3. So…. Why do we need Brushless motors?
The very first and foremost point is that the brush assembly, being made from
carbon gets rubbed and wears off. So, they need to be updated often.
Next, due to the friction between the rotating ROTOR and the stationary
BRUSHES, friction losses occur and some amount of input power gets wasted
in this.
Often possibility of sparking occurs between the brush contacts with the
rotating coil which damages the critical parts of the motor.
4. WHAT’s BLDC???
We all know that motors have brushes to commutate the current from supply to the
coil… So how could this commutation be possible without the brushes?
In BLDC motors, the entire ideology of using a brush is blown off. Here, the stator is
the one that is the current carrying coil (stator is electromagnet) and the rotor is made
up of permanent magnet (High grade magnets like NiFeB or an alloy of Nd).
Brushless DC electric motor (BLDC motors, BL motors) also known as electronically
commutated motors (ECMs, EC motors) aresynchronous motors that are powered by a
DC electric source via an integrated inverter/switching power supply, which produces
an AC electric signal to drive the motor (AC, alternating current, does not imply a
sinusoidal waveform but rather a bi-directional current with no restriction on
waveform); additional sensors and electronics control the inverter output amplitude
and waveform (and therefore percent of DC bus usage/efficiency) and frequency (i.e.
rotor speed).
(Courtesy : Wikipedia)
5. Using BLDC….
First of all, there is no brush assembly present in the motor, so no loss due to
friction at the brush contacts. Also no brush means no maintenance with
brush point of view, thus increasing the efficiency of the motor while
decreasing the mechanical wear.
An electronic controller replaces the brush/commutator assembly of the
brushed DC motor, which continually switches the phase to the windings to
keep the motor turning. The controller performs similar timed power
distribution by using a solid-state circuit rather than the brush/commutator
system.
6. Brushless vs Brushed…
more torque per weight
more torque per watt (increased efficiency)
increased reliability
reduced noise
longer lifetime (no brush and commutator erosion)
elimination of ionizing sparks from the commutator
overall reduction of electromagnetic interference (EMI)
With no windings on the rotor, they are not subjected to centrifugal forces, and
because the windings are supported by the housing, they can be cooled by
conduction, requiring no airflow inside the motor for cooling. This in turn means
that the motor's internals can be entirely enclosed and protected from dirt or
other foreign matter.
Brushless motors are more efficient at converting electricity into mechanical
power than brushed motors. This improvement is largely due to the brushless
motor's velocity being determined by the frequency at which the electricity is
switched, not the voltage. Additional gains are due to the absence of brushes,
alleviating loss due to friction.
Source : Wikipedia
7. BLDC – The dark side….
The maximum power that can be applied to a brushless motor is limited
almost exclusively by heat; too much of which weakens the magnets, and may
damage the winding's insulation.
A brushless motor's main disadvantage is higher cost, which arises from two
issues.
Firstly, brushless motors require complex electronic speed
controllers (ESCs) to run. In contrast, brushed DC motors can be regulated by
a comparatively simple controller, such as a rheostat (variable resistor).
However, this reduces efficiency because power is wasted in the rheostat.
Secondly, some practical uses have not been well developed in the
commercial sector. For example, in the radio control (RC) hobby arena,
brushless motors are often hand-wound while brushed motors are usually
machine-wound.
8. Controller Implementations…
Generally Hall effect sensors(or rotary sensors) are used.
http://en.wikipedia.org/wiki/Hall_effect_sensor
Advanced controllers employ a microcontroller to manage acceleration,
control speed and fine-tune efficiency.
Source : Wikipedia
9. Electronic Commutation…
There are different systems. maxon uses the following three:
•Block commutation with or without Hall sensors
•Sinusoidal commutation. As you can see the different maxon controller
families perform different commutation types. Common to all these systems
is that they should apply the current in a way, that the generated torque is as
high as possible. As we have learned this is achieved by a perpendicular
orientation of the magnetic fields of permanent magnet and winding.
10. The SENSORED Block Communication…
The actual position of the control magnet
in the diagram generates the following signals:
• The blue Hall sensor sees the
north pole. Thus the signal output level is
high and will remain high for the next 120°.
• The green Hall sensor is close to
the south pole. The output level is low for
the next 60°.Then the north pole approaches
and the output signal will switch to a high state.
• The red Hall sensor has just switched
from high to low where the signal level will
stay for the next half a turn. The combination
of the three Hall sensor signals is unique for each 60° of rotation. Looking at these signals
allows to know the rotor position within 60°.That exactly what we need for commutation.
Remember there were 6 different ways of current flow through the motor at a
commutation angle of 60°.
11. First we have to look at the Hall sensor feedback signals. In the back of the
motor there are three Hall sensor mounted on the PCB at an angle of 120°.
The Hall sensor detect the magnetic poles of the control magnet which is
mounted on the shaft. The control magnet exhibits the same two magnetic
poles in the same orientation as the power magnet. (Basically the Hall sensors
could monitor the power magnet directly but the control magnet offers two
advantages: The magnetic transitions between north and south pole are more
precisely defined. And an angular misalignment and tolerances between the
relative position of winding and Hall sensors can be adjusted.) The digital Hall
sensors used probes. They generates a high output signal (5V) if the north
pole of the control magnet is close to them. A south pole produces a low level
(Gnd).
13. There is another way of getting the necessary position information from the motor. Let's consider a
motor with one pole pair with a winding in star configuration.
There is always one phase of the winding which is not powered. But this phase will see the rotating
permanent magnet which induces a sinusoidal voltage in this phase – the back EMF. One can show that
exactly in the middle of the 60° of block commutation (when the phase is not powered) the induced
voltage crosses zero. This voltage crossing can be detected if the star point of the winding is accessible
as well.
Then one has to wait until 30° of rotation have passed and do the next switching of block
commutation. (The tricky thing is to have speed information as well in order to know when the 30° have
passed. But this can be done more or less precisely, e.g. from the time distance of the previous zero-
crossings).
During the next commutation interval one looks at the next phase that is not powered and so on.
There is one problem. When speed is low the amplitude of the EMF becomes smaller and smaller. The
slope of the EMF voltage becomes flatter and flatter and it is difficult to determine exactly the zero
crossing. Even worse, at zero speed (e.g. during start-up) there is no back EMF at all!
This means that sensorless commutation does not work well at low speeds (typically below approx. 1000
rpm for motors with 1 pole pair) and it needs a special starting procedure which is done similar to a
stepper motor. I.e. the windings are powered according to the block commutation sequence without
taking note of the EMF. The commutation frequency is enhanced and if anything goes well the rotor will
speed up. Once a certain minimum speed is reached the back EMF is taken into consideration and the
real sensorless block commutation is established.
In order to get a reliable starting up the parameters of the start-up procedure must be selected
carefully depending on motor characteristics and load (friction, mass inertia, …).
