01-10-2012, 04:16 PM
Current control removes brushless DC motor commutation spikes
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Successful removal of the excessive
current and consequent
back EMF present at the end of
the commutation period for
brushless DC motors results in
improvements in efficiency, reduced
component cost and
lower acoustic noise. A technique
of ‘tail-end’ current control
integrated within a motor
pre-driver IC provides a solution
that supports existing half
and full-bridge speed control
circuitry.
The problem
The significance of the excessive
current is illustrated in Figure
1. At the start of the commutation
cycle, when the currents
are first switched on in the
coil, the rotor and stator are of
the same polarity so they repel
each other in the desired direction
of rotation.
At this time, the current in
the coil quickly rises and then
levels off to a peak at approximately
one quarter of the way
through the commutation
cycle after which point it starts
to fall slightly to around 90 percent
of the peak value at around
three quarters of the way
through the cycle. After this
low, there is then a sharp rise
in current flow to a level above
that of the first peak
Current practice
A number of methods are used
to compensate for these effects.
One method is to over-specify
the power switching devices
driving the coil. This enables
them to withstand the severe
voltages and currents generated
as a result of the back EMF.
However, use of such overly robust
components generally results
in a more expensive solution
than should be the case.
A disadvantage of over-specifying
the voltage rating of power
devices is that for each doubling
of the voltage rating of the device,
the RDS(on)—in the case of
MOSFETs—will also double, resulting
in higher power dissipation
in the devices.
Tail-end current control
In the early part of the commutation
cycle, when the poles of
the rotor and stator are the same
polarity, there is considerable
work done as the two poles repel
each other and are attracted
to the neighbouring poles. It is
in this early stage that most of
the torque is generated. However,
towards the end of the
commutation cycle, as the opposing
poles become increasingly
aligned because they are
attracted to each other.
Advantages
As this technique reduces the
excessive current flow in the
most inefficient part of the commutation
cycle, it has an immediate
knock-on effect on efficiency.
As described, 10 percent
reduction in current consumption
has been achieved with a
minimal, and in other cases,
zero reduction in speed. Refinement
of the technique should
see improvements beyond this.
The resulting ability to keep
to a realistic voltage rating for
the components also reduces
the internal motor dissipation,
which is most welcome in the
cramped conditions inside a
BLDC motor. And finally, since
the current waveform generated
by this new technique is
more sinusoidal, the movement
of the stator plates becomes
quieter, reducing high speed
commutation noise.