Abstract

In this paper, focuses on a low cost DC motor speed control system using a

microcontroller logic pwm controller to control brushless DC Motor. In a

digital controller of brushless DC Motor, the control accuracy is of a high level,

and it has a fast response time. Here a microcontroller of 8-bit type (80CL580)

will be used as the system’s core controller in other to acquire an accurate

logic control algorithm. The DC motor speed control has found vast

application in the day to day application of modern day engineering.

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CHAPTER ONE

1.0 INTRODUCTION

Direct current (DC) motors have been widely used in many industrial

applications such as electric vehicles, steel rolling mills, electric cranes, and

robotic manipulators due to precise, wide, simple, and continuous control

characteristics. Traditionally rheostatic armature control method was widely

used for the speed control of low power DC motors. However the

controllability, cheapness, higher efficiency, and higher current carrying

capabilities of static power converters brought a major change in the

performance of electrical drives. The desired torque-speed characteristics

could be achieved by the use of conventional pulse width modulation

controllers (PWM).

In recent years PWM were effectively introduced to improve the performance

of nonlinear systems. The application of PWM is very promising in system

identification and control due to learning ability, massive parallelism, fast

adaptation, inherent approximation capability, and high degree of tolerance. A

constant-power field weakening controller based on load-adaptive multi-

input multi-output linearization technique has been proposed to effectively

operate a separately excited DC motor in the high-speed regimes. A single-

phase uniform PWM DC-DC buck-boost converter with only one switching

device able to produce a controllable DC voltage ranging from zero to more

than the maximum value of input dc voltage has been used for armature

voltage control method of a separately excited DC motor the drives using poly-

phase brushless DC motors fed by a PWM inverter with current regulation.

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The novelty of this paper lies in the application of a “dc motor speed control

using pwm” for the speed control of a DC motor.

The purpose of a dc motor speed controller is to take a signal representing the

demanded speed, and to drive a motor at that speed. The controller may or

may not actually measure the speed of the motor. If it does, it is called a

Feedback Speed Controller or Closed Loop Speed Controller, if not it is called

an Open Loop Speed Controller.

Recently, a brushless DC Motor (BLDC) has been rapidly demanded due to

preciseness of industrial technology and increase of various kind of control

device. Because a brushless DC Motor is suitable as a servo motor because of

its high efficiency and excellent control character.

Fig 1.0 Brushless DC (BLDC) motor

The 80CL580 that is 8 bit microcontroller will be used to produce the system

so as to monitor a digital speed change in analog signal for efficiency.

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Dc motors come in a variety of forms, and the speed controller's motor drive

output will be different dependent on these forms. The speed controller

presented here is designed to drive a simple cheap DC motor, which can be

purchased from any electrical/electronic store. These motors are generally

series wound, which means to reverse them, they must be altered slightly.

Below is a simple block diagram of the speed controller.

Fig 1.0 Block diagram of a microcontroller fan speed controller using pwm

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CHAPTER TWO

LITERATURE REVIEW

In the past, when only partial power was needed (such as for a sewing

machine motor), a rheostat (located in the sewing machine's foot pedal)

connected in series with the motor adjusted the amount of current flowing

through the motor, but also wasted power as heat in the resistor element. It

was an inefficient scheme, but tolerable because the total power was low. This

was one of several methods of controlling power. There were others—some

still in use—such as variable autotransformers, including the trademarked

'Autrastat' for theatrical lighting; and the Variac, for general DC power

adjustment. These were quite efficient, and also relatively costly.

For about a century, some variable-speed electric motors have had decent

efficiency, but they were somewhat more complex than constant-speed

motors, and sometimes required bulky external electrical apparatus, such as a

bank of variable power resistors or rotating converter such as Ward Leonard

drive .

However, in addition to motor drives, pumps and robotic servos, there was a

great need for compact and low cost means for applying adjustable power for

many devices, motors, such as electric stoves and lamp dimmers.

One of early applications of PWM was in the Sinclair X10, a 10 W audio

amplifier available in kit form in the 1960s. At around the same time PWM

started to be used in DC motor control.

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For some time ASD has been in use for motor speed control. An adjustable-

speed drive (ASD) or variable-speed drive (VSD) is an interconnected

combination of equipment that provides a means of driving and adjusting the

operating speed of a mechanical load. An electrical adjustable-speed drive

consists of an electric motor and a speed controller or power converter plus

auxiliary devices and equipment. In common usage, the term “drive” is often

applied to just the controller.

In this seminar, a different approach will be used. This technique involves the

use of a microcontroller for the control of a dc motor speed by generating a

PWM signal that is used to control the speed. This is done by modifying the

width of the conduction time of the power mosfet motor driver.

This technique give superior power control of the motor because there are no

moving or passive parts involved hence power consumption is reduced.

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CHAPTER THREE

3.0 BRUSHLESS DC MOTOR FUNDAMENTALS

Brushless DC motors consist of a permanent magnet rotor with a multi-phase

stator winding. As the name implies, BLDC motors do not use brushes for

commutation; instead, they are electronically commutated.

