Author Topic: Why is BLDC Stall Current Changing Over 50% PWM Duty Cycle?  (Read 849 times)

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Offline hakansensoyTopic starter

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Why is BLDC Stall Current Changing Over 50% PWM Duty Cycle?
« on: October 30, 2019, 11:03:44 am »
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 Edit: October 30, 2019, 11:06:27 am by hakansensoy »
 

Online Siwastaja

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Re: Why is BLDC Stall Current Changing Over 50% PWM Duty Cycle?
« Reply #1 on: October 30, 2019, 11:11:47 am »
Depending on the exact ratings and circuit used, either of the following:

1)

Transition from DCM to CCM. In DCM, the PWM cycle is short enough so that while the current rises during on-time, the stored energy in the inductance fully discharges during off-time, to zero. Each cycle starts from zero - easy to understand. There is no carryover state.

In CCM, the current rise during on-time is more than the current fall during off-time, so the current doesn't reach 0. Instead, current value is stored from the previous cycle. On each cycle, it gets higher and higher.

You need current feedback, instead of dumb PWM. The feedback can be a simple "terminate the cycle when current limit is reached", or a PI control loop controlling the PWM value, based on current measurement. Either way, it uses actual current measurement, hence understand the already stored energy.

2)

Saturation of the motor inductance. When the current increases, the effective inductance drops. After a certain point, the inductance drops very quickly. This means, the same on-time will cause much higher current rise, if the current is already high.

--

Stalling the rotor exposes the winding inductance because the back-EMF voltage is simply zero. The circuit can be analyzed as a typical buck converter.
« Last Edit: October 30, 2019, 11:14:23 am by Siwastaja »
 

Offline hakansensoyTopic starter

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Re: Why is BLDC Stall Current Changing Over 50% PWM Duty Cycle?
« Reply #2 on: October 30, 2019, 11:20:53 am »
Thaks for your reply.

2) My motor's phase to phase L is 0.0843mH
                 phase to phase R is 0.209 ohm.
to=L/R=0.4ms

And my PWM freq is 40kHz and one PWM duty cycle time is 25us.

I think there is no saturation.

1) I will try the dumb PWM and write it here.

Thanks a lot.
 


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