Week-7 Challenge: DC Motor Control

Aim:

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.

2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

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

1A:

An H-bridge is a simple circuit that lets us to control a DC motor to go backward or forward.

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration:

The switching elements (Q1..Q2) 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..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.

Though the load can in be anything we want, by far the most pervasive application if H-bridges is with a brushed DC or bipolar stepper motor (steppers need two H-bridges per motor) load. In the following we will concentrate on applications as a brushed DC motor driver.

Static Operation

The basic operating mode of an H-bridge is fairly simple if Q1 and Q4 are turned ON, the left lead of the motor will be connected to the power supply, while the rnight lead is connected to ground. Current starts flowing through the motor which energizes the motor in (lets say) the forward direction and the motor shaft starts spinning.

If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards.

In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time If you did that, you just have created a really low- resistance path between power and GND, effectively short-circuiting your power supply. This condition is called shoot-through and is an almost guaranteed way to quickly destroy your bridge, or something else in your circuit.

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

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device

The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink signal (1/0).

MODEL Description:

• The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.
• The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models).
• Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off.

• When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off

RESULT:

Thus from above graph we can see that, for the current flow through diode D3 and from 0.5 sec to 1 sec , the current flow through Q3.

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

the waveform for speed, armature current and load torque is shown in above graphs.

from armature current graph, we can see that, at starting there is sudden spike increase in current at O sec also and at 0.5 sec also.

We can see that, as the voltage gets changed in the circuit, the speed of motor also changes.

In this transition of positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occuring in the circuit.

As there is already current flowing in the circuit & on changing of switches after 0.5 sec, the current in the circuit adds to the changed direction of current and we observe a surge.

1B:

from armature current graph, we can see that at starting there is sudden spike increase in current at O sec also and at 0.5 sec also means at transition from positive to negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the circuit.

As there is already current flowing in the circuit & ON changing of switches after 0.5 sec, the current in the circuit adds to the changed direction of current and we observe a surge in current.

The surge in the current can be reduced by reducing the duty cycle.

In above results, the duty cycle selected as 75%. by changing the duty cycle from 75% to 50%, the current surge can be reduced tremendously.

1C: Refer to the help section of The Four-Quadrant Chopper DC Drive (DC7) block. Compare it with the H-bridge model.

Four Quadrant Operation of DC Motor:

Four Quadrant Operation of DC Motor means that the Machine Operates in Four Quadrants. They are Forward Motoring Forward Braking Reverse Motoring, and Reverse Braking.

A Motor Operates in two modes Motoring and Braking, a motor dive is capable of operating in both Direction (forward and reverse) of rotation and of producing both motoring and regeneration is called as Four Quadrant Operation of DC Motor.

QUADRANT - I

• When the motor draws power from supply, the Voltage and Current are in Same Direction.

• Now the Motor is said to be in Motoring Operation, its Operation correspond to 1st.

• In the Quadrant power developed is positive and Machine is working as Motor Supplying Mechanical energy.

• The 1st Quadrant Operation is called as Forward Motoring

QUADRANT-II

• IF there is a possibility of reversing the current direction keeping the Voltage Polanty same, the motor is said to be operating in 2nd

• This happens when regenerative braking is applied to motor.

• During regenerative braking operation, reverse power flow will happen Le Power will be feed back to supply system.

• 2nd quadrant operation is known as braking.

• In this Quadrant the direction of rotation is positive and the torque is negative, and thus. the machine operates as Generator developing a negative Torque, which opposes the motion.

• The Kinetic Energy of the rotating parts is available as Electric energy which may be supply back to mains. In Dynamic braking energy id dissipated in the resistance.

QUADRANT-III

• If the voltage polarity and load current are reversed simultaneously, the motors operation will be in the opposite direction (but the power is consumed by load).

• The operation is corresponding to 3rd Quadrant and even it is said to be in Motonng operation (in reverse direction) • The 3" Ouadrant operation is known as the Reverse Motoring.

• The Motor works, in the reverse direction. Both the Speed and Torque have Negative Values while the power is positive.

QUADRANT-IV

• If the voltage polarity is reversed maintain the current direction source to Load, the power is reversed and the operation corresponding to 4th

• Now the Motor is said to be in reverse Regenerative operation (e regenerating in reverse directionl.

• In the 4h Quadrant the Torque is positive and Speed is negative This quadrant corresponds to brakng in the reverse regenerative braking mode.

FOUR QUADRANT CHOPPER CIRCUIT DIAGRAM

• Four Quadrant Chopper Circuit Diagrarns also dlassified as Class E Chopper .

• It functions in 1st , 2nd , 3rd , 4th.

• It is a parallet combination of two classes C Chopper.

• CH1, CH2, CH3, CH4, are Chopper switch and DI, D2, D3, D4 are Diode.

