See discussions, st ats, and author pr ofiles f or this public ation at : https:www .researchgate.ne tpublic ation324725356 [623403]

See discussions, st ats, and author pr ofiles f or this public ation at : https://www .researchgate.ne t/public ation/324725356
Speed control and electrical braking of axial flux BLDC motor
Conf erence Paper · Oct ober 2017
DOI: 10.1109/CER A.2017.8343344
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5 author s, including:
Some o f the author s of this public ation ar e also w orking on these r elat ed pr ojects:
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Axial Flux Brushless DC Mot or View pr oject
Pooja v asant A wari
Shri R amdeob aba Kamla Nehru Engineering Colle ge
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Pank aj Saw arkar
Shri R amde vbaba Colle ge of Engineering & Manag ement Nagpur
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Anur ag V idyadhar Kher gade
Visvesvaraya National Instit ute of T echnolog y
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Sanjay Bodkhe
Shri R amdeob aba Kamla Nehru Engineering Colle ge
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978-1-5090-4874-8/17/$31.00 ©2017 IEEE Speed Control and Electrical Braking of Axial Flux
BLDC Motor

1Pooja Awari, 1Pankaj Sawarkar, 1Rupal Agarwal, 2Anurag Khergade, 1Sanjay Bodkhe
1Shri Ramdeobaba College of Engineering and Management, Nagpur, India 440013
2Visvesvaraya National Institute of Technology, Nagpur, India 440010
[anonimizat], [anonimizat], agarwalr7@ rknec.edu, [anonimizat], [anonimizat]

Abstract —Axial flux brushless direct current motors
(AFBLDC) are becoming popular in many applications including
electrical vehicles because of their ability to meet the demand of high power density, high efficiency, wide speed range, robustness, low cost and less maintenance. In this paper, AFBLDC motor
drive with single sided configuration having 24 stator poles and
32 permanent magnets on the rotor is proposed. It is driven by six pulse inverter that is fed from a single phase AC supply through controlled AC to DC converter. The speed control and braking methods are also proposed based on pulse width
modulation technique. The overall scheme is simulated in
MATLAB environment and tested under different operating conditions. A prototype of proposed AFBLDC motor drive is designed and fabricated. The control methods are implemented
using DSC dsPIC33EP256MC202 digital signal controller. Tests
are performed on this prototype to validate its performance at different speeds with and without braking mode. It is observed that the proposed scheme works effectively and can be used as
wheel direct driven motor for electrical vehicle.
Index Terms— Axial flux machine, BLDC motor, speed
control, electric braking, Hall sensor, microcontroller, permanent
magnet motor.
VI. INTRODUCTION
Axial Flux Brushless Direct Current motors are one of the
motors rapidly gaining popularity. They are used in industrial
applications such as appliances, automotive, aerospace,
consumer, medical equipment, automation and instrumentation. As the name implies, these motors do not use mechanical commutator and brushes for commutation, instead they are electronically commutated. They have many advantages over brushed DC motor and induction motors. A
few of these are: better speed versus torque characteristics,
high dynamic response, high efficiency, long operating life, noiseless operation and higher speed range [1]. In addition, the ratio of torque delivered to the size of the motor is higher, making it useful in applications where space and weight are critical factors. One of the prime reasons for above
advantages is the use of permanent magnet on the rotor due to
which both, radial flux and axial flux BLDC motors have earned a pronounced position in existing as well as new applications [2]-[3]. Because of high efficiency, it draws less electrical power and imposes less stress on the power
electronic controller. In electric vehicles, shaft of the machine
is directly coupled to the wheel and gives high compactness to the machine. However it requires high torque capability to
overcome stiction and to run at very low speeds [4]-[5].
AF BLDC motor can be designed as double sided or single
sided machine with internal or external rotor. Based on mounting of permanent magnet, the rotor could be of surface
mounted type or interior type. The single sided machine
consists of single stator and single rotor. With single sided topology it is possible to get a very high ratio between machine diameter and length. The bearing and axel must be properly dimensioned to withstand the axial force between two sides. Double sided machine consists of two possible
configurations such as interior rotor and interior stator. In
interior rotor, as the rotor is between the two stators and in the case when the distance from the rotor to each stator is equal, the attraction forces are equilibrated, avoiding possible stress in mechanical parts. Interior stator axial flux machines can be configured in two ways, namely, North-North (N-N)
configuration and North-South (N-S) configuration [5]. The
axial flux motor topologies proposed in literature are mostly double rotor with internal stator [3]. This requires more axial space and cost. The motor proposed in this paper is of single sided type with concentrated stator coils as discussed in
section II. This allows trapezoid al waveform of back emf in
addition to reduction of space requirement and cost. In electrical vehicle, precise control over speed and
stopping of machine is important along with start. In order to control the speed and acceleratio n as well as deceleration,
speed control method and braking is necessary. In mechanical
braking, motion is diminished by the friction applied by
brakes which is preferred during low speeds where mechanical brake dissipated energy. The purpose of electrical braking is to restrict the motion of the machine gradually or as desired without wear and noise. Various speed control techniques and braking method are proposed in literature [6]-[10]. They fall in
two categories; one is by controlling the input dc voltage and
other by controlling PWM duty cycle. In order to implement braking, different methods are suggested by researchers such as using additional boost conve rter, ultra-capacitor or by
changing the switching sequence of the inverter [7]-[10].
Reference [10] has suggested the use ultra-capacitor in place
of battery for hybrid electrical vehicle. A boost converter is used to enable the regenerative action during braking mode in [8]. These methods are effective, however they add to the cost and complexity of overall drive. This limitation is addressed

