In power electronics chopper circuits, unidirectional power semiconductors are used. f these semiconductor devices are arranged appropriately, a chopper can work in any of the four quadrants. can classify chopper circuits according to their working in any of these four quadrants as type A, type B, type C, type D and type E. Now take a look at these classifications and their characteristics of various classifications.

Type A Chopper or First–Quadrant Chopper

This type of chopper is shown in the figure. t is known as a first-quadrant chopper or type A chopper. hen the chopper is on, v_{0 =} V_S as a result and the current flows in the direction of the load. when the chopper is off v0, is zero but I₀ continues to flow in the same direction through the freewheeling diode FD, thus the average value of voltage and current say V₀ and I₀ will be always positive as shown in the graph.

In a type A chopper the power flow will be always from the source to the load's the average voltage V₀ is less than the dc input voltage V_s

Type B Chopper or Second-Quadrant Chopper

In type B or second quadrant chopper, the load must always contain a dc source E. en the chopper is on, v₀ is zero but the load voltage E drives the current through the inductor L and the chopper, L stores the energy during the time T_on of the chopper. W en the chopper is off, v₀ =( E+ L . d dt ) will be more than the source voltage V_s. Because of this the diode D2 will be forward biased and begins conducting and hence the power starts flowing to the source. No matter whether the chopper is on or off the current I₀ will be flowing out of the load and be treated negatively. Sine V_O is positive and the current I₀ is negative, the direction of power flow will be from load to source. The load voltage V₀ = (E+L .di/dt ) will be more than the voltage V_s so the type B chopper is also known as a step-up chopper.

Type -C chopper or Two-quadrant type-A Chopper

Type C chopper is obtained by connecting type –A and type –B choppers in parallel. We'll always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper is on 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 and it will be positive regardless the chopper is on or the FD conducting. The l d current will be negative if the chopper or the diode D2 conducts. We can ay the chopper and FD operate together as a type-A chopper in the first quadrant. In the second quadrant, the chopper and D2 will operate together as type –B choppers.

The average voltage will be always positive but the average load current might be positive or negative. The pow r flow may be living the first quadrant operation ie from source to load or from load to source like the second quadrant operation. The two hoppers should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring, this type of chopper configuration is used.

Type D Chopper or Two-Quadrant Type –B Chopper

The circuit diagram of the type D chopper is shown in the above figure. When the two choppers are on the output voltage v₀ will be equal to V_s. When v₀ = – V_s the two choppers will be off but both the diodes D1 and D2 will start conducting. V₀ the average output voltage will be positive when the choppers turn on the time T_on will be more than the turn off time T_off its shown in the waveform below. As the d odes and choppers conduct current only in one direction the direction of load current will be always positive.

The power flows from source to load as the average values of both v₀ and i₀ are positive. From the waveform, it is seen that the average value of V₀ is positive thus the fourth quadrant operation of the type D chopper is obtained.

From the waveforms, the Average value of output voltage is given by
V0= (V_s T_on-V_sT_off)/T = V_s.(T_on-T_off)/T

Type –E chopper or the Fourth-Quadrant Chopper

Type E or the fourth quadrant chopper consists of four semiconductor switches and four diodes arranged antiparallel. The 4 choppers are numbered according to which quadrant they belong. Their operation will be in each quadrant and the corresponding chopper only is active in its quadrant.

First Quadrant

During the first quadrant operation, the chopper CH4 will be on. Chopper CH will be off and CH1 will be operated. AS the CH1 and CH4 are on the load voltage v₀ will be equal to the source voltage V_s and the load current i₀ will begin to flow. v₀ and i₀ will be positive as the first quadrant operation is taking place. As soon as the chopper CH1 is turned off, the positive current freewheels through CH4 and the diode D2. The type E chopper acts as a step-down chopper in the first quadrant.

Second Quadrant

In this case, the chopper CH2 will be operational and the other three are kept off. As CH2 is o negative current will starts flowing through the inductor L . CH2, E and D . Energy is stated in the inductor L as the chopper CH2 is on. When CH2 is of the current will be fed back to the source through the diodes D1 and D4. Here (E+L.di/ t) will be more than the source voltage V_s. In the second quadrant, the chopper will act as a step-up chopper as the power is fed back from load to source.

Third Quadrant

In the third quadrant operation, CH1 will be kept off, CH2 will be on and CH3 is operated. For this quad ant working the polarity of the load should be reversed. As the chopper CH3 is on, the load gets connected to the source V_s and v₀ and i₀ will be negative and the third quadrant operation will take place. This chopper acts as a step-down chopper.

