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Can we use 2 legs of three phase power for a heat pump?

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#61 ·
CHAINIK said:
THERE IT IS. THE FIRST CORRECT ANSWER IN THIS THREAD.
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Only if you are considering voltage level, and that is not the question being discussed.
 
#62 ·
I will concede that was not the original question, but at the beginning, there were lots of statements like, "I don't know, but here's what I think"- kind of answers, that kinda spun off from the original question into the area of 240 vs 208, and stuff like that. That's what I meant.

I will freely admit, I don't know as much as I should. I have an intuitive understanding of how AC power in general works, including poly-phase systems, but, with no formal schooling, and math skills that have never been worth a damn, I'm unable to articulate to anyone else something that has always made perfect sense to me. I'd make a terrible teacher. I'm sure I'm not alone in thinking that I'm glad those like you and Lahrs are here, to "put it in layman's term" for me. I really do appreciate it.
 
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#63 ·
jayguy said:
the problem with viewing 2 phases on a 3 phase system is that we are getting hung up on the whole "degrees" thing. when you put your meter on the terminals of a single phase or three phase system, it looks at the 60 Hz sine waves and reports back on the DIFFERENTIAL voltage between the 2 peaks...not the 3 peaks. even though the 2 peaks on a 3 phase system don't "look right" they really are "right" to the motor. the DIFFERENTIAL voltage at the motor is still 208 VAC between the 2 phases...power goes in one of the terminals and goes out on the other terminal...then it switches direction...however, the DIFFERENTIAL voltage is still the same....no phase angle. phase angles only really matter in three phase systems since they use all three phases...the angles do need to be 120 degrees apart in a 3 phase system.
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Let's try to clarify this a little.

If a system is only designed to respond to instantaneous voltage value, the voltage diagram is all we need to worry about.

However, the phase relationship determines how much current flows, and at which moment in time.

If a designer of an inverter drive does not use a good rectification system to create the DC needed for the switching circuits, then a partially rectified system could potentially (no pun intended) have unusual currents.

Cheaper electronic designs do not use full wave rectification, and we as techs would have a difficult time determining the approach being used, because manufacturers like to keep their design ideas a secret.


This is why it is always a good idea to not assume that everything will be just fine, and instead, CYA by getting the manufacturer to approve the power connections you are envisioning for the installation.

One more thing.

The term "phase," as it is used, does not describe the number of conductors. It instead describes the number of windings that are producing the currents.

A single phase transformer uses a single winding. Even a center tapped residential pole transformer can be seen as two single phase windings connected together, so either the 120 volt or the 240 volt configuration is still "single phase."

Questions?
 
#64 ·
CHAINIK said:
I will concede that was not the original question, but at the beginning, there were lots of statements like, "I don't know, but here's what I think"- kind of answers, that kinda spun off from the original question into the area of 240 vs 208, and stuff like that. That's what I meant.

I will freely admit, I don't know as much as I should. I have an intuitive understanding of how AC power in general works, including poly-phase systems, but, with no formal schooling, and math skills that have never been worth a damn, I'm unable to articulate to anyone else something that has always made perfect sense to me. I'd make a terrible teacher. I'm sure I'm not alone in thinking that I'm glad those like you and Lahrs are here, to "put it in layman's term" for me. I really do appreciate it.
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I am only here to help. I will admit that I am getting ready for calculus (again) so I can get a formal degree for the day when I can no longer climb ladders all day long. I can see how my dad moves now, and I want to be able to do something of value every day.

That said, I just want to make clear that the conclusion of the system in question being just fine on the two legs being discussed is not as simple as the additive value of instantaneous voltage of two phases. Even if the system works, there are forces that we should seek to understand, or at least acknowledge, that are always in play, and which can affect the system.
 
#65 ·
timebuilder said:
....However, the phase relationship determines how much current flows, and at which moment in time. ....
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Admittedly, the idea of a VFD has been forgotten and the manufacturer does need to be consulted, however, degrees of phase has nothing "directly" to do with how much current flows...only voltage differential (among other things including motor design, inductance, etc) determines current flow. Now, in single phase systems, as the phase angle between 2 phases change, it can change the voltage difference that the motor sees...so, in a way, phase angle does affect the voltage differential...but it doesn't directly determine how much current flows. The phase angle (which we won't be changing unless you install a phase angle transformer) is really only important to 3 phase motors and possibly some drives. It may also have an effect on 2 phase motors, however, I am not familiar with these older 2 phase systems.
 
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#66 ·
Actually, that is incorrect.

Current flows according to its own phase relationship, which we know can be offset from the voltage relationship. We see this in power factor when discussing single phase. When using conductors not connected in the normal 180° configuration, current flow is affected by the interaction of both the voltage AND the current waveforms, and their relationship which is based on different timing.

