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One Cycle Control of Buck-Type, Current Source Inverter-
Fed, Brushless DC Motor Drive
Omid Mohammadpour Abolfazl Halvaei Niasar
Department of Electrical and Computer Engineering, University of Kashan,
Kashan, Iran
omid.mohammadpour67@gmail.com halvaei@kashanu.ac.ir
Abstract— Traditional BLDC motor drives use voltage source
inverters (VSI) that utilize PWM and hysteresis hard switching
methods. It leads to generate switching losses and need to use of
large heat sinks. VSIs are costly because of low reliability and
need to large DC link capacitor. Moreover, common control
methods such as hysteresis current control can create variable
range of high frequency noises. This paper overcomes the
problems of VSIs and reduces the volume and cost, by using
current source inverter (CSI) which replaces an inductor instead
of DC link capacitors. Also, to solve the problems of hysteresis
control method, one-cycle control (OCC) technique has been used
for soft switching of the inverter. Moreover, to reduce the
difficulties and cost of installing electromechanical sensors in
BLDC motors an appropriate method for rotor position
estimation has been presented. Simulation results in
Matlab/Simulink verify the effectiveness of OCC-based control of
CSI-BLDC motor drive comparing to hysteresis-based VSI-
BLDC motor drive.
Keywords—BLDC Motor Drive; Current Source Inverter; One
Cycle Control; Sensorless; Buck converter
I. INTRODUCTION
Because of permanent-magnetic on the rotor, brushless DC
(BLDC) motors possess higher efficiency than the popular
induction motors. Therefore more and more BLDC motors are
used in various high-efficiency variable-speed applications,
such as fan motors, compressor motors, vehicle motors and
home appliances [1]. A brushless dc motor is a dc motor
turned inside out, so that the field is on the rotor and the
armature is on the stator. In fact BLDC motor is a modified
PMSM motor with the modification being that the back-EMF
is trapezoidal instead of being sinusoidal as in the case of
PMSM. Voltage source inverters are widely used in BLDC
motor drives but due to the low reliability of the motor which
is because of the high dv/dt that comes from output voltage
and bulky capacitor in inverter DC link, drives cost have been
increased. Current Source Inverters (CSIs) could potentially
address some of the mentioned issues due to their ruggedness
to over current/short circuit and low dv/dt voltage over the
stator windings [2]. Also CSI uses an inductor as the energy
storage component, and thus avoids many drawbacks of VSI.
The inductors which have been used in DC link have longer
lifetime than the capacitors. All these features make the CSI
attractive for some high power applications [3, 4].
BLDC motors are usually controlled via dc-link current
regulation method like to DC motors. For more simplicity,
hysteresis current controller is usually used for current
regulation instead of PI controller. It is simple in
implementation, but it has variable switching frequency and
leads to high frequency ripple. To improve the performance of
current regulation, some fixed-frequency PWM methods have
been proposed [5]. One-cycle control (OCC) technique is a
nonlinear control method with a simple topology that operates
in constant frequency and does not require any complex power
processing circuits [6]. OCC is a simple control technique that
has advantages of both PI and hysteresis controllers where in
this study it has been employed for current regulation in
current inverter source of BLDC motor drive.
Furthermore, knowing the angular position of the rotor in
BLDC motor is essential to control of BLDC motor drive. The
most solution is employing the low-cost Hall Effect position
sensors. Using electro mechanical sensors increases the cost
and hardware complexity of drive. As a result mounting the
sensor on the motor shaft can cause problems and create high
speed limitation. These problems which have been faced to
rotor position estimation methods should be considered by
researchers. The major sensorless methods have been
discussed in [7]. Sensorless techniques based on back EMF
are the most popular due to their simplicity, ease of
implementation, and lower cost [8]. In conventional sensorless
methods, zero-crossing points of the phase back-EMFs can be
detected from the terminal voltages. Therefore delaying these
zero-crossing points by 30° is required to obtain the correct
commutation instants [9], but in proposed method the line-to-
line back-EMFs can be easily sensed from the terminal
voltages, and thus the commutation instants can be obtained
directly without the processing of the additional 30° delay and
virtual neutral voltage point [10,11].
