power factor in the electrical power

[Herro]

Hallo everybody,

i am measuring the current and voltage in a Brushless motor. the motor with its electronic is to a battery connected. I am measuring the current and voltage between the Battery and the motor. Since the Brushless motor with its electronic is a DC Device do i need to include the power factor in the electrical power consumption calculation?
Thank you for your Help

 

[itsmoked]

Not typically in the case you're describing. If you have a scope look at the power across the input leads from the battery. If the waveform is not radically non-DC then just measure the current and voltage and P=VxI.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

NO. Power factor applies only to AC circuits.

 

[Skogsgurra]

This is a really unexpected and interesting question with som deep and mind boggling consequences.

The obvious answer is NO. You don't have power factor in a DC circuit.

The afterthought is Why not?

There are two power factors: the displacement power factor aka cos(phi) and the generalised power factor defined as P/S.

Since P is power developed in a load and S Is A times U, you could actually have something like a PF in a DC circuit.

I have never thought of it that way. How about other members?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Gunnar, only you could make something complicated out of something simple like this!

Technically, I think you are right as far as the distortion pf where an AC line is being fed to a DC power supply. In fact I think it is something often overlooked when considering generator sizing on loads where the pf will make a significant difference.

But in this case, he specifically said he is feeding it with a battery which is a purely DC source to start with. So how is your voltage "wave form" going to be leading anything?

 

[waross]

Hi Gunnar;
With all respect, are we not using different words to describe efficiency when we talk about power factor in a DC circuit?
However itsmoked may have a point. I will spend a few days running over in my mind the the possible consequences of applying a PWM potential to the inductive components in a DC circuit. Will the charging charging and discharging of the magnetic fields result in the DC equivalent of a power factor or will the charging component of the current be cancelled by the discharging component of the current?
Interesting!
Respectfully

 

[fsmyth]

I would tend to leave the words "power factor" to apply to
standard low frequency (50/60 Hz) systems, and use efficiency
or "Q" in relation to other circuits. DC just has losses.
<als>

 

[Herro]

Yeah the things is that by using an electrical Network (not a battery) to drive the motor, we get a lower value of the power factor. It means the current has a significant oscillations ...

 

[Skogsgurra]

My idea didn't catch on. Did it?

I did stress that I was not talking about displacement power factor. So no need for a voltage waveform to lead or lag. Also, I did not think of efficiency.

As I said. I hadn't thougt about it that way before. And I probably shouldn't onwards either.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[stevenal]

The pf is 1, whether you are speaking of displacement or general pf in a purely DC circuit. But if you have pulsing DC or ripple from a rectifier, you might have a pf less than 1.

 

[davidbeach]

I think that if you are going to talk about pf of less than unity you need to be able to assign it as either leading or lagging. How that distinction might be made in DC is beyond me.

 

[Skogsgurra]

Don't forget the general PF definition PF=P/S.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[stevenal]

David,

Leading/lagging only applies to displacement pf. Distortion pf cannot be described as leading or lagging.