A Prototype Implementation of a Digital Controlled

Rectifier-Based Wireless PMDC Motor Drive Muhammad Usman Rafique, Umama Rashid, Saba Iqbal, Qasim Shafique, Sehrish Jabeen

Department of Electrical Engineering

COMSATS Institute of Information Technology

Lahore, Pakistan

usmanrafique@ciitlahore.edu.pk, umamarashid@ymail.com

sabaiqbal2012@gmail.com, qasim_shafique@ymail.com, sehrishjabeen53@yahoo.com

Abstract --- A working model of wirelessly controlled

permanent magnet DC motor (PMDC) drive is proposed in this

paper. Proposed design helps enhancing the control of DC

motors that are installed in industries from a central location

though wireless medium, hence replaces dedicated control units

and human supervision. Proposed design is based on the

spectrum analysis carried out on DC motor supply voltage

waveform resulted by half-controlled rectifier circuit. This

methodology not only reduces harmonics in motor voltage and

current but also reduces losses that occur due to high switching

frequencies in conventional pulse-width modulation (PWM)

based drives. Proposed design communicates with a remote

control unit through RF channel. A prototype of proposed design

is implemented to control a medium power PMDC motor and

results are presented. Proposed design may be extended to single-

phase or three-phase AC-DC converter fed DC drives and to

single-phase or three-phase AC motor drives. Proposed design is

fully compatible with 50Hz and 60Hz power systems. Proposed

drive is implemented using a high-power microcontroller unit.

Keywords --- Full-wave controlled rectifier, Spectrum

analysis, Microcontroller unit, DC motor, Thyristor, Wireless.

I. INTRODUCTION

Microprocessor-based power electronic drives have been a stimulating area of research since the advent of the microchip

fabrication technology [1]. In this paper, a wireless DC motor

drive is proposed in order to provide a cost-effective and

remote accessible control over the permanent magnet DC

(PMDC) motor for home and industry applications. Proposed

DC drive is implemented using controlled rectifier-based

converter that produces fewer harmonics as compared to other

variable speed motor drive methodologies such as PWM-

based variable speed drives [2][3]. DC motors are in common

use where high speed and precise motion is required [4].

Modern electrical vehicles make use of DC motors in order to

provide motion to the vehicle [5][6]. Hence the door to carry

out more research in the control and applications of DC

motors is still open.

In the proposed design of digital wireless PMDC motor

drive the parameters such as average value of rectified

waveform and speed of the motor are controlled by a

microcontroller unit (MCU). Proposed design consists of

command and control mechanism where communication

between command unit and control unit is made possible

through a radio frequency (RF) channel. Novelty of the

proposed design lies in that the drive can be used to control

more than one PMDC motors from a central command unit

through wireless medium. Hence a more rigid and economical

solution is resulted that improves industry throughput by

automating the control over machines. Implementation of the

proposed design is not limited to industry and home

applications only but it can also be extended to control DC

motors installed in autonomous vehicles.

Pulse-width modulation (PWM) is amongst the common

techniques that are used to control the speed of different DC

motors such as brushless DC (BLDC) motor and PMDC motor

[9][10]. PWM, consisting of digital pulses, provides easy

control over DC as well as AC machines [11][12]. A simple

control algorithm and a medium power digital controller are

adequate to implement PWM-based variable speed motor

drive [13]. Frequency-domain analysis of PWM waveform

shows that a considerable number of harmonic components

are present in it. These high frequency components are the key

factor to impart negative effects on DC machine such as

heating and excessive current drawing from the power supply.

Currents produced due to high frequency voltages in the DC

motor circuit cause heat and eddy current losses that result in

poor performance of motor. Due to the switching nature and

the existence of high frequency harmonics, PWM technique is

not an ideal solution to implement modern DC drives. Fig. 1

shows the spectrum of 8kHz PWM waveform. It can be

inferred from the Fig. 1 that high frequency components are

extended to a considerable band with considerable amplitude.

Fig. 1. Spectrum of 8kHz PWM waveform.

A full-wave rectified waveform has less number of harmonics

as compared to the PWM waveform having same average

value [15]. In this proposed design the full-wave rectification

is achieved using thyristor-based controlled-rectifier AC-DC

converter [19]. Thyristors are smaller in size and can handle

large values of voltage and current [19].

978-1-4673-4451-7/12/$31.00 ©2012 IEEE

Fig. 2 Simplified block diagram of proposed digital controlled rectifier-based wireless DC drive.

Modern techniques for driving thyristors make use of digital

control circuits in order to control the conduction angle with

more accuracy. Fig. 2 shows a simplified block diagram of

proposed digital controlled rectifier-based wireless DC drive.

