Week-7 Challenge: DC Motor Control

Q1 A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

Objective 1 :-

• BJT (Bipolar Junction Transistor):-

A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector, and the emitter. A signal of a small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process.

BJT is a semiconductor device that is constructed with 3 doped semiconductor Regions i.e. Base, Collector & Emitter separated by 2 p-n Junctions.

The BJT is operating in three regions, Active Region, Saturation Region, and the Cut-off region.

Active Region :-

In this mode, one junction (emitter to base) is forward biased and another junction (collector to base) is reverse biased. This mode is used for amplification of current.

Saturated Region :-

In this mode, both junctions are reversed biased so no current flows through the device. Hence, transistor is in off mode and acts like open switch. This mode is used for switch OFF application.

Cut-off Region :-

In this mode, both junctions are reversed biased so no current flows through the device. Hence, transistor is in off mode and acts like open switch. This mode is used for switch OFF application.

There are two typpes of BJT.

1. PNP BJT

2. NPN BJT

PNP BJT :-

In PNP BJT, the n-type semiconductor is sandwiched between the two p-type semiconductors. The two p-type semiconductors act as emitter and collector respectively while the n-type semiconductor acts as a base.The current enters the transistor through the emitter such that the emitter-base junction is forward biased and the collector-base junction is reverse biased.

PNP BJT figure :-

NPN BJT :-

In NPN BJT, p-type semiconductor is sandwiched between the two n-type semiconductors. The two n-type semiconductors act as emitter and collector respectively while the p-type semiconductor acts as a base.Current entering the emitter, base, and collector has the sign convention of positive while the current that leaves the transistor has the sign convention of negative.

NPN BJT figure :-

Application of BJT :-

• It is used as an amplifier
• It is used as an oscillator
• It is used as a demodulator
• BJT is used in clipping circuits so that the waves can be shaped.
• Logic circuits and switching circuits use BJT.

H-Bridge :-

H-bridge is a rather simple circuit, containing four switching elements, with the load at the center, in an H-like configuration.

The switching elements (Q1, Q2, Q3, and Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs.

Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not

relevant for the most part for this discussion. The diodes (D1, D2, D3, and D4) are called catch diodes and are usually of a

Schottky type.

The top end of the bridge is connected to a power supply (battery for example) and the bottom end is

grounded.

In general, all four switching elements can be turned on and off independently, though there are some obvious

restrictions.

H-Bridge Circuit is shown below figure :-

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

open the matlab and then enter the command POWER_Hbridge to open the Demo model of speed control of DC motor using BJT H-bridge.

the demo model simulink will be open .

Operation :-

If the switches Q1 and Q4 are turned ON the left lead of the motor will be connected to the power supply.

Current starts flowing through the motor which energies it in the forwarding direction and the motor starts spinning.

If Q2 and Q3 are turned ON, then the reverse will happen.

The motor is energized in the reverse direction and the shaft will start spinning in the backward direction.

In a bridge, we should never ever close both Q1 and Q2 or Q3 and Q4 at the same time.

If we did that, a low resistance path is created between power and GND effectively short-circuiting the power supply.

This condition is called Shoot through the condition. It will quickly damage or destroy the bridge.

Adding diodes across the switches is protect the electricity that may flow backward from the motor which makes damage to the transistor. When we disable the transistors to stop running the motor the energy needs to be released. If diodes are connected in the reverse direction to the transistors, then it will give a path for the current to release this energy. Without them, the voltage rises and damages the transistors.

From the demo Simulink model, The IGBT Switch is controlled by a Simulink signal that is 1 or 0. The DC motor uses in the pre-set model which is 5HP, 24V, 1750RPM. It simulates the load as fan type and where the load torque is directly proportional to v^2. The armature voltage is varied from 0-240V when the duty cycle is varied from 0% to 100%. The duty cycle value is specified in the Pulse Generator block.

This demo Simulink model is run for 1 second with default values.

Total Cycle Time = 1 second

Duty Cycle = 75%

Results:-

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

case 1 :-

Total time cycle :- 1

Duty Cycle :- 50

Results :-

Case 2 :-

In this case, the Duty Cycle is changed from 50% to 30% in the pulse generation block. then run the simulation.

Total Cycle Time = 1

Duty Cycle = 30%

Results :-

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

A chopper is a basically static power electronics device that converts fixed DC voltage/power to variable DC voltage or power.

