Variable Speed Control in BLDC fan using external MOSFET

[sravan_rikka]

Hi,
I am using 12V BLDC Fan and external MOPSFET to control the speed of Fan, i generated the 1.22KHz PWM signal using microcontroller.
I am varying the duty cycle of PWM signal for variable speed control of Fan, when i am starting from the 0% duty cycle and then increasing the duty cycle then Fan is is starts running at 60% duty cycle. I want to know why it is on at 60% duty cycle when i starting from 0% duty cycle.

 

[chuckey]

There is a lot of discussion on this subject, use the search facility.
Frank

 

[100k]

bldc motors are basically synchronous motor so like synchronous motor they would need a starter if the permanent magnets weren't there. Basically, what I see is that at duty cycles below 60% there aren't enough magnetic interaction between rotor and stator fields to magnetically lock them. Or your pwm frequency is much higher than the maximum inertia of your motor. You can either do the analysis/experiment to find out the time constant of your machine or you should try with a low frequency pwm.

One more thing I wan't to know what pwm technique are you using?

 

[sravan_rikka]

Hi sir,
How do we calculate the inertia and time constant of machine.
I am using square wave PWM technique.

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Hi sir,
How do we calculate the inertia and time constant of machine.
I am using square wave PWM technique.

 

[100k]

For calculating the machine constant you can use a LCR meter and find L/R ratio, it can be considered as machine constant for time being your frequency should be less than this frequency otherrwise your machine will undergo into saturation and with increase in frequency you will observe decrease in speed. Also, you shouldn't operate the a machine beyond its rated voltage and frequency.

How many phases your motor has?
Also, I would like to look into your code for pwm because many a time the pwm code is wrong because its quite a tricky one.
For time being you can set your frequency at 50Hz(if your mains are 220V, 50Hz) or 60Hz(if your mains are 110V, 60Hz) then vary the duty cycle and reply your observation.

 

[sravan_rikka]

For calculating the machine constant you can use a LCR meter and find L/R ratio, it can be considered as machine constant for time being your frequency should be less than this frequency otherrwise your machine will undergo into saturation and with increase in frequency you will observe decrease in speed. Also, you shouldn't operate the a machine beyond its rated voltage and frequency.

How many phases your motor has?
Also, I would like to look into your code for pwm because many a time the pwm code is wrong because its quite a tricky one.
For time being you can set your frequency at 50Hz(if your mains are 220V, 50Hz) or 60Hz(if your mains are 110V, 60Hz) then vary the duty cycle and reply your observation.

Hi,
I dont know the inductance of coil and also dont know resistance, can you tell me how to calculate L and R.
Yah i observed by incresing frequency, i used 19.61Khz frequency then the fan started rotating at 95% duty cycle untill 95% duty cycle it is not come into motion.
The microcontroller i am using is PIC16F1824, which provides least frequency of 1.22kHz, So UNABLE TO USE 50Hz.

 

[chuckey]

Brushless DC motors (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are synchronous motors which are powered by a DC electric source via an integrated inverter, which produces an AC electric signal to drive the motor; additional sensors and electronics control the inverter output - WIKIPEDIA
NOTE "additionel sensors", "electronics control the inverter".
You are using the wrong technique on the wrong sort of motor.
Frank

 

[sravan_rikka]

Brushless DC motors (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are synchronous motors which are powered by a DC electric source via an integrated inverter, which produces an AC electric signal to drive the motor; additional sensors and electronics control the inverter output - WIKIPEDIA
NOTE "additionel sensors", "electronics control the inverter".
You are using the wrong technique on the wrong sort of motor.
Frank

Hi,
Is BLDC motor is works with AC supply, then why we are calling it as BLDC motor.

 

[100k]

How many phases are there in your motor?
You can drive your BLDC by a microcontroller and a power electronic circuitry its no doubt but only if you microcontroller is generating suitable pulses .

The name BLDC ia a misnomer as it is said it need electronic communtation you are providing electronic commutation by your pic. It is said as BLDC because you feed the electronics commutator with a DC supply, a BLDC also have other names for marketing. But the basic principle of operation is based on synchronous motors.

You can measure the L and R by a LCR meter but thats not necessary for time being.

What you have to do is instead of using the inbuilt pwm channel write your own pwm code, I am not familiar with pic (I m 8051 guy) so I can't help yo with inbuilt pwm channels. If you are uncomfortable with writing your own pwm code you can use a timer ic like 555 and adjust the duty cycle. I personally suggest you to operate at 50Hz and also use the optocoulers in between the controller o/p and mosfet.

 

[chuckey]

sravan_rikka you said its a BLDC motor, as it works on 12V, this is extremely likely. However They are difficult to control externally, a, because of the inbuilt electronics and b, because they rely on the flywheel effect of the fan to get the rotor from pole to pole. They won't run with the impeller missing. So if they try to run too slowly they stall. To get reduced airflow, it is probablely better to pulse the motors on/off at a low speed, say over a 5 Second period.
Test the fan on an ordinary power supply, find the minimum voltage for it to start, you can not go any lower then this voltage - else it will not start. This then sets the minimum pulse width ratio - depending on applied voltage.
Frank

 

[sravan_rikka]

Hi,
Yes i observed that minimum voltage to turn on BLDC Fan is 3.2V, it is not rotating for voltage blow 3.2V. I am using single phse motor.
And by applying 60% duty cycle only it is turning ON. At the time of 60% duty cycle voltage across fan is 5.38V, At 50% duty cycle the voltage across fan is 3.61V.
Without any circuit only with power supply it is turning on for 3.2V then it should be on for 50% duty cycle also but it is not turning on why? and why it is turn on for 60% only.

 

[chuckey]

This is because the motor takes its current as pulses, from a PSU, the volts do not drop. With your PWM, you are measuring the mean voltage not the actual voltage when the motor is trying to take current, try putting a 1000MF cap across the motor, it might hold the voltage up while the motor takes current.
Frank

 

[100k]

I think I understood your problem
Now answer this
Do your motor has a in-built power electronic circuit?

If so then there is nothing conceptual explanation needed, just make sure that whatever the value of frequency or duty cycle you apply you get the sufficient rms output voltage that should be above the 3.2V.
You have to make sure that average value i.e ton*frequency*Vcc >=3.2V. Also insert a large capacitor in between like chuckey said.

And please reply with your observation.

 

[yousuf21]

I think it is better to use electronic speed controller to control the BLDC. Though I haven't use it but you will find literature about it on internet.