Fig. 2 shows that proposed system of digital controlled

rectifier-based wireless PMDC motor drive can be divided

into two main units: the command unit and the control unit

that communicate each other through RF link. Command unit

consists of an MCU that is interfaced with an RF module that

transmits and receives data wirelessly. User interface is

provided in form of an LCD and a matrix keypad in order to

view and input data to operate the drive. On the control unit

side, an MCU is interfaced with RF module and thyristor drive

circuitry. Thyristor drive circuitry controls the gate triggering

of thyristors in the controlled rectifier that in turn, controls the

speed of PMDC. A tachometer consisting of a digital encoder

is attached with shaft of PMDC to measure the speed of

motor. Tachometer outputs speed in terms of digital pulses

that are converted in the numerical value by MCU.

Proposed wireless PMDC drive works on the principle of

command and control mechanism. Commands are issued from

the command unit that controls the start, stop and speed of

motor. Control unit receives data through RF link and drives

the motor according to the information in the data. Upon the

reception of appropriate command issued by the user, it

transmits motor’s speed and current firing angle of controlled

rectifier circuitry to the command unit. Data are interchanged

through RF channel between command unit and control unit

that provide easy and precise control over the machine.

Proposed drive is implemented to control a single PMDC

motor but design can be extended to control multiple motors

via a single command unit. Implementation of proposed digital

controlled rectifier-based wireless drive is discussed in the

following sections.

II. COMMAND UNIT IMPLEMENTATION

Command unit consists of three main hardware components:

MCU, RF module and user interface. MCU that is used in the

implementation of command unit is industry standard MCS51

family member. Due to simplicity of architecture of command

unit, selected controller is AT89C51 that is an 8-bit MCU.

AT89C51, also referred to as C51 hereinafter, is low power

CMOS controller that is commonly used for control-oriented

applications [8]. It has 4kB Flash memory, 128 bytes RAM

and two timer/counter modules. In addition to that, it has

versatile interrupt logic and an RS232 standard UART that can

be operated in several modes [8]. For this proposed design,

C51 is programmed in C language and firmware is compiled

on industry-standard Vision compiler. Fig. 3 shows a

simplified architecture of AT89C51 MCU.

Fig. 3. Simplified architecture of AT89C51 MCU

A. Interfacing RF Module

Cost is a vital parameter in designing an embedded system.

Proposed design is implemented with such components that

are less expensive and provide high performance. RF module

that is used in the implementation of this proposed design

works in 433MHz RF band that is free of licence. RF module

has a simple UART interface for data exchange to and from a

Fig. 4. Simplified schematic diagram of interfacing

RF module with AT89C51

microcontroller. It has maximum 9.6kbps bit-rate and uses

frequency shift keying (FSK) technique for digital data

communication [20]. Along with many other features, it has

power save modes that helps save power when module is not

communicating. Four digital I/O lines of RF module named

(1) EN, (2) RXD, (3) TXD and (4) AUX are used to

communicate with an MCU [20]. EN line is used to enable or

disable the RF module. RXD and TXD receive and transmit

digital bit stream, respectively. AUX line informs about the

exchange of data either to or from the RF module. RF module

is interfaced with C51 through on-chip UART. MCU firmware

enables or disables RF module according to the status of

system by placing appropriate logic level on this line. A

simplified schematic diagram of interfacing RF module with

C51 is shown in Fig. 4.

B. Interfacing LCD and KeypadProposed wireless DC motor drive has user interface in form

of an LCD, a keypad and a pushbutton in order to facilitate

user to control the operation of DC motor. User can input

required value of motor speed in terms of revolution per

minute (RPM) from keypad and can view the information such

as current RPM and current firing angle of thyristors on LCD.

A pushbutton interfaced with MCU is used to stop the motor

under emergency conditions. This feature may prove helpful

when DC motor is installed to drive a vehicle [5]. LCD that is

used in the prototype implementation of the proposed design is

controlled by HT44780 controller. LCD is interfaced with

MCU in 4-bit mode of communication.

Fig. 5. Simplified schematic diagram of interfacing

LCD and keypad with MCU.

A matrix keypad is interfaced with MCU in order to input

desired information. MCU I/O lines continuously scan keypad

and firmware of the MCU detects the pressing of any key from

keypad. A simplified schematic diagram of interfacing LCD

and keypad with MCU is shown in Fig. 5.

III. CONTROL UNIT IMPLEMENTATION

Half of the proposed wireless drive consists of control unit

that communicates with command unit through RF link.

Control unit performs most of the prime functions of the drive

such as thyristor triggering control, RPM measurement and

transferring and receiving information to and from the remote

command unit. Control unit is implemented around AT89C51

microcontroller that performs all the control tasks. Control

unit consists of three major hardware units: (1) RF module, (2)

thyristor driver and (3) tachometer. Interfacing methodology

for RF module is same as that presented in Section-II.A.

