Brushless DC Motors
Brushless DC Motors
Confused about motors? Understand the
principle and applications of these high efficiency motors.
What is a Brushless DC Motor?
A Brushless DC Motor (BLDC) is a brushless
motor which is powered by a direct current voltage supply. In conventional DC
motors, brushes are used to transfer the power to the rotor as they turn in a
fixed magnetic field, while the brushless DC motors are commutated
electronically, thereby eliminating the use of brushes.
BLDC motors are broadly used in various
applications owing to their high efficiency and exceptional controllability. It
also has power-saving advantages as compared to other types of motor.
Construction
of BLDC Motors
In a BLDC motor, the mechanical commutator used in the conventional DC motor is replaced with an electric switch circuit. A brushless DC Motor is basically a type of synchronous motor. Therefore, the magnetic field generated by the stator and the rotor revolve at the same frequency.
A BLDC motor consists of the following parts :
- Stator
The stator of a BLDC motor
is similar to the one used in an induction motor. The stator is built of steel
laminations that are stacked together with slots for winding which are axially
cut.. Most BLDC motors consist of three stator windings that are connected in a
star or ‘Y’ arrangement.
- Rotor
The rotor of a BLDC motor
consists of permanent magnets (Samarium Cobalt (SmCo), Neodymium (Nd), alloy of
Neodymium, Ferrite and Boron (NdFeB)). The arrangement of poles can vary
according to the number of poles depending on the application of the motor.
- Position Sensors (Hall Sensors)
Since BLDC motors are
commutated electronically, the windings of the stator must be energized in a
sequence and the position of the rotor must be known to precisely energize a
particular set of stator windings, in order to rotate the motor.
A Position Sensor or a Hall
Sensor is a sensor that works on the principle of Hall Effect. Its function is
to detect the position of the rotor and transform it into an electrical signal.
These sensors are embedded into the stator to detect the rotor’s position.
Usually, three Hall sensors are required in a BLDC motor.
Advantages of BLDC Motors
Following are some of the advantages of BLDC
motors :
●
High efficiency
●
Reduced motor size
●
Superior thermal characteristics
●
Light weight
●
Longer lifetime
●
No maintenance due to lack of
brushes
●
No sparking
●
Steady operation
●
Variable speed
●
Excellent controllability
●
High output power to size ratio
●
Higher speed range
● Lower electric noise generation
BLDC motors can be controlled using feedback mechanisms to give
specifically the required torque and rotation speed. Precision control, in
turn, lessens energy utilization and heat generation. In certain cases, where
motors are powered by battery, the precision control enhances the battery life.
BLDC motors also offer high durability and low electric noise
generation owing to the lack of brushes. With brushed motors, the brushes and
commutator wear down because of the continuous moving contact, and also produce
sparks at the point of contact. Particularly, electrical noise is a consequence
of the strong sparks that happen to take place in the areas where the brushes
pass over the spaces in the commutator. Therefore, BLDC motors are often
considered preferable in applications where it is important to avoid electrical
noise.
Disadvantages of BLDC Motors
Following are some of the disadvantages of
BLDC motors :
●
Cost
●
Limited Constant power range
●
Safety
●
Demagnetization
●
High-Speed Performance
The cost of a BLDC motor
is relatively higher when compared to a conventional brushed DC motor. The
electronic controller also elevates the overall cost of the complete setup, as
in a traditional motor. A less expensive
mechanical commutation setup involving brushes is used. When a BLDC motor is
operated at low speed, slight vibrations take place during low-speed rotation.
However, the vibrations are reduced at high speed. Due to the inherent natural
vibration frequency of the BLDC motor, sometimes this natural frequency is
capable of matching or coming closer to the vibration frequency of the body or
plastic parts resulting in the occurrence of a phenomenon, known as resonance.
However, this resonance can be minimized by adjustment. The resonance phenomenon
is commonly observed in various BLDC motor based devices. The brushed DC motors
are easy to operate as they have simple wiring. The positive terminal is
connected to the positive wire and the negative terminal is connected to the
negative wire and the motor starts rotating. However, in the case of a BLDC
motor, wiring and operation of the motor are not that smooth and effortless.
This is because of the involvement of electronic control and its connection to
all the electromagnets.
A large constant power range is important for
achieving a high vehicle efficiency. It is impossible for a BLDC motor to
achieve a maximum speed greater than twice the base speed. During the
manufacturing process of the BLDC motor, it might result in being dangerous
because the large rare earth permanent magnets can attract scattered metal
objects. In case of a vehicle crash, if the wheels spin freely and the motor is
still excited by permanent magnets, high voltages will appear at the motor
terminals. This can endanger the passengers or rescuers. The permanent magnet
can be demagnetized by applying a huge reverse magnetomotive force and
increasing the temperature. The critical demagnetization force is distinct for
each permanent magnet material. Surface mounted motors of permanent magnets
fail to achieve high speeds. This is because of the limitation by the
mechanical strength of the assembly between the rotor yoke and the permanent
magnets.
Considering the long list of advantages, BLDC
motors can be used in numerous applications, ranging from automotive to
industrial applications. They can be used in medical equipment, home
appliances, building controls and industrial automation. They can also be used
to drive vacuum machines. These brushless motors can also be used to spin hard
disc drives. Here, their durability keeps the drives operating dependably over
the long term. The power efficiency of these motors contributes to energy
reduction in an area where this is becoming increasingly important. Some
applications that use the BLDC motors are :
● Consumer electronics
● Transport
● Heating and ventilation
● Industrial engineering
●
Model engineering
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