On another thread a conversation is unfolding where a comment was made about using BEMF to drive a brushless 3 coil motor. Can't hijack that thread so I'm just starting one. One that will get few responses, but hopefully someone will educate me in understanding how BEMF can be used in this situation.

Or use a sensorless controller that relies on back EMF

I'd like to learn how BEMF is used.

No, I don't directly have a project, but I DO have a motor I'd like to run.

 

Will check in come morning.

 

Hey, @Tonyr1084.

Back EMF control is simple in principle. When the rotor of the motor spins, it induces a reverse current into the coils as they cut the B fields. By using the unpowered phase as a sensor, you can detect the zero crossing point and infer the position of the motor from it.

The simplest implementation is to create a trapezoidal drive. It uses six step commutation and gets its name from the trapezoidal waveform of the back EMF. Usually, two of the three phases are powered and one is not—that one is used for the zero crossing point detection.

Speed control is PWM-based and usually it is applied to one of the two powered phases while the other receives full power. You can implement the scheme on an MCU that has good peripherals for it, or you can use an MCU and external peripherals.

You need something with 3 ADCs (higher the resolution the better) and six PWMs—high and low side for each phase. Computationally, probably any modern MCU will do it. Obviously you also need motor control (MOSFETs, &c) as well.

The TI DRV11873 is an ASIC that does the job with on-board MOSFETs as well. The development board from TI is only about 30 bucks, so you could even skip tying to breadboard it. This seems like the easiest way to experiment.

 

Depends on the bldc type. The asic chip above needs this 4 wire type coil.
https://www.electroschematics.com/hdd-bldc-motor/
Oh, it’s a 4-Wire HDD BLDC motor!
As you might have noticed, your salvaged HDD BLDC motor has four connection leads instead of the said three wires A-B-C (often marked as U-V-W). Your 4-pin BLDC motor is just a normal 3-phase BLDC motor with the center tap brought out. If your motor has phases that are 1Ω each then it should measure 1Ω from center tap to each phase, but 2Ω between phases. Certain BLDC motor drivers use the center tap to help measure BEMF for commutation!



But many bldc types are like this 3 wire coil.
https://www.digikey.com/en/articles/controlling-sensorless-bldc-motors-via-back-emf

 

The motors I have are 3 wire. One motor is from a dish washer. It has its own control board. Two wires are heavier and I'd assume are main power for the board. There are three smaller wires and would assume they control the motor speed. I have never been able to find any information on how to repurpose this pump motor. I WANT to use it in a pond application hopefully this year. The OTHER motor is an air pump from a C-PAP machine. Its control board is integral to the controls and sensors. If breathing is not detected it shuts down. Using just the motor would require a complete new control method. Off hand I don't know what voltages either of those motors run on or if they're AC or DC. I'd assume they're DC. Their voltage is likely (but not necessarily) 12 or 24VDC. This air pump is something I'd like to use to increase air flow into a fire pit for better ventilation. Control of air flow means I don't have to force ember lifting winds into the pit. THIS project is very low on the list of things I'd like to accomplish.

 

The motors I have are 3 wire. One motor is from a dish washer. It has its own control board. Two wires are heavier and I'd assume are main power for the board. There are three smaller wires and would assume they control the motor speed. I have never been able to find any information on how to repurpose this pump motor. I WANT to use it in a pond application hopefully this year. The OTHER motor is an air pump from a C-PAP machine. Its control board is integral to the controls and sensors. If breathing is not detected it shuts down. Using just the motor would require a complete new control method. Off hand I don't know what voltages either of those motors run on or if they're AC or DC. I'd assume they're DC. Their voltage is likely (but not necessarily) 12 or 24VDC. This air pump is something I'd like to use to increase air flow into a fire pit for better ventilation. Control of air flow means I don't have to force ember lifting winds into the pit. THIS project is very low on the list of things I'd like to accomplish.

One downside of the Back EMF technique is that it doesn’t work at very low speeds. This is because the back EMF is proportional to rotation speed and it must be moving fast enough to get a stable signal.

If you need to drive the motor very slowly, you would need to use HFI (High Frequency Injection) instead—which is somewhat more complex.

 

Some BLDC motors have hall-cell sensors to determine the actual rotor position. The algorithm for these doesn’t rely on back-EMF for control.
Could you take some clear and focused photos of the motor, wiring and control board?
That would help enormously in our estimates.

 

On another thread a conversation is unfolding where a comment was made about using BEMF to drive a brushless 3 coil motor.
I'd like to learn how BEMF is used.
No, I don't directly have a project, but I DO have a motor I'd like to run.

There are two methods of commutating each styles of motor, construction of each type is practically identical to each other.
The difference is in the commutation, i.e. One is BLDC (Brushless DC) where just two windings out of the 3 are powered at any one time, and typically uses three sensors for rotor position, or other methods as posted.
The second motor style is 3ph AC, where the same constructed motor, powered using 3 phase sine wave sequentially in the same manner as most 3ph AC motors.
. ,

 

Could you take some clear and focused photos of the motor, wiring and control board?

Yes, I will, later, when I have some time. Right now I'm just goofing off with the computer. I really need to get back to work.

 

Got a bit distracted. Here's the photos of both motors.
Motor #1 (one picture) is an air pump from a C-PAP machine. I don't have the controller board. Even if I did - the motor would run for a number of seconds then shut down if no "Breathing" was detected. This motor has 3 wires.


Motor #2 is a water pump from a dishwasher. The control board failed and has long since been scrapped for materials.
Photo #1: The unit as pulled from the DW.


#2 Connection view (A)


#3 Connection view (B) (same as A just different angle)


#4 Board component side view with wires plugged in
View attachment 4.jpg

#5 Solder side. Note the 3 contact pads lower left corner. These mate to the 3 spring contacts in photos #7 & #8


#6 Closeup of contact pads


#7 Contact springs (3 of them) upper left side. What appears as possible contacts is just a jumper. This is where all three coil sets are joined at what is the center of the motor. I probably need to explain this better. Ask if you don't understand.


#8 Better view of spring contacts. Note the two pin junction. Look close at #7 and you can see three copper wires stabbed into what is the center point of the three coil segments.

So these are the two motors I have that I would like to use later this year. I do not have a project underway right now, largely because I don't know how to connect them or power them.

 

Seems like a strange commutation method used? Generally there is 3 power and 3 sensor conductors??
(shows 5 conductors?)

 

Having issues uploading pictures again. As for the communication method - it's proprietary. I think the board controls the motor based on input from the three grey wires.

I'll have the photos reloaded shortly. I hope.

 

The diagram below is from nsaspook and shows the three coil point in my motor. "O" is this jumper.


There are no other connections to the motor other than A, B & C.

Picture curtesy of nsaspook

 

OK, at last I'm done with editing.