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Brushless DC Motor Application

What is a Brushless DC motor ?

Brushless DC electric motors (BLDC motors, BL motors) (Figure 1) also known as electronically commutated motors (ECMs, EC motors) are synchronous 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. In this context, 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).

Efficiency is a primary selling feature for BLDC motors. Because the rotor is the sole bearer of the magnets, it requires no power, i.e., no connections, no commutator, and no brushes. In place of these, the motor employs control circuitry. To detect where the rotor is at certain times, BLDC motors employ, along with controllers, rotary encoders or a Hall sensor.

BLDC motor
Figure 1 - BLDC motor

How it works ?

BLDC motors can be thought of as an inside out DC motor. The coils of a BLDC are located on the stator and the permanent magnet is now on the rotor. Commutation is done by switching current into different phases in the stator. As current enters one phase the rotor aligns itself with the active pole and is rotated by applying current in succession to each pole.

The bi-directional current is controlled by using a micro controller and transistors to supply current into the correct phase. To determine which phase should be active, the angular position of the rotor must be known at all times. To do this, most motors leverage the Hall Effect to determine where the rotor is in its rotation. A Hall Effect sensor works by detecting disturbances in its magnetic field caused by a moving rotor. The position is then relayed to the micro-controller as a binary number based on which sensors are active and which are inactive. The micro-controller then decides which phase current should enter based upon the angular position, which causes commutation.

Principe of BLDC motor
Figure 2 - Principe of BLDC motor

Brushless Vs. Brushed DC Motor

Brushes require frequent replacement due to mechanical wear, hence, a brushed DC motor requires periodic maintenance. Also, as brushes transfer current to the commutator, sparking occurs. Brushes limit the maximum speed and number of poles the armature can have. These all drawbacks are removed in a brushless DC motor. Electronic control circuit is required in a brushless DC motor for switching stator magnets to keep the motor running. This makes a BLDC motor potentially less rugged.
Advantages of BLDC motor over brushed motors are: increased efficiency, reliability, longer lifetime, no sparking and less noise, more torque per weight etc.

Application :

Brushless DC motors (BLDC) are used for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments, etc. Some specific applications of BLDC motors are :

  • Computer hard drives and DVD/CD players
  • Electric vehicles, hybrid vehicles, and electric bicycles
  • Industrial robots, CNC machine tools, and simple belt driven systems
  • Washing machines, compressors and dryers
  • Fans, pumps and blowers.

Description

The motor being considered here has a rotor containing 8 permanent magnets and a 12-coil stator as shown in Figure 3. The rotor is driven to turn by magnetic forces resulting from the excitation coils and the permanent magnets. By creating multiple studies, the user can change the materials, the number of turns, the current through each turn, and the geometry of each part. EMS allows the user to keep the same assembly file and associate each study with a design table. All these features are very helpful for designers and can be used to determine the Brushless DC motor parameters which must be changed in order to optimize the Motor performances.

3D model of BLDC 3D model of BLDC

Figure 3 - 3D model of BLDC

Study

The Magnetostatic module of EMS is used to compute and visualize the magnetic flux and the magnetic intensity in the motor. It is also used to calculate the inductance of the coil and the electromagnetic force applied in the load (the rotor). After creating a Magnetostatic study in EMS, four important steps shall always be followed: 1 - apply the proper material for all solid bodies, 2- apply the necessary boundary conditions, or the so called Loads/Restraints in EMS, 3 - mesh the entire model and 4- run the solver. Moreover, a magnetostic study can be coupled to thermal which can give user an idea about the thermal behaviour of the motor.

Materials

In the Magnetostatic analysis of EMS, the following properties of material are needed (Table 1).

Components / Bodies Material Relative permeability Conductivity (S/m) Thermal conductivity (W/m*k)
Rotor AISI 1010 Steel Non linear 6.9e+006 65.2
Outer Air Air 1 0 0.024
Inner Air Air 1 0 0.024
Coils Copper 0.99991 57e+006 401
Stator AISI 1010 Steel Non linear 6.9e+006 65.2
Permanent magnets S2818 1.0388 0 69
 
Table1 - Table of materials
 
Permanent magnets of the rotor

Figure 4 - Permanent magnets of the rotor : Coercivity : 819647 A/m , Remanence : 1.07 T

AISI 1010 Steel BH Curve
Figure 5 - AISI 1010 Steel BH Curve
 

ElectroMagnetic Input

In this study, 8 coils (Table 2) are applied and the rotor (Table 3) where we need to calculate the virtual work.
 
Name Number of turns Current excitation
Wound Coil (1-8) 200 1 A
Table 2 - coils information
 
Name Torque Center Components / Bodies
Virtual Work At origin Rotor and Permanent Magnets
Table 3 - Force and Torque information.

Meshing

Meshing is a very crucial step in the design analysis. EMS estimates a global element size for the model taking into consideration its volume, surface area, and other geometric details. The size of the generated mesh (number of nodes and elements) depends on the geometry and dimensions of the model, element size, mesh tolerance, and mesh control. In the early stages of design analysis where approximate results may suffice, you can specify a larger element size for a faster solution. For a more accurate solution, a smaller element size may be required.

Mesh quality can be adjusted using Mesh Control (Table 4), which can be applied on solid bodies and faces. Below (Figure 6 ) is the meshed model after using Mesh Controls.
 
Name Mesh size Components /Bodies
Mesh control 1 1.200 mm Coils
Mesh control 2 7.000 mm Rotor
Mesh control 3 1.000 mm Magnets
Mesh control 4 5.000 mm Stator
Mesh control 5 0.6699000 mm Inner air

Table 4 - Mesh control
 
Meshed model
Figure 6 - Meshed model

Results

After running the simulation of this example, Magnetostatic Module coupled to thermal solver generates the results of: Magnetic Flux Density (Figure 7,8), Magnetic Field Intensity (Figure 9) , Force density (10,11), Temperature Distribution (Figure 12) and a results table which contains the computed parameters of the model, the force and the torque(Figure 13) .
 
Magnetic Flux Density, fringe plot
Figure 7 - Magnetic Flux Density, fringe plot


Magnetic Flux density, vector plot
Figure 8 - Magnetic Flux density, vector plot

Magnetic Field Intensity, fringe plot

Figure 9 - Magnetic Field Intensity, fringe plot


Lorentz Force density

Figure 10 - Lorentz Force density


Force density (virtual work) in the magnets and rotor
Figure 11 - Force density (virtual work) in the magnets and rotor


Temperature distribution in the motor with inner air
Figure 12 - Temperature distribution in the motor with inner air


Results table
Figure 13 - Results table

Conclusion

The Magnetostatic Module of EMS gives all needed results of a BLDC for a good dimensioning and a better efficiency.

Videos

Magnetostatic Analysis of a DC Motor 2/2

Magnetostatic Analysis of a DC Motor 2/2

Magnetostatic Analysis of a DC Motor 1/2

Magnetostatic Analysis of a DC Motor 1/2