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troubles with inverter currents

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Newbie level 5
Aug 24, 2009
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Hi everybody,

I´ve got a three phase inverter circuit to control a BLDC motor. I´ve put current sensors in the DC bus and in two of the three phases. The DC bus sensor shows a current that is the same as the power supply ( for example 2A), but the two phase sensors show a current that is much higher ( about 5A).

Does somebody have any idea about what is happening? Thanks very much

(Sorry for my english)

Sounds like a correct working PWM drive. Also reactive phase current components don't contribute to the bus current.

thanks FvM,

I think it works OK too, but I don´t understand why the phase current is much higher than DC bus current. What do you mean with "reactive phase current components don't contribute to the bus current"? Why don´t they contribute?

Thank you for your time.

Are you driving the BLDC motor in a way that only 2 phases are active at any one time. This is a pretty common way to drive BLDC motors. Usually the top side switch of the inverter is turned on and the bottom switch of another phase gets the PWM signal. Thus when the bottom switch is turned on the phase current rises and is equal to the Bus current. When the bottom switch turns off, the bus current drops to zero while the inductance of the motor causes the phase current to decay linearly. So, since the bus current drops to zero for a portion of the cycle and the phase current doesn't, the phase current has a much higher average value than the bus current. Which is normal.

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