Week-7 Challenge: DC Motor Control

OBJECTIVE 1: Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge

ANS: DC motors are used commonly at lots of electrical application. DC drive systems are often used in many industrial applications such as robotics, actuation and manipulators.

• These motors are easy to drive, fully controllable and readily available in all sizes and configurations When examined these applications dc motors are needed to be operated on variable or constant speed with forward or reverse operation.
• There are many control techniques to obtain control of different speeds.
• For this controlling process the electric drive systems are used frequently. Industrial applications are increasingly required to meet higher performance and reliability requirements.
• DC motors have to be controlled at the DC motor applications
• DC motor is needed to be operated with desired speeds, reverse or forward motion for different time periods To control these types of parameters of the motor, the drive circuit is needed to design.
• A brushed DC motor is very easy to control and DC motor does not require an external driver to operate.
• Speed control of brushed DC motor is also easy because the voltage-speed characteristic of brushed DC motor is relatively linear.
• H bridge DC motor driver is designed and implemented. H bridge curcuit is used for controlling DC motor speed and rotating side.
• The H bridge driver Mosfets are driven by a high frequency PWM signal.
• Controlling the PWM duty cycle is equivalent to controlling the motor terminal voltage.
• which in turn adjust directly the motor speed & DC motor driver is controlled.
• Microcontrollers has been investigated for different speed control of DC motor.

Generally, DC motors try to control with a variable potentiometer or variable resistor connected to a transistor.

In the control method where transistors are used the main problem is heating of these transistors.

Because of heating the transistors system waste the power to outside so this situation is decreasing the efficiency of this motor driving system.

To remove this disadvantage Pulse Width Modulation PWM DC motor control method can be used.

PWM DC motor control method adjusts the motor speed by short pulses.

These PWM pulses fill rate is named as “Duty Cycle”.

According to Duty Cycle speed of the motor can be changed.

If the Duty Cycle is high, the motor turns faster.

If it is low then motor turns more slowly

Generally, DC motors direction of rotation and speed are controlled with using PWM signals.

By means of PWM signals applied voltage to DC motor windings can be changed. This changing is seen as a speed on the motor.

The voltage increasing causes the speed increasing.

In many applications simple voltage regulation would cause a lot of power loss in control circuit, so Pulse Width Modulation (PWM) is used in many DC motor controlling application.

The rapid rising and falling edges ensure that the semiconductor power devices are turned on or turned off as fast as practically possible to minimize the switching transition time and the associated switching losses.

The average value of voltage fed to the load is controlled by turning the switch between supply and load ON and OFF at a fast pace.

The longer the switch is ON compared to the OFF periods, the higher the power supplied to the load.

The term duty cycle describes the proportion of ON time to the regular interval or period of time, a low duty cycle corresponds to low power, because the power is OFF for most of the time.

Duty cycle is expressed in percent 100% being fully ON.

This “on” time is referred to as the “duty cycle” and is stated as a percentage.

To change the direction of rotation of the DC motor, it is necessary to change the polarity of the voltage applied to the winding coil.

The most common way is using “H bridge” configuration for changing easily DC motor direction.

The shape of the circuit resembles an “H”, so it was named H bridge.

For ‘forward’ rotation Q1 and Q4 are switched on while Q2 and Q3 are off. For ‘reverse’ rotation Q2 and Q3 are on while Q1 and Q4 are off

WORKING OF H-BRIDGE

• Designed H bridge circuit contains two NPN (Q1, Q2) and two PNP (Q3,Q4) type Power Mosfets.
• Direction of rotation and DC motor speed can be varied these mosfets.
• PWM signals are applied by means of mosfet driver TLP250 to NPN type Q1 and Q2 mosfets.
• According the duty rate of PWM signals DC motor speed can be increased or decreased. And steady +5V applied to PNP type Q3 and Q4 mosfets.
• While DC motor is rotated to right direction Q3 and Q2 mosfets have to be swithed. If DC motor want to rotate left direction, then Q4 and Q1 mosfets have to be switched.
• When H1 signal applied Q5 BC337 transistor is switched and +24V is grounded. So PNP type Q3 mosfet is switched.
• At the same time when PWM signal is applied to L2 NPN type Q2 is switched.
• So, DC motor is turned right direction.
• The same operations are performed to when Q4 and Q1 switched. And DC motor current can be flow on ACS-712 current sensor.
• By means of this sensor DC motor current level can be determined.
• Such as rush current and short circuit situations are can be identified and blocked.
• Hence DC motor can be protected.

SPEED CONTROL OF A DC MOTOR USING BJT H-BRIDGE:

THE ARMATURE CURRENT SHOOT-UP FROM THE SCOPE RESULTS:

• The spike shown at initial t=1sec. during starting od DC motor is shown above in the scope results.
• The armature current spike is observed due to initial starting torque required to startup the DC motor shaft from rest.
• Starting current is high because motor is initially at rest, and due to the supply of constant voltage & due to its inertia & mass of armature shaft, it requires high torque, which results in high starting current.

zoomed view of armature current is also shown , it is observed the dynamic nature of current.

THE HELP SECTION OF ‘THE FOUR-QUADRANT CHOPPER DC DRIVE (DC7) BLOCK’. COMPARE IT WITH THE H-BRIDGE MODEL:

Step 1:

Voltage and current are positive and power is also positive.

Power direction: source to load

IGBT 1: ON

IGBT 4: ON

IGBT 2: OFF

IGBT 3: OFF

Step 2:

Voltage positive, current negative, power negative.

Power direction: load to source

IGBT 1: OFF

IGBT 4: ON

IGBT 2: ON

IGBT 3: OFF

Step 3:

Voltage & current negative: power is positive.

