Brushless DC Motor

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Transformers & Power Engineering

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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.

Figure 1 - BLDC motor

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 2. 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

Figure 2 - 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). Figure 4 shows the B-H curve of the used steel.

Components / Bodies	Material	Relative permeability	Conductivity (S/m)	*Thermal conductivity (W/mk)**
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

Figure 3 - Permanent magnets of the rotor : Coercivity : 819647 A/m , Remanence : 1.07 T

Figure 4 - 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

Figure 4 - Meshed model

Results

After running the simulation of this example, Magnetostatic Module coupled to thermal solver generates the results of: Magnetic Flux Density (Figure 5,6), Magnetic Field Intensity (Figure 7) , Force density (8), Temperature Distribution (Figure 9) and a results table which contains the computed parameters of the model, the force and the torque(Figure 10) .