Week-7 Challenge: DC Motor Control

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

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

BJT is a power transistor or BJT is bipolar device. The power transistor blocks high voltage in the OFF state and has high current carrying capacity in the on state.

It is current controlled device(BJT)

It requires sufficient base current for keeping it in on state.

The transistor is operated in 3 different regions:

Region	Emitter Base Junction	Collector Base Junction	state
cutoff	Reverse bias	Reverse bias	Transistor is OFF
active	Forward bias	Reverse bias	Transistor is amplifier
saturation	Forward bias	Forward bias	Transistor is ON

Power BJT used for:

1) Signal processing.

2) Static switches in power electronic converters and choppers.

Switched mode power supplies

MATLAB demo model of the Speed control of a DC motor using BJT H-bridge:

The Bipolar Junction Transistor BJT when used for power switching applications, operates as an IGBT.

The DC motor uses the preset model 5 HP 24V 1750 rpm with a field voltage of 300v.

The DC motor is connected actoss the four switches.

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%.

H-bridge CKT working:

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..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..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.

Operation:

if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground.

Current starts flowing through the motor which energizes the motor in 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.

n 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.

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

OUTPUT RESULTS:

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.

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

Working Principle:

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4.

The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

In the above circuit diagram, the choppers are numbered CH1 to CH4.

First Quadrant Operation:

CHH1 and CH4 turn on the supply entry into CH1 to motor terminal to CH4 is connected in negative of the supply.

The positive of supply is connected to the positive of the motor and negative is connected to the negative of the supply.

In this condition the motor current is positive and voltage also positive.

CH1 OFF then CH4-D2 conductes.

when CH1 is off there is current not flowing through the motor terminal in this case D2 is forward bias the energy stored in inductor.

The current flow through CH4 to D2 terminal and D2 terminal is connected to motor terminal.

this operation is First Quadrant.

Second Quadrant operation:

second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF.

When CH2 is ON at this condition back emf is produce the energy.

The current flow from the back emf CH2 to D4.

D4 is coms in the forward bias the current enterd into the D4 and get back to the motor.

in this condition L will be energized due to the back emf of the motor.

Third Quadrant Operation:

To obtain third quadrant operation, both the load voltage and load current should be negative.

The current and voltage are If the direction is opposite to the polaritys it is considered negative.

Fourth Quadrant operation:

fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF.

The polarity of load emf E needs to be reversed in this case too like third quadrant operation.

When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON.

When CH4 is made OFF, current is fed back to the source through diodes D2, D3.

Here load voltage is negative but the load current is always positive.

ouput result:

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

2-quadrant chopper simulink block:

Input dc source is 24v.

There is second dc source 12v.

This is pulse generator inputs.

Output result:

2-quadrant chopper:

Whem CH2 is on diode conducts i.e on,output voltage Vo=0 and load current Io, is negative.

When CH1 is on diode conducts i.e D-on,output voltage Vo=Vs and load current Io is positive.

Average load voltage is always positive but average load current may be positive or negative.

Therefor power flow may be from source to loaad first quadrant or from load to source second quadrant.

CH1 and CH2 should be simultaneously turned on it leads to short circuit on supply lines.

3) AIM: Explain in a brief about operation of BLDC motor

INTRODUCTION OF BLDC MOTOR(Brushless DC Motor):

BLDC motors are superior to brushed DC motors in many ways, such as 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:

A brushless DC motor is a permanent magnet synchronous electric motor which is deiven by direct current (DC) electricity and it accomplishes electronically controlled commutation s/m instead of a mechanically commutation s/m.

BLDC motor 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 rotates and current carrying conductors are fixed.

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.

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 part:

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.

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 and repulsion forces .

By 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 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