by a new cost effective method which is based on modification
of switching sequence and do not use any additional converter
or ultra-capacitor [7]. However, the above methods have been
executed on radial flux type BLDC motor and none are found implemented on AFBLDC motor.
This paper presents the proposed AFBLDC motor drive
with speed control and electrical braking implemented by using dsPIC33EP256MC202 digital signal controller. The braking is performed by modification of switching sequence and do not suggest the use of additional boost converter or ultra-capacitor. The paper is organized as follows. Section I introduces the research motivation and existing state of technology. Section II & III briefly describes the design and operating principle of AFBLDC motor. Section IV presents the proposed scheme along with speed control and braking methods. Simulation and experimental results are illustrated in section V and section VI respectively. Section VII present conclusion.
VII. D
ESIGN OF AFBLDC MOTOR
Fig. 1 shows structure of proposed single rotor-single stator
AFBLDC motor [5]. Procedure of the motor design and fabrication is based on [5]-[6].
A. Stator Design
The primary purpose of the stator is to provide a structure
that allows the flux and the current to interact and to produce usable torque. Stator designs are of two types, one is slotted stator design and another is slotless stator design. The slotted stator design is associated with few disadvantages such as cogging. Due to saliency effect of the slotted stator it increases
the cogging torque.
A circular back plate of aluminum is used which supports the laminated Si-steel core. The laminations are circular sheets of 300 mm outer diameter. Twenty four concentrated coils are fixed on the stator core. 106 turns of 20 gauge copper wire are wound on the former to form these stator coils on T-shaped
stampings. The back Si-steel core provides a return path for
the magnetic flux. The stator coils are grouped in three single phase circuits each consisting of eight coils connected in series. Fig. 2 shows the disassembled view of stator. The other details are as given below.

The number of stator coils per rotor pole per phase (N
spp),

25.0328= = = =
NN
NNN
phsp
ms
spp (1)

The number of coils per magnet pole (N sm),

75.03224= = =
NNN
ms
sm (2)

Fig. 1. Prototype of single sided AFPM BLDC motor.

Fig.2. Stator design of proposed prototype

Fig.3. Front view of rotor.

B. Rotor Design
The rotor is composed of thirty-two NdFeB permanent
magnets alternating in polarity separated by non-magnetic spacers which is air in this case and attached to ferromagnetic back iron. Rotor core or rotor back iron of Si-steel laminations
is used to provide a return path for magnetic flux. It is
supported by aluminum back plate of 300 mm diameter. Fig. 3 shows front view of rotor.

