converting 220 volt single phase to 440 3 phase
- Thread starter a.bisnath
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Quite interesting to find such a definitive calculation, especially given that others state that there will always be some derating required at full load, not to mention potential problems with starting under load at greatly reduced torque.
That formula may give the C value for minimum derating, not for zero derating.
That formula may give the C value for minimum derating, not for zero derating.
Here is an attachment wherein (see box) the only requirement for running three phase motor on single phase supply is keeping the voltage across each winding of the three phase motor at its rated value and there is no requirement of de-rating mentioned in it.
Last edited:
kwired
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You think you will get the same performance out of that situation as you will with true three phase supply with 120 degree phase angles? I have no doubt it can make the motor turn. I will not say it can produce full motor output rating and at same time keep all the same heating effects within the motor as a true three phase supply would. Will it get you by with a motor that has been oversized for the driven load? Many times it does.
kwired:
The problem with the ordinary static converters in
the market is the use of smaller size capacitor
with the result rated voltage does not appear ine
each winding of the motor and so it can not take
rated load. But if a capacitor of suitable size is
used as mentiod in my last POST, the motor can take
rated load on simgle phase supply.
The problem with the ordinary static converters in
the market is the use of smaller size capacitor
with the result rated voltage does not appear ine
each winding of the motor and so it can not take
rated load. But if a capacitor of suitable size is
used as mentiod in my last POST, the motor can take
rated load on simgle phase supply.
kwired
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Was not how things worked with my latest run in with a static phase converter.
It did have multiple capacitors installed and was intended to move jumper wires to adjust the connected capacitor value. The most you could do was try to get the applied voltage as balanced as possible - and even that changed depending on how loaded the motor was. No matter what capacitance I connected though the motor current was very unbalanced - the "manufactured phase" being the lowest current lead in all instances, and the supply lead that was "common" to the other two output phases always carried a current that was the equal to the sum of the other two output leads. My conclusion was you do not have three phases you have single phase that is split a couple different ways and then returns back to the second supply conductor. It doesn't do this all in the converter it does part of it in the converter and part of it in the driven motor. A true conversion to three phase would have all conversion done in the conversion unit like would be done in a VFD. A rotary converter may not be a true converter either but has much more of the conversion taking place in something other then the driven motor.
kwired
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OK, these figures are from memory and could be slightly off but lets start with them.
This was a 10 HP 240 volt motor application.
Can't recall exact nameplate full load current values, but NEC tables say 28 amps - I believe this one was a little less which is typical maybe 24 to 26 amps.
I am going to disregard the start capacitors as we had no troubles with starting this motor, a control relay switched out the start capacitors and left the unit to run on (again from recollection) several dual value capacitors with ratings of 10/60 mfd. I think there were 8 of them.
It was designed to change connection to these capacitors to adjust the capacitor value to fit the application needs. I increased capacitance as well as decreased it. What made for better voltage and current balance at lower load may have been worse at higher load, but I never came close to achieving voltage and current balance, and there was always one motor lead that carried the sum of the other two leads, which makes total sense if you draw out a schematic of how things are connected, input current comes in one line splits two ways in the converter, those two paths lead to two motor input leads and join back together via the third motor lead and then back to the other source lead. Changing capacitor values only changes how much current is split between two motor leads but it all comes back on the third motor lead no matter what.
My conclusion is this is not really a true phase converter, it just tricks the motor into developing torque but will have the need to derate the motor.
This kind of phase conversion may work great for a fixed load application with an overrated motor but not so well when the load needs to vary.
kwired:
There were two formulas given in the attachment of post#23-one for star connected motor and other for delta connected motor. The authors of that reference stated that to keep phase voltage of each winding of motor at its rated value during single phase operation, a capacitor of value approximately as given in the attachment is to be used. If what they stated to be true, then rated current would flow through each winding and so there would be no over load in any winding at the rated output of the motor. I tried to analyse the formula for star connection as below:
C=2800*In/V
C is in MFD. So for C in Farad,
C=2800*(In/V)*0.000001
For more accuracy, the factor 2800 is taken as 2745. So
C=2745*(In/V)*0.000001
2*3.14*f*C=2*3.14*f*2745*(In/V)*0.000001, where f=frequency=50Hz (mains frequency in Russia
)
The reactance Xc of the capacitor =1/2*3.14*f*C. Therefore
Xc=(V/In)*1000000/2*3.14*50*2745=1.16*(V/In). So
In*Xc=1.16*V, the voltage across the capacitor.
As the rated current 'In' flows through the series motor phase winding, the voltage across it is 0.58*V. So the voltage across the capacitor and phase winding series combination is (1.16-0.58)*V=0.58*V,the voltage across the second phase winding of the motor. So this second motor phase winding would also take the rated current. It appears that the two currents would sum up to be double the rated current through the third phase winding of the motor. But that is not the case: the capacitor introduces necessary phase shift so that the vectorial sum of those two rated motor currents is equal to the rated current of the motor through the third winding.
