# Calculating 3 phase AC motor HP

wrote:
|> Simple trignometry. Do they need a web based helper program to do it? |> | | You missed the point.
| | The design was two inverters, a master and a slave feeding into a fixed | transformer configuration. | | Sure the *math* is easy. But you have to get the inverters to *actually* | perform the way the 'math' expects. That is, you have to get the slave | inverter to be delayed behind the master by exactly 1/4 cycle. In order to | get the output to be exactly 120 VAC 400 Hz and all three phases be 120 | degrees and 120V, you have to control the master and slave inverters to a | high degree of precision and know exactly how to adjust them when you aren't | getting the output you want. | | If you didn't understand the whole setup, you could spend all day going back | and forth tweaking 'R-17' or 'R-22' or 'R-33' to get one voltage in spec | only to find that another phase was now out of spec. | | But understanding how the voltage output of the primary inverter fed line | A-B directly, and how the voltage output of the secondary inverter added to | both A-C and B-C at the same time, while the phase delay added to B-C and | subtracted from A-C made it easy. You measured all three phases L-L and | tuned A-B to exact spec. Then you tuned the phase delay to get B-C and A-C | to be exactly equal. Finally, you adjusted secondary inverter to | raise/lower B-C and A-C at the same time until they were both in spec (since | they are now equal after step two). | | Considering this stuff was early 1960's, all before ARPANET, much less the | WWW, no 'web-based helper program' was involved.
And that's probably the real issue ... the technology of the day. Today, a waveform can be precisely generated from a chip, in terms of some form of digital output (numerically coded, or pulse width, or diversity pulsed). Today, lots of stuff, even analog stuff, gets modeled in a computer, and comes out usually damned close to what was intended, if the model was correct.
One nice way to make an analog, or even multi-level digital, transmitter, is to have 2 separate constant level sine wave carriers that vary in phase. They can be amplified more cheaply with class C/D or other PA designs, filtered, and combined passively with the power difference lost being less than the loss of doing a class A or A/B design. I know this has been done for AM radio transmitters. For something like 8VSB digital, this only involves 8 discrete levels, so only 8 discrete phase angles are needed in 2 separate carriers. OTOH, I'd probably do a design with 7 or 8 PAs "fired" in a round robin that balances the work load on each PA (which individually would just be going on and off like fast CW). It would be a more complex combiner.
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------------ Agreed -It is possible and is done- to go from n phases to m phases as long as n,m>=2 (and ruling out the Edison system in which the "2 phases" are 180 degrees apart) . This is done over and over in machine analysis. You can also represent unbalanced systems in terms of a set of balanced polyphase systems as in using symmetrical components for power system analysis.
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Don Kelly snipped-for-privacy@shawcross.ca
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| Agreed -It is possible and is done- to go from n phases to m phases as long | as n,m>=2 (and ruling out the Edison system in which the "2 phases" are 180 | degrees apart) . This is done over and over in machine analysis. You can | also represent unbalanced systems in terms of a set of balanced polyphase | systems as in using symmetrical components for power system analysis.
As long as you have a 2-D vector space instead of a 1-D vector space, you are good to go.
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wrote:

------------ And a one=D vector space is a line, so all you have is + or - along this line. Not what I would call a true vector space. Also note that when we use normal AC analysis, we are in a 2D space. After all, a rms voltage doesn't actually exist and a real voltage is time varying- hence the mathematical transform from a time varying sinusoid to a complex number representation. The actual voltage is in a 2 D space- one of which is time, while the rms voltage is in a 2D space neither of which is time.
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Don Kelly snipped-for-privacy@shawcross.ca
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| And a one=D vector space is a line, so all you have is + or - along this | line. Not what I would call a true vector space. Also note that when we use | normal AC analysis, we are in a 2D space.
That's because there will be some reactance in all by the most perfectly non-reactive case. Only if the reactance will never be anything but zero can you get away with using a 1D space. And using a 2D space still works, so it's not practically worth bothering with 1D.
| After all, a rms voltage doesn't actually exist and a real voltage is time | varying- hence the mathematical transform from a time varying sinusoid to a | complex number representation. | The actual voltage is in a 2 D space- one of which is time, while the rms | voltage is in a 2D space neither of which is time.
But when I look at JUST the voltage phase angle vectors, if they are all in 1D, then I can't get them out of 1D without employing some time base reactance (a real world example being a shaded pole motor).
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daestrom wrote:

This sort of thing has long been used in suppling the high voltage DC for the output stages of AM transmitters. The unfiltered ripple shows up as modulation in the RF output.
It's common to use multi-phase dc-dc converters. Of course the "phases" are interleaved pulses used to control the switching power transistors. It's not common to go above four "phases" as circuit complexity overcomes filtering ease.
Here's a controller for 2, 3, or 4 phases.
http://www.primarion.com/PrimarionMain/Main.aspx?screen=Product&view=VRMVRD
It's interesting to see digital solutions overtaking the classical hardware ones!
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Virg Wall

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Steve wrote:

