Different motor power factor.
In the case of a VFD, differences in efficiency will change the input current for any given load. Usually the current taken by the motor is higher than the input current to the VFD.
High amps on transformer primary
- Thread starter dave121
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In the case of a VFD, differences in efficiency will change the input current for any given load. Usually the current taken by the motor is higher than the input current to the VFD.
kwired
Electron manager
- Location
- NE Nebraska
Time to verify some of what is installed as some are speculating what may or may not be there to come up with their answers.
I read this as meaning we have a 240 volt supply and are powering a transformer designed to be a 480 to 240 step down transformer and back feeding it to step up to 480.
Questions here: is the X0 terminal of the (now primary side) bonded or connected to the supply neutral? You should leave it float in this kind of application. Also what has been done to ground the secondary (480 volt) side?
I further read we are supplying a VFD which supplies a 75 hp pump motor.
As has been asked already, what exactly is the drive rating? It not only needs increased rating because it is taking in single phase and outputting three phase, but it may also not like being on either an ungrounded system or a system that is 480 volts to ground - back to a previous question of how is the 480 volt side of transformer grounded
You further said if the motor is drawing 100 amps the input of transformer is drawing 300 amps.
Are you measuring the 100 amps at the 480 volt secondary or at the VFD output? Disregarding power factor and efficiency you should have 1.73 times more current on the VFD input than the output. As others have said if you are measuring the primary/secondary currents then you are losing about 24 kW somewhere and if that is the case there likely should be smoke.
mivey
Senior Member
How are you going to float a terminal on one side of a single-phase transformer that only has two terminals to start with?
I don't know the answers. Resonance on the primary side could result in currents which are then not just a reflection of secondary current.
The measurements might be suspect but if the same measurement techniques were used as on other installations, then the results would be consistent even if not a true reflection of actual currents.
Or it might be, as I hinted at in post #7 that the measurement was not of what it purported to be.
dave121
Member
- Location
- Central Wisconsin
High amps on trans. primary
High amps on trans. primary
The trans. pri. side is straight 240v in. Can't confirm an xo connection. There is a strap coming
from inside the core to the gr stud. X1 to gr=120v, x4 to gr= 120v, x2 & x3 together and not
grounded. Secondary, h1 to h4 480v with h1 being grounded. H1 to gr= 0v, h4 to ground=
480v.
The VFD is 480 1 ph. input, and 480v 3 ph. out to run the motor.
No matter how fast the motor is run the Transformer ( before the vfd ) primary amps @ 240v is 3 times the times the secondary @ 480 v, transformer amps. Should be only 2 times.
Moving the trans. further from the po. co. trans was not my idea. Some one else heard that the 2 diff. magnetic fields too close to each other could cause impedance? in one or both windings?
The amp readings were done with a Fluke multi. with a clamp on probe.
Another of our electricians has been on this job, I will be going to confirm his findings and check the basics like wiring.
Have not considered the vfd as a possible problem. Assumed if the trans. secondary amp draw
was good, no problem down stream.
High amps on trans. primary
The trans. pri. side is straight 240v in. Can't confirm an xo connection. There is a strap coming
from inside the core to the gr stud. X1 to gr=120v, x4 to gr= 120v, x2 & x3 together and not
grounded. Secondary, h1 to h4 480v with h1 being grounded. H1 to gr= 0v, h4 to ground=
480v.
The VFD is 480 1 ph. input, and 480v 3 ph. out to run the motor.
No matter how fast the motor is run the Transformer ( before the vfd ) primary amps @ 240v is 3 times the times the secondary @ 480 v, transformer amps. Should be only 2 times.
Moving the trans. further from the po. co. trans was not my idea. Some one else heard that the 2 diff. magnetic fields too close to each other could cause impedance? in one or both windings?
The amp readings were done with a Fluke multi. with a clamp on probe.
Another of our electricians has been on this job, I will be going to confirm his findings and check the basics like wiring.
Have not considered the vfd as a possible problem. Assumed if the trans. secondary amp draw
was good, no problem down stream.
kwired
Electron manager
- Location
- NE Nebraska
If it is a back fed step down transformer (which I kind of think that is what OP tried to tell us he had) you don't think the low voltage side didn't have X1, X0, and X2 terminals so that 120/240 volts could be supplied? That also means there is a chance the high voltage side is two wire and that means either grounding one side and having 480 volts to ground or use as an ungrounded system. Small transformers would be common to have dual voltage ability on high side but I really am not all that sure if that would be very common on a 100 kVA unit.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
One little symbol error.
Damn, it all worked out so nice and clean too...