Brushless DC (BLDC) motors are rapidly gaining popularity. They offer longer

life and less maintenance than conventional brushed motors. Some other

advantages over brushed DC motors and induction motors are: better speed

versus torque characteristics, noiseless operation and higher speed ranges.

And in addition, the ratio of torque delivered to the size of the motor is higher,

making them useful in applications where space and weight are critical

factors. The electromagnets do not move; instead, the permanent magnets

rotate and the phase stator windings remain static.

Fig 3.0 Brushless DC motor

The speed and torque of the motor depend on the strength of the magnetic

field generated by the energized windings of the motor, which depend on the

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current through them. Therefore adjusting the rotor voltage (and current) will

change the motor speed.

3.1 HOW TO CONTROL A BRUSHLESS DC MOTOR

A BLDC motor is driven by voltage strokes coupled with the given rotor

position. These voltage strokes must be properly applied to the active phases

of the phase winding system so that the angle between the stator flux and the

rotor flux is kept close to 90° to get the maximum generated torque.

Therefore, the controller needs some means of determining the rotor's

orientation/position (relative to the stator coils.)

3.2 SPEED CONTROL

By simply varying the voltage across the motor speed of the motor can easily

be controlled. When using PWM outputs to control the six switches of the

three-phase bridge, variation of the motor voltage can be achieved easily by

changing the duty cycle of the PWM signal.

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Fig 3.1 Brushless DC motor switching using power mosfet

3.3 THEORY OF SPEED CONTROL

The speed of a DC motor is directly proportional to the supply voltage, so if we

reduce the supply voltage from 12Volts to 6 Volts, the motor will run at half

the speed. How can this be achieved when the mains voltage is fixed at

12Volts?

Fig 3.1 Relationship between supply voltage and speed

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The speed PWM controller works by varying the average voltage sent to the

motor. It could do this by simply adjusting the voltage sent to the motor, but

this is quite inefficient to do. A better way is to switch the motor's supply on

and off very quickly. If the switching is fast enough, the motor doesn't notice

it, it only notices the average effect.

When you watch a film in the cinema, or the television, what you are actually

seeing is a series of fixed pictures, which change rapidly enough that your

eyes just see the average effect - movement. Your brain fills in the gaps to give

an average effect.

Now imagine a light bulb with a switch. When you close the switch, the bulb

goes on and is at full brightness, say 100 Watts. When you open the switch it

goes off (0 Watts). Now if you close the switch for a fraction of a second, and

then open it for the same amount of time, the filament won't have time to cool

down and heat up, and you will just get an average glow of 50 Watts. This is

how lamp dimmers work, and the same principle is used here to drive a

motor. When the switch is closed, the motor sees 12 Volts, and when it is open

it sees 0 Volts. If the switch is open for the same amount of time as it is closed,

the motor will see an average of 6 Volts, and will run more slowly accordingly.

As the amount of time that the voltage is on increases compared with the

amount of time that it is off, the average speed of the motor increases.

This on-off switching is performed by power MOSFETs. A MOSFET (Metal-

Oxide-Semiconductor Field Effect Transistor) is a device that can turn very

large currents on and off under the control of a low signal level voltage.

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The time that it takes a motor to speed up and slow down under switching

conditions is dependent on the inertia of the rotor (basically how heavy it is),

and how much friction and load torque there is. The graph below shows the

speed of a motor that is being turned on and off fairly slowly:

From the above, the average speed is around 1500, although it varies quite a

bit. If the supply voltage is switched fast enough, it won’t have time to change

speed much, and the speed will be quite steady. This is the principle of switch

mode speed control. Thus the speed is set by PWM – Pulse Width Modulation.

The microcontroller generates the PWM control signal that is used to control

the fan speed in this regards the motor. Because there can be no fan without

the motor.

SUMMARY OF OPERATION

From the fore going, the speed of the motor is directly proportional width of

the pulse. That is to say the wider the width of the pulse the greater the

maximum power transferred to the motor hence the greater the revolution or

speed of the motor thus resulting in an increased fan speed. Also, when the

width is reduced the fan runs at a low speed owing to a less power transfer to

the motor.

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Motor Speed Response to PWM Control Input Signal

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CHAPTER FOUR

CONCLUSION

This seminar note demonstrates the use of a low cost NXP Semiconductors

80CL580`microcontroller for a motor speed control. It may be used as a

starting point for motor control system designers using it as the master

controller.

The 80CL580 is based on an 8bit ARM7 CPU combined with embedded high-

speed flash memory. A superior performance as well as their tiny size, low

power consumption and a blend of on-chip peripherals make these devices

ideal for a wide range of applications. Its PWM features through output match

make them particularly suitable for industrial control of motor speed.

RECONMENDATION

In this seminar, microcontroller has been used solely as the master controller.

In other to enhance the efficiency through put, I suggest the speed control unit

be interfaced with a computer for further data processing.

Also, wireless control should be given a second thought as this will increase

remote controllability so that users can seamlessly control their gadget with

ease.

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