BLDC motor

Reluctance that the magnetic flux through the steel, air gap and magnet composed of magnetic circuit of the degree of difficulty. Magnetic flux is a very good source, the equivalent current source. Phase winding MMF is a good source, a voltage source. At low magnetic load, steel reluctance is small, and not in magnetic saturation. In high magnetic loading (typically> 1.5 T), the reluctance of steel will start with the rapid increase in magnetic saturation. Air has a very high reluctance, it has nothing to do with the magnetic load. Magnetic materials have similar characteristics.

Reluctance with the position change, so can be used as the basis of sensor operation. In all the BLDC motor, the reluctance of some changes will occur with the point of view. From the motor point of view, the reluctance of change and change in inductance as obvious. Reluctance variation has obvious advantages, because at zero speed can be detected. However, you need to know to control the motor L (i, θ) characteristics.

Unfortunately, for many BLDC motors, reluctance changes with location is too small, can not be reliably measured. Especially for surface mounting of the magnet motor, the large effective air gap, so this feature in this type of motor has been particularly evident. Therefore, the reluctance of the main part of the same, which makes the remaining changes occurred with the location is difficult to measure. In specially designed for low torque ripple of the motor, the reluctance changes will be small because of the reluctance change will produce extra torque. Magnetic or magnetic motor hidden inside the angle changes with great reluctance, but they are generally powered with sinusoidal voltage and therefore not be discussed here.

Motor BEMF waveform changes with position and velocity. It is not possible at zero speed and low speed with the BEMF detection position. However, there are many applications (such as fans and pumps) at low speed or position control loop does not need to run. For these applications, BEMF method is very suitable. There are many ways to use BEMF, the main method can be summarized as follows:
• The motor terminal voltage detection
- By direct measurement or estimated (state and DC bus voltage switching is known).
• midpoint voltage detection
- Applies only to special characteristics of Y-connection BEMF motor.
- Does not really need the first four lines. Using the resistor network and differential operation, you can re-establish the center of a star connection.
• bus current gradient detection
- Because of the change go hand in hand ahead of or behind the rotor change, bus current thus depends on the particular shape.
- Can not use the fast bus current control.