Implementation of controlled rectifier, thyristor driver and

tachometer is given below.

A. Controlled Rectifier Three major topologies of AC-DC converters exist in order to

convert AC mains voltage to a DC voltage. These topologies

are: (1) uncontrolled rectifier, (2) half-controlled rectifier

(HCR) and (3) fully-controller rectifier (FCR). Out of these

three, HCR and FCR topologies make use of controlled power

switches such as insulated gate bipolar transistor (IGBT) or

silicon controlled rectifier (SCR) [19]. HCR is simpler to

implement as it uses two uncontrolled switches such as diodes

and two controlled switches such as SCRs.

Proposed methodology of digital controlled rectifier-

based wireless PMDC motor drive implements HCR topology

due to the following major advantages.

Easy to control conduction

Less count of components

Easy commutation control

Less software overhead on digital controller

Simplified schematic diagram of HCR is show in Fig. 6. From

Fig. 6 it can be inferred that HCR topology consists of two

sets of power switches. One set consists of D1 and T1 that is

referred to as SET1. Other set consists of D2 and T2 and is

named as SET2. SET1 and SET2 alternately conduct during

the positive and negative half cycles of AC mains supply,

respectively.

Fig. 6. Simplified schematic diagram of HCR.

Both diodes and thyristors are naturally commutated elements

[19]. Hence T1 and T2 require a triggering pulse on gate each

time the conduction is required.

MCU generates precisely timed pulses in order to fire the

thyristor at desired firing angle. A point on input sinusoidal

waveform when AC voltage passes from zero voltage point

and enters positive half cycle is called “zero-crossing point”.

A circuit that detects this zero-crossing point is called “zero-

crossing detector (ZCD)”. On detection of zero-crossing point,

MCU generates gate triggering pulses that control firing angle

of thyristors and hence average power to the motor.

Implementation of ZCD for this proposed design is simple

and consists of LM339 voltage comparator. Mains voltage is

scaled down through a voltage divider that provides a safe

margin against the voltage variations that may occur in mains

supply [7]. Schematic diagram of ZCD and its output

waveform are shown in Fig. 7.

Fig. 7. Schematic diagram of ZCD and its output waveform

AC mains are half-wave rectified by diode and are compared

against zero voltage. Output of ZCD is a square wave whose

negative-going edge is synchronized with starting of new

cycle of sinusoidal voltage. This square wave is input to INT0

of C51 that issues an interrupt to the CPU [8]. Time period of

50Hz AC is 20msec. Therefore, in order to cover 360 degrees

on sinusoidal waveform, 20msec are to be elapsed since the

detection of zero-crossing point.

B. Thyristor Driving Circuitry

Thyristor firing is controlled by MCU of the control unit.

MCU receives input from ZCD and calculates the firing angle

for the controlled rectifier. CPU of C51 receives an interrupt

request (IRQ) when a negative-going edge of ZCD output

square wave is detected. A timer is started in response to this

interrupt whose expiry period depends upon the firing angle.

User can input RPM of the motor from keypad. A look-up

table contains the values of average DC voltage required to

run a motor at a specific speed. Firing angle ‘ ’ can be

computed from (1) to produce desired average value of DC

voltage for the motor.

(1)

Timer’s reload value ‘T’ is computed from (2).

(2)

MCU generates a firing pulse for thyristor in SET1 of

controlled rectifier. Another timer is started which expires

after every 10msec interval in order to generate firing pulse

Fig. 8. Schematics of interfacing thyristor driver with MCU

through optocouplers.

for thyristor in SET2. Hence both thyristors are alternately

fired at required firing angles. MCU transmits this value of

firing angle to command unit through RF transceiver that is

shown on LCD. It is important to keep the gate of thyristor

electrically isolated from MCU circuitry [7]. This precaution

is necessary to take in order to avoid any high voltage spike or

electromagnetic noise that may damage controller circuitry.

Any high voltage spike passed to the controller circuitry may

cause permanent damage to rectifier and motor. Therefore,

gate driving circuitry is optically coupled with MCU output

for each thyristor in rectifier. Fig. 8 shows the schematics of

interfacing thyristor driver with MCU through optocouplers.

MCU writes logic 0 on its output pin connected with cathode

of optocoupler LED. This causes saturation on output bipolar

of optocoupler and current flows into the gate of thyristor.

Two identical gate driving circuits are implemented to

individually fire thyristors in rectifier.