There were two formulas given in the attachment of post#23-one for star connected motor and other for delta connected motor. The authors of that reference stated that to keep phase voltage of each winding of motor at its rated value during single phase operation, a capacitor of value approximately as given in the attachment is to be used. If what they stated to be true, then rated current would flow through each winding and so there would be no over load in any winding at the rated output of the motor. I tried to analyse the formula for star connection as below:
C=2800*In/V
C is in MFD. So for C in Farad,
C=2800*(In/V)*0.000001
For more accuracy, the factor 2800 is taken as 2745. So
C=2745*(In/V)*0.000001
2*3.14*f*C=2*3.14*f*2745*(In/V)*0.000001, where f=frequency=50Hz (mains frequency in Russia
)The reactance Xc of the capacitor =1/2*3.14*f*C. Therefore
Xc=(V/In)*1000000/2*3.14*50*2745=1.16*(V/In). So
In*Xc=1.16*V, the voltage across the capacitor.
As the rated current 'In' flows through the series motor phase winding, the voltage across it is 0.58*V. So the voltage across the capacitor and phase winding series combination is (1.16-0.58)*V=0.58*V,the voltage across the second phase winding of the motor. So this second motor phase winding would also take the rated current. It appears that the two currents would sum up to be double the rated current through the third phase winding of the motor. But that is not the case: the capacitor introduces necessary phase shift so that the vectorial sum of those two rated motor currents is equal to the rated current of the motor through the third winding.
kwired
Electron manager
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- NE Nebraska
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- EC
Sounds like a good theory, why did it not work that way when applied? BTW I believe the motor in my case was delta connected if that makes any difference.kwired:
There were two formulas given in the attachment of post#23-one for star connected motor and other for delta connected motor. The authors of that reference stated that to keep phase voltage of each winding of motor at its rated value during single phase operation, a capacitor of value approximately as given in the attachment is to be used. If what they stated to be true, then rated current would flow through each winding and so there would be no over load in any winding at the rated output of the motor. I tried to analyse the formula for star connection as below:
C=2800*In/V
C is in MFD. So for C in Farad,
C=2800*(In/V)*0.000001
For more accuracy, the factor 2800 is taken as 2745. So
C=2745*(In/V)*0.000001
2*3.14*f*C=2*3.14*f*2745*(In/V)*0.000001, where f=frequency=50Hz (mains frequency in Russia
The reactance Xc of the capacitor =1/2*3.14*f*C. Therefore
Xc=(V/In)*1000000/2*3.14*50*2745=1.16*(V/In). So
In*Xc=1.16*V, the voltage across the capacitor.
As the rated current 'In' flows through the series motor phase winding, the voltage across it is 0.58*V. So the voltage across the capacitor and phase winding series combination is (1.16-0.58)*V=0.58*V,the voltage across the second phase winding of the motor. So this second motor phase winding would also take the rated current. It appears that the two currents would sum up to be double the rated current through the third phase winding of the motor. But that is not the case: the capacitor introduces necessary phase shift so that the vectorial sum of those two rated motor currents is equal to the rated current of the motor through the third winding.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Maintaining the same voltage on the derived leg is not at all the same as getting the correct magnitude and phase, nor does it guarantee the same current in the winding since that is the result of the vector difference between. the applied voltage and the back EMF, not the value of the applied voltage.
The back EMF, because of the mechanical orientation of the windings, must to first order be exactly 120 degrees apart for the three windings.
The back EMF, because of the mechanical orientation of the windings, must to first order be exactly 120 degrees apart for the three windings.
glene77is
Senior Member
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- Memphis, TN
converting 220 volt single phase to 440 3 phase
Kwired,
Good to see a "real world' response.
"""Sounds like a good theory, why did it not work that way when applied?"""
Have run across a similar application, and it worked only with a fixed load.
As you cautiously pointed out, there are more factors than included in the equations,
and that is where a good experienced Electrician has his merit.
Kwired,
Good to see a "real world' response.
"""Sounds like a good theory, why did it not work that way when applied?"""
Have run across a similar application, and it worked only with a fixed load.
As you cautiously pointed out, there are more factors than included in the equations,
and that is where a good experienced Electrician has his merit.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Kind of what I ran into, I could get better balance by changing capacitor values, but the load in my case was not fixed and the balance changed as load changed. But no matter what load was or what capacitor value was being used I still had sum of two "phase leads" current on the third "phase lead", and when the motor was running at higher load the current in that one lead was well above motor namplate rating, up to 40 amps or so on a motor rated for somewhere around 25 amps.
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