It is nothing but a high-speed switch that connects and disconnects the load from the source at a high rate to get variable or chopped voltage at the output.

The four-quadrant chopper DC drive (DC7):-

A four-quadrant chopper is a chopper that can be operated in all four quadrants. The power can flow either from source to

load or load to source in this chopper. In the first quadrant, a Class-E chopper acts as a Step-down chopper whereas in the

second quadrant it behaves as a Step-up chopper. This type of chopper is also known as Class-E or Type-E chopper.

Block diagram of four-quadrant chopper DC drive (DC7):-

To open the dc7 reference model enter the command dc7_example in the command window.

Then simulation model will open.

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 converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 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 DC7 block.

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 regulatory controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behavior, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

Quadrant -1 operation when switch s1 turned on:-

When the switch s1 is operated, switches s1 and s4 conduct, the output voltage Vo and the output current io both are positives, power flows from source to load and inductor stores energy, the motor rotates in the forward direction hence called forward motoring.

Quadrant -1 operation when switch s1 turned off:-

Switch s1 turned off but switch s4d and diode D2 conducts, output current io is positive and the output voltage Vo becomes zero, inductor release energy and freewheeling action using diode D2 takes place, the motor rotates in the forward direction hence called forward motoring.

Quadrant -2 operation when switch s2 turned on:-

Let us assume that the motor is running in the forward direction. Switch s2 operated, switch s2 and diode D4 conducts, the output voltage Vo is zero and Eb is responsible for the negative output current io, machine behave as generator and inductor stores energy.

Quadrant -2 operation when switch s2 turned off:-

Switch s2 turned off, diode D1 and diode D4 conducts, output voltage Vo becomes positive and the output current io is negative, inductor release energy using diodes D1 and D4, power flows from load to source and hence called as reverse braking.

Quadrant -3 operation when switch s3 turned on:-

The polarity of back emf Eb must be reversed. Here switch S3 operated, switches S3 and S2 conducts, the output voltage Vo and the output current io both are negatives, power flows from source to load and inductor stores energy the motor rotates in the reverse direction hence called as reverse motoring.

Quadrant -3 operation when switch s3 turned off:-

The polarity of back emf Eb must be reversed. Switch s3 turned off but switch s2 and diode D4 conducts, output current io is negative and the output voltage Vo becomes zero, inductor release energy and freewheeling action using diode D4 takes place, the motor rotates in the reverse direction hence called as Reverse motoring.

Quadrant -4 operation when switch s4 turned on:-

The polarity of back emf Eb must be reversed. Let us assume that the motor is running in the reverse direction. Here switch s4 operated, switches s4 and diode D2 conduct, the output voltage Vo is zero and Eb is responsible for the positive output current io, machine behave as generator and inductor stores energy.

Quadrant -4 operation when switch s4 turned off:-

The polarity of back emf Eb must be reversed. Switch s4 turned off, diode D2 and diode D3 conducts, output voltage Vo becomes negative and output current io is positive, inductor release energy using diodes D2 and D3, power flows from load to source and hence called as reverse braking.

Comparison of H-bridge vs Four-Quadrant chopper DC7:-

H-Bridge Four Quadrant Chopper

Here we need to change the polarity to change Here we need to interrupt the signals to another

the motion (forward or backwar)

Regenerative braking is not possible Regenerative braking is possible

Simulation will be Continuous Simulation will be Discrete

Takes more time to simulate Comparatively, takes less time to simulate

Efficiency will be less compared to four Efficiency will be more compared to H-Bridge

quadrant choppers

Q2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )

A two-quadrant chopper is a category of chopper that can operate in the first as well as a second quadrant. This basically means that the power can either flow from source to load or load to source in this chopper. This kind of chopper is also known as the Two quadrant Class-A chopper.

1st quadrant will work when CH1 or FD is ON and 2nd quadrant will work when CH2 or D2 is ON. We should not turn ON CH1 and CH2 simultaneously, because the DC supply will get short circuit.

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
• scope
• NOT gate
• Battery

Contruction :-

Given input DC voltage source of 240V, here we have used MOSFET for switching purpose, Measurement’s block to measure the current and voltage across the points, diodes, logical operator (NOT), pulse generator, powergui, inductor and scope blocks.

