Week-7 Challenge: DC Motor Control

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

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

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

ANS:

In the MATLAB/SIMULINK documentation, we can search for the simulation model of the 'Speed control of a DC Motor using BJT H-bridge'. It is used to generate a chopped voltage and to control the speed of the DC motor. This model can be opened in SIMULINK from the documentation itself. As the name suggests, it uses a BJT (Bipolar Junction Transistor which is a type of transistor that uses both electrons and holes as charge carriers, hence the name bipolar. Other transistors such as FETs (Field Effect Transistors) are unipolar transistors meaning that they only use one type of charge carrier. BJTs are used for switching and amplifying applications. 

BJTs have 3 terminals which are: the emitter (E), the base (B) and the collector (C). Current flows in only the emitter and the collector and the amount of current that flows through them is controlled by the base. BJTs use semiconductors in their working, and so there are 2 types of BJT: NPN type and PNP type. Their circuit symbols are shown below:

• PNP Transistor

This type uses 2 p-type semiconductors and one n-type semiconductor. The n-type semiconductor will be the base of the transistor. The emitter terminal is more positive in relation to the collector and base terminals. The collector is connected to positive supply and emitter is connected to negative supply. The base terminal is supplied with voltage and it operates the transistor by switching between ON or OFF states. When the base voltage is the same as the emitter voltage, the transistor is in the OFF state. When the base voltage decreases with respect to the emitter voltage, the transistor is in the ON state. 

• NPN Transistor

This type uses 2 n-type semiconductors and one p-type semiconductor. The p-type semiconductor will be the base of the transistor. Construction wise it is exactly the same as the PNP type having 3 terminals. Just as in the PNP type, the collector terminal is conneced to the positive voltage supply and the emitter terminal is connected to the negative voltage supply. The base terminal is supplied with a voltage and controls the transistor being either in an ON or OFF state. When the base voltage is the same as the emitter voltage, the transistor will be in the OFF state. When the base voltage increases with respect to the emitter voltage, the transistor will be in the ON state. 

BJT H-Bridge

When the BJT is conducting i.e. operating in the saturated region, there will be a forward voltage developed between the collector and the emitter (in the range of 1V). An IGBT is used to model the BJT because of this. The switch is controlled by a SIMULINK signal. The DC motor used in this model is preset to with specifications: 5HP, 24V, 1750rpm. It simulates a fan type load where the load is proportional to the square of the speed. The mean armature voltage can be varied between 0 and 240V when the duty cycle is changed anywhere from 0 to 100%. The duty cycle can be changed from the pulse generator block. 

The H-bridge consists of 4 BJT and Diode pairs. Two transistors are switched on simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and the diodes, D2 and D3 operate as free wheeling diodes when Q1 and Q4 are switched off. Similarly, when Q2 and Q3 are fired, a negative voltage is applied to the motor and the diodes, D1 and D4 operate as free wheeling diodes when Q2 and Q3 are switched off. 

By default, the motor will start in the positive direction and has a preset duty cycle of 75% with a mean DC voltage of 180V. At t = 0.5 seconds, the armature voltage will reverse and the motor will run in the negative direction. 

This is the SIMULINK model for the BJT H-Bridge used in the speed control of a DC motor:

Here, we can see the 'Pulse Generator 500Hz' block. If we open it, we will get a dialog box with its settings which is shown below:

As we can see, the pulse generator is already preset with some settings. The pulse width field allows us to modify the duty cycle of the motor and hence, we can prevent the motor current from shooting up when the motor changes its direction from forward to reverse. By default the duty cycle is 75% meaning that the voltage is supplied to the motor for 75% of the time for one cycle. For the remaining 25%, the motor is off and receives no voltage from the source. It is this alternation between the ON and OFF states, that allows us to vary the voltage that is supplied to the motor and thereby control the speed of an electric vehicle. 

The image below shows the output of the scope when the simulation is run using the preset values in the pulse generator block. There are 3 output graphs of angular speed, armature current and the load torque all against a time axis.

For this question, we are interested in modifying the simulation in order to prevent the initial shoot up of the armature current at the start of the cycle. This shootup occurs whenever the armature voltage reverses direction which is every 0.5 seconds. In the second graph above (which shows the armature current vs. time) we can see that at the start of the cycle, there is a sharp increase in the armature current that then decreases and comes to a more stable level where it remains until 0.5 seconds. When the next cycle begins, the exact same effect occurs but in the negative axis because in this cycle, the armature voltage is reversed. 

This frequency of reversing the voltage direction has been preset in the 'armature voltage control' block. Opening it will give you a dialog box of its setting where the 'step time' field controls the frequency at which the voltage is reversed. 

If we change the duty cycle i.e. the pulse width, we can control this initial shoot up of the current that occurs every time the armature voltage direction is reversed. When the duty cycle is decreased to 50% the output graph that is obtained is shown below:

 For the 50% duty cycle, we can see that armature current does not shoot up initially for the positive direction of the armature voltage. However, after the 0.5 second mark when the armature voltage is reversed, the armature current shoots up in the negative direction. So, we need to decrease the duty cycle further to prevent this. 

This time, I have reduced the duty cycle to 40%. The output graph is shown below:

Here, we can see that at the start of the cycle in the positive armature voltage direction, the armature current does not shoot up like it did before. The armature current increases immediately to approximately 7A and it maintains that level until the next cycle where the armature voltage is reversed to -7A. 

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

ANS:

The four quadrant chopper DC drive block in MATLAB/SIMULINK represents a four quadrant, DC supplied chopper drive for DC motors. It is also referred to as the 'DC7' motor drive in SIMSCAPE. It features closed loop speed control with four quadrant operation. The closed loop speed control will output the armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. The duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper. 

