Sensorless motor control BLDC

[RenesasT]

Hello Guys,

I am properly able to drive BLDC motor using hall sensors. The motor is working fine.

Now the project is different. I want to drive BLDC motor without using sensors(i.e. using back emf) . For this operation, I am using Renesas Controller. The controller works with 5 V and the back emf signals are coming with high peaks.

Can you please explain me, how I can be able to get motor position. Programatically and Hardwarely what I have to do. I am driving motor which requires 12V.

Please help me.

Awaiting for your reply.

 

[BradtheRad]

Detecting the magnet's approach sounds similar to one of those 'perpetual pendulum' desktop gadgets. It's really a motor made from just a few components. The circuit is simple but ingenious. It maintains the action until the battery depletes.

(1) A magnet is hidden inside the pendulum.
(2) Magnet approaches circuit (hidden in enclosure).
(3) Induces flux in tapped inductor, resulting in current flow,
(4) biasing the transistor, triggering it to conduct.
(5) Transistor delivers pulse to the inductor.
(6) Inductor becomes an electromagnet,
(7) Gives momentary 'kick' to the magnet (inside the pendulum or porpoise, etc.).
(8) Pendulum gets sufficient energy so swinging continues instead of diminishing.

The concept is similar to what you want to do in the sense that this circuit has the ability to detect the magnet's approach.

Although I have not built a brushless motor myself, I have done experiments holding a coil of wire close to magnets spinning around a shaft. The aim is to detect the rise or fall in voltage at the coil. As far as I can tell, generated emf has small amplitude, very small if you are simultaneously sending 12V to the coil. However it is true that higher speed generates greater amplitude. So my observations are not the final word, of course.

 

[KlausST]

Hi,

There are documents in the internet discussing how to do sensorless BLDC control. Read through them.
In short:
They give power to two wires of the motor and observe the voltage on the third wire.

I assume there are BLDC controllers doing this for you.

Klaus

 

[RenesasT]

Hello,

KlausT,

I have BLDC motor and I wanted to ru by using my controller. I have read many articles, but still I am not getting how to sense the position of the magnet.

Pleased if you will resolve it. As its a last stage of my project. If you are with me , I will feel like some support is there for me. Please help.

 

[KlausST]

Hi,

I took 10 seconds to google for "sensorless BLDC".

The first hit is this document: https://www.st.com/resource/en/application_note/cd00020086.pdf

It describes all in detail. 35 pages full of schematics, technical description, measurement description, scope pictures...

I think it´s very complete. I can´t think of what is missing...

Klaus

 

[BradtheRad]

I am trying to clarify in my mind how the concept works. I think it is easiest for a coil to detect a near approach of a magnet when the coil is idle. Therefore wait until a coil starts to generate a waveform which signals a near approach. Then send a repelling pulse through the coil. This sends the magnet toward the next coil.

Use repelling pulses only, while testing this method. Also use light loads only.

I have been experimenting with ways to make a brushless motor (1 coil, 3 magnets). The led trigger method works reliably. Then I have been trying to amplify a waveform from a coil. What makes it difficult is that I also amplify noise on the power supply. The noise is worse when drawing current. That is why I think it is easier to detect the coil signal during an idle moment in the cycle.

 

[RenesasT]

Hello Brad and KlauST,

Thanks for your replies. I made the sensor less BLDC motor possible.

I am making application of Ceiling FAN using sensorless BLDC motor. My operation is perfect as I am getting perfect variable speed with perfect wattage.

But I am not sure about my first time rotation of ceiling FAN. I am using normal open loop method for running first time but it is sounding at first 3 or 4 rotations.

The sound is like "khat khat khat khat".

Please suggest me a fine start method for my ceiling FAN.

 

[BradtheRad]

I'm pretty sure you're hearing sound of sharp pulses going through your motor. From what I heard there were good reasons for me to choose to purchase a true sinewave inverter for my backup power system... Because motorized appliances are known to produce a hum when powered by square-ish waveforms.

 

[KlausST]

Hi,

maybe you are running into a current limit. This often generates sounds.

Klaus

 

[RenesasT]

No, I am not running in current limit.

I am running the same as we are running in open loop control method(or we can say manual run).

The sound is coming for only 3-4 rotations (when motor is on manual control).

Once motor left the manual control or we can say switches from manual control to back emf control no sound is coming i.e perfectly rotating.

Please help me.

 

[RenesasT]

The sound is coming only at start .