Week-7 Challenge: DC Motor Control

1.Objective:

•   Speed control of induction motor using BJT H-bridge.

Bipolar Junction Transister:

1. BJT is a three terminal semiconductor device.It can be used as either amplifier or as a switch.The three terminals are base(B),emitter(E) and collector(C).
2. There are two types of BJT NPN and PNP.

Construction:

                                                  

• Emitter is highly doped 
• Base is lightly doped 
• Collector is mediumly doped

Operation:

There are three operating regions of a bipolar junction transistor:

• Active region: The region in which the transistors operate as an amplifier.
• Saturation region: The region in which the transistor is fully on and operates as a switch such that collector current is equal to the saturation current.
• Cut-off region: The region in which the transistor is fully off and collector current is equal to zero.

H-Bridge:

• Case1: FORWARD Motoring

                               In this case Switch s1 and s4 are on state

                                 switch s3 and s2 are in off state

                               Motor runs in one direction.

• Case2: REVERSE motoring

                              In this case Switch s3 and s2 are on state

                              switch s1 and s4 are in off state

                              Motor runs in another  direction.

Simulation:

 

The model contain several blocks:

• Input voltage 240v
• H-bridge and Motor block

                

                          The switches are IGBT and motor is separately exicited dc brushed motor whose intput is mechanical torque.

                                              

• The input to the motor is torque, for FAN type of load T = K w^2;

   

• The IGBT switches are controlled by a technique called pwm technique.

• Time period = 0.002s
• duty cycle = 0.75

Simulation Results:

 

Observations:

•  The motor is run as a fan type load.
• In simulation time of 1 sec 0.5 motor act a load ie the supply is given to the motor in order to rotate in forward direction after 0.5 the  supply to the motor is reversed to rotate inreverse direction.
• 
•  The above plot shows the armature current and freewheel diode current.

 

                              speed vs torque plot

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

For a DC shunt Motor 

                                       

Vt = armature voltage

Eb = Back emf

wkt        Vt = Eb + IaRa   -----------------------------1

                -----------------------------2

thus Eb is propotional to speed

so from eqn 1

• Ia = (Vt - Eb)/Ra

Reason for current shootup:

• In the Dc motor speed  control the armature voltage is 240 v and duty cycle is 75% so Vt = 240*0.75=180v
• At starting speed will be low,so back emf will be low so current will be high,as speed increases the back emf get increases thus the armature current reduces.
• At the time of reversal the armature current shoots up more than starting current 
• 
• Starting Ia =38A                At reversal Ia = 61 A
• If we change the duty cycle to 50% the current shootup reduces.
• 
• here we can able to see that only at the direction of reversal the current shootup in negative direction.At starting the apply voltage is less so starting current is less. 
• If duty cycle is 90% then starting current increases.
• 

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

Working:

               The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

               The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

               The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

              At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.

              At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.

               At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

comparision:

H-Bridge	Four quadrant operation
·        The motor can operate in two quadrants only in forward motoring and reverse motoring.	·        The motor can operate in four quadrants only in forward motoring and braking reverse motoring and braking.
·        Here regenerative is not possible	·        Regenerative is possible
·        Speed control is not performed	·        Better speed control can be achived

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2.TWO QUADRANT CHOPPER:

• Two quadrant chopper or Class C chopper works on forward motoring and forward braking or forward regenerative braking.

Case 1: Forward Motoring

• CH1 will be on and CH2 will be in off state
• When CH1 is ON the current flows from source->CH1->load
• when CH1 is in OFF then Load->FDdiode.

Case 2:  Forward braking or Forward Regenerative braking.

• CH2 will be on and CH1 will be OFF
• when CH2 is ON then load ->Ch2->load
• if ch2 off Load->D2-> source.

Result:

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3.BRUSHLESS DC 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 coil 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 automotive 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 (NdFeB)).

Based on the application, the number of poles can vary between two and eight with North (N) and South (S) poles placed alternately. The following image shows three different arrangements 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 case, the magnets 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 (i.e. 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.

Read about HOW TO USE HALL EFFECT SENSOR WITH ARDUINO?

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 C one 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 same time so that one coil will attract the magnet and the other coil will repel it. During this time, the third will be idle.

For a complete 360⁰ 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 (or 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)
• High efficiency
• 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