mivey
Senior Member
Jraef referenced the original thread. In there was the beginning of all this where he stated:
Jraef said:
Single Phase current draw for a 3 phase output VFD: technical discussion
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But you can get to motor FLA if you "oversize" the VFD compared to a minimally-rated VFD with 3Ø input. Is that not the gist of the thread?
Yes. But not just —diode— current, i.e. any one diode. Jraef even used the plural sense in his statement. We want input current, i.e. the summed RMS current through the rectifier stage.
I don't believe so. There was much discussion on what factor to de-rate a 3ph drive if you give it a single phase input.
I think it's time to draw a line under this discussion.
I do not recall anyone saying you can't get motor FLA with an "oversized" 3Ø VFD with 1Ø input.
Nor do I. Are you done?
mivey
Senior Member
Singular? Plural? Is there a point to your distinction?
Input current: are you supporting the premise that the 1ph input current is sqrt(3) times the output current or what? What point are you trying to make?
mivey
Senior Member
The 1st gist was the OP had a 3ph drive and 1.5 HP motor that would run at less than 50% load for 95% of the time and apparently full load or so for 5% of the time so the VFD would not be totally "oversized".
It was suggested that the 1ph current (diode left out just for you) would be sqrt(3) times the 3ph output current, which is not a given.
The gist of the 2nd thread was is the sqrt(3) multiplier a given or not. It is not.
As long as no provides further impetus, yes.
Only that the current we are comparing to the output is not the current through a single diode.
I support the premise the VFD's input power is not less than the output power. We all know what the associated factor is.
mivey
Senior Member
Ok. Not that anyone has been claiming that such that it would require you raising that point. But ok.
Ok. Not the topic of this thread. But ok.
I glad we could not see eye to eye on that, too. :angel:
mivey
Senior Member
Your happiness is my primary goal.
junkhound
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Just to be contentious.....
If one describes 'output' as the feed connecting with the motor and the input as connected to the bus, and the VFD happens to be a matrix converter, and you drive the motor above synchronous speed,
THEN the 'input' is LESS than the 'output'!:lol:
If one describes 'output' as the feed connecting with the motor and the input as connected to the bus, and the VFD happens to be a matrix converter, and you drive the motor above synchronous speed,
THEN the 'input' is LESS than the 'output'!:lol:
Perpetual motion strikes again................Just to be contentious.....
If one describes 'output' as the feed connecting with the motor and the input as connected to the bus, and the VFD happens to be a matrix converter, and you drive the motor above synchronous speed,
THEN the 'input' is LESS than the 'output'!:lol:
At any point in the cycle, the current flows through two diodes. That applies to both single phase and three phase.
Yes. Pi/Po > 1I support the premise the VFD's input power is not less than the output power. We all know what the associated factor is.
Job done. Except it's not what the topic is about.
Please accept a belated thank you for that..........
Incorrect. With 3Ø current could be flowing through up to 6 diodes concurrently.
Also, "at any point in the cycle" is inaccurate because of zero-current crossings. With single phase there is likely two points in a cycle with no current through any diode.
Additionally, your statement assumes the internal circuitry calls for current draw on "average". If the DC bus filtering is "full", i.e. capacitor not discharged, no current flows at that point in time.
I ask you to revisit the thread title once again. I'll even quote it here for you. "Single Phase current draw for a 3 phase output VFD: technical discussion". Especially note the last two words.
Is current draw not directly related to power draw? Seems to me, it is damn well related to what the topic is about. :happyyes:
As such, Ii1Ø/Io3Ø > 1.732
And for the record, you provided further impetus. :blink:
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That's incorrect. Possibly not your field of expertise. No problem.
I know I've done this before. Maybe you will understand it this time.
Call the three phases A, B, and C. Call the diodes in the top half of the Bridge A+ etc and those in the bottom A- etc. Current conducts first from A+ to B- then from A+ to C-.
Only two devices at any one time. If the DC is reasonably smooth/continuous each device conducts for 120 degrees.
Draw the circuit out. A+ and A- cannot conduct at the same time.
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