VIII. OPERATING PRINCIPLE
To drive the motor, a six step inverter is used as shown in Fig.
4. The switches can be either MOSFET or IGBT which are driven by gate pulses. But pulses cannot be given directly
from microcontroller to the gate of a switch because of low
magnitude. Therefore it is passed through a driver circuit. The driver circuit involves a driver and optocoupler. Driver is used for fast turn on and turn off of switches and optocoupler is used to provide isolation. Fig. 5 shows the block diagram of driver circuit for a single switch. To avoid burden on
controller, buffer IC is used which strengthens the pulses
coming from controller and then this is given to the optocoupler driver IC. The commutation of three phase windings on the stator is done electrically. The switching in stants are decided on the
basis of rotor position identified by Hall sensors. For rotation,
the stator winding should be energized in sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. The Hall Effect sensors are embedded into the stator at 120 degrees interval on the non-driving end of the motor.
Whenever the rotor magnetic poles pass near the Hall sensors,
they generate high or low signals, indicating the North or South Pole as shown in Fig. 6 [1]. The switching sequence is as given in Table 1. Each switching state has one of the windings energized to positive power (current enters into the winding), the second winding is
negative (current exits the winding) and the third is in a non-
energized condition. Torque is produced because of the
interaction between the magnetic field generated by the stator coils and the permanent magnets . Ideally, the peak torque
occurs when these two fields are at 90° to each other and falls
off as the fields move together. In order to keep the motor
running, the magnetic field produced by the windings should shift position, as the rotor moves to catch up with the stator field. The Hall sensor signal has the rising edge and the falling edge for each phase. That is, the six trigger signals are generated per cycle. Using these trigger signals, motor control
is carried out.

IX. PROPOSED SCHEME
The proposed scheme of AFBLDC motor drive is presented
in Fig. 7. The power electronic circuit consists of a controlled single phase rectifier which feeds DC power to the six step
inverter. The three phase AC output of the inverter is used to
drive the motor. With the help of Hall sensors the rotor position is detected and the signal that is generated from Hall sensor is given to the controller which energizes appropriate winding of the motor and it starts running at speed ω
r.
To control the rotor speed, the output voltage of rectifier is
controlled by varying the duty cycle and applying appropriate
triggering pulses. The actual speed is compared with speed command and error signal is processed by PI controller as shown in Fig. 8. The constant PI controller is selected by trial and error. A 10k Ω potentiometer is linked up with PWM
generator to vary pulse width of the signal.

Fig.4. Three-phase equivalent circuit of inverter fed BLDC motor.

Fig.5. Block diagram of driver circuit for single IGBT switch

Fig.6. Back emfs and Hall sensor signal for each phase

Table 1. Switching Sequence
Switching
sequence
Hall Sensor Switch
Position Phase Voltage
Ha H b H c A B C
1 1 0 1 Q 1 Q6 DC+ DC- OFF
2 1 0 0 Q 1 Q2 DC+ OFF DC-
3 1 1 0 Q 3 Q2 OFF DC+ DC-
4 0 1 0 Q 3 Q4 DC- DC+ OFF
5 0 1 1 Q 5 Q4 DC- OFF DC+
6 0 0 1 Q 5 Q6 OFF DC- DC+

During the motoring mode, as seen from Table 1, only two
switches will conduct simultaneously. The equivalent circuit
during switching state (101) is shown in Fig. 9. The methodology used for electrical is based upon generation of

Fig.7. Block diagram for closed loop speed control and electrical braking of
AFBLDC motor.
Fig 8: Speed controller details.

negative torque by switching ON appropriate switch of lower
group. During braking period all upper switches of inverter are
turned OFF. The inductive energy stored in stator winding is
dissipated through the winding, lower switch and freewheeling diode of adjacent leg [7]- [10]. Hence rotor decelerates rapidly
and stops running. The equivalent circuit during braking mode is shown in Fig. 10. By using this braking technique, the problem of reverse motoring during plugging is overcome.
Fig.11 shows waveform of hall sensors, back emf, switching
pulses of inverter during motoring mode and braking mode.

Fig.9. Equivalent circuit during motoring mode.