• It we tun on Switch CH1 and CH2 Simultaneously output will get short circuited, similar case is with CH3 and CH4.

• When Switch CH1 and CH4 is Turned ON at the same time, the inductor (L) will STORE its energy and this chopper will function in 1st Quadrant.

• When Switch CH1 and CH4 is Turned OFF at that time Inductor(Polarity changes) will Release its energy conducting Diode D2 and D3 and this chopper will function in 4th-Quadrant.

• When Switch CH3 and CH2 is Turned ON at the same time we will observe that chopper will be having negative voltage and negative current and it will function in 3d Quadrant.

• But when Switch CH3 and CH2 is Turned OFF at the same time, then charge inductor would release its energy conducting Diode D1 and D4 and function in 2nd- Quadrant.

• When Chopper works in 1st and 3d Quadrant, average output Voltage is less than input voltage, which means it's a Step down Chopper.

• When Chopper works in 2nd and 4h Quadrant, average Output Voltage is more than input voltage, which means it's a Step Up Chopper.

DC7 - FOUR QUADRANTCHOPPER 200 HP DC DRIVE

• 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 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 four IGBT devices (Pulse Width Modulation).

• For proper system behaviour, 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.

OUTPUT:

COMPARISON OF FOUR-QUADRANT CHOPPER DC DRIVE (DC7) WITH H-BRIDGE:

1. In four Quadrant Chopper DC drive distributes required current supply for each sector and it consist of 4 quadrants like Forward Motoring. Regenerative Braking. Reverse Motoring and Reverse Regenerative Braking. Whereas in H-bridge supplies an irregular current for each sector that causes voltage Fluctuation in armature current; it consist of 2 Operation namely they are Forward Motoring, Reverse Motoring

2. In Modern Battery Technology Charger use for EV charging uses H-bridge circuit. Whereas a Four Quadrant Operation is required in Industrial as well as Commercial application.

3. The efficiency is high in Chopper compare with H-bridge and Maintenance is less than H-bridge.

4. In Robotic Technology, H-bridge circuit is used; some other application of Four Quadrant Operation is Engine Test Loading Systems 5. In H-bridge we need to change Polanty which turns into opposite direction to control the motor whereas in chopper model we need to interrupt one signal under control of another.

2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

CIRCUIT DIAGRAM FOR TWO QUADRANT CHOPPER:

Two quadrant chopper is also called as type c chopper. Type C chopper is obtained by connecting type -A and type -B choppers in parallel. We will always get a positive output voltage Vo as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage vo will be equal to V,. The direction of the load current io will be reversed The cumenti wil be flowing towards the source and it will be positive iegardless the chopper is on or the FD conducts. The load current will be chopper is or the diode D2 conducts. We can say the chopper and FD operate together as type-A chopper in first quadrant in the second quadrant, the chopper and D2 will operate together as type -B chopper.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be quadrant operation ie from source to load or from load to source like the second quadrant operation. The two choppes should on simultaneously as the combined action my cause a short circuit in supply lines. For regenerative braking and motoring these type of configuration s used.

TWO QUADRANT CHOPPER MODEL USING SIMULINK:

Let us take two mosfet blocks and diodes blocks and connect it as shown in circuit diagram.

The dc voltage source is connected as shown.

Let the voltage of dc voltage source be 24 volts.

Let us consider an series RLC branch block and select inductor L. thus block works as a load.

To consider LE load ie motor load, and dc voltage source of 12 volt is also connected in series with inductor.

The pulse generator block is used to create gate pulse.

The not gate is used in the model so that the two mosfet will not get closed at the same time otherwise short circuit will occur due to use of not gne when one mosfet sON the other mosfet will get off and when other mosfet is ON the first mosfet will get off.

The ampitude of pulse generator is kept 10 so that we can clearly see the pulses at scope and let period be 0.02 seconds and let pulse wedth be 50%

SCOPE RESULT:

The voltage waveform is not going less than zero, the minimum value of voitage is zero and maximum value is always in the positive direction

However in current waveform, the current increases and decreases. the current may be positive as well as negative.

and the pulse amplitude is 10 thus from pulse waveform we can see that the pulse amplitude is 10.

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

Introduction:

Brushless DC Motors or BLDC Motors have become a significant contributor of the modern drive technology Their rapid gain in popularity has seen an increasing range of applications in the fields of Consumer Appliances, Automotive Industry, Industrial Automation, Chemical and Medical, Aerospace and Instrumentation.

Even though they have been used for drives and power generation for a long time, the sub kilowatt range, which has been dominated by Brushed DC Motors, has always been a grey area. But the modern power electronics and microprocessor technology has allowed the small Brushless DC Motors to thrive, both in terms price and performance.

