23-05-2014, 12:26 PM
A novel sensorless control method for brushless DC motor
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Abstract:
The authors present the theory and implementation of a novel sensorless control method for the
interior permanent magnet (IPM) brushless DC motor (BLDCM). The proposed new sensorless technique can
accurately detect the zero-cross point (ZCP) of back electromotive force (BEMF), which is based on a
comparison of the terminal voltage of the unconducting phase during the first and second part of a pulse
width modulation (PWM) cycle. Compared with the conventional BEMF sensorless approach, the proposed
new sensorless method solves the problem of the sensorless BLDCM drives at very low speeds. Experimental
results confirm the validity of the new method.
Introduction
Brushless DC motor (BLDCM) can be broadly categorised
into interior permanent magnet (IPM) motors and surface-
mounted permanent magnet (SPM) motors. Compared
with SPM motors, IPM motors have a mechanically robust
and solid structure, because the magnets are physically
contained and protected. In addition, because of their
reluctance to torque production, the IPM motors are more
suitable for traction applications, which require constant
power output at high speeds over a wide range. So, IPM
motors are more practical than SMP motors in various
industrial and other applications.
Model of inductance of
IPM BLDCM
Generally, BLDCM is driven by a three-phase inverter
with what is called a six-step commutation. Each
conducting phase is called one step. The conducting
interval for each phase is 120 electrical degrees.
Therefore only two of the three phases are excited at any
instant, and a phase remains open in BLDCM. Fig. 1
shows an equivalent circuit of BLDCM and inverter
system.
The amplitude of BEMF is proportional to rotor speed,
but the ZCP of the unconducting phase is invariant. In a
BLDCM, the BEMF induced by the rotary permanent
magnet excitation field is approximately trapezoidal, as
shown in Fig. 2. The BEMF is a function of rotor
position.
Experimental results
Fig. 7 shows the prototype of the proposed sensorless drive
and the adopted BLDC machine (five poles, rated power
3 kW and rated speed 2600 rpm). To ascertain the
effectiveness of the technique, there are two commutation
methods in Fig. 7. So, we can compare the commutation
signals from the proposed technique and the Hall sensor.
Unlike conventional solutions, analog filter circuit and the
phase shift circuit are not required in the proposed method.
It can be seen that only six additional resistors have been
included for the proposed control method. The entire drive
system is controlled by a low-cost, fixed-point digital signal
processor (DSP), Dspic6010.
Conclusion
A novel approach to the position-sensor elimination of an
IPM BLDCM is presented in this paper. Both theoretical
analysis and experimental results verify that satisfactory
performance can be achieved with the proposed sensorless
commutation method. When compared with the
conventional solutions, the proposed method has several
advantages and disadvantages, including the following.
1. This method does not depend on the BEMF, and so it
can be operated at very low speeds. DC bus voltage Ud ,
quadrature inductance Lq , direct inductance Ld and
component of the self-inductance Lg2 are very important to
the sensitivity of the proposed method.
2. The process of estimating ZCP is implemented by the
software, and so this method does not need any sensor.
Moreover, the parameter in the software is important to
commutation error.