International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Analysis of Closed loop Current controlled BLDC
Motor Drive
P.Sarala
Dr. S. F Kodad
Dr. B. Sarvesh
Research Scholar
Professor & Head, Dept of EEE
Professor, Dept of EEE
JNTUA, Ananthapur, A.P India. PESITM, Sivmogga, Karnataka, India JNTUA, Ananthapur, A.P, India
dilip1.eee@gmail.com
kodadsf@rediffmail.com
b_sarvesh@yahoo.com
Abstract—DC motors are simple choice for drive systems
having very easy speed control techniques. DC motors are used
where DC supply is available. Commutation in the conventional
DC motors is carried out by commutator which is rotating part
placed on the rotor and brushes. Due to these mechanical parts,
conventional DC motor consist high amount of losses. Brushless
DC (BLDC) Motors are very extensively used motors these days
because of its advantages over conventional DC motors.
Commutation is carried out with the help of solid-state switches
in BLDC motor instead of mechanical commutator as in
conventional DC motor. This improves the performance of the
motor. BLDC motor shows good speed control characteristics
especially in low power applications like hard-disk drives, laser
printers and many more. BLDC with current control scheme was
discussed in this paper. BLDC drive with fixed speed and
variable speeds are developed using Matlab/Simulink and results
were shown for both fixed and variable speed applications. Basic
operation of BLDC motor was explained with open loop and
closed loop speed control. Matlab results were shown for BLDC
motor with open loop speed control, BLDC motor for fixed speed
closed loop control and BLDC motor with variable speed closed
loop control.
Keywords—BLDC, Conventional, DC, commutator, solid-state
switch, hall sensor.
I.
INTRODUCTION
Motor is an electromechanical machine which converts
electrical energy to mechanical energy. Basically, every motor
works on the same conversion principle. The electrical energy
which is given as an input produces torque due to interaction
of current carrying coils with the magnetic field. In the process
of conversion, the electro-magnetic field (EMF) inside the
conductors will be an alternating quantity. For a DC motor,
supply will be DC type but the EMF should be AC type. This
operation is done by commutator and brushes in a
conventional DC motor. Commutator is mechanical part
placed on the rotor segment for the purpose of commutation.
This commutator along with brushes produces wear and tear
on the commutator surface and hence commutation might not
be effective. Also this mechanical commutator produces high
amount of losses. Since both brushes and commutator are
good conductors, they produce copper losses. The wear and
tear of the commutator surface produces sparks due to uneven
current distribution. Sparks produces heat which is a major
drawback. The above said disadvantages are mainly due to the
presence of commutation process by commutator and brushes.
Thus the disadvantages in a conventional DC motor can be
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
overcome by eliminating brushes. This led to the realization of
motors without brushes called brushless DC (BLDC) motor
[1-2]. Electrical commutation in BLDC motor is carried out by
electronic solid-state switches. Due to the usage of electronic
switches for commutation, the drawbacks in conventional DC
motor are eliminated thus improving the system performance.
Though conventional DC motors show very good speedtorque characteristics and can be used for servo applications,
consequences due to brush and commutator requires
maintenance. DC motors have very good speed control and
especially BLDC exhibits many advantages [3-4] over
conventional DC motor like high efficiency, reliability, low
acoustic noise, good dynamic response, lighter, improved
speed-torque characteristics, higher speed range and requires
very less maintenance.
The type of supply given decides whether the motor is AC
or DC motor. BLDC motor is a type of AC synchronous motor
[5] with small constructional variations. A synchronous
machine consists of both stator windings and rotor windings
whereas BLDC motor has stator windings but the rotor
consists of permanent magnets with no windings wound on
rotor. BLDC motor is something different from AC
Synchronous motor where the earlier integrates hall sensors
[6] to sense the rotor position.
Fig 1: Typical BLDC Motor
BLDC motor shown in figure 1 is typically a combination of
permanent magnet (PM) AC machine with electronic
commutator. Sensor less operation [7] of BLDC is also
possible with the help of monitoring back EMF signals. Back
EMF is proportional to the speed of the rotor. So, at starting
condition of the motor or low speeds, sensor less operation
needs additional set-up to control the rotor position.
