The Structure and Performance Analysis of BLDC Motor

The brushed DC Motor has the advantage of speed control and start-up characteristic, high stall torque. However, because of the mechanical commutation of brush and commutator, it affects the life span, performance, and reliability of the motor, and the associated fire will also interfere with other electric devices. For the complex structure, high noise and difficult maintenance of the commutator, people are looking for the DC motor which doesn’t have commutator and brush and have similar characteristic.

With the rapid development of electronic technology, this idea has become true. From the principle to the technology, the technology, uses the electronic switch circuit and position sensor instead of a brush and the commutator has become mature.

The brush motor not only has the characteristic of DC motor but also has the character of simple structure, reliable operation, easy maintenance that the AC motor has. It can replace general DC motor, single-phase AC motor, and stepper motor and be used in the field such as automatic mechanical arm, CNC machine, medical and chemical equipment.

1, The Characteristic of BLDC Motor:

The BLDC motor consists of a motor and electronic driver, whose structure is composed of the stator, rotor, and position sensor. The stator includes the shell, stator iron core, and armature winding; the rotor includes magnetic steel, shaft, and tracking rotor; position includes Hall sensor and printed circuit board, etc. The armature of the general brushed DC motor is the rotary armature, and the current commutation is realized by commutator and brush. While the BLDC motor is rotary magnetic steel, the current commutation in the armature is realized by detecting the phase relationship between the rotor magnet steel and armature, and by the logic transformation of the driver to control each phase electronic switch, whose effect is the same as brushed dc motor.

Because the armature goes through the electronic commutation, so the motor has a similar operation characteristic of DC motor. It is easy to control the output characteristic of the motor and has a relatively wide range of speed control. Using high-performance magnet steel will generate a strong air gap field, which improves the efficiency of the motor and reduce the loss of the rotor and heat of stator. By optimization, in the same condition, compared with other motors, the efficiency of BLDC motor is relatively high. Because the maximum speed is not restricted by the mechanical commutation but determined by the performance of bearing and electronic components, so it greatly improves the maximum speed and power destiny. For high speed and long life, the BLDC has an obvious advantage.

2, Stator Structure and Performance

Generally, the armature windings of the motor are embedded in the slot of the stator core, which consists of silicon steel sheets. But the core of the hollow cup motor has not a slot, which is easy to assemble the winding into the motor, but the making of hollow cup winding is very hard. BLDC motor can also be made into a hollow cup and the difficulty of the making of winding is lower, the quality is higher and the life is longer. Stator core and armature winding of Slotted BLDC Motor has a certain impact on the performance of the motor.

2.1, The Stator Core Structure

The core structure of the BLDC motor has two types: integral type and assembled type. The integral iron core is formed at one time. The assembled type needs to be assembled into an integral iron case. The main feature of the assembled type is less waste, and improve the material utilization rate of silicon steel strip; whether manual or mechanical, the embedded wire of integral winding is very hard. When the assembled stator is winding, the gear block is wound separately, and the coils are arranged orderly, which not only improves the visible full slot rate, but also benefits the winding heat dissipation, and shorten the end length of armature coil, reduce leakage resistance and save copper material.

2.2, The Stator Winding Structure

An important index of motor winding is the number of slots per pole per phase q when q is an integer, the winding is an integer slot winding when q is the fraction, it is a fractional slot winding.

Z is the number of the slot of the stator, p is the number of pole pairs of the permanent rotor, m is winding phases of the stator.

(1) Integral slot winding. The relationship between the electromagnetic volume and output performance parameter is :

Because the design of the integral slot winding end motor is simple, the winding coefficient is larger than fractional slot winding, so the power of the motor is larger.

(1)  Fractional slot winding. The number of slots under each pole of BLDC motor with fractional slot winding is relatively reduced, which can be replaced by a small number of large slots to reduce the space occupied by slot insulation and improve the full slot rate, improve the performance of motor; at the same time, it increases the short distance and distribution effect and improves sinusoidal waveform of back EMF. The design of fractional slot winding can be centralized winding. One stator tooth and a coil can be automatically wound by a winding machine, which can improve the production efficiency. The stator core can be designed as an assembled structure to save material and reduce cost.

3 Rotor Structure and Performance

3.1 Rotor Type:

The rotor type of BLDC motor is magnetic steel surface mount type and magnetic steel insert type. The surface mount type magnetic steel is pasted on the surface of iron; insert type magnetic steel is inserted into the inside of the iron core. For preventing the magnetic steel from loosening and falling under the centrifugal force, surface mount type should wrap high-strength glass fiber or carbon which is soaked by epoxy resin. Under the special condition, the armature reaction will lead to the demagnetization of magnetic steel. By magnetic focusing, the insert type will get higher air gap magnetic destiny, but there is magnetoresistance torque for salient pole effect. The design of the rotor of the insert type structure is complex, if the detail is not properly treated, it will affect the performance. The common magnetic circuit is hard to meet the requirement. Most electromagnetic design is carried out through finite element software, which reduces the design intensity, shorten the design time, improve the design precision.

3.2Magnetic Steel Type:

The commonly used magnetic steel has ferrite, NdFeB, and SmCo. Ferrite is divided into injection molding and sintering, NdFeB is divided into bonding and sintering. Samarium cobalt has strong performance and corrosion resistance, but its price is expensive; ferrite has a low price and corrosion resistance without surface treatment, but its magnetic properties are low, requiring the magnetic steel to be thickened, resulting in a relatively larger motor volume under the same performance; neodymium iron boron magnetic steel has the strongest performance and high-cost performance, which is widely used in high-power permanent magnet motor, but the surface must be treated. Injection ferrite magnetic steel is very special. Many pairs of magnetic poles can be integrally formed and magnetized at the same time, which is very suitable for mass production. To improve the dynamic performance of the motor, when magnetizing, it can be directly charged into the oblique magnet, without the need of stator core oblique teeth, so it is easy to manufacture.

4, The Impact of Driver Type on Performance:

Only equipped with driver, the Brushless DC motor can operate. The quality of the driver has an impact on the operation of the motor. The mode of inspecting the position of the rotor by the driver has two types: sensor driver that needs an additional position sensor. The sensor-less driver that doesn’t need an additional position sensor.

4.1, Drive Type of Induction Motor

Hall Sensor is commonly used in the inductive motor. The motor with the inductive drive type starts rapidly and has a large starting torque. But the installation of the hall sensor will occupy a relatively large space. In the condition such as high temperature, high vibration, high interference, the hall sensor will fail, leading to the failure of the motor.

4.2, Drive Type of No Induction Motor

Because there is no position sensor in the sensor-less driver, it calculates the rotor position by indirect detection method, and the typical is a three-stage start. When positioning, the pulse is injected into the two-phase winding of the motor, the motor vibrates. By detecting the feedback waveform of the other phase, the position can be calculated. But this method is not suitable for the condition that requires large load inertia and starting torque.