Fig.10. Equivalent circuit during braking mode.
\
Electromotive force ea
Time<Stator back EMF e_b (V)>

Fig.11. Waveform of hall sensor, back EMF, switching pulses of inverter
during motoring mode and braking mode.
V. SIMULATION STUDY
To study the performance of proposed drive of Fig. 7 during
motoring and braking operation, simulation is carried out in MATLAB/Simulink software. The study is carried out for three operating conditions. They are (1) free acceleration, (2)
step change in load, and (3) fo rced deceleration by electrical
braking. A fixed-step ode4 (Runge-Kutta) solver with fixed
step size of 5×10
-6s was used.
1. Free Acceleration:
On application of AC supply from the inverter, the motor accelerates freely at no-load and it’s speed reaches steady state
in 0.05s. The back emf is trap ezoidal due to concentrated
windings. The simulation results are shown in Fig. 12 & 13. 2. Step Change in Load:
In this case, mechanical step load of 1Nm is applied at 0.06 s
on the rotor shaft. Due to application of load on motor, the
equilibrium state between electromagnetic torque and load torque gets altered resulting in a short time dip in actual speed. Due to closed loop control, error between the actual speed and reference speed activates the PI controller that provides appropriate triggering to the six switches to reduce this speed
error. From Fig.14 & 15, it is observed that the drive takes
about 0.03 s to regain its original speed after a dip due to application of load.

3. Electrical braking:
In this case, a comparative study between two performances
was carried out. In one case, the input supply to the motor was
disconnected at 0.06 s and it was allowed to decelerate freely depending on its mechanical time constant. In the second case, braking signal was applied at the same instant and deceleration time was noted. This is shown in Fig. 16. It is observed that the time required by the motor to come to rest when braking is
applied is about one-third to that during free deceleration.

-20020
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-5000500
Fig.12 Stator current and back emf during free acceleration at no-load

020004000Stator current ia
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-50050Electromotive force ea

Fig.13 Rotor speed and torque during free acceleration at no-load

-20020
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-5000500

Fig.14.Stator current and back emf during step change in load.

020004000
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-50050

Fig.15 Variation in speed and torque on application of load. -500005000
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-101

Fig.16 Variation of speed & toque during braking and free deceleration

VI. EXPERIMENTAL RESULT S
The hardware set up of proposed drive is shown in Fig. 17. It
includes prototype of AFBLDC motor, three phase inverter, and dsPIC33EP256MC202 digital signal controller with hall
sensor circuitry. The control code was developed in MPLAB
and dumped on microcontroller by using Pikit3 dumper. The performance was evaluated at three operating conditions (1) motoring, (2) change in reference speed, (3) free deceleration and (4) forced deceleration by electrical braking. Fig. 18 presents the experimental result when the motor
was excited and allowed to run freely upto 1000 rpm at no-
load. The speed was then reduced to 50% by changing the reference speed. The result is shown in Fig. 19. To test the effectiveness of electrical braking, the motor was first de-energized and allowed to decelerate freely. Thereafter, it was again energized and after attaining the rated speed, electrical
signal was applied to the microcontroller. The difference
between deceleration times was noted. The results are presented in Fig. 20 and 21 respectively.

Fig.17. Complete experimental set up of AFBLDC motor

Fig.18 Experimental result during motoring mode.

Fig.19 Experimental result during reduction in reference speed

Fig.20 Experimental result during free deceleration

Fig.21 Experimental result during forced deceleration by electrical braking

VII. CONCLUSION
This paper has proposed a simple but effective method of
speed control and braking technique for AFBLDC motor.
Prototype of a single sided, 24/32 poles, AFBLDC motor has been designed and fabricated. The simulation results of the proposed scheme prove its effectiveness at different operating conditions. By using PWM technique for speed control power
loss in the switching devices is low. Speed reversal of motor
during plugging is overcome using proposed braking technique. After performing the simulation study, a power electronic drive along with microcontroller based control
circuit is designed and fabricated. The experimental results
validate the effectiveness of proposed scheme.
A
PPENDIX
DC link voltage = 48 V
Rated Speed = 1000 rpm Stator resistance/phase R
s = 0.50 ohm
Stator inductance/phase L s = 0.005 H
Flux linkage established by magnets = 0.057 V.s Torque Constant = 0.34 Nm/A
Friction factor = 0.001 Nm.s
Pole pairs = 4
R
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