Fig 2: Diagram of BLDC Motor with AC Supply
in this paper, needs to be controlled. In this paper, a
current controlled BLDC drive was discussed with fixed speed
and also with variable speed. The basic operation of BLDC
motor without speed control was discussed and later the same
was discussed with speed control. Simulation work was
carried out by Matlab/Simulink and the results were discussed
in detail for each case.
II. BLDC MOTOR FED WITH AC SUPPLY
BLDC motor requires DC supply but generally available
supply is of AC type. To feed the BLDC, this available AC
supply is to be rectified and then fed to the BLDC motor
which is commutated electronically. The basic diagram of
BLDC motor fed with AC supply is shown in figure 2. BLDC
is similar to that of AC Synchronous machine. Stator consists
of windings to produce flux but the rotor does not have any
windings wound on them instead permanent magnets on rotor.
The rotation of the rotor depends on the attraction or repulsion
of stator poles and rotor poles. Rotor consists of one or
multiple poles. When a pair of poles of stator windings are
excited, this attracts the nearest opposite pole of rotor. By
continuously switching the poles of stator attracts or repulses
the nearest poles and thus the rotor continuously rotates
producing torque.
The general available supply is AC. But the supply
required for BLDC motor should be DC. So when AC supply
is given, this AC supply should be rectified and sent to the
BLDC motor. This rectification is done by a simple diode
bridge rectifier. The internal electronic commutation of BLDC
motor takes this DC supply through a DC link capacitor. This
DC link capacitor maintains constant DC voltage at the input
of inverter circuit before actually exciting BLDC motor. By
proper switching of inverter switches, the current in the stator
windings can be controlled. The rotor position is sensed by
using the hall sensors. This is open loop type of BLDC motor
without current control. The speed and remaining parameters
pertaining to the motor were not fed back for proper driving of
BLDC. But open loop control is very simple in construction
and low cost.
III. CLOSED-LOOP CONTROL OF BLDC DRIVE
BLDC motor without speed control was discussed in the
previous section. BLDC motor drive needs speed control for
smooth control of drive to run the motor at variable speeds.
BLDC motor drive with closed loop speed control is shown in
figure 3. This ensures that the drive can operate at any desired
speed. In open loop control the speed is set to the reference
value and in closed loop operation; this reference speed can be
changed at any desired time for the drive to operate at desired
speed. In closed loop speed control drive, the actual motor
speed is fed back to the input. By using a proper control
system, this speed can be varied and brought to desired speed.
The actual speed is fed back to the input and is measured with
the reference speed value producing an error signal. This is fed
to a PI controller which controls the speed with proper
proportional and integral gain value. This PI controller
nullifies the present error and the past error. Speed when
passed through PI controller produces reference torque value.
This reference torque value was compared with the actual
ωref
ωact
Fig 3: Diagram of closed-loop speed control of BLDC Motor
motor torque which was fed back from the output with the
help of hall sensors. The error signal produced due to the
measurement of actual torque with reference torque produces
reference current. This reference current is again measured
with actual stator current and produced error is fed to the
hysteresis current controller. Hysteresis current controller
produces gate pulses to the VSI (voltage source inverter)
switches to turn ON or OFF. By proper switching operation,
the VSI is controlled thus controlling the stator currents to the
BLDC motor. VSI is operated with a DC link capacitance
which acts as a source to the voltage source converter. All the
feed backs fed to produce the error signals are sensed by hall
sensors. The closed loop speed control gives a very smooth
sped control and can be run at any desired speeds. Current
control method for speed control yields better results.
IV. SIMULATION RESULTS
Matlab models with simulation results were discussed for
the BLDC motor without current control, BLDC motor with
closed loop control run at fixed speed and BLDC motor with
closed loop operation run at variable speed.
