Brushless motor controller

[Njguy]

I was wondering if anyone knew where I could get more information on how brushless motors are timed. If you look at this webpage, **broken link removed** there is a decoder circuit that times the motor. How does the circuit know what position the rotor is in? I see something with H1-3 on the stator case, which matches the number on the circuit but it is unclear. Any help would be appreciated. I always learn more from you guys.

 

[kinarfi]

probably a hall effect device. maybe built into an IC

 

[Njguy]

^ That's what I mean nobody really seems to have much info on brushless electric motors. It's like a damned secret

 

[shortbus=]

Brushless motors are similar to a stepper motor, they take a driver chip. A lot of the newer ink jet printers use BLDC motors now instead of steppers.
Here is a link to one; https://www.st.com/stonline/products/literature/an/9214.pdf

 

[Grossel]

The controller doesn't need to know what position/angle the motor has. Well that's almost true.

A reluctance motor differs from a stepper in the way that a steppers control circuit will output four different states that just repeat as the stepper moves around. The controller to a three terminal (four with center tap) reluctance motor will output three different states instead of four. So that means a reluctance motor need only three sensors atached.

It doesn't need to know the angle of the motor, but it need to know wich of three states it is into.


If this doesn't make sense it's just because of my poor english. God luck

 

[DamoRC]

For smaller brushless motors and controllers, there are some good discussions linked from this RC thread which cover design etc:

five(5) diy brushless ESC designs - RC Groups

 

[dknguyen]

If no encoder or explicit sensor is used (ie. sensorless) it is measuring the back EMF of the floating phase on the BLDC. There are 3 phases, and only two are connected to the power source at any one time, the third is floating and a measurement can be made (through methods that are sometimes no so obvious) to detect the rotor state (or more specifically the zero-crossing which is a marker for when to commutate the phases +/- some amount of time depending on whether you want advance or delayed timing for your motor). Note: t is not actually detecting the actual rotor position- it is actually detecting position of the upcoming phase to be energized (for which there are multiple valid rotor positions as multiple rotor position correspond to the same phase).

THere are various approaches to doing so:

https://www.electro-tech-online.com/custompdfs/2010/03/01083a.pdf

https://www.electro-tech-online.com/custompdfs/2010/03/T.pdf

 

[Njguy]

So what would you use to measure the position of the rotor to precisely flip the required switches in the circuit?

 

[dknguyen]

So what would you use to measure the position of the rotor to precisely flip the required switches in the circuit?

Did you bother to read the application notes I linked? (One application note has just the theory, but from that you should be able figure out your own circuit anyways. THe other application note compares two possible methods with each having very clear advantages and disadvantages and levels of complexity depending on what you are looking for). The traditional 3-resistor virtual ground method is a very very simple circuit, then again so is the other method. The real trick is understanding the theory to make the MCU be able to interpret the circuit output properly.

Are you aware of how phases are actually arranged in a motor? A 3 phase motor doesn't just have 3 coils (like you see in a model whch is drawn for simplicity). You actually have many coils (in multiples of 3), but every coil is wired to one of the three phases to produce three groups of coils energizing one coil in that group energizes all the coils in the group. Sure, you could give each coil have it's own wire running out of the motor, but then you have a motor with 12 or more wires and the controller would be just as complex. WIth just 3 phases, you get the vast majority of the same performance with only a fraction of the complexity- it's a matter of diminishing returns as you add on more phases.

You don't actually need to know the rotor position. You only need to know which two phases are currently energized, which phase is not, the direction of rotation, and when the BEMF zero-cross occurs. WHen the zero cross occurs, it tells you when to commutate (energize new phases and denergize old ones). You know that the denergized phase must be the one that becomes energized, and in turn one of the energized phases must become denergized. You know which of the two currently energized phases to denergize from the direction of rotation (CCW is one of the energized phases that needs to be denergized, and CW is the other phase that needs to be denergized).

FOr example:
https://www.aerodesign.de/peter/2001/LRK350/LRK_in_action.gif

Notice that it doesn't matter the angle of the rotor? It only matters that the red (or blue) winding is the next to be energized.