Only the manufacturer who has tested their equipment for this power connection can tell if these phase relationships will have any negative impact on their designs.
 
#67 ·
timebuilder said:
Actually, that is incorrect....
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what is not correct? i know we are starting to get off-topic and a little theoretical here.


timebuilder said:
...When using conductors not connected in the normal 180° configuration, current flow is affected by the interaction of both the voltage AND the current waveforms, and their relationship which is based on different timing....
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When using conductors not connected in the normal 180° configuration, current flow is affected by its own current waveform? so, current flow ISN'T affected by its own current waveform when in a 180 degree configuration? this doesn't make any sense.

current flow is a byproduct (a useful one at that). a motor (single phase, three phase, transformer, light bulb, whatever) has a design voltage, phase and frequency input. due to how it is made or designed (and the actual voltage, number of phases and frequency you give it), the current will flow based on these items (and the resultant load). the current isn't based on itself.

a single phase current also isn't directly based on the phase angle between the 2 phases. it is based on voltage difference (primarily...and the previously mentioned items). as i mentioned before, as the phases get more and more in synchronous with each other (closer to 0 degrees), the voltage difference with be less and less and the current will rise to compensate but at some point, it will not be able to make shaft horsepower and just be a toaster...now i am rambling.

if you look at a 115 VAC single phase circuit, there is only 1 phase to work with...no second phase to deal with...well, there is, however, it is always at 0 VAC (neutral) so there aren't any phase angles to look at...yet the motor still turns.
 
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#68 ·
Darn. Only one cup of coffee and less than an hour of wakefulness and I have to discuss current dynamics. Ouch.




jayguy said:
what is not correct? i know we are starting to get off-topic and a little theoretical here.

When using conductors not connected in the normal 180° configuration, current flow is affected by its own current waveform? so, current flow ISN'T affected by its own current waveform when in a 180 degree configuration? this doesn't make any sense.
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Current is always affected because its waveform is an analogy of its flow by amplitude and time. When you combine two waveforms, current changes by amplitude and time. In a 180° configuration, there is a slight time shift measured by angle theta that we use to calculate power factor. When we have two combined waveforms,there is both that time (angle theta) and an amplitude distortion cause by the additive instantaneous waveform sum.

jayguy said:
current flow is a byproduct (a useful one at that). a motor (single phase, three phase, transformer, light bulb, whatever) has a design voltage, phase and frequency input. due to how it is made or designed (and the actual voltage, number of phases and frequency you give it), the current will flow based on these items (and the resultant load). the current isn't based on itself.
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See above.

jayguy said:
a single phase current also isn't directly based on the phase angle between the 2 phases. it is based on voltage difference (primarily...and the previously mentioned items). as i mentioned before, as the phases get more and more in synchronous with each other (closer to 0 degrees), the voltage difference with be less and less and the current will rise to compensate but at some point, it will not be able to make shaft horsepower and just be a toaster...now i am rambling.
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And I am not able to follow that.

jayguy said:
if you look at a 115 VAC single phase circuit, there is only 1 phase to work with...no second phase to deal with...well, there is, however, it is always at 0 VAC (neutral) so there aren't any phase angles to look at...yet the motor still turns.
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The neutral is one end of the effective coil. The coil dictates the phase, not the number of conductors.

The single phase device is designed on the expectation that both voltage and current will be provided in a sinusoidal waveform that is relatively close to unity, or a PF of 1. What is shown in the diagram above is that the expected resultant timing and amplitude of both voltage and current have been changed.

My point is that only the manufacturer can tell us if this change in the supplied power will have any impact on their design as far as long lasting and efficient operation is concerned.

While the device imposes a demand, it can only take from what is being supplied. Waveforms being supplied to the unit in question determine how much is available, and at what moment in time that amount is available. Then, based on that availability, the unit can draw what it can, based on what is presented to it.

Depending on the design, this may have no impact at all. At least, that would be our hope. If a very good rectification section is a part of the unit, then a smooth DC output is available, with any sinusoidal aspect being wholly eliminated, and at that point, there is no concern at all. My guess is that the only DC section is that which supplies power to the inverter drive.
 
#69 ·
Imagine two voices, one is the device, and the other is the power connection.

I want 3.5 amps right now.
I have 4 amps available now.
I want 5 amps now,
I have 4.9 amps available now.
I want 2 amps now.
I have 6 amps available now.

Ect.

Of course, the availability is determined by the nature of the power generator and all of the interconnected substations and devices.

In Philly, they take three phase from the mains and connect it to a Scott T array and make 2-phase for the paces in town that still use it. Some convert it right back to three phase to avoid having a new service dug in from the modern mains.
 
