Sunshine, I am not arguing against the use of RMS values.
Single Phase current draw for a 3 phase output VFD: technical discussion
- Thread starter Jraef
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I agree in general, but in the specific case of a VFD controlling a 3Ø motor, is the VFD output linear? I'm not real familiar with VFD technicalities and have always assumed the output to be sinusoidal (or at least a near approximation).
No need to answer. I looked it up myself. Some VFD's have approximated sinusoidal outputs. Many do not.
Most do but that's altogether relevant to the topic since it's the input that is being considered
Y'all:
We can take the ratio between the DC output voltage of a 3-phase bridge versus a full-wave, single-phase rectifier --> this configuration will be the same 3-phase bridge rectifier with one leg not supplied!
The DC output (RMS) of a 3-phase rectifier will be 1.732 x 3/pi x Vac-in = 2.34Vac-in
The DC output of a full-wave, single-phase rectifier will be = 2 x 1.414 x Vac-in/pi = 0.9Vac-in
The ratio between the two voltage outputs = 2.34/0.9 = 2.6! Since the DC link bus has capacitors, that would make the determination of the DC output very much dependent on the rectifier circuit parameters and whether or not the same VFD can be loaded to 1/1.732 times its rating when the 3-phase VFD supplied with single-phase.
We can take the ratio between the DC output voltage of a 3-phase bridge versus a full-wave, single-phase rectifier --> this configuration will be the same 3-phase bridge rectifier with one leg not supplied!
The DC output (RMS) of a 3-phase rectifier will be 1.732 x 3/pi x Vac-in = 2.34Vac-in
The DC output of a full-wave, single-phase rectifier will be = 2 x 1.414 x Vac-in/pi = 0.9Vac-in
The ratio between the two voltage outputs = 2.34/0.9 = 2.6! Since the DC link bus has capacitors, that would make the determination of the DC output very much dependent on the rectifier circuit parameters and whether or not the same VFD can be loaded to 1/1.732 times its rating when the 3-phase VFD supplied with single-phase.
Dzboyce
Senior Member
- Location
- Royal City, WA
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- Washington 03 Electrician & plumber
You guys are giving me a headache!
I am installing a 7 1/2 horsepower submersible pump right now. The motor is a Franklin 230 volt 3 phase. Full load amps is 23.0. Service factor amps are 26.4. It will be run on a vfd. The incoming power will be 230 volt single phase. The VFD will be a Franklin (Cerus) 20 hp drive rated at 60 amps. I also called another of my distributors, they quoted me a Yaskawa IQ1000 drive, which is what I usually use. They quoted me a 69 amp drive.
you guys can talk theory all day long. But this is how much we oversize drives in the real world every day, when we have single phase input.
I am installing a 7 1/2 horsepower submersible pump right now. The motor is a Franklin 230 volt 3 phase. Full load amps is 23.0. Service factor amps are 26.4. It will be run on a vfd. The incoming power will be 230 volt single phase. The VFD will be a Franklin (Cerus) 20 hp drive rated at 60 amps. I also called another of my distributors, they quoted me a Yaskawa IQ1000 drive, which is what I usually use. They quoted me a 69 amp drive.
you guys can talk theory all day long. But this is how much we oversize drives in the real world every day, when we have single phase input.
mivey
Senior Member
That goes for the both of you really. Some very nice posts from you and gar. Should be clear by now. It can be easy to forget fundamental constraints and mis-apply something you use regularly within the constraints: been there, done that.
mivey
Senior Member
Ditto. It makes an enjoyable read.