Power direction: source to load

IGBT 1: OFF

IGBT 4: OFF

IGBT 2: ON

IGBT 3: ON

Step 4:

Voltage: negative, current: positive, power: negative.

Power direction: load to source

IGBT 1: OFF

IGBT 4: ON

IGBT 2: ON

IGBT 3: OFF

OBJECTIVE 2: 2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

ANS:

The two-quadrant chopper operates in the first and second quadrants, allowing positive and negative output current.

The Control subsystem implements a simple PI-based control algorithm for controlling the output current.

The load of the system is considered constant throughout the simulation.

In two quadrant chopper, the average voltage will be always be positive but the average load current might be positive or negative.

Two switches CH1 & CH2 should not be turned on simultaneously as the combined action may cause a short circuit in supply lines.

For regenerative braking & motoring these types of chopper configuration is used.

SIMULINK MODEL:

Blocks to be used:

• Power GUI
• Voltage measurement Block
• Current measurement block
• DC voltage source
• MOSFET
• Pulse generator
• Diode
• Series RLC band
• Logical Operator

• The Two - quadrant chopper DC drive block represents a two-quadrant, DC - supplied, chopper drive for DC motor.
• This drive features closed-loop speed control with a two-quadrant operation. The speed control loop output thereference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal, to obtain the required PWM signals for the chopper.
• The main advantages of this drive, compared with other DC drives, are its implementation simplicity and the that it can operate in two quadrants. In addition due to the use of a high switching frequency DC-DC converter, a lower armature current ripple is obtained. However, for all two-quadrant DC drives reversible and regenerative operations, which are required in most DC drives cannot be obtained.

OUTPUT:

3

OBJECTIVE 3: Explain in a brief about operation of BLDC motor.

A BLDC motor controller regulates the speed and torque of the motor; it can also start, stop, and reverse its rotation.

To understand the working principles of the controller, let us start first with the construction of a brushless motor. Its major components comprise:

An ARMATURE or ROTOR made of permanent and in many cases neodymium magnets:

• A STATOR with windings that create a magnetic field when energized.
• The rotor’s magnets and stator’s windings provide the rotation of the motor.

They attract each other with opposite poles and repel each other with the same poles. A similar process takes place in a brushed DC motor.

The essential difference is in the method of switching the current applied to the wire windings.

In a BDC motor, this is a mechanical process triggered by a commutator with brushes. In a BLDC motor, it happens electronically with the help of transistor switches.

A BLDC motor controller detects the position of the rotor either by using sensors (for example, a Hall-effect sensor) or sensorlessly.

The sensors measure the rotor’s position and send out this data.

The controller receives the information and enables the transistors to switch the current and energize the required winding of the stator at the right time.

Types of BLDC Motors and Controllers:

Depending on the rotor’s placement, BLDC motors can be of two types:

• Inrunner motor (the rotor is internal, and the stator is on the outside of the motor);
• Outrunner motor (the rotor is external, so the permanent magnets spin around the stator together with the motor’s case).

In runners have a more lightweight construction and a better rotational speed because of their smaller rotating diameter.

In their turn, outrunner motors have a higher torque because of the longer arm and greater electromotive force applied to the rotor.

Three-phase brushless DC motors can have two different types of winding connections:

• wye (Y) or star connection (windings meet at the center forming a wye);
• delta (Δ) connection (windings are connected in series forming a triangle).
  
  

The wye configuration has a neutral wire connected to ground. This can protect the motor from overvoltage and overload. The delta connection has no neutrals, so it works better for motors with a balanced load. However, each of these types can show efficient performance depending on your requirements.

BLDC Motor Controller Circuit Design:

A typical BLDC motor controller has a half-bridge or half-H bridge circuit. Unlike an H bridge, this circuit configuration has only two switches - one high-side and one low-side transistor.

Most brushless motors use two or three-phase power systems. So in a BLDC motor controller circuit diagram, this will look like two or three half-bridges (depending on the number of phases) with a pair of switches each.

Let’s take a closer look at a 3 phase brushless DC motor controller with Hall-effect sensors to view the basic principles of its circuit design.

The stator has three-phase windings located at 120° to one another.

Each winding has a vector representation of voltage and current applied to the stator.

The BLDC motor controller Hall sensors identify the rotor’s position.

Upon receiving the sensor data, the power MOSFETs switch the current, injecting it into the right winding

In a high power brushless DC motor controller, IGBTs and GaN switches can replace MOSFETs.

Either integrated or discrete gate drivers can control the transistors.

The drivers of a brushless motor controller schematic act as intermediaries between the switches and a microcontroller (MCU).

The three-phase BLDC motor controller circuit includes six steps necessary to complete a full switching cycle (that is to energize all the three windings of the stator).

By turning the high-side and low-side transistors on and off, the current flows through the stator windings in sequence.

Designing a BLDC motor controller, you can consider different approaches to current switching, including trapezoidal and sinusoidal commutation.

The names of these methods relate to the signal waveforms.

Conclusion:

Brushless DC motors have been in use for over fifty years.

Their application area ranges from a small-sized consumer device to a complicated industrial automation system.

The all-electronic control system increases torque, improves wide-range speed regulation, and enhances other characteristics of the motor.

Despite the reliability and high efficiency, such motors are not universal. First of all, they have a high price and a complex controller’s implementation.

Thus, for some projects, a brushed DC motor controller could become a reasonable option.

One of the key design challenges of a BLDC motor controller lies in determining the position of the rotor. You can accomplish this in different ways, such as:

• using a suitable sensing device;
• measuring back electromotive force created in the stator’s windings;
• combining various methods to achieve the desired result.