Fourth Quadrant

CH4 will be operated and CH1, CH2 and CH3 will be off. When the chop er CH4 is turned on positive current starts to flow through CH4, D2, and E and the inductor L will store energy. As the CH4 is turned off the current is feedback to the source through the diodes D2 and D3, the operation will be in the fourth quadrant as the load voltage is negative but the load current is positive. The chopper a ts as a step-up chopper as the power is fed back from load to source.

SIMULINK MODEL:

• First, create the Simulink model as shown in the above figure.
• Next, set the voltage values in DC Source to 24 and 12 V and set the pulse width to 50 % with Amplitude to be 10 and period to be 0.02 secs.
• Next, run the simulation and plot the graphs.

Plots and Graphs:

Conclusion:

• The pulse generator value was given as 10, so the waveform is set to be between 0 and 10.
• The current waveform is fluctuating between the positive and the negative values.
• The minimum value for the voltage is 0 and the maximum value keeps changing but it's always on the positive side.

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Q) Explain in a brief the operation of the BLDC motor.

A) Brushless DC Motor (BLDC) – Construction, Working & Applications:

Brushless DC motors (BLDC) have been a much-focused area for numerous motor manufacturers as these motors are increasingly the preferred choice in many applications, especially in the field of motor control technology. BLDC motors are superior to brushed DC motors in many ways, such as the ability to operate at high speeds, high efficiency, and better heat dissipation.

They are an indispensable part of modern drive technology, most commonly employed for actuating drives, machine tools, electric propulsion, robotics, computer peripherals and also for electrical power generation. With the development of sensorless technology besides digital control, these motors become so effective in terms of total system cost, size and reliability.

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 an 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 mechanical commutation system. BLDC motors are also referred to as trapezoidal permanent magnet motors.

Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with a 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 a permanent magnet rotor and a stator with a sequence of coils. In this motor, a 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 the armature field is in space quadrature with the rotor field poles. Hence the for e acting on the rotor causes it to rotate. Hall sensors 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 per cent, whereas brushed type DC motors are 75 to 80 per cent efficient. There are wild 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 rotors are most commonly used.

The construction of this motor has many similarities to a three-phase induction motor as well as a conventional DC motor. This motor has stator and rotor parts like all other motors.

The stator of a BLDC motor is made up of stacked steel laminations to carry the windings. These winding are placed in slots which are axially cut along the inner periphery of the stator. These winding 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.

The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred. For industrial applications and automation systems, 100 V or higher rating motors are used.

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 the required magnetic field density.

Ferrite magnets are inexpensive, however, they have a low flux density for a given volume. Rare earth al oy 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 the permanent magnet on the periphery, a circular core with rectangular magnets, etc.

Hall Sensors:

The Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the computation of the BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energising a particular stator winding, acknowledgement of the rotor position is necessary. So the Hall Effect sensor embedded in the stator senses the rotor position.

Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor gnerates Low and High signals whenever the rotor poles pass near it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensors' responses.

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 is placed in a magnetic field it experiences a force. As a consequence of the reaction force, the magnet will experience an equal and opposite force. In the case, f the BLDC motor, the current-carrying conductor is stationary while the permanent magnet moves.

When the stator coils are electrically switched by a supply 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 the permanent magnet rotor, the rotor continues to rotate.

Consider the figure below in which the motor stator is excited based on different switching states. 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 aligns with stator poles causing the motor to rotate.

Observe that the motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). In this way motor moves in a clockwise direction.

Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor's continuous rotation depends on the switching sequence around the coils. As discussed above Hall sensors give shaft position feedback to the electronic controller unit.

Based on this signal from the sensor, the controller decides on particular coils to energize. Hall-effect sensors generate Low and High-level signals whenever rotor poles pass near them. These signals determine the position of the shaft.

Brushless DC Motor Drive:

As described above that the electronic controller circuit energizes appropriate motor winding by turning transistors or other solid-state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of a MOSFET bridge (also called an inverter bridge), electronic controller, hall effect sensor and BLDC motor.

Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.

In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on the required application. These speed control units are generally implemented with PID controllers to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives.

Advantages of BLDC Motor:

BLDC motor has several advantages over conventional DC motors and some of these are:

• 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
• Quiet operation (or low noise) due to the absence of brushes

Disadvantages of Brushless Motor:

• These motors are costly
• The electronic controller required to control this motor is expensive
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
• Requires complex drive circuitry
• Need for 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 types of equipment, etc. Some specific applications of BLDC motors are

• 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