19-10-2012, 05:52 PM
Improved Direct Back EMF Detection for Sensor less Brush less DC (BLDC) Motor Drives
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Abstract—
Improved back EMF detection circuits for low
voltage/low speed and high voltage sensorless BLDC motor drives
are presented in this paper. The improvements are based on the
direct back EMF sensing method from our previous research
work described in reference [10], which describes a technique for
directly extracting phase back EMF information without the
need to sense or re-construct the motor neutral. The reference
method is not sensitive to switching noise and requires no
filtering, achieving much better performance than traditional
back EMF sensing scheme. A complementary PWM
(synchronous rectification) is proposed to reduce the power
dissipation in power devices for low voltage applications. In order
to further extend the sensorless BLDC system to lower speed, a
pre-conditioning circuit is proposed to amplify the back EMFs at
very low speed. As a result, the brushless dc motor can run at
lower speed with the improved back EMF sensing scheme. On the
other hand, another improved detection circuit is presented for
high voltage applications to overcome the delaying problem
caused by large sensing resistors. The detailed circuit models are
analyzed and experimental results verify the analysis.
INTRODUCTION
Brushless DC (BLDC) motors are becoming widely used as
small horsepower motor controls due to their high efficiency,
reliability, low maintenance and low cost.
For three-phase BLDC motors, six-step commutation with
120o conduction time allows for current to flow in only two
phases at any one time. This leaves the third phase available
for sensing back EMF. The back EMF zero crossing of this
third floating phase can be detected to determine the
commutation sequence [1][2]. Erdman and Uzuka originally
proposed the method of sensing back EMF to build a virtual
neutral point that will, in theory, be at the same potential as the
center of a Y wound motor and then to sense the difference
between the virtual neutral and the voltage at the floating
terminal [3][4]. However, when using a chopping drive, the
PWM signal is superimposed on the neutral voltage, inducing a
large amount of electrical noise on the sensed signal.
DIRECT BACK EMF SENSING FOR SENSORLESS BLDC DRIVES
Generally, a brushless dc motor is driven by a three-phase
inverter with what is called six-step commutation. The
conducting interval for each phase is 120o by electrical angle.
Therefore, only two phases conduct current at any time, leaving
the third phase floating. This opens a window to detect the back
EMF in the floating winding.
In the reference scheme [10], the PWM signal is applied on
high side switches only, and the back EMF signal is
synchronously detected during the PWM off time. The low
side switches are only switched to commutate the phases of the
motor. Fig.1 shows the inverter configuration, current
commutation sequence, and the PWM algorithm from [10].
The true back EMF 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.
IMPROVED BACK EMF SENSING FOR HIGH VOLTAGE APPLICATIONS
It is shown in Fig.3 (B) that three phase terminal voltages
are directly fed into the microcontroller through resistors,
which limit the injected current. For some household
appliance applications, the dc bus voltage can be as high as
300 V. The resistor value is fairly high, 160kΩ. We find that
we have wrong zero crossing detection when PWM duty
cycle is high. The problem is caused by the large time
constant of the current limit resistors. Inside the
microcontroller, there is some parasitic capacitance. Since
the outside resistance is high enough, even though the
capacitance is low, the effect of RC time constant will show
up.