This study presents a sensorless BLDC motor drive based
on the one-cycle control (OCC) strategy for regulation the
current of buck-type current source inverter. After introducing
the basic concept of OCC technique, employed CSI is
investigated. Then, the suggested position sensorless
technique based on line-to-line terminal voltages is described.
Finally, simulation results of the proposed BLDC motor drive
are presented.
II. ONE-CYCLE CONTROL THEORY
The theory of one-cycle control is shown in Fig. 1 and Fig.
2. The controller consists of integrator, SR flip-flop,
comparator and pulse generator. In fact this method is based
on the control keys K1 and K2. In each cycle, the switch is on
for duration of and is off for duration of ,
where
and
.
The 6th International Power Electronics Drive Systems and Technologies Conference (PEDSTC2015)
3-4 February 2015, Shahid Beheshti University, Tehran, Iran
978-1-4799-7653-9/15/$31.00 ©2015 IEEE 113
Fig. 1. One-cycle controlled constant frequency switch [12]
Fig. 2. Switch input and output
As shown in Fig. 2;
. (1)
that,
1, 0
0,
(2)
And the average of the switched variable is
y(t) =
= (3)
If the duty ratio of switch is modulated; such that the
integration of the switched variable at the switch output is
exactly equal to the integration of the control reference in each
cycle, i.e.
=
(4)
then,
y(t) =
= (5)
Since the switching frequency is much higher than the time-
varying reference voltage, the reference can be seen as
constant in one period, thus simplifying the above equation to,
= (6)
and, when the clock pulses activate the input set of flip-flop
the integrator starts to work according to the equation (7) and
when reaches the control reference the integrator
resets.
1
.
(7)
Then, according to equation (6), the key can completely
eliminate all the input disturbances, independent of the input
signal [12].
III. POWER INVERTERS
A. Voltage source inverter
Voltage source inverter has small or negligible impedance
at dc side, in other words a voltage source inverter has stiff dc
source voltage at its input terminal. A large capacitor is
connected at the input terminal tends to make the input dc
voltage constant. However, it has some conceptual and
theoretical barriers and limitations [4]. Additional power
converter stage increases system cost and lowers the
efficiency. Fig. 3 shows the topology of voltage source
inverter.
B. Current source inverter
Current-source based systems have the requirement for
continuous DC link current, without which, the voltage at the
DC link inductor would rise to possibly damaging levels
during the periods of discontinuous current. In practical
systems, the constant current source is replaced by a DC
voltage supply and a DC link inductor, as detailed in Fig. 4.
Because of large capacitor in VSIs DC link, the size and cost
of the drive should be reduced. For this purpose, it has been
proposed to use buck converter in DC link as in fig 5 to have
further reduction in value of current source inverter inductor.
The gate signal of the controllable switch is obtained from
the one cycle controller.
114
Fig. 3. Voltage source inverter
Fig. 4. Current source inverter
Fig. 5. Buck-type current source inverter [1]
IV. PROPOSED SENSORLESS METHOD
The major problem of the conventional back EMF sensing
techniques is that they require noisy motor neutral voltage and
a fixed phase shift circuit. And also there are several practical
problems when using the phase to neutral zero crossing
detection method. In order to cope with the aforementioned
problems, a proper sensorless commutation method is
proposed. The proposed method extracts the commutation
points directly from the motor terminal voltages with simple
comparators and a single stagelow pass filter. Fig 6 shows the
conventional sensorless commutation circuit and proposed
commutation circuit [11].
(a)
(b)
Fig. 6. (a) propsed commutation circuit. (b) Conventional commutation
circuit [11].
In this method, inherently there is 30 degree phase delay
between motor terminal voltages and back EMF voltage and
the estimated commutation signals are well in phase with the
ideal commutation points as shown in Fig. 7 and Fig. 8.
Fig. 7. Back emf voltage and line to line voltage
Fig. 8. Commutation points
115
The voltage spikes shown in Fig. 9 are created by the residual
current when the armature current is blocked by the power
switches. The voltage spike is the main cause for the
commutation error in the back-EMF based method. By using
proposed method the spikes will be eliminated according Fig.
10.