Assuming the voltage and current numbers are for line voltage and current (usually are), the formula you want is:
hp = sqrt(3)*V*I*Effic*pf / 746
The term you're missing is 'power factor'. It is *not* the same thing as efficiency. If this data is accurate, then the power factor would have to be 0.78. That's not too bad for a small more such as this. For motors in this range a 'rule of thumb' power factor is between 0.8 and 0.9. For really large motors, the power factor can be as high as 0.95. For fractional single-phase motors I've seen it as low as 0.5
daestrom
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The system in this building is 550volts and has three wires (obviously for different phases) and a ground.
If I understand correctly, the voltage was measured across two wires at a time. Two wires had 550V, another two had 550V, and the remaining two had nearly 0V.
I'm not sure why this could be but an old electrician was working on the system and informed his younger crew that it's normal for this type of system.
Does this make sense to anyone?
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Steve wrote:

Steve You keep adding new information. These voltages do not make sense, and they would certainly cause the motor that you described to blow fuses (which you said was the problem). The motor is rated 230 or 460 volt, three phase, which will not run on 550 volts. Even if the voltage was normal, no three-phase motor will stay on line, as that is a "single-phasing" condition. A high leg system still has normal line-line voltage between all three pairs of wires. I can't help but think there is some information either missing or inaccurate, or the electrical system there is really messed up.
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Benjamin D Miller, PE
www.bmillerengineering.com
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From my understanding, 550VAC three-phase motors were common in the old days. Any new motors have been rewired by a proffesional service that specializes in this. They take 460VAC three-phase motors and wire them for 550VAC three-phase.
The building has 460VAC entering and going into a step-up transformer to supply 550VAC. There is a mixture of rebuilt 460VAC motors and old standard 550VAC motors - this was easier than replacing every motor in the building.
The electricians crews measaured the conditions I mentioned above and the "old timer" said this is normal for this type of set-up. Another person I spoke to informed me it's called a 'Bastard Leg".
I apologize if I confused anyone or left out information. Some of the stuff I assumed was common knowledge for anyone familiar with this setup while the other parts were confusion at my end.
Thanks for everyones help so far.
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Steve wrote:

Steve It sounds like they were measuring L-G from each phase, not L-L, and it is a 550V corner grounded delta. Then those numbers make sense. The line that is grounded measures zero, and the other two measure full L-L voltage, with respect to ground.
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Benjamin D Miller, PE
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What about the 3rd line measuring zero?
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------- It is the grounded line.
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Don Kelly snipped-for-privacy@shawcross.ca
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Steve wrote:

We are trying to figure out what "this setup" is!

That is the grounded line. It will measure zero volts to ground, since it is connected to ground. See the fourth diagram down at http://bmillerengineering.com/elecsys.htm
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Benjamin D Miller, PE
www.bmillerengineering.com
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Except that Steve's description:
"If I understand correctly, the voltage was measured across two wires at a time. Two wires had 550V, another two had 550V, and the remaining two had nearly 0V."
made it sound like they measured between each possible pair of wires. A 550V delta (corner grounded or not) should measure 550 volts for each pair of wires.
If each measurement was from each of the 3 conductors to ground, and not to each other, then it is consistent with a corner grounded delta.
Also this has nothing to do with a "bastard leg" configuration. AFAIK, that configuration (delta with a grounded center tap) exists only in the 120V/240V form. Has that configuration ever been used for other voltages?
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Michael Moroney wrote:

I agree. If they measured it that way then it makes no sense that there would be readings near zero between any two.

That is what leads me to believe that he may have gotten confusing information from the electrician about the measurements.
> Also this has nothing to do with a "bastard leg" configuration.
I agree. My experience is that the terms "bastard leg"/ "high leg"/ "wild leg" etc.refer to the center-tap delta. I have never heard them used for a corner ground, although the electricians on the group may prove me wrong.

Not that I know of. However, a 550 volt center-tapped delta would give 277 to ground (okay, 275.00000 to make Phil happy!) on two phases, which is usable for lighting the same as a 480 wye. It would have 480 to ground on the high leg. This would be a "bastard leg" system, but as you pointed out, it should measure 550 between all three pairs of phase conductors with no measurements at or near zero. This does not agree with what he reported. There must be some other information we don't have yet.
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Benjamin D Miller, PE
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I agree with Ben that we don't have all the facts.
Another possibility is that the 3rd wire isn't the 3rd phase. Either it's not connected to the same system or it's connected to one of the other phases and the OP has only a single phase of the supposed 3 phase system. Since it gave 550V to one other leg, I'd think that the second situation is a possibility.
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Fred Lotte
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writes:

I believe I've given every piece of information explained to me.
If the system is a 'corner ground', how does the third phase come into play?
Thanks for the help so far.
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Steve wrote:

Steve Have you looked at the diagrams on my web page? If you connect loads line to line on a corner grounded system, they don't know the difference. Each load sees the L-L voltage (550) between all three pairs of wires, and it is happy. You could, in theory, disconnect the ground (ungrounded delta), connect it to a different corner, or center tap it to one winding (center-tap grounded delta) and there is no difference as far as a three-phase three-wire load is concerned. Any one point on a system can be grounded. The location of the ground affects the voltage to ground from various points, but it does not affect the voltage line to line.
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Benjamin D Miller, PE
www.bmillerengineering.com