Do brushless motors have back EMF?
The primary distinction between brushless DC (BLDC) motors and brushless AC (BLAC) motors is their back EMF waveform. BLDC motors typically have a trapezoidal back EMF, while BLAC motors (also referred to as permanent magnet synchronous motors, or PMSM) have a sinusoidal back EMF.
What does a Hall effect sensor do?
The Hall-effect Sensor is able to distinguish between the positive and negative charge moving in opposite direction. The magnetic field detected by the hall-effect sensor is converted to the suitable analog or digital signal that can be read by the electronic system, usually a motor control system.
Are there Hall effect sensors on a BLDC motor?
In the sensorless variant of the BLDC motor, there are no Hall-effect sensors. Instead, as the motor rotates, the back EMF in the three coils varies in a trapezoidal waveform (long-dashed lines) shown in Figure 2. For comparison, the same figure also shows the outputs from the Hall sensors of a similarly configured motor.
How to control a sensorless BLDC motor via back EMF?
Figure 3 shows a control circuit for a sensorless three-phase BLDC motor. In this case, the circuit uses a Microchip PIC18FXX31 8-bit MCU to generate the pulse width modulated (PWM) outputs to trigger the IGBTs or MOSFETs in the three-phase inverter bridge. The MCU reacts to input from a back EMF zero-cross detect circuit.
How is a commutation encoder aligned to a BLDC motor?
To properly align a commutation encoder or even Hall effect sensors to a BLDC motor, the resulting commutation waveform should be aligned to the back EMF. Traditionally, this results in an iterative process requiring a second motor to drive the first, and an oscilloscope to observe the waveforms.
What are the zero crossing points on a BLDC motor?
A combination of all three zero crossing points for the coils is used to determine the coil energizing sequence. Note that there is a phase difference between an individual Hall sensor changing output in a conventional BLDC motor, and the back EMF zero crossing point for an individual coil in a sensorless unit of 30 degrees.