What is a BLDC Motor? A Brushless DC Motor is similar to a Brushed DC Motor but as the name suggests, a BLDC doesn't use brushes for commutation but rather they are electronically commutated In conventional Brushed DC Motors, the brushes are used to transmit the power to the rotor as they turn in a fixed magnetic field.

As mentioned earlier, a BLDC motor used electronic commutation and thus eliminates the mechanically torn brushes.

Construction of BLDC Motor

The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. Keeping that in mind, 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.

Brushless Motors are available in three configurations: single phase, two phase and three phase. Out of these, the three phase BLDC is the most common one.

The following image shows the cross-section of a BLDC Motor.

As you can see in the image, a BLDC Motor consists of two main parts a stator and a rotor.

Stator

The structure of the stator of a BLDC Motor is similar to that of an induction motor, It is made up of stacked steel laminations with axially cut slots for winding. The winding in BLDC are slightly different than that of the traditional induction motor.

Generally, most BLDC motors consists of three stator windings that are connected in star or Y fashion (without a neutral point). Additionally. based on the col interconnections, the stator windings are further divided into Trapezoidal and Sinusoidal Motors.

In a trapezoidal motor, both the drive current and the back EMF are in the shape of a trapezoid (sinusoidal shape in case of sinusoidal motors). Usually, 48 V (or less) rated motors are used in autormotive and robotics (hybrid cars and robotic arms).

Rotor

The rotor part of the BLDC Motor is made up of permanent magnets (usually, rare earth alloy magnets like Neodymium (Nd), Samarium Cobalt (SmCo) and alloy of Neodymium, Ferrite and Boron (NidFeB)).

Based on the application, the number ot poles can very between two and eight with North (N) and South (S) poles placed alternately. The following image shows three different arrangenments of the poles. In the first case, the magnets are placed on the outer periphery of the rotor.

The second configuration is called magnetic-embedded rotor, where rectangular permanent magnets are embedded into the core of the rotor. In the third cae the magnetic are inserted into the iron core of the rotor.

Position Sensors (Hall Sensors)

Since there are no brushes in a BLDC Motor, the commutation is controlled electronically. In order to rotate the motor, the windings of the stator must be energized in a Sequence and the position of the rotor (ie. the North and South poles of the rotor) must be known to precisely energize a particular set of stator windings.

A Position Sensor, which is usually a Hall Sensor (that works on the principle of Hall Effect) is generally used to detect the position of the rotor and transform it into an electrical signal Most BLDC Motors use three Hall Sensors that are embedded into the stator to sense the rotor's position.

The output of the Hall Sensor will be either HIGH or LOW depending on whether the North or South pole of the rotor passes near it. By combining the results from the three sensors, the exact sequence of energizing can be determined.

Working Principle

Consider the following setup of three windings in the stator designated A, B and C. For the sake of understanding, let us replace the rotor with a single magnet.

We know that when a current is applied through a coil, a magnetic field is generated and the orientation of the field lines i,e the poles of the generated magnet will depend on the direction of the current flowing through the coil.

Using this principle, if we supply current to the coil. A so that it will generate a magnetic field and attract the rotor magnet. The position of the rotor magnet will shift slightly clockwise and will align with A.

If we now pass current through coils B and Cone after the other (in that order). the rotor magnet will rotate in clock wise direction.

To increase efficiency, we can wind the opposite coils using a single coil so that we get double attraction Further increasing the efficiency, we can energize two coils at the sarne time idle. that one coil will attract the magnet and the other coil will repel it. During this time, the third will be For a complete 360° rotation of the rotor magnet, six possible combinations of the coils A, B and C are applicable and are shown in the following timing diagram.

Base on the above diagram, we can confirm that at any time, one phase is positive, one phase is negative and the third phase is idle.

for floating So. based on the inputs from the Hall Sensors, we have two switch the phases as per the above diagram.

Driving Brushless DC Motors

If stator and rotor are essential parts of a BLDC Motor that are integral to it, then the driving electronics are equally essential. Block diagram of a typical Brushless DC Motor control or drive system is shown in the following image.

This drive circuitry is often known as Electronic Speed Controller System or simply an ESC. One common setup is called the Full Bridge Drive Circuit. It consists of an MCU with PWM outputs, six MOSFETS for the three phases of the stator windings, feedback from the Hall sensors and some power supply related components.

The MCU can be programmed to appropriately switch the MOSFETS based on the data from the Hall Sensors

Advantages of BLDC Motors

• Since BLDC Motors are electronically commutated, there are several advantages over traditional Brushed DC Motors. Some of them are

• No wear and tear (due to absence of brushes)

• Hgh efficency

• Better speed vs torque characteristics Long life

• Less noise or noiseless operation Significantly higher RPM

Applications of Brushless DC Motors

Some of the areas of applications of BLDC Motors are mentioned below:

• Single speed applications

• Adjustable speed applications

• Position control

• Low noise applications

• High speed applications