Sensorless Control of BLDC Motor with Back
EMF Detection Method using ATmega328
Rahul Thorat1, Sagar Salunke2,Shubham Meshram3 and Prof. P. M. Mainkar 4
Department of Electronics and Telecommunication, Maharashtra Institute of Technology Pune,
Address - S.No.124, Paud Road, Kothrud, Pune, Maharashtra 411038
Email- rthorat10@gmail.com 1 , mysjs44@gmail.com 2 , shubham.mms786@gmail.com 3 ,
prakash.mainkar@mitpune.edu.in4
1,2,3,4
Household appliance have traditionally relied on historical
Abstract -Brushless dc (BLDC) motors and their drives
are penetrating the market of home appliances, HVAC
industry, and automotive applications in recent years
because of their high efficiency, silent operation, compact
form, reliability, and low maintenance. Traditionally,
BLDC motors are commutated in six-step pattern with
commutation controlled by position sensors. To reduce
cost and complexity of the drive system, sensor less drive is
preferred. The back EMF of the floating motor winding
can be detected during off time of PWM because the
terminal voltage of the motor is directly proportional to
the phase back EMF during this interval. Also, the back
EMF voltage is referenced to ground without any common
mode noise. Therefore, this back EMF sensing method is
immune to switching noise and common mode voltage. As
a result, there are no attenuation and filtering necessary
for the back EMFs sensing.
classic electric motor technologies such as single phase AC
induction, including split phase, capacitor-start, capacitor–run
types, and universal motor. These classic motors typically are
operated at constant-speed directly from main AC power
without regarding the efficiency. Consumers now demand for
lower energy costs, better performance, reduced acoustic
noise, and more convenience features. Those traditional
technologies cannot provide the solutions.
In the same time, automotive industry and HVAC industry
will also see the explosive growth ahead for electronically
controlled motor system, the majority of which will be of the
BLDC type For example, at present, the fuel pump in a car is
driven by a dc brushed motor. A brush type fuel pump motor
is designed to last 6,000 hours because of limit lifetime of the
brush. A BLDC motor life span is typically around 15,000
hours, extending the life of the motor by almost 3 times. It is
in the similar situation for the air-conditioning blower and
engine-cooling fan. The cost effective and high performance
Keywords -BLDC motors, Sensorless motors, back EMF,
motor control
transition. There are two methods of BLDC control,sensor and
I.
INTRODUCTION
Brushless dc (BLDC) motors have been desired for small
horsepower control motors due to their high efficiency, silent
operation, reliability, and low maintenance. However, the
control complexity for variable speed control and the high cost
of the electric drive hold back the widespread use of brushless
dc motor.
BLDC motor drive system will make big contribution for the
sensorless.[1,2,3] The BLDC motor is usually operated with
one or more rotor position sensors since the electrical
excitation must be synchronous to the rotor position. For
reasons of cost, reliability, mechanical packaging and
especially if the rotor runs immersed in fluid, it is desirable to
run the motor without position sensors – so called sensorless
operation.[4]
II.
TYPICAL BLOCK DIAGRAM
current is in phase with the back EMF. The commutation
timing is determined by the rotor position, which can be
detected by Hall sensors or estimated from motor parameters,
i.e., the back EMF on the floating coil of the motor if it is
sensorless system. Basically, two types of sensorless control
techniques, The first type is the position sensing using back
EMF of the motor, and the second one is position estimation
using motor parameters, terminal voltages, and currents.[6]
The second type scheme usually needs DSPs to do the
complicated computation, and the cost of the system is
relatively high. So the back EMF sensing type of sensorless
scheme is the most commonly used method.
IV. BACK EMF DETECTION
Fig.1
In brushless dc motor, only two out of three phases are excited
at one time, and the third winding floating. The back EMF
III. COMMUTATION
A transition from one step to another step is called
commutation. Brushless dc motor is driven by a three-phase
voltage is in the floating winding and that can be measured to
establish a switching sequence for commutation of power
devices in the three-phase inverter.[7,8]
inverter which is called as six-step commutation.[5] The
conducting interval for each phase is 1200 by electrical
angle.The commutation phase sequence is like AB-AC-BCBA-CA-CB. Each conducting stage is called one step.
Fig.3
Back EMF zero crossing detection scheme
If the proper PWM strategy is selected, the back EMF voltage
referred to ground can be extracted directly from the motor
terminal voltage. In the proposed scheme, the PWM signal is
applied on high side switches only, and the back EMF signal
Fig.2
is detected during the PWM off time.
Therefore, only two phases conduct current at any time,
Assuming at a particular step, phase A and B are conducting
leaving the third phase floating. In order to produce maximum
current, and phase C is floating. The upper switch of phase A
torque, the inverter should be commutated every 600 so that
is controlled by the PWM and lower switch of phase B is on
during the whole step. The terminal voltage Vc is measured.
V. CONCLUSION
In this thesis direct back EMF sensing, without motor neutral
voltage for BLDC drives is proposed, analyzed, and extended,
overcoming the drawbacks of the conventional scheme. In this
scheme, the PWM is applied to high side switches of the
inverter, the back EMF is measured during the PWM off time
in the floating winding. It is proved that the terminal voltage
of the floating winding is directly proportional to the back
EMF of that phase. Several advantages of the direct back EMF
sensing scheme are summarized as follows:
Fig.4
Fig.4 Shows Circuit model of Back EMF detection during the
PWM off time moment.
When the upper switch of phase A is turned on, the current is
flowing through the switch to winding A and B. When the
i. The scheme can detect the back EMF with very high
resolution, because it doesn’t have signal attenuation;
ii. The switching noise is rejected by synchronous sampling;
iii. There is no filtering to cause phase shift or delay;
iv. There is no common mode voltage noise issue.
upper transistor of the half bridge is turned off, the current
freewheels through the diode paralleled with the bottom
REFERENCES
switch of phase A. During this freewheeling period, the
terminal voltage Vc is detected as Phase C back EMF when
[1] Boldea and S.A Nasar, “Electric Drives”, CRC Press,1999
there is no current in phase C.
[2] M. H. Rashid, Power Electronics, Prentice Hall, 1993.
From the circuit, it is easy to see Vc = ec + Vn , where Vc is
[3] R. Krishnan, Electric Motor Drives: Modeling Analysis,
the terminal voltage of the floating phase C, ec is the phase
and Control, Prentice Hall, 2001.
back EMF and Vn is the neutral voltage of the motor.
[4] AVR444:Sensorless control of 3-phase brushless DC
motors.
[5] Amol Sutar, SatyawanJagtap“Advanced Three Phase
PWM Inverter Control Using Microcontroller”
[6] Ward Brown, “Brushless DC Motor Control Made Easy”,
Application Note Microchip Technology Inc, 2002.
[7] Reston Condit, “Sensor less BLDC Control With BackEMF Filtering”, application note AN1083 by Microchip.
[8] Jianwen Shao, “Direct Back EMF Detection Method for
Sensorless Brushless DC (BLDC) Motor Drives”
Fig.5
Back EMF waveforms for phase A,B and C