Why is BLDC Stall Current Changing Over 50% PWM Duty Cycle?

Thread Starter

hakansensoy

Joined Oct 30, 2019
1
Hello,

I am in trouble to drive a BLDC Motor -170W with a 3-phase driver circuit, which is designed by me.
My PWM frequency is 40 kHz and I just try to get a graph about the relationship between the stall current (I mean the current with blocked motor shaft) over PWM duty cycle (from %10 to %70)
In trapezoidal driving mode; why the stall current is rising linear up to %50 PWM duty cycle and rising exponentially over %50 PWM duty cycle? What difference happening over %50 PWM duty cycle?

PWM/Current
100 / 0.5A
200 / 1A
300 / 1.5A
400 / 2A
500 / 2.5A
600 / 4A
700 / 15A

What could be the possible reason for it?
Is this normal?

Thanks and looking forward to your reply.
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
3,676
Welcome to AAC. It's apparent english is not your first language. English is my only language, and I appreciate your efforts to be clear for people like me to understand your question. I hope my answer is clear enough to be of help.

From what I see in the numbers you post - as you approach full power the current goes up much faster. Consider Ohm's law.

At 10% you see a current of 0.5 amps being used. You didn't specify the voltage so I'll assume you're using 12 volts. At 10% you should be pushing 1.2 volts. For 0.5 amps, it would be assumed to be a reactive load of 2.4Ω. Since the load doesn't change, changing the effective voltage will change the current.

@12V, 2.4Ω
10%V ÷ 2.4Ω = 0.5A
20%V ÷ 2.4Ω = 1A
30%V ÷ 2.4Ω = 1.5A
40%V ÷ 2.4Ω = 2A
50%V ÷ 2.4Ω = 2.5A
60%V ÷ 2.4Ω = 3A
70%V ÷ 2.4Ω = 3.5A
80%V ÷ 2.4Ω = 4A
90%V ÷ 2.4Ω = 4.5A
100%V ÷ 2.4Ω = 5A

Those numbers are theoretically linear. However, you must consider the load your motor is driving. Spinning at higher RPM's will encounter higher resistance, and therefore cause the motor to draw more current.

I'm not certain this is the answer, but it's worth considering. And you're discussing a 3 phase motor - something I have no experience with. So I could be entirely wrong. If I am then someone here will be sure to point out my error(s).
 

dendad

Joined Feb 20, 2016
3,084
You could well be driving the motor into magnetic saturation when you go over 50%PWM. So then, the current will increase just limited by the resistance, no longer by the inductance reacting to the 40KHz signal.
 
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