Brushless Direct Current (BLDC) motors are one of the motor types
rapidly gaining popularity. BLDC motors are used in industries such as Appliances, Automotive,
Aerospace, Consumer, Medical, Industrial Automation Equipment and Instrumentation.
As the name implies, BLDC motors do not use brushes for commutation;
instead, they are electronically commutated. BLDC motors have many advantages over brushed DC
motors and induction motors. A few of these are:
BLDC motors are a type of synchronous motor. This means the magnetic
field generated by the stator and the magnetic field generated by the rotor rotate at the same
frequency. BLDC motors do not experience the “slip” that is normally seen in induction motors.
BLDC motors come in single-phase, 2-phase and 3-phase configurations.
Corresponding to its type, the stator has the same number of windings. Out of these,
3-phase motors are the most popular and widely used. This application note focuses
on 3-phase motors.
Stator
The stator of a BLDC motor consists of stacked steel laminations with
windings placed in the slots that are axially cut along the inner periphery (as shown in Figure 3).
Traditionally, the stator resembles that of an induction motor; however, the windings are
distributed in a different manner. Most BLDC motors have three stator windings connected in star
fashion. Each of these windings are constructed with numerous coils interconnected to form a
winding. One or more coils are placed in the slots and they are interconnected to make a winding.
Each of these windings are distributed over the stator periphery to form an even numbers
of poles.
There are two types of stator windings variants: trapezoidal and
sinusoidal motors. This differentiation is made on the basis of the interconnection of coils
in the stator windings to give the different types of back Electromotive Force (EMF).
As their names indicate, the trapezoidal motor gives a back EMF in
trapezoidal fashion and the sinusoidal motor’s back EMF is sinusoidal, as shown in Figure 1
and Figure 2. In addition to the back EMF, the phase current also has trapezoidal and sinusoidal
variations in the respective types of motor. This makes the torque output by a sinusoidal motor
smoother than that of a trapezoidal motor. However, this comes with an extra cost, as the
sinusoidal motors take extra winding interconnections because of the coils distribution on the
stator periphery, thereby increasing the copper intake by the stator windings.
Depending upon the control power supply capability, the motor with
the correct voltage rating of the stator can be chosen. Forty-eight volts, or less voltage
rated motors are used in automotive, robotics, small arm movements and so on. Motors
with 100 volts, or higher ratings, are used in appliances, automation and in industrial
applications.
Rotor
The rotor is made of permanent magnet and can vary from two to eight pole
pairs with alternate North (N) and South (S) poles.
Based on the required magnetic field density in the rotor, the proper
agnetic material is chosen to make the rotor. Ferrite magnets are traditionally used to make
permanent magnets. As the technology advances, rare earth alloy magnets are gaining popularity.
The ferrite magnets are less expensive but they have the disadvantage of low flux density for a
given volume. In contrast, the alloy material has high magnetic density per volume and enables the
rotor to compress further for the same torque. Also, these alloy magnets improve the
size-to-weight ratio and give higher torque for the same size motor using ferrite magnets.
Neodymium (Nd), Samarium Cobalt (SmCo) and the alloy of Neodymium,
Ferrite and Boron (NdFeB) are some examples of rare earth alloy magnets. Continuous research
is going on to improve the flux density to compress the rotor further.
Hall Sensors
Unlike a brushed DC motor, the commutation of a BLDC motor is controlled
electronically. To rotate the BLDC motor, the stator windings should be energized in a sequence.
It is important to know the rotor position in order to understand which winding will be energized
following the energizing sequence. Rotor position is sensed using Hall effect sensors embedded
into the stator.
Most BLDC motors have three Hall sensors embedded into the stator on
the non-driving end of the motor.
Whenever the rotor magnetic poles pass near the Hall sensors, they give
a high or low signal, indicating the N or S pole is passing near the sensors. Based on the
combination of these three Hall sensor signals, the exact sequence of commutation can be
determined.
Note:
Hall Effect Theory: If an electric current carrying conductor is
kept in a magnetic field, the magnetic field exerts a transverse force on the moving charge
carriers which tends to push them to one side of the conductor. This is most evident in a thin
flat conductor. A buildup of charge at the sides of the conductors will balance this magnetic
influence, producing a measurable voltage between the two sides of the conductor. The presence
of this measurable transverse voltage is called the Hall effect after E. H. Hall who discovered
it in 1879.
Figure 5 shows a transverse section of a BLDC motor with a rotor that
has alternate N and S permanent magnets. Hall sensors are embedded into the stationary part of
the motor. Embedding the Hall sensors into the stator is a complex process because any
misalignment in these Hall sensors, with respect to the rotor magnets, will generate an error
in determination of the rotor position.
To simplify the process of mounting the Hall sensors onto the stator,
some motors may have the Hall sensor magnets on the rotor, in addition to the main rotor magnets.
These are a scaled down replica version of the rotor. Therefore, whenever the rotor rotates,
the Hall sensor magnets give the same effect as the main magnets.
The Hall sensors are normally mounted on a PC board and fixed to the
enclosure cap on the non-driving end. This enables users to adjust the complete assembly of Hall
sensors, to align with the rotor magnets, in order to achieve the best performance.
Based on the physical position of the Hall sensors, there are two versions
of output. The Hall sensors may be at 60° or 120° phase shift to each other. Based on this,
the motor manufacturer defines the commutation sequence, which should be followed when controlling
the motor.
Note:
The Hall sensors require a power supply. The voltage may range
from 4 volts to 24 volts. Required current can range from 5 to 15 mAmps.
While designing the controller, please refer to the respective motor technical specification
for exact voltage and current ratings of the Hall sensors used. The Hall sensor output is
normally an open-collector type. A pull-up resistor may be required on the controller side.
Theory of Operation
Each commutation sequence has one of the windings energized to positive
power (current enters into the winding), the second winding is negative
(current exits the winding) and the third is in a non-energized condition.
Torque is produced because of the interaction between the magnetic field generated by the stator
coils and the permanent magnets. Ideally, the peak torque occurs when these two fields are at 90°
to each other and falls off as the fields move together. In order to keep the motor running,
the magnetic field produced by the windings should shift position, as the rotor moves to catch
up with the stator field. What is known as “Six-Step Commutation” defines the sequence of
energizing the windings.