C. Measurement of RPMRPM of motor is measured using an optical rotary encoder

(ORE). A disc with a notch at its boundary is attached with the

shaft of motor. A pulse is generated when notch on disc passes

through the slot of ORE. Number of pulses in one second

gives count of revolutions per second of motor. These pulses

are counted by MCU in an interval of 50msec. Firmware of

MCU converts this figure into RPM by multiplying it with a

constant factor of 1200. Hence if five pulses are received in

50msec, then RPM of motor will be 6000. After calculating

the value of RPM, MCU transmits it to the command unit.

IV. DATA COMMUNICATION

Data are exchanged between command unit and control unit

by the means of an RF transceiver hardware unit. Both

command unit and control unit are equipped with an RF

transceiver. Command unit transmits data and commands to

the control unit in order to control the speed of PMDC motor.

The value of firing angle is wirelessly transmitted to the

control unit that controls the conduction of thyristors in HCR.

Data that are transmitted by the control unit are categorized in

two classes (1) the data and (2) the command. Each class of

data consists of one byte.

Data bytes have decimal value from 0 to 180. This value

is the firing angle in degrees at which thyristors are to be fired.

Command bytes have values in the range of 240 to 255.

Hence, 16 different command values are available to perform

different operations. MCU in the control unit determines the

received bytes as either data byte or command byte, depending

upon its decimal value. For the purpose of prototype

implementation, only four command bytes are verified. Upon

reception of command byte, MCU in control unit takes action

accordingly such as to turn the motor on, off or transmitting

the current RPM back to the command unit. TABLE-I shows

the four command bytes that are implemented and the action

that is performed against each byte.

TABLE I

COMMANDS FOR MOTOR CONTROL

Sr. No Value Action

1 240 Send RPM

2 244 Turn On Motor

3 248 Turn Off Motor

4 252 Send Firing Angle

V. EXPERIMENTAL RESULTS

Proposed digital controlled rectifier-based wireless PMDC

motor drive is implemented as a prototype using discrete

components. A 160V/100W PMDC motor is wirelessly

controlled by implementing the proposed drive circuitry and

results that were obtained are presented in the following

subsections.

A. Range of Wireless Transmission

Proposed wireless drive successfully performed in the average

ranges of 350 meters under line-of-sight conditions and 120

meters under reflection by hurdles. This range of wireless

communication is sufficient for an industrial setup and

commercial building. Communication range can be enhanced

by incorporating more powerful and long-range RF modules.

B. Power Utilization

Proposed design of digital wireless PMDC motor drive utilizes

little amount of power that makes it operable by a battery or

solar panel. TABLE-II shows power consumption of the

proposed drive under different modes of operation.

TABLE II

POWER CONSUMPTION OF PROPOSED SYSTEM

Unit Mode Power Consumed (mW)

Command Unit LCD off, RF off 152

Control Unit RF off, MCU Sleep 184

C. Frequency Contents

Voltage waveforms that were applied across the PMDC motor

and their corresponding fast Fourier transform (FFT) are

shown in Fig. 9 to Fig. 11. Firing pulses with different firing

angles that were generated by the MCU are also shown below

the voltage waveforms. The controlled rectifier waveforms of

motor voltage show that motor supply is free of any high

voltage spike and noise that eliminates electromagnetic

interference in the mains lines and reduces losses in the DC

motor. Moreover, FFT images show that frequency contents of

rectified waveforms are congested nearer the low frequency

area of spectrum that proves the minimized harmonics in the

resulting voltage.

Fig. 9 a. Motor voltage waveform at 45 degrees firing angle

Fig. 9 b. FFT of voltage waveform at 45 degrees firing angle

Fig. 10 a. Motor voltage waveform at 60 degrees firing angle

Fig. 10 b. FFT of voltage waveform at 60 degrees firing angle

Fig. 11 a. Motor voltage waveform at 90 degrees firing angle

Fig. 11 b. FFT of voltage waveform at 90 degrees firing angle

VI. CONCLUSIONS

A digital controlled rectifier-based wireless PMDC motor

drive is proposed and implemented. Proposed design is tested

on a 160V/100W PMDC motor. Proposed drive provides an

economical and easy to operate solution for industry as well as

home application to control PMDC. Drawbacks associated

with PWM-based variable speed DC motor drives such as

high-frequency harmonic contents and switching losses are

minimized in this proposed design that results in longer life of

motor and minimized power consumption. Proposed wireless

motor drive is based on a low-cost microcontroller unit that

provides a compact and energy-conscious design. Design

presented in this paper can be easily enhanced to provide

wireless control over the inverter-fed three-phase induction

motors also. Proposed design can control more than one DC

motors by a single command unit over the RF link. Design can

also be applied to drive electrical vehicles for telemetry

purposes and remote control operations.

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