Fig 5: Simulation result of stator current and back EMF for open loop BLDC
motor
Case 1: BLDC motor without current control
Fig 4: Simulation model of open loop BLDC motor
Fig 6: Simulation result of speed for open loop BLDC motor
Fig 7: Simulation result of torque for open loop BLDC motor
The model of BLDC motor drive with open loop control is
shown in figure 4. The model discussed is without current
control. The simulink results of stator current and back EMF
of BLDC motor with open loop control was shown in figure 5.
Trapezoidal back EMF can be observed at the output of motor.
The speed of the open loop control for BLDC motor without
current control was shown in figure 6 and its corresponding
torque waveform was shown in figure 7. There is a high
content of ripple in torque which can be observed from the
torque waveform.
Case 2: Closed loop BLDC motor with fixed speed
Fig 11: Simulation result of torque for open loop BLDC motor drive with
fixed speed
The model of BLDC motor drive with closed loop control
and running with fixed speed was shown in figure 8. The
model discussed is with current control. The simulink results
of stator current and back EMF of BLDC motor with closed
loop control was shown in figure 9. Trapezoidal back EMF
can be observed at the output of motor. The speed of the
closed loop control for BLDC motor with current control was
shown in figure 10 and its corresponding torque waveform
was shown in figure 11. In speed waveform, the reference
speed wave is shown in green and the blue represents actual
speed of the motor. Torque contains very less ripples when
compare to open loop control.
Case 3: Closed loop BLDC motor with variable speed
Fig 8: Simulation model of closed-loop BLDC motor with fixed speed
Fig 12: Simulation model of closed-loop BLDC motor with variable speed
Fig 9: Simulation result of stator current and back EMF for closed loop BLDC
motor drive with fixed speed
Fig 10: Simulation result of speed for closed loop BLDC motor drive with
fixed speed
Fig 13: Simulation result of stator current and back EMF for closed loop
BLDC motor drive with variable speed
Fig 14: Simulation result of speed for closed loop BLDC motor drive with
variable speed
brushes and mechanical commutator for the commutation
purpose called brushless DC (BLDC) motors. BLDC motors
eliminate all the disadvantages in conventional DC motors due
to the absence of brushes and can give better performance
characteristics with smooth speed torque characteristics.
BLDC motors use electronic commutator for the purpose of
commutation. A converter with solid-state switches was
employed to convert DC to AC EMF inside the machine. The
operation along with the diagrams of BLDC was discussed.
The operation of BLDC with open loop control without
current control was explained. Closed loop operation of BLDC
motor with current control was also explained. Closed loop
BLDC motor with current control running at fixed speed and
variable speed was simulated and their corresponding
characteristics for speed, torque, back EMF and stator currents
were discussed. Results obtained were simulated using
Matlab/Simulink. Results validate the use of BLDC motor
with closed loop operation running with fixed and variable
speeds.
REFERENCES
[1]
Fig 15: Simulation result of torque for open loop BLDC motor drive with
variable speed
The model of BLDC motor drive with closed loop control
and running with variable speed was shown in figure 12. The
model discussed is with current control. The simulink results
of stator current and back EMF of BLDC motor with closed
loop control was shown in figure 13. Trapezoidal back EMF
can be observed at the output of motor. The speed of the
closed loop control for BLDC motor with current control was
shown in figure 14 and its corresponding torque waveform
was shown in figure 15. The variation of speed at 0.35 sec and
0.65 sec can be observed from the speed waveform. The
reference speed wave is shown in green and the blue
represents actual speed of the motor. The variation of speeds
at different time can be observed from the speed
characteristics but still maintaining constant torque. Table 1
represents the torque ripple content which shows the BLDC
drive without current control contains more torque ripples than
in closed loop current control BLDC drive system.
Table 1: Torque ripple
Torque ripple
Without current
control
With current
control
68 %
4.5 %
V. CONCLUSION
DC motors have very good speed-torque characteristics. DC
motors are very much accommodated in many of the industrial
drives. Commutation in conventional DC motors was carried
out by mechanical parts like brushes and commutator.
Presence of brushes for commutation can lead to sparks,
losses, reduced efficiency. DC Motors were realized without
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