# Get RPM of BLDC motor using Hall effect sensors

1. ## Get RPM of BLDC motor using Hall effect sensors

Hi
How do I measure the RPM of a 3 phase BLDC motor using Hall effect sensors? I need to feedback the RPM to a PI controller. How do I get the time it takes for the commutation to be done? I know the idea, but don't know how to build it as a circuit.

2. ## Re: Get RPM of BLDC motor using Hall effect sensors

Hi,

what´s the problem?

RPM measurement is just the same as frequency measurement.
You need to multiply the frequency to get the correct RPM value.
The factor depends on motor/gear.

Klaus

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3. ## Re: Get RPM of BLDC motor using Hall effect sensors

A bare Hall sensor is going to give a pretty weak signal
and a sensor packaged with its own amplifier tends to
be pretty slow. I'd bet however that there are purpose-
made Hall speed sensors out there (Allegro, maybe?) or
motor controllers with Hall inputs.

If you find such parts, behind them should be a good set
of app notes, usually the most educational material out
there (better than textbooks, because details).

4. ## Re: Get RPM of BLDC motor using Hall effect sensors

Get RPM of BLDC motor using Hall effect sensors
For obtaining motor speed, mechanics matters for choosing the appropriate solution; in general you dont measure anyting on the motor itself (unless when sensor is coupled to its axis), therefore you should consider the problem first and then select the solution after; for most cases an optic approach suffice.

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5. ## Re: Get RPM of BLDC motor using Hall effect sensors

So the motor I am using already has Hall effect sensors in it. I know the mathematics, but I can't figure out the hardware for it yet. How should I make the electronic circuit to measure the frequency? I don't want to use an arduino. I am now thinking of using an integrator so that corresponding to different speeds I can get a different proportional voltage, will that work?

6. ## Re: Get RPM of BLDC motor using Hall effect sensors

Hi,

As long as you don't tell us .... we don't know the motor type and how it's hall signal look like.
We don't know how many pulses per revolution they give, nor do we know the signal levels.

You say you don't want to use arduino, but you don't say why not, and you don't say what you want to use.

We don't know whether your PI controller already has hall signals inputs, and if it has how many and what signals it needs.
It can be one digital, three digital, one analog or something else.

How can we help?

Klaus

7. ## Re: Get RPM of BLDC motor using Hall effect sensors

The motor is a 3 phase BLDC motor with 4 pole pairs and 3 inbuilt hall effect sensors. I know it can easily be done using an arduino, but I wanna build an electronic circuit without the Arduino to measure rpm of the motor. You asked me what I want to use, but that's what I need help to figure out. Is there a way to do this using an integrator?
The PI controller isn't finalized yet.

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8. ## Re: Get RPM of BLDC motor using Hall effect sensors

A question is, whether the Hall sensor changes amplitude,
frequency or both with RPM. If only frequency, then an
integrator is not going to help; average DC value would
be little or none affected.

A F-V converter (or a timer and math) could be a way
to go. Have to get at the Hall output attributes.

9. ## Re: Get RPM of BLDC motor using Hall effect sensors

Hi,

The motor is a 3 phase BLDC motor with 4 pole pairs and 3 inbuilt hall effect sensors. I know it can easily be done using an arduino, but I wanna build an electronic circuit without the Arduino to measure rpm of the motor. You asked me what I want to use, but that's what I need help to figure out. Is there a way to do this using an integrator?
The PI controller isn't finalized yet.
Still the hall sensor signal are not clear. You know an electrical signal is defined with: volts, current, power supply, frequency...and so on

With all this being undefined your question is similar to ask for the price to travel to your uncle.
* it's not defined where to start (undefined motor)
* it's not defined where your uncle lives (undefined PI controller)
* it's not defined what vehicle to use, like car, ship, aeroplane (undefined signals)

There are many different ways to travel. And there are many different ways to build a motor controller system.

The motor should be clear. And it has a datasheet with all the necessary motor informations.
Then you need to decide which power stage and which speed controller to choose. You need to decide what parameters are important for you. Voltage and current need to meet the motor specifications, but with others like: cost, size, power dissipation, precision, self made vs ready to buy, time to market .... we can't help you.
Then you will see which input signals it needs. It may be analog, it may be digital. There are many controller modules already have built in the hall interface, then it'just the cabling you need. No need to worry about an integrator...

I recommend to look for similar projects in the internet. Look how they made it. Find out what you need.

Klaus

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