#70 ·
Looking back over this thread, I can see that I have not done a good job of explaining what I was trying to share,

The availability I am speaking of is the normal amplitude and time of the normally shaped sine waveform. Yes, the load can demand currents to flow at times other than this ideal time, and that actual demand WILL track the instantaneous voltage available to the load.

My original point is that we now have a distorted waveform of both voltage and current, as shown in the diagrams pictured above.

I tried to explain this above using a voice of availability and a voice of demand, but that did not provide clarity. The voice of availability is the "normal" trace of available current in an undistorted sine wave, and the voice of demand is the unit trying to pull current when it wants it, and the result is the equally distorted current waveform.

If it seemed that the unit would not get current when it wants it, that is my fault for not being more clear. It will get the current, but that waveform is distorted, like the voltage waveform,.

We can't predict if this distortion of amplitude and time will cause any problems. I want to lean toward saying "no, it won't," but I would want to get that from the manufacturer.
 
#71 ·
timebuilder said:
Darn. Only one cup of coffee and less than an hour of wakefulness and I have to discuss current dynamics. Ouch....
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many reasons to like you TB...add this to the list!:grin2:
 
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#72 ·
Sometimes, I think I am crazy for going back to math again for pre-calculus. Maybe I am crazy.....:putergreet:
 
#73 ·
agree thank you for the in depth summary tb.

interesting fact, the apple falling on newtons head was not only the cause of defining gravity it was also the birth of calculus. he determined a way to measure the apples speed at anygiven point during the fall.
 
#74 ·
Core_d said:
agree thank you for the in depth summary tb.

interesting fact, the apple falling on newtons head was not only the cause of defining gravity it was also the birth of calculus. he determined a way to measure the apples speed at anygiven point during the fall.
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Yes, that guy was either a straight ahead genius or an ET. The real chore is to take complex ideas and make them digestible in small blocks of learning. That is truly an art, and one that I aspire to.
 
#75 ·
Keep going Guys,don't worry about me,I'll catch up sooner/later after I find out what some of those words/phrases(not phases,lol) mean.I think I'll be a lot late to the party.

Very interesting,all that stuff!
 
#76 ·
I feel like throwing a wrench in this interesting thread about phase angles. Have any of you guys heard of a "matrix" type VFD drive? If you go trying to figure out how those things work, you will blow your friggin' mind. They don't have a DC rail at all, and they have 9 transistors for a 3 phase supply!
 
#78 ·
To me, the most piratical point of this most interesting subject comes from the damn dam. I don't know of a dam generation station that does not produce 3 separated sources of EMF from said generators. So that is what is made & that is what we have to work with. Now the motor manufactures step in & build single phase motors (using two of those generated sources oddly enough) & they think if we want to sell electric motors we have to make due with what is supplied out there in the field. So the end result is that single phase motors are mechanically built to accept this rather out of phase EMF & those motors work obviously. We are living proof of it cause we use them every day. Are they perfectly in line, in phase or in tune? No. But that fault is built into the mechanical operation of the electric motors that we use. What doesn't work mechanically within those motors makes them less efficient in the final analysis and that less efficiency is shown by the production of heat & not mechanical advantage in the end. That's the way I've known it to be since the end of ice blocks for refrigeration to the buzzy motors that operate all of our HVAC/R systems.
 
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#79 ·
DeltaT said:
To me, the most piratical point of this most interesting subject comes from the damn dam. I don't know of a dam generation station that does not produce 3 separated sources of EMF from said generators. So that is what is made & that is what we have to work with. Now the motor manufactures step in & build single phase motors (using two of those generated sources oddly enough) & they think if we want to sell electric motors we have to make due with what is supplied out there in the field. So the end result is that single phase motors are mechanically built to accept this rather out of phase EMF & those motors work obviously. We are living proof of it cause we use them every day. Are they perfectly in line, in phase or in tune? No. But that fault is built into the mechanical operation of the electric motors that we use. What doesn't work mechanically within those motors makes them less efficient in the final analysis and that less efficiency is shown by the production of heat & not mechanical advantage in the end. That's the way I've known it to be since the end of ice blocks for refrigeration to the buzzy motors that operate all of our HVAC/R systems.
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But the 3 phase generated goes through a step down transformer to become the 3 phase and single phase service that we work with.

Sent from my SCH-I545 using Tapatalk
 
#80 ·
shellkamp said:
But the 3 phase generated goes through a step down transformer to become the 3 phase and single phase service that we work with.

Sent from my SCH-I545 using Tapatalk
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Unless we are talking about something different a step down/up transformer only changes the output voltage, not the phase.
 
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