Brushless DC Motors.

Confused about motors? Understand the principle and applications of these high efficiency motors.

What is a Brushless DC Motor?

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.

Construction of BLDC Motors

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

A BLDC motor consists of the following parts :

1. 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.

2. 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.

3. 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   

·        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

BLDCmotors 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.

·        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.