diagram. Vo, the output voltage, is at zero at this point. Diode D2 becomes forward biassed when CH2 is turned OFF, which causes the current from the load to enter the source. As the load is linked to the source through D2 during the CH2 chopper's OFF period, the output voltage at this point is Vs. Thus, the load current is always negative i.e. functioning of chopper is within second quadrant.

While the average load current can be positive or negative, the average load voltage is always positive. As a result, power might flow from the source to the load (first quadrant operation) or from the load to the source (second quadrant operation). The hatching area below represents the Class-C or Type-C chopper's operational area.

Shown below is the simulink model of the Type-C chopper 

The Values of Resistor is 5 ohm and Inductor is 1 mH.

The Parameters of the continuous Pulse generator is

An amplitude of 20 is chosen so that we could see the pulses clearly.

DC Voltage Source Parameters:

Source side: 24V

Load side: 12V

Thus, it was evident that while the current might be both positive and negative, the voltage is always positive. The Type C chopper is therefore simulated and confirmed.

Reference to the circuit of the simulink is attached in the files section along with the provided link.

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3)

BLDC motor 

Electronically commutated motor is another name for brushless DC electric motors (ECMs, EC motors). For BLDC motors, primary efficiency is a crucial quality. because the rotor is the only device carrying the magnets and needs no power. i.e., no commutator, no brushes, and no connections. The motor uses control circuitry in its place. BLDC motors use controllers, rotary encoders, or a Hall sensor to determine where the rotor is at specific times.

Construction

The rotor of this motor is where the permanent magnets are attached. The armature windings, or conductors that carry current, are found on the stator. To transform electrical energy into mechanical energy, they employ electrical commutation. The fundamental structural distinction between brushed and brushless motors is the electric switch circuit used in place of the mechanical commutator. Insofar as the magnetic field produced by the stator and the rotor revolve at the same frequency, a BLDC motor is a form of synchronous motor. There are no current-carrying commutators in brushless motors. A brushless motor's internal field is switched using an amplifier that is activated by a commutating device, such as an optical encoder.

Descriptive view of BLDC motor.

Working on BLDC motor

A brushed DC motor and a BLDC motor operate on a similar theory. The current-carrying conductor suffers a force anytime it is placed in a magnetic field, according to the Lorentz force law. The magnet will experience an equal and opposite force as a result of the reaction force. The permanent magnet is moving while the current-carrying wire is stationary in a BLDC motor.

When the stator coils receive power from a source, they become electromagnets and begin to create a consistent magnetic field in the air gap. Despite the supply coming from a DC source, switching causes an AC voltage waveform with a trapezoidal shape to be created. Rotor rotation is maintained by the electromagnet stator's interaction with the permanent magnet stator.

The corresponding windings were activated as the North and South poles when the windings were switched between High and Low signals. The motor rotates because the North and South poles of the permanent magnet rotor align with the poles of the stator.                                       

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