Table I summarizes the three line to line voltages for the
proposed sensorless commutation approach. According to the
properties of the average line to line voltage, the ideal
commutation points can be obtained directly from the three
motor terminal voltages without the knowledge of the neutral
voltage. The estimated signal and hall signals showed in Fig.
11 have negligible difference which proofs that proposed
sensorless method acts like Hall Effect sensor.
Fig. 9. Line to line voltage of phase a and c
Fig. 10. Filtered line to line voltage of phase a and c
TABLE I. THE RELATIONSHIP BETWEEN ESTIMATED SIGNAL AND LINE TO
LINE VOLTAGES
Virtual hall signals Line to line voltages
H1 V
A - VC
H2 V
B - VA
H3 V
C - VB
Fig. 11. Estimated hall signal and hall sensor output
V. SIMULATION RESULTS
In this section, some simulation results are provided and
one cycle control technique has been compared with hysteresis
method. To drive the BLDC motor using current source
inverter, at first a constant current should be provided. This
current is the DC link current of the current source inverter
which is output current of the buck converter that shown in
Fig. 12. Then by using the difference between the terminal
voltage and back-EMF, zero crossing points redetected and the
commutation points are estimated. Finally the designed drive
is controlled with one cycle control technique. The inputs of
OCC block according to Fig. 14 are DC link current and speed
error. DC link current has been multiplied by
2 and
summed with output signal from the integrator which has also
been multiplied by
, then integral of the summation has
been compared with speed error and creates a Flip-Flop reset
pulses. Also the square wave pulse generator generates 25
KHz frequency signals to activate the Flip-Flop in each
period. So selection of proper values of R and C is very
important and has great impact on the performance of the
controller.
The one cycle control method for buck-type CSI fed
BLDC motor is shown in Fig. 13 where a buck converter is
connected in front of the common inverter. The gate signal of
switch is generated from the OCC block as shown in Fig.
14 and the inverter switches signals (named PULSE in Fig.
13) are obtained from three commutation signals that
described in previous section. The stator current, back EMF
voltage, rotor speed and electromagnetic torque are shown in
Fig. 15 to Fig. 18 respectively. Fig. 15 and Fig. 16 shows that,
as predicted motor current is square and back-EMF waveform
is trapezoidal. Also Fig. 17 shows that motor reachs to rated
speed after 0.04 seconds. And Fig. 18 shows the motor torque
ripple, wich load applied in t=0.1 .
Fig. 12. Constant current in DC link
116
Fig. 13. Simulation circut of Buck-type CSI fed BLDC motor drive
Fig. 14. Structure of One-cycle controller
Fig. 15. Stator cu rrents (isa,isb,isc)
Fig. 16. Back emf voltages (Ea,Eb,Ec)
Fig. 17. Rotor speed
Fig. 18. Electromagnetic torque
In this step, motor torque in one cycle control method and
hysteresis control method has been compared and their ripple
has been provided in Table II. As shown in Fig. 19 and Fig.
20, one cycle control method can considerably decrease the
torque ripple. Also dynamic responses of one cycle control
method and hysteresis control method have been compared in
Fig. 21 and Fig. 22 when a disturbance occurs at the input
voltage of the motor. As can be seen from the figures, one
cycle control method has faster dynamic response than
hysteresis control method.
117
TABLE II. COMPARISON OF TORQUE RIPPLE
Control method Hysteresis OCC
Torque ripp le (%) 32% 20%
Fig. 19. Torque ripple in OCC technique
Fig. 20. Torque ripple in hysteresis method
Fig. 21. Speed tracking in OCC technique
Fig. 22. Speed tracking in hysteresis method
VI. CONCLOSION
In this paper, the behavior of sensorless CSI fed BLDC
motor drive using one cycle control (OCC) technique drive
has been studied. OCC controller has been used instead of
traditional hysteresis and PI controllers for current regulation
of current source inverter. Developed CSI fed drive enjoys the
soft switching control and then the switching loss has been
reduced considerably. Moreover, simulation results have
demonstrated that torque ripple caused of hysteresis controller
is reduced from 32% to 20% of rated torque with saving the
dynamic performance of hysteresis control and zero steady-
state error of PI controller. Developed drive is a reliable drive
due to short circuit of DC bus, and has low-cost control
algorithm and components where it can be used for cost
sensitive demands such as home applications.
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