Hardware Reference
In-Depth Information
4.4.2.4 Determining the Speed in Sensorless BLDC Drive
Detection of speed in BLDC mode can be implemented using numerical dif-
ference method and can be realized using a counter or timer. The counter is
incremented by clock pulse from a precise crystal oscillator. If the change in
counter value between two consecutive ZCPs is N, then the mechanical speed
(Ω) of the rotor can be calculated as
Ω =
60f
e
p
=
60f
c
6Np
=
10f
c
Np
,
(4.87)
where, p is the number of pole-pair in the motor, f
c
is the frequency of the
crystal oscillator, and f
e
is the electrical frequency. If a timer is used instead
of the counter to measure the time difference ∆t between two consecutive ZCP
events then the mechanical speed is
Ω =
60f
e
p
6∆tp
=
10
60
=
∆tp
,
(4.88)
These methods of speed detection assumes the absence of voltage noise
causing error in ZCPs and the effects of pole jitter (discussed in section 4.3.2).
Unfortunately, pole jitter is unavoidable as neither the magnetic
fi
eld is per-
fectly symmetric nor the dimensions of the components and quality of the
assembling process can be controlled perfectly. While realizing the closed loop
speed control, an average of several values of the measured speed can be taken
into consideration to reduce the
fl
uctuations caused by errors.
Once the rotating speed of the motor is obtained, the drive voltage can be
controlled to maintain precise and accurate speed of the motor under closed
loop control. In the CV-BLDC mode, the voltage regulation is realized using
an adjustable DC regulator to modify the DC link voltage. In the PWM-BLDC
mode, the drive voltage can be controlled by adjusting the PWM duty cycle.
For the CC-BLDC mode, the current reference is set for a certain speed and
the drive current is controlled to meet the reference value.
4.4.3 Starting of Spindle Motor
There exists a problem in using the back-EMF detection method for
fi
nding
the rotor position. The back-EMF is either zero or very low at standstill or
during low speed operation, and the rotor position cannot be detected. For
the PMSM driven by the sensorless method mentioned above, the motor must
be
fi
rst started from standstill using some special procedures and brought up
to a speed suitable for sensing back-EMF. Therefore, in the application of
sensorless BLDC drive mode, how to start the motor effectively without a
sensor detecting rotor position is an important concern.