As my late, great friend Tommy was fond of saying, "The man who never made a mistake never made anything".
junkhound
Senior Member
- Location
- Renton, WA
- Occupation
- EE, power electronics specialty
Fluke
Fluke
Fluke multi
Which model? Models such as the very common 77 do not read rms amps, they measure average and the display shows 1.11 times the average, which is rms for a sine wave.
Very different reading than true rms for high harmonic content waveform results.
Also, is a delta-wye poco xfmr feeding your single phase xfmr? If so, check the temperature of that xfmr, if the VFD is bad as surmised, all the tripplet harmonic currents are going into heat in the poco xfmr.
Fluke
Fluke multi
Which model? Models such as the very common 77 do not read rms amps, they measure average and the display shows 1.11 times the average, which is rms for a sine wave.
Very different reading than true rms for high harmonic content waveform results.
Also, is a delta-wye poco xfmr feeding your single phase xfmr? If so, check the temperature of that xfmr, if the VFD is bad as surmised, all the tripplet harmonic currents are going into heat in the poco xfmr.
See post#24.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
130804-1414 EDT
Some basic practical transformer theory.
For a closely coupled iron core transformer (low leakage flux), and only slightly saturating, the open circuit voltage ratio will be closely related to the turns ratio, and the same for the current ratio. The input current waveform will be very close to the output waveform.
Full load output current will reflect to the input very closely to the turns ratio. Full load output voltage will be substantially lower than predicted by the turns ratio because of the transformer internal impedance. Thus, in many transformers the actual turns ratio will be adjusted to a value that provides the desired output voltage at full load.
Consider an example:
Input voltage 240 to an output of 120 at full load. Suppose output drops 5% at full load. The theoretical turns ratio for no load is primary to secondary, 2 to 1. The current ratio will be 1 to 2.
Adjust the ratio to compensate for the 5% voltage drop. Thus, turns ratio becomes about 2 to 1.05 = 1.90 vs 2.00. Now the input current = output current / 1.90 or 0.525 times output current.
Turn the power flow around. The output voltage at full load is about Input voltage * 1.90 * 0.95 = 1.81 * Input voltage. Input current is about 1.9 * Output current.
If the same kind of meter is used to measure input and output current, then it does not matter whether it is RMS or full-wave rectified average reading. The waveform is the same at both the input and output.
There is something wrong with the information in the original post, or there is insufficient information, or additional needed measurements were not made.
.
Some basic practical transformer theory.
For a closely coupled iron core transformer (low leakage flux), and only slightly saturating, the open circuit voltage ratio will be closely related to the turns ratio, and the same for the current ratio. The input current waveform will be very close to the output waveform.
Full load output current will reflect to the input very closely to the turns ratio. Full load output voltage will be substantially lower than predicted by the turns ratio because of the transformer internal impedance. Thus, in many transformers the actual turns ratio will be adjusted to a value that provides the desired output voltage at full load.
Consider an example:
Input voltage 240 to an output of 120 at full load. Suppose output drops 5% at full load. The theoretical turns ratio for no load is primary to secondary, 2 to 1. The current ratio will be 1 to 2.
Adjust the ratio to compensate for the 5% voltage drop. Thus, turns ratio becomes about 2 to 1.05 = 1.90 vs 2.00. Now the input current = output current / 1.90 or 0.525 times output current.
Turn the power flow around. The output voltage at full load is about Input voltage * 1.90 * 0.95 = 1.81 * Input voltage. Input current is about 1.9 * Output current.
If the same kind of meter is used to measure input and output current, then it does not matter whether it is RMS or full-wave rectified average reading. The waveform is the same at both the input and output.
There is something wrong with the information in the original post, or there is insufficient information, or additional needed measurements were not made.
.
sparkyrick
Senior Member
- Location
- Appleton, Wi
Yup. been seeing a lot of these at dental clinics for the vac system. Single phase input to the freq drive, three phase out the the motor.
kwired
Electron manager
- Location
- NE Nebraska
Are they driving 75 Hp motors though? Most POCO around here will want to supply three phase service to that kind of load. I have seen a couple 50 HP irrigation wells supplied via phase converters on single phase supply but they are all older and today POCO would never supply those services as single phase and do have intentions to eventually supply them with three phase someday.
delayed response by IM:
I apologize for not responding 8-13-13. the problem was a faulty VFD drive.
All of your help is appreciated, and I will try to be more responsible in the future as I know you would like to know the out come of problems posed to you. thanks again.
dave121
I apologize for not responding 8-13-13. the problem was a faulty VFD drive.
All of your help is appreciated, and I will try to be more responsible in the future as I know you would like to know the out come of problems posed to you. thanks again.
dave121
mivey
Senior Member
I hate it when that happens. I went all-in the other day and found out my answer was based on an alternate but also valid unit conversion. Stupid international units cost me a face.
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