U.S. residential power questions, - Page 1
Forum section: electronics / projects

author: CaptDon
author_rank: Super Contributor
timestamp: July 16, 2022, 01:05:11 pm
content: This is a theoretical set of questions. 1. If I have 100 feet of 10/4 being used as 120-N-120 (240) and I have a 120 volt (can't be converted to 240) device with high starting current which causes let's say 15 volts of loss during startup it occurred to me that I have a 1KVA control transformer of the typical 120-C-120 primary and 120-C-120 secondary if I paralleled up the primary and secondary in phase and connected it out at the load end as a 3 wire 120-N-120 it would help reduce the startup loss of my 120 volt device because I would be getting power from both sides of the cable run instead of just one side. Does that sound reasonable? 2. If I had a synchronis sizable motor (1 to 5 horsepower) single phase or perhaps 3 phase in a bigger application and it was simply coupled to a large flywheel running in place, if I were to suddenly apply a heavy startup current electrical load near this motor/flywheel apparatus it would help reduce momentary line sag by pulling energy from the flywheel as sort of a dynamic braking effect? It would seem to only work with a synchronis motor to maintain the phase relationship. I don't think it would work with an induction motor that has 'slip'. It seems I have heard of this type of apparatus used in large factories? Any thoughts or experience?

author: Ian.M
author_rank: Super Contributor
timestamp: July 16, 2022, 03:17:02 pm
content: 10 AWGcopperwire is about 1 milliohm per foot, so your 100 foot cable has about 0.2 ohms resistance for the circuit out and back. Lets assume that 2/3 of your 15V startup 'sag' is due to the 100 foot cable. This would imply an 'in the ballpark' estimate for the startup surge current is around 50 A.Unfortunately a 1KVA transformer is only good for 8.33 A continuous @120V assuming secondaries wired in parallel, and at 100% overload (OK for a few minutes) 16.66A. It probably cant deliver enough extra current to do much to tame a 50A surge. I suspect you'd need a 3KVA autotransformer to make a big difference balancing the feed.Your transformer's primary to secondary turns ratio is almost certainly *NOT* 1:1 (to partially compensate for voltage droop and improve the nominal regulation) so if you attempt to rewire it as an autotransformer you'll probably get excessive circulating current between the two windings. If you've got a Variac, try it with a clamp meter on one of the wires bridging primary to secondary (without bridging the center taps), and wind it up gently watching the no-load supply and circulating currents. If its acceptable, bridge the center taps and repeat with the clamp meter on that wire.

author: andy3055
author_rank: Super Contributor
timestamp: July 16, 2022, 03:54:04 pm
content: " if I paralleled up the primary and secondary in phase and connected it out at the load end as..."Is this a typo or are you really trying to tie up the primary and secondary together? I have not heard of a thing like this.

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2022, 04:09:09 pm
content: You have the right idea about using a transformer to balance the load across a split phase, or a synchronous motor to absorb vars, harmonics, and surges. But like Ian.M points out, sizing matters for capacity, and primary and secondaries may not match exactly.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 16, 2022, 04:15:17 pm
content: 15 volt sag at startup is no big deal I wouldnt worry about it. As explained you likely cannot connect a standard isolation transformer as an autotransformer without problems as the windings are not matched.If your line voltage is already at the low end (say 110 volt with no load ) and the 15 volt drop is actually causing problems starting you could use a smaller buck/boost transformer to raise that up to 125 or so. That would give you a bit more headroom. I've had to do that to run European equipment that wanted 230 V on US 208. Our 208 was actually running 197 and that was too far out of spec for the instrument. Unless this is causing actual problems I wouldn't mess with it.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 16, 2022, 05:18:47 pm
content: Soft starter might be a better option?Some wild edge case aside I can't imagine it's worth the idle current of a big transformer or idler motor.Or if we're going to get into surplus industrial stuff, get a pair of 480 primary, 120-0-120 secondary, 15+kva and use them to convert your feed to 30 amps of 480, doubling the power in this hypothetical outbuilding. I'm not 100% sure that would work, be safe, be legal, or potentially cost less than a new, heavier, aluminum feed run, but its an idea.

author: CaptDon
author_rank: Super Contributor
timestamp: July 16, 2022, 07:09:28 pm
content: The transformer is exceptionally well balanced and I was wrong to say it is 120-C-120 primary with 120-C-120 secondary. The voltages are true but it is wired as X1-X2 / X3-X4 dual primary and H1-H2 / H3-H4 secondary. I suppose it was meant to be a 120 to 240 or 240 to 120 control transformer. Anyway, as one poster correctly identified that paralleling the primary and secondary was to increase the 'auto-transformer' current capacity.

author: bdunham7
author_rank: Super Contributor
timestamp: July 16, 2022, 07:22:45 pm
content: Quote from: CaptDon on July 16, 2022, 07:09:28 pmYes, but it doesn't appear to be big enough to do the job. Perhaps it will help somewhat, but I'd make sure to have plenty of fuses on each winding. Assuming this is an inductive motor starting, two simple things you can do are momentarily switching in an appropriate capacitor directly across the line (or across the run capacitor if it is a PSC motor) and putting a significant load on the other phase during startup to reduce the sag on the neutral side. Your autotransformer would actually put some load on that side and shift the neutral voltage, so perhaps all 3 in some combination might get the job done.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 16, 2022, 08:09:34 pm
content: What is the winding resistance of the transformer?

author: Gregg
author_rank: Super Contributor
timestamp: July 16, 2022, 08:56:40 pm
content: I fully agree with the boost transformer solution as being the simplest and possibly the most efficient solution.I have made several boost transformer systems for contractor friends that alleviate the exact symptoms you describe. Most induction motors are quite happy running with an over voltage and start much faster if the voltage doesn’t sag beyond the nameplate nominal voltage.Just find a transformer with the appropriate input voltage and 6 to 12 volt secondary with sufficient amperage rating. Landscape transformers are often 12V and sometimes 24V center tapped for dual output voltage. These are often available at thrift stores like Habitat for Humanity ReStores. There used to be a lot of 6.3V filament transformers around, but times have changed.