I have a situation where I have to power a boat lift motor. The motor is rated for 240v, but the avaliable power is 208v. I was planning on using a buck/boost transformer to get up to 240v. The lift owner wants a remote controller on the boat lift, this requires a neutral for 120v to power the electronics. I was thinking of running two 208v hot wires and a neutral to the buck/boost and then bringing two 240v hot wires and the same neutral (no connection to the transformer) to the controller. Will this work? Will I get 120v to neutral and 120V to ground?
Neutral for Buck/Boost Transformer
- Thread starter cosmo
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kwired
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One of those conductors should be common to the input of the auto transformer (buck boost) and will be 120 volts to ground.
The boosted output lead will be more than 120 to ground - likely 138 like Jim said.
The boosted output lead will be more than 120 to ground - likely 138 like Jim said.
gar
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cosmo:
Why do you need a neutral? How much power does the electronics require? What connects to the electronics to control it? Can the lift motor work on 208? Is it possible that the electronics has a universal power supply that works from maybe 90 to 260 V?
To get your 120 V from either 208 or 240 use a small control transformer. Solve your grounding problems by whatever NEC requires.
.
cosmo:
Why do you need a neutral? How much power does the electronics require? What connects to the electronics to control it? Can the lift motor work on 208? Is it possible that the electronics has a universal power supply that works from maybe 90 to 260 V?
To get your 120 V from either 208 or 240 use a small control transformer. Solve your grounding problems by whatever NEC requires.
.
kwired
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I would guess this motor would run just fine on 208 volt system, will draw a little more current than it would on 240 but the type of application is short and infrequent duty cycles and will likely last for a very long time. JMO.
masterinbama
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Just use the 1 leg that passes the transformer and the neutral.
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What values did you use?
The actual answer depends on which transformer ratio and configuration is used.
Standard boost configuration for 120V will provide either 10% (132V) or 13.3% (136V). Connect two of these in open wye and you will have L-L voltages of 228V and 236V respectively.
An alternate arrangement would be to use the existing neutral and just boost the 208V using a single transformer. The standard 10% L-L output would be 228.8V and at 13.3% it would be 236V. But if these new "L" are referenced back to the original neutral the voltages become roughly 129V L-N at 10% and 132V L-N at 13.3%.
hardworkingstiff
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Is it 120/240 or just straight 240? If it's 120/240, it may be less expensive and less upkeep if you pulled some larger conductors and wired the motors 120 as a MWBC.
Are you providing a ground fault breaker or is the controller providing the ground fault?
If you are, would that transformer cause any nuisance tripping?
hardworkingstiff
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Does that apply when the source is a wye instead of having the phases 180 out from each other (from the neutral perspective)?
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The OP said he had 208V, therefore he is dealing with an open wye.
As I said the exact voltages will depend on the exact transformer configuration (e.g. phase angles and additive or subtractive autotransformers).
hardworkingstiff
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I misread your post, so my post just looks stupid now. :ashamed1:
hardworkingstiff
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If the transformer in post 2 is used, and 208V is brought to the LV side, I'm assuming the HV side that was not boosted remained 120V to neutral, but what I don't know is what happens to the phase angle. Would it continue out the direction of the 208V line (a line drawn from the 2 ends of the 120V vectors? Does that make sense?
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Yes, the side not boosted would remain 120V to neutral. And yes, the "added" voltage continues in the same direction. If you put 208<0V on the H winding side, you should see 27.7<0 at the X winding(assuming a 13.3% boost), and the LV voltage would be 208<0 + 27.7<0 = 236V<0. With 120V from H1 to neutral and 120V from H4/X1 to neutral, I should think you would see 145V from X4 to neutral.
hardworkingstiff
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Thanks David! I drew it out correctly, because 145 is what I got.
That is correct. Because your calculation simply used 240V, so did I, and based on the configuration described by kwired. Using a 13.3% boost from 208V yields a voltage of ~145V (as described by david luchini).
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My example was based on a connection of X1-X2H1-H2H3-H4X3-X4. Effectively I was moving both L-N voltages.
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The motor is rated 120/240. It is at the end of a 1500' run. I am concerned about the voltage drop on the motor.
The motor is 120/240. I planned on coming out of the transformer to a 6/12 space MLO panel with a GFI breaker.
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