All About Circuits Forum How to add 3 phase currents for total?
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#1
03-23-2011, 03:30 PM
 bart_dood Junior Member Join Date: Jul 2010 Location: Norcal Posts: 22
How to add 3 phase currents for total?

Hi Folks,

I am working on a 3 phase motor controller, one each phase leg of the motor I have current sensors, they are transformer coupled and nominally float at 2.5 volts, they then swing between 0 volts and a maximum of 5 volts as they detect the alternating current flowing through.

The scaling is something like 85amps for 1 volts.

What I'm trying to do is get the total current value. From what I understand I should add all three current values together in my microcontroller, this should equal the total current.

Of course I'll have to do some stuff on my microcontroller to do this, first of all subtract the 2.5 volt offset from each sensor, then remove any negative signs, then multiply the voltage by the 85 scalar and then add them all together.

Does this make sense?
Attached Images
 3 phase currents.jpg (85.5 KB, 16 views)
#2
03-23-2011, 11:23 PM
 bart_dood Junior Member Join Date: Jul 2010 Location: Norcal Posts: 22
Another image

Here is what I'm talking about adding, the green lines represent the instant currents at one point.

So the absolute values of 1, 2 and 3 would be added together.
Attached Images
 3 phase currents2.JPG (27.5 KB, 20 views)
#3
03-24-2011, 08:46 AM
 eblc1388 Senior Member Join Date: Nov 2008 Location: UK Posts: 1,542

Quote:
 Originally Posted by bart_dood What I'm trying to do is get the total current value. From what I understand I should add all three current values together in my microcontroller, this should equal the total current.
No. Ever heard of vector sum?

The vector sum of all three phase current in a balanced load is always zero.

There is no total current for a motor as currents vectors are not in phase but 120 degree apart. It is common to use line current value of one phase to specify the current of a motor.
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#4
03-24-2011, 06:42 PM
 bart_dood Junior Member Join Date: Jul 2010 Location: Norcal Posts: 22

Quote:
 Originally Posted by eblc1388 No. Ever heard of vector sum? The vector sum of all three phase current in a balanced load is always zero. There is no total current for a motor as currents vectors are not in phase but 120 degree apart. It is common to use line current value of one phase to specify the current of a motor.
But if I take one phase and read it as the current, it will be going up and down? I thought the whole point of 3 phase systems was that the current load is effectively constant, eliminating ripple and increasing efficiency?
#5
03-27-2011, 02:34 PM
 bart_dood Junior Member Join Date: Jul 2010 Location: Norcal Posts: 22
Use RMS chip

I had the idea of using an off the shelf RMS to DC analog chip, I would input one of the phase voltages and read the DC output with my microcontroller.
Would this work?
#6
03-28-2011, 01:52 AM
 bart_dood Junior Member Join Date: Jul 2010 Location: Norcal Posts: 22
Ok I think I got it now

From reading online it seems the DC component of the 3 phases is the sum of the squares, so I just square each current and add them together.

Can anyone confirm this is correct?
#7
03-28-2011, 04:14 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,962

Quote:
 Originally Posted by bart_dood From reading online it seems the DC component of the 3 phases is the sum of the squares, so I just square each current and add them together. Can anyone confirm this is correct?
Probably incorrect.

What do you mean by DC component? You seem to have some mistaken ideas about AC.

If the phase currents to the AC motor are balanced, then the instantaneous current summation is zero - as eblc1388 has pointed out. There is normally no DC current in an AC motor fed by a 3-phase mains supply.

If you want the power input to the motor, then the 3-phase input power under balanced conditions would be

Pin=3xVrmsxIrmsxcosθ

Where Vrms and Irms are the phase voltage and current and θ is the phase different between the voltage & current waveforms.

Again as eblc1388 already suggested you would typically use a current measurement on one of the motor phases to monitor current and assume the other phases are equal in magnitude.

You could certainly use an RMS to DC converter to obtain a simple measurement of the motor line current(s). One might measure all line currents where the dynamic control of an AC motor requires extremely tight control of performance or where motor protection is critical, and so on. This would typically be of interest in a variable speed drive (VFD) system.
#8
03-28-2011, 04:31 PM
 bart_dood Junior Member Join Date: Jul 2010 Location: Norcal Posts: 22

Quote:
 Originally Posted by t_n_k Probably incorrect. What do you mean by DC component? You seem to have some mistaken ideas about AC. If the phase currents to the AC motor are balanced, then the instantaneous current summation is zero - as eblc1388 has pointed out. There is normally no DC current in an AC motor fed by a 3-phase mains supply. If you want the power input to the motor, then the 3-phase input power under balanced conditions would be Pin=3xVrmsxIrmsxcosθ Where Vrms and Irms are the phase voltage and current and θ is the phase different between the voltage & current waveforms. Again as eblc1388 already suggested you would typically use a current measurement on one of the motor phases to monitor current and assume the other phases are equal in magnitude. You could certainly use an RMS to DC converter to obtain a simple measurement of the motor line current(s). One might measure all line currents where the dynamic control of an AC motor requires extremely tight control of performance or where motor protection is critical, and so on. This would typically be of interest in a variable speed drive (VFD) system.
When I talk about DC component I'll explain a little more.. my system is a set of batteries driving a 3 phase inverter that drives a brushless permanent magnet motor.
On each phase leg between the inverter and motor there are current sensors (essentially little transformers around the phase legs) that pick up the current flowing through each leg as an AC waveform, I am in the process of figuring out the scale, IE how many volts output from the sensor equates to amps in the leg.

I am designing an LCD readout which has all the various parameters on it and to calculate power used I need the current draw. I will have a real time reading of the battery voltage, so once I have the current draw I'll simply multiply the voltage by the current.
Of course the inverter is not 100% efficient so the DC side will be slightly higher power draw, I can actually calculate this real time as I track the efficiency based off a look up table.

I could add a shunt resistor on the battery side and design a circuit to read in the voltage drop and equate it to current, however this adds more electronics and there are a few other reasons I'm trying to avoid doing this. It should be possible to use the 3 phase currents to calculate this, once I have the scalar (volts to amps).

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