Here pulse generated signals are given to the port-g in CH1 block, by connecting to the Logical operator (NOT), other port of NOT block is connected to the port-g in CH2 block, this NOT block takes care to do not operate the switches simultaneously (for example: if the CH1 is ON then CH2 will be OFF and vice-versa). Inductor acts as a load here.

The simulation is run for 1second with Amplitude of 5, time period of 0.03 seconds and Pulse width of 50%.

First Qaudrant Working :-

When the CH1 is ON the current flows asper the connections and charges the inductor with polarity (+ and –) so, the voltage and current are positive.

When the CH is OFF the inductor changes its polarity from (+ and –) to (- and +) where current flows from Battery to D2 (ON position) to inductor here discharging of the inductor takes place. So, the voltage and current are positive.

Second Qaudrant Working :-

When the CH2 is ON the current flows from ‘E’ to inductor and CH2 so, the inductor will get charged (stored energy) with (+ and -) polarity. so, the voltage is positive and current are negative.

When the CH2 is turned OFF, the inductor will release the stored energy via D1 (ON position) so, the voltage is positive and current are negative.

Results :-

We can observe that Voltage is positive in both the quadrants were as Current is positive in 1st quadrant (CH1 and D2) and negative in 2nd quadrant (CH2 and D1).

Q3. Explain in a brief about operation of BLDC motor.

What is a Brushless DC motor (BLDC)?

• A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system.
• BLDC motors are also referred as trapezoidal permanent magnet motors.
• Unlike conventional brushed type DC motor, where in the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils.
• In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed.
• The armature coils are switched electronically by transistors or silicon controlled rectifiers at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate.
• Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.

• This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds.
• The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

Construction of BLDC Motor:-

• BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.
• Permanent magnets are mounted on the rotor of a BLDC motor, and stator is wound with a specific number of poles.

• The stator of a BLDC motor is made up of stacked steel laminations to carry the windings. These windings are placed in slots that are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have a three-phase star-connected stator.
• Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.

Rotor :-

• BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement.
• In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.
• Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB).
• The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.

Hall Sensors :-

• Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.
• Most BLDC motors incorporate three Hall sensors which are embedded into the stator.
• Each sensor generates Low and High signals whenever the rotor poles pass near to it.
• The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.

Working Principle and Operation of BLDC Motor :-

• BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which 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 case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.
• Stator windings of a BLDC motor are connected to a control circuit (an integrated switching circuit). The control circuit energizes proper winding at proper time, in a pattern which rotates around the stator.
• The rotor magnet tries to align with the energized electromagnet of the stator, and as soon as it aligns, the next electromagnet is energized. Thus the rotor keeps running.
• Commutator helps in achieving unidirectional torque in a typical dc motor.
• Obviously, commutator and brush arrangement is eliminated in a brushless dc motor. And an integrated inverter/switching circuit is used to achieve unidirectional torque.
• That is why these motors are, sometimes, also referred as electronically commutated motors .

Advantages of BLDC Motor :-

• It has no mechanical commutator and associated problems.
• High efficiency 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.
• Quite operation (or low noise) due to absence of brushes.

Disadvantages of Brushless Motor :-

• These motors are costly.
• Electronic controller required control this motor is expensive.
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors.
• Requires complex drive circuitry.
• Need of additional sensors.

Applications of Brushless DC Motors (BLDC) :-

• Brushless DC Motors (BLDC) are used 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

Refernce:-

• (https://r.search.yahoo.com/_ylt=AwrKDxvMdb1j0AgPEGy7HAx.;_ylu=Y29sbwNzZzMEcG9zAzIEdnRpZAMEc2VjA3Ny/RV=2/RE=1673389645/RO=10/RU=https%3a%2f%2fwww.electricaltechnology.org%2f2016%2f05%2fbldc-brushless-dc-motor-construction-working-principle.html/RK=2/RS=W51l3m7t2tatoqcZbOpiJgbODZY)
• (https://r.search.yahoo.com/_ylt=AwrKDxvMdb1j0AgPDmy7HAx.;_ylu=Y29sbwNzZzMEcG9zAzEEdnRpZAMEc2VjA3Ny/RV=2/RE=1673389645/RO=10/RU=https%3a%2f%2fwww.aarohies.com%2fconstruction-working-principle-of-bldc-motors%2f/RK=2/RS=xYAbHmYQvDl_6b_CYaH9utOR4mw)