On a buck/boost xfmr installation, the drawings indicate a single phase buck transformer [208-220] however the load requries a neutral also to control some small valves, sensors, can a buck boost still be installed? From previous experience I have only seen buck boost xfmr's on straigh single / three phase loads w/o a neutral conductor since the buck transofmrer does not derive a neutral it would not work for the instlaltion. I have asked the question to the engineer but wanted to get some others thoughts / opinions.
buck / boost & single phase neutral
- Thread starter Mike01
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gar
Senior Member
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- Ann Arbor, Michigan
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- EE
130401-1118 EDT
Mike91:
Describe your circuit more completely. 208 implies the line to line voltage of a 3 phase wye system. You seem to indicate a single boost transformer from the 208 line to line (A to B for example) to get 220 (why not 240). Call this new line E. None of the lines A, B, or E are connected to earth. E to A is in phase with B to A.
You say a neutral is required for control circuitry. This equipment was originally 120-0-120 (maybe 110-0-110). Several questions arise. Are there any transformers in the equipment? Was this equipment designed or rated to have what was connected to neutral to be operated at an elevated voltage relative to earth?
If there are no problems with what you can do with this neutral, then you could put an auto-transformer (divide by 2) (a single winding transformer with a center tap) across the 220 (240) to create a potential equivalent of the original neutral. Neutral is the wrong word to use here, but it describes a point in the original system.
If the control circuit is fed by an internal transformer, then that transformer might be connected anywhere you could get 110 (120). But this probably creates complications.
.
Mike91:
Describe your circuit more completely. 208 implies the line to line voltage of a 3 phase wye system. You seem to indicate a single boost transformer from the 208 line to line (A to B for example) to get 220 (why not 240). Call this new line E. None of the lines A, B, or E are connected to earth. E to A is in phase with B to A.
You say a neutral is required for control circuitry. This equipment was originally 120-0-120 (maybe 110-0-110). Several questions arise. Are there any transformers in the equipment? Was this equipment designed or rated to have what was connected to neutral to be operated at an elevated voltage relative to earth?
If there are no problems with what you can do with this neutral, then you could put an auto-transformer (divide by 2) (a single winding transformer with a center tap) across the 220 (240) to create a potential equivalent of the original neutral. Neutral is the wrong word to use here, but it describes a point in the original system.
If the control circuit is fed by an internal transformer, then that transformer might be connected anywhere you could get 110 (120). But this probably creates complications.
.
Equipment Specs.
Equipment Specs.
Gar,
The equipment in question is a piece of kitchen euipment [coffee] The manufacturers literature indicates a 220V circuit, the engineer designed in a buck / boost xfmr. to achieve the 220V, [not sure if they were aware of the neutral at the time]. The approved equipment cut sheets indicate the equiment to require a 220V single phase branch ciruict with 2Ph, 1N, 1EG.
This would be from the panel 2-pole breaker [208V Lines A & B] + 1N & 1 EG, to
Buck xfmr. located at equipment 208V[A&B in] and 220V[C&D] out to receptacle.
The euqipment requries a neutral the 220V is for the heating elements, and the 120V connected L1 - to N with a small cap between the two lines [C & N] line C also serves as the phase conductor for the 120V to a control module for some valves, sensors. The neutral is not derived directly from the buck boost [auot xfmr. 208 / 240 primary with 12V secondary .750kVA].
I am not sure at the time of the design if it was known that the equiment requried the neutral [120V ciruict] and have not seen this before where the buck / boost was used in conjuction with a neutral not derived from the xfmr. from prevuious experience the buck boost was unsed strictly on single phase / three phase loads where no neutral was requried.
Equipment Specs.
Gar,
The equipment in question is a piece of kitchen euipment [coffee] The manufacturers literature indicates a 220V circuit, the engineer designed in a buck / boost xfmr. to achieve the 220V, [not sure if they were aware of the neutral at the time]. The approved equipment cut sheets indicate the equiment to require a 220V single phase branch ciruict with 2Ph, 1N, 1EG.
This would be from the panel 2-pole breaker [208V Lines A & B] + 1N & 1 EG, to
Buck xfmr. located at equipment 208V[A&B in] and 220V[C&D] out to receptacle.
The euqipment requries a neutral the 220V is for the heating elements, and the 120V connected L1 - to N with a small cap between the two lines [C & N] line C also serves as the phase conductor for the 120V to a control module for some valves, sensors. The neutral is not derived directly from the buck boost [auot xfmr. 208 / 240 primary with 12V secondary .750kVA].
I am not sure at the time of the design if it was known that the equiment requried the neutral [120V ciruict] and have not seen this before where the buck / boost was used in conjuction with a neutral not derived from the xfmr. from prevuious experience the buck boost was unsed strictly on single phase / three phase loads where no neutral was requried.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
If the heating element and control configuration of the machine requires 120-0-120 split phase, then to derive that from 208 line to line will require a 208-220 isolation transformer with a center tapped secondary. Or an autotransformer with a 208 tap on a 220 winding and also a center tap at the 120 point. You cannot do itwith a buck or boost transformer alone.
Whether the equipment would be happy with ~70 volts between neutral and ground and 208 volts line to ground is another question.
If the 120V portion of the circuit only uses one line, there should be no problem. Using your terminology of A&B in, C&D out, one of the ins and one of the outs are unchanged. Let's say L1 is connected to A and C is the output... technically this is just a splice, so it is L1 in and out. You'd have L2 connected to B, and the boosted voltage at D (let's call it L2'). L1 and N are essentially unchanged and carried through the BBTx.
Need to be certain the 120V portion is to one line only...
Need to be certain the 120V portion is to one line only...
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
130401-1724 EDT
Mike01:
Your circuit description is not clear. Is your source two legs of a three phase wye with 208 leg to leg? Thus, 120 V leg to neutral.
If so then you could wire this as you would for a split single phase 120-0-120 system. But now there is only 208 V to the heater. Thus, no boost transformers, just somewhat longer heating time, and longer heater element life time.
For a voltage ratio of 240/208 = 1.15 . The heater will take about 1.33 times the heating time at 240 to do the same heating when at 208.
For a voltage ratio of 220/208 = 1.06 . The heater will take about 1.12 times the heating time at 220 to do the same heating when at 208.
.
Mike01:
Your circuit description is not clear. Is your source two legs of a three phase wye with 208 leg to leg? Thus, 120 V leg to neutral.
If so then you could wire this as you would for a split single phase 120-0-120 system. But now there is only 208 V to the heater. Thus, no boost transformers, just somewhat longer heating time, and longer heater element life time.
For a voltage ratio of 240/208 = 1.15 . The heater will take about 1.33 times the heating time at 240 to do the same heating when at 208.
For a voltage ratio of 220/208 = 1.06 . The heater will take about 1.12 times the heating time at 220 to do the same heating when at 208.
.
- Occupation
- Licensed Electrician
I agree with those guys. As long as you don't boost the leg that has the 120V controls it will be fine.
I scavenged a homemade cord that powered a coffee machine in a diner that we remodelled. It started with a male cord cap + BB transformer and ended with a female cord cap. It lives on the shelf of "Oddities that may come in handy for something, or at least be something cool to show somebody."
I scavenged a homemade cord that powered a coffee machine in a diner that we remodelled. It started with a male cord cap + BB transformer and ended with a female cord cap. It lives on the shelf of "Oddities that may come in handy for something, or at least be something cool to show somebody."
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