How a BLDC work

Question

I am working on a controller for a Brushless dc motor and I watched a tutorial about this (the only one that i found and that wasn't just a demonstration), but the problem is that some thing aren't explained:

1. I need 2 mosfets for every coil in the motor. Why? One side and one low side for each but how does this affect the current trough the wire? Why isn't one enough?

2. In parallel with the mosfets were some capacitors. The circuit is DC , what is the role of the capacitors?

3. What is the difference between sensorless and sensored control?

Sorry if the questions are dumb but I watched one hour of this tutorial and everything was confusing. Graphics with no references, jump from one explanation to another. Doesn't explain why is he doing some of the things. Here is the tutorial, maybe I am dumb, but I ca;t find a tutorial that explains for my understanding.

Answer 1

A BLDC motor is actually a multiphase AC motor. The DC is with regard to the power supply, which is DC. The driver electronics set up a (usually) 3 phase AC system between driver and motor to drive the AC motor. Basically, a BLDC system consists of a inverter system and a motor. The inverter system in this case is basically three synchronized PWM sine generators or similar. In the tutorial, three H-Bridges made from two FETs each are chosen to apply the PWM signal to the three motor phases.

With regard to your Q2, there are diodes in parallel to the FETs, not capacitors. See this diagram. It is the same in the video at 4:08. These diodes are there to protect the FETs from the inducted voltage in the motor coils when turning of the coil current.

To transfer any amount of useable power, you have to make a AC system that is synchronous with the rotation of the motor. A sensorless system just starts with an arbitrary speed. If the motor does not need significant power output, it will start spinning asynchronously, producing next to no torque. At some rotational speed, inducted voltages from the motor can be interpreted by the controller, setting up a synchronous system, now producing plenty power.

A sensored motor makes its position measurable to the controller. In this case, the controller can set up a synchronous field from the get-go, having torque at startup.

This is relevant for some power applications.

In an airplane or boat, there is no resting torque, the propeller can spin more or less freely at startup. No need for sensors here: just spin the motor up, eventually get a reading, get power going.

In a wheeled vehicle, be it pedelec or RC truck, the motor has to provide a starting torque to get to any kind of speed. So in these cases, sensored motors are chosen.

Answer 2

Q1 The MOSFETS form a half bridge. The three half bridges combined allow the coil polarity to be inverted.

Q2 The capacitors in parallel withe MOSFETS were most likely snubbers. Snubbers allow the MOSFETS to shut off cleanly by redirecting the current over the capacitor for a split second.

Q3 The problems with a BLDC is that the rotor has magnets. If comparing a BLDC to a synchronous AC motor, the AC motor has a metal slug for a rotor. When the metal slug is acted up on by rotating magnetic field the slug becomes magnetized in the exact opposition to the field. This allows the AC motor to always spin in the correct direction. With a BLDC because of the magnets in the rotor the field rotation has to occur in the correct sequence to make sure that the motor spins in the correct direction. This sequence can be timed in one of two ways. The first is using a quadrature encoder on the motor. This gives the controller the ability to monitor the direction of the motor. The second is by using the back emf of the coils to monitor the rotation for sequence the coils.