Compared to other DC drives, the main advantage of using the four quadrant chopper DC drive is that it can operate in all 4 quadrants. Each quadrant in its characteristics represents a different mode of operation. The first quadrant is forward motoring, the second is reverse braking, the third is reverse motoring and the forth is forward braking. Another advantage is that it uses high switching frequency DC-DC converters resulting in a lower armature current ripple is occurs than compared to thyristor based DC drives. 

The image below shows the DC7 block:

The detailed circuit diagram for the DC7 block is shown below:

The four quadrant chopper DC drive uses these 4 main blocks from the fundamental drive blocks library:

• speed controller
• current controller
• regulation switch
• chopper

dc7_example 

Entering this code shown above in the command window will open the SIMULINK model for the 4 quadrant chopper DC drive. 

4 quadrant chopper DC drives are used mainly due to their ability to work in the 4 different quadrants of operation. It allows the controller to command the DC motor to rotate in clockwise and counter clockwise directions and to perform braking. Traditional DC motors would rely on the innate friction and air resistance forces or actual physical brakes to bring the motor to rest. The other simpler option is to switch off the motor till it comes to rest. 

Operation

• Forward Motoring: motor rotates clockwise and controller applies a clockwise drive.
• Reverse Motoring: motor rotates anti-clockwise and controller applies an anti-clockwise drive.
• Forward Braking: motor rotates clockwise and controller applies anti-clockwise drive.
• Reverse Braking: motor rotates anti-clockwise and controller applies a clockwise drive.

Advantages

4 quadrant choppers have more control over the deceleration of the motor and can more accurately brake the motor. This will make the deceleration more smoother. The second benefit is that it reduces the risk of current spikes (and possibly damage the motor and the controller) which can occur when the motor is made to brake while running at a high speed. So the controller life is generally longer with 4 quadrant choppers. A third advantage is that 4 quadrant choppers are more efficient because lesser energy is wasted as heat and lesser current spikes occur which means lesser power dissipation. Finally, the 4 quadrant chopper makes regenerative braking possible so that when the motor is made to brake, the kinetic energy of the rotor moving is transferred back as electrical energy which can then be used to charge a battery. 

Comparison to H-Bridge

Both BJT H-bridges and 4 quadrant choppers have their advantages and disadvantages. H-Bridges like the 4 quadrant choppers are also capable of the braking operation without using air resistance or friction or physical brakes to bring the motor to rest. 4 quadrant DC choppers are more commonly used in automobiles whereas BJT H-bridges are commonly used in robotics and electro-mechanical devices.

Unlike 4 quadrant DC choppers, BJT H-bridges are not capable of performing regenerative braking but they can perform dynamic braking. It is this difference that makes 4 quadrant DC choppers useful in hybrid and electric vehicles.

In 4 quadrant DC choppers, the current signals are interrupted with one another to change the direction of rotation of the motor. This is done by a speed reference or a drive cycle from which the speed values are converted to current signals which interrupt one another to change the rotation of direction of the motor. However in BJT H-bridges, the polarity of the current is reversed (depending on the duty cycle) to change the direction of rotation of the motor.

Also, the current signals in 4 quadrant DC choppers are chopped meaning that they are not continuous, they are discrete and so take less time for simulations. But in BJT H-bridges, the current values are not chopped, meaning that they are continuous and so take more time for simulations. 

Finally, in 4 quadrant DC choppers, a PI controller is used to control the speed and the current and in the H-bridge, this is not implemented. 

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 )

ANS:

Two Quadrant Chopper 

The Two-Quadrant Chopper DC Drive (DC6) block represents a two-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with a two-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 advantages of this drive, compared with other DC drives, are its implementation simplicity and the that it can operate in two quadrants (forward motoring and reverse 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, for all two-quadrant DC drives, reversible and regenerative operations (reverse motoring and forward regeneration), which are required in most DC drives, cannot be obtained.

The Two-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:

In this type of Chopper, the average voltage will be always positive but the average load current might be positive or negative. The two Switches CH1 and CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used.

We will always get a positive output voltage V₀  as the freewheeling diode FD is present across the load. When the chopper CH1  is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF  the freewheeling diode starts conducting and the output voltage v₀ will be equal to V_s. The direction of the load current i₀ will be reversed. The current i₀  will be flowing towards the source. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source.

The armature voltage is provided by an IGBT buck-boost converter controlled by two PI regulators.

The buck-boost converter is fed by a 630 V DC bus obtained by rectification of a 460 V AC 60 Hz voltage source.

In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC6 block.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.

The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system.

The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the two IGBT devices (Pulse Width Modulation).

For proper system behavior, the two IGBT devices have opposite instantaneous pulse values. This generates the average armature voltage needed to obtain the desired armature current.

Scope result of IGBTs, Armature voltage of DC Motor, Armature current and Motor speed.

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

ANS:

BLDC motor is an Electronically commutated Motor.

Primary efficiency is the most important feature for BLDC motors.

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 sens

Construction:

Rotor:

The permanent magnets attached to the rotor.

The armature windings are located on the stator.

They use electrical commutation to convert electrical energy into mechanical energy. 

The main design difference between brushed and brushless motors is the replacement of a 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.

The 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.

BLDC motor works on a principle similar to that of a Brushed DC motor.

The 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.

When the stator coils get a supply from the 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. Due to the force of interaction between the 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.

BLDC motor is used in Electric vehicles like Two-wheelers and Three-wheeler vehicles.