The vfd can be considered a linear black box for i or P
with a factor for losses
Vi x Ii = sqrt3 x (1/eff) x Vo x Io
(vi x Ii) / ((1/eff) x Vo x Io) = sqrt 3
Pi / (1/eff x Po) = sqrt 3
ratio becomes sqrt 3 / eff, if eff is 0.9 factor is 1.93
doesn't get more linear than that
although the output is pwm a fft will show >95% of its P is at 60 Hz (where ever set)
factor in the L of filtering and you have a good approximation of a sine wave
anyways f does not show up in the power equations anyways
http://www.dartcontrols.com/wp-content/uploads/2012/01/De-Rating-VFD-for-Single-Phase-Power.pdf
http://www.hitachi-america.us/suppo.../AN032404-1_Rev_A_Sizing_for_Single-Phase.pdf
most start with the 1.732 as the basis for derating but use 2 or nexr larger std size
with a factor for losses
Vi x Ii = sqrt3 x (1/eff) x Vo x Io
(vi x Ii) / ((1/eff) x Vo x Io) = sqrt 3
Pi / (1/eff x Po) = sqrt 3
ratio becomes sqrt 3 / eff, if eff is 0.9 factor is 1.93
doesn't get more linear than that
although the output is pwm a fft will show >95% of its P is at 60 Hz (where ever set)
factor in the L of filtering and you have a good approximation of a sine wave
anyways f does not show up in the power equations anyways
http://www.dartcontrols.com/wp-content/uploads/2012/01/De-Rating-VFD-for-Single-Phase-Power.pdf
http://www.hitachi-america.us/suppo.../AN032404-1_Rev_A_Sizing_for_Single-Phase.pdf
most start with the 1.732 as the basis for derating but use 2 or nexr larger std size
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Something is off, I get ~1.65 from that equation, not 2.34.....But I remember DC output voltage of a three phase bridge being ~1.35.
My bad. Should have been Vdc = 1.732 x 3 x Vm/pi, where Vm (peak voltage).
Vm = 1.4141 x Vrms; so Vdc = 2.34 Vrms
It's not linear. Just look at the current waveforms. They are harmonic rich.
Donn't make the same mistake, as others have, of trying to apply linear analysis to non-linear systems.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
TG, Maybe I am misunderstanding the proposition or we are talking about two different things. DC voltage after rectification clearly cant be more than the AC peak voltage and will be less without an infinite number of phases, so what is the 2.34 again?
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
So I guess your answer to my earlier question is, "Never talk about the relationship of input current to motor current on a 3 phase VFD fed by a single phase source, because you have no way of knowing exactly what it will be since it is a non-linear system; it's unknowable." Is that right? :blink:
I think I'll just try to skirt the subject to avoid sounding pedantic (or more pedantic than I already sound to most electricians), and simply stick to telling them to double the current rating of the VFD to the motor FLA and leave it as a DC bus ripple issue, since that's a better reason anyway. Then when they ask me what the input current will be, I'll tell them I have no way of knowing; could be 1.5x, could be 2.43x, could be 1.732x, who knows...
I think I'll just try to skirt the subject to avoid sounding pedantic (or more pedantic than I already sound to most electricians), and simply stick to telling them to double the current rating of the VFD to the motor FLA and leave it as a DC bus ripple issue, since that's a better reason anyway. Then when they ask me what the input current will be, I'll tell them I have no way of knowing; could be 1.5x, could be 2.43x, could be 1.732x, who knows...
- Occupation
- Licensed Electrician
Compared to the lot your up against here you sound positively flippant.
So I guess your answer to my earlier question is, "Never talk about the relationship of input current to motor current on a 3 phase VFD fed by a single phase source, because you have no way of knowing exactly what it will be since it is a non-linear system; it's unknowable." Is that right? :blink:
I think I'll just try to skirt the subject to avoid sounding pedantic (or more pedantic than I already sound to most electricians), and simply stick to telling them to double the current rating of the VFD to the motor FLA and leave it as a DC bus ripple issue, since that's a better reason anyway. Then when they ask me what the input current will be, I'll tell them I have no way of knowing; could be 1.5x, could be 2.43x, could be 1.732x, who knows...
There is a way where everyone will be happy with.
Will somebody do a load test with a VFD supplied just single phase and compare when supplied with three-phase?
I understand the difference
you have non linear devices
but for our purposes the input/output relationship can be approximated and treated as linear
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