Challenge

Week-7 Challenge: DC Motor Control

SShailendra Yadav22 Mar 2023

SOLUTION [1]:

An H-bridge is built of four switches that control the flow of current to a load(Motor load)
IGBT acts as BJT in power switching devices.
Four IGBT switches are used for inverting the DC voltage 0 to 240 v supply.The current flows from collector(C) to emitter(E).
The gate terminals are controlled by the giving pulses(0 & 1) according to the armature voltge of motor given by pulse generator.

Above image shows the value of voltage from initial to final in the step time. According to the pulses on and off this voltage supplied to the IGBT's.

Here in model four switches(IGBT) are used Q1,Q2,Q3,Q4. Pulses P1,P2,P3,P4 are conected with corresponding switches.
Two state operation is performed in model
In first state, Q1 and Q4 are closed by giving 1V to IGBT'S,curre nt from Source(Battery) passes through Q1 and Q4.Diode D2,D3 are used as free wheeling diode in this state.
This is a case of Forward motoring and forward breaking
In Second state,Q2 and Q3 are closed by giving 1V to IGBT's,current from Source(Battery) passes through Q2 and Q3.Diode D1,D4 are used as free wheeling diode in this state.
This Is a case of a reverse motoring and reverse breaking.The current direction is opposite to the first state.
Both the states are occuring in 0.5 seconds in their sample time.Thus DC supply converted and Supplied to the armature of the motor.
Here free wheeling diodes are used as a safety purpose of this inverter circuit as when the flow of current on low values protects the circuit and machine by feedback of the current or in high voltage fluctuations voltage get splited & gives protection to the motor and circuit components.
In motor operation, armature terminals are connected with a 240V along the inverter circuit and field windings are connected with 300 V DC suppy creates electromagnets in field winding
The load torque is given as mechanical input to the motor for its reference
In generated magnetic fields, variable current carring conductors in rotor armature produces forces according to fleming left hand rule principle. Due to that the motor starts to rotate.
At the end run this model and scope result are shown below.

Scope Result:the speed of the motor, armature current and load torque of the motor is given below.

We can se that in above result the armature current shoots up when the motor is made to change direction at 0.5s of the simulation.

SOLUTION [2]:

To see what will happen if we reduce the width from 75 to 50 in pulse generator

at T=0.5 of the simulation the motor changes direction and the current shoot is reduced accordingly. Due to the reduced pulse width, the speed of the motor and the load torque is also reduced this can be seen in above scope result.

Conclusion:Hence from all the simulation results it is concluded that, when the duty cycle of the pulse generator block is low then the speed, current and load torque is also low accordingly.

SOLUTION [3]:

The Four-Quadrant Chopper DC Drive (DC7) block

The Dc7-four quadrent chopper 200 HP DC drive model is opened BY Searching in Help window of MATLAB.The simulation window of the model is given below.

The model consists of a 380V 50Hz three phase AC supply with phase angle 0 which is connected to a converter block. In the converter block it has a rectifier bridge and a braking chopper. The output of the rectifier bridge is connected to the braking chopper. The braking chopper is used to limit the bus voltage during braking mode with a frequency of 4KHz.

A DC7 block situated near the converter block takes inputs such as speed reference, voltage input from AC to DC converter block and load torque value or speed of motor or mechanical rotation port from the motor.

The Four-Quadrant Chopper DC Drive

Inputs and output connections

Armature voltage ,speed referance ,motor torque referance are given as inputs.

Motor,convertor,controller,motor angular speed are get as outputs.

Comparison Table

The Four-Quadrant Chopper DC Drive (DC7)

H-Bridge

1. Speed controlled by varying current pulses from PI current controller

2. In four quadrantchopper DC Drive regenerative breaking takes place by changing polarity of current flow

3. In this the operation is discreat

4. simulation time is more comapred to H-Bridge

1. speed controlled by varying duty cycles voltages.

2. In H-bridge model braking is occured by changing polarity of current flow,dynamic breaking occurs in it.

3. In H-Bridge the operation is Continuous

4. simulation time is les

Q2: SOLUTION :CIRCUIT DIAGRAM OF TWO QUADRANT CHOPPER

Working of circuit

Case-1: When Mosfet1 is switched ON / OFF

When chopper Mosfet1 is switched ON, source V_s directly gets connected to the load and hence, load voltage V_o is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.

When Mosfet1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage V_o becomes zero. However, the i_o continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage V_o and current I_o are positive and hence operation of chopper is in first quadrant.

Case-2: When Mosfet2 is switched ON / OFF

When chopper Mosfet2 is switched ON, load DC source E drives current through Mosfet2 and load. The direction of this current i_o will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage V_o is zero during this time. When Mosfet2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is V_s in this time as the load is connected to the source through D2 during OFF time of chopper Mosfet2. Thus, the load current is always negative i.e. operation of chopper is within second quadrant.

-TWO QUADRANT CHOPPER MODEL USING SIMULINK

Simulink model explanation:

Taken two mosfet blocks and diodes blocks and connected it as shown in circuit diagram. The dc voltage source is connected as shown. Let the voltage of dc voltage source be 24 volts & consider an series RLC branch block and select inductor L . thus block works as a load. To consider LE load i.e 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 to avoid switching at the same time otherwise short circuit will occur .By using NOT gate alternate switching condition achieved.

The amplitude 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 weidth be 50%

Value given to blocks are shown in below ss

SIMULATION SCOPE RESULT

Hence, The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation)

Q3: SOLUTION:-

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, wherein 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 SCR 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.

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature winding are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

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

Construction of BLDC Motor:

In this motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

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.

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.

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.

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.

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.

Consider the figure below in which 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.

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

Applictaion of BLDC Motors

Electric vehicles, hybrid vehicles, and electric bicycles
Computer hard drives and DVD/CD players
Industrial robots, CNC machine tools, and simple belt driven systems
Washing machines, compressors and dryers
Fans, pumps and blowers

Refrences

https://www.electricaltechnology.org & https://robu.in/brushless-dc-motor-working-principle-construction-applications