Week-7 Challenge: DC Motor Control

1.

A. Speed control of a DC motor using BJT H-bridge:-

Construction:-

• Simulink model is constructed as per the circuit diagram.
• H- Bridge consists of 4 BJT and Diode pairs.
• Input voltage to the circuit is given from the pulse generator.
• Pulse width(%) value is given as per the required duty cycle.
• Duty cycle=Ton/(Ton+Toff).
• The switch block is connected to Pulse generator through armature voltage control block.
• This switch seperates the voltage and given to four BJT's using From and Goto blocks.
• The DC Motor is connected to battery and the Mechanical Input of DC Motor is Initiated as torque.

Working:-

• Set the simulation time as 1 sec and run the simulation.
• When Q1 and Q4 are fired, a positive voltage is applied to 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 Motor and Diodes D1 – D4 operates as free-wheeling diode, when Q2 and Q3 are switched OFF.
• Q1 and Q2 – Forward Motoring.
• Q2 and Q3 – Reverse Motoring.
• Load Torque is proportional to the square of speed. T∝ω²

OUTPUTS:-

B. Armature Current:-

• From the above output results, the scope shows IGBT current and Diode current passed in the Quadrant Q3 and D3.
• Pulse width is 75%.
• Where Q3 represents as IGBT current and D3 is represents as Diode current in 'currents' scope.
• Graph shows the condition where Q3 is in OFF state and D3 is ON state for the time upto 0.5sec.
• After the time 0.5sec Q3 is in ON state and D3 is in OFF state.
• From the scope, at starting there is sudden shoot-up in Armature current and is in positive upto 0.5sec.
• When armature current is in positive, the load torque is in forward motoring mode and speed is in positive.
• After 0.5sec there is sudden shoot-up of armature current in negative direction till 1sec.
• When armature current is in negative, the load torque is in reverse motoring mode and speed is in negative.

To reduce the shoot-up of armature current the duty cycle should be reduced.

By changing the pulse generator input value that is pulse width as 50%.

OUTPUTS:-

Here we see that the sudden shoot up is not acted by comparing with 75 of pulse width. Same cycle time is followed in this experiment also.

From the above observation the shoot-Up is reduced when motor changes direction from Forward to Reverse compared to 75% of Pulse width.

C.

Simulink Model for Four-Quadrant Chopper DC Drive (DC7):-

OUTPUT:-

Comparison between H-Bridge and Four-Quadrant Chopper DC Drive (DC7):-

H-Bridge	Four-Quadrant Chopper DC Drive (DC7)
The source is of Fixed DC supply.	The Source is an AC supply which can be rectified to DC varying supply.
It’s used for changing the direction of the Motor (forward and reverse).	Here the Motor can be Operated in 4 different Mode (Forward Motoring and Braking, Reverse Motoring and Braking)
Regenerative Braking cannot be available.	Regenerative Braking can be achieved.
The motor speed reference cannot be fed.	The motor speed reference can be fed.
Signals are continuous.	Signals are not continuous and they are discrete.
Less control of the Vehicle.	Better control of the Vehicle.
The armature current shoots up during the phase transition.	The armature current doesn’t shoot up to the value of BJT H-bridge but for low voltage value it’s negligible.
Maintanence is less compared to H-Bridge and efficiency is lower compared to Chopper.	Maintanence is more compared to Chopper and efficiency is higher compared to H-Bridge.
Used in EV Charging.	Used in Industrial and Commercial applications.

2.

Two-Quadrant Chopper:-

Circuit Diagram:-

Simulink model:-

Link for the Simulink Model:- https://drive.google.com/file/d/1CerkgiiU7ag8qp18h1GQBiR9ZRroeKwA/view?usp=sharing

Explanation of Working of 2-Quadrant Chopper:-

• Open Library browser and select the required components for the construction of 2-Quadrant Chopper.
• Required blocks are MOSFET, Diodes, Pulse Generator, Logical Operator, Voltage and Current Measurement, RLC Branch, Scope and DC Voltage Source blocks.
• Take Powergui block and keep it in continuous mode.
• Arrange the block as per the circuit diagram.
• Untick "Show Measurement Port " for MOSFET and Diode blocks.
• Connect the MOSFET and Diodes by changing the required direction.
• In RLC Branch set the branch type as 'L' i.e Inductor which is considered as Load.
• Open Pulse Generator and give values, amplitude as 5, Time Period as 0.02 and Pulse Width as 50.
• Open logical operator and select NOT in Operation options, connects its Input to output of Pulse Generator block. The reason we use NOT gate to ensure both the Switches does not conduct simultaneously if they do so supply will be short-circuited.
• We will be giving the tapping for the Pulse gate terminal for 1 switch CH1and from NOT gate we are giving it to other switch CH2. So if CH1 is conducting CH2 will not conduct because we are using the NOT gate.
• Voltage Measurement Block is connected between the input of load and DC Voltage Block 2.
• Connect the Voltage Measurement, Current Measurement and Pulse Generator Blocks to the scope to get the required outputs graphs.
• Change the DC Voltage Source1 amplitude to 24V and DC Voltage Source2 amplitude to 12V.
• Take the simulation time as 1sec and Run the simulation.

OUTPUT:-

3.

Brushless DC Motor:-

Definition:-

A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply.
It uses an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows.
The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor.
This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

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

Stator:

Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which 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 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.Stator of BLDC Motor
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 required magnetic field density.Rotor of brushless motor
Ferrite magnets are inexpensive, however they have a low flux density for a given volume. 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.

When the stator coils are electrically switched by a supply source, it becomes 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 trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

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 align with stator poles causing motor to rotate.

Observe that 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). By this way motor moves in a clockwise direction.BLDC motor operation

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 continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

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

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.
• Quite operation (or low noise) due to absence of brushes

Disadvantages of Brushless Motor:

• These motors are costly.
• 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 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.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.