@jmkasunich
It is pure physics truth that the power relationship between line voltage and phase voltage is always
√3, even for a motor in USA.
Agreed. The US uses different terms: "line-to-line" and "line-to-neutral" voltage. I would claim that the US terminology is clearer, it spells out exactly where both terminals of the voltmeter are connected.
This is turning into a dissertation on power systems and motor wiring that most people won't care about, but I'm going to put the info out for the few that might find it informative. Source: I'm an electrical engineer, and I spent about 30 years of my career designing VFDs. Most of my experience is with larger drives. The smallest thing I've personally designed was rated at over 100 amps at 480V, or about 80kW/100HP. The largest was 1.5MW/2000HP. I've worked with (but not designed) "medium voltage" drives rated to 7500 volts and over 25 megawatts, as well as tiny ones rated at a fraction of a kW.
The subtle point here is that if a VFD has to live with a single phase input, the highest bus voltage that can be wrung out of a 240VAC input without a boost inverter is 240 x √2 =339.4V, or in practice, a bit less than that.
The maximum DC bus voltage of the VFD is equivalent to the peak voltage of the supplied input. For either single phase 240V _or_ three phase 240V, the DC bus voltage is 240 * sqrt(2) = about 340V. The three-phase version has smaller dips between the peaks and a slightly higher average DC voltage compared to the single phase, but with suitably sized bus capacitors both can produce almost the same AC output voltage - just under 240V RMS.
To start with a single phase 110VAC, it ends up as little as 155.5V, which is probably why using both 110V phases in a USA house to get the 220V is recommended.
There are probably a few VFDs designed for 120V single-phase in and 120V three-phase out, but they are uncommon - mostly because 120V (line-to-line) three-phase motors are vanishingly rare.
By far the most common VFD rated for 120V input will have a voltage doubling input rectifier which gives it the same ~340V DC bus voltage as a 240V input VFD, albeit with even more ripple voltage and even bigger bus capacitors needed. That's how you get VFDs that can run on 120V single phase and deliver almost 240V three phase output. For example:
https://www.amazon.com/Single-Output-Variable-Frequency-Control/dp/B0899T3QQ4
The voltage doubling rectifier doesn't scale well to high power, so you'll rarely see them rated more than a few HP/kW. (Also, 120V single phase is a terrible source for any load requiring more than a couple kW.)
If the VFD used such voltages to make it's version of 3-phase, on a motor that started out connected in series (ie. "star" connection), there would be a dramatic loss of power. Changing to the delta connection recovers much of that
We have a potential confusion about terms we use. It's about what is a "parallel" connection. Yes - the low voltage connection in your second diagram does make a motor into a lower voltage, higher current type, but both connections are "star", with a central neutral point. For me, the delta connection
is the parallel form of connection, regardless that windings within it may have been further locally paralleled.
The real series connection, compared to the parallel form (delta) is shown here..
View attachment 401395
Changing to the delta connection is the immediate convenient way to make a old style induction motor better suit a VFD offering 3-phase at about the (artificial) 330VAC line-to-line. One can, of course, take it further by splitting the windings, and connecting them in parallel. This is why we find motors with multiples of 3 connections, like 6, or 12.
There is certainly confusion here.
Series and parallel are one thing, start and delta are a completely different thing, and they should never be confused. That is why I posted in the first place. Whenever a person on one side of the Atlantic tries to answer a motor question from someone on the other side, this issue almost inevitably arises because each person is making assumptions based on what they have seen, without realizing that things are totally different on the other side of the pond.
Your EU/UK motor has three windings and six terminals. Series and parallel connections are NOT available. Your ONLY options are star and delta, and the required voltages vary by a factor of sqrt(3).
Our US motor has six windings and nine terminals. Star and delta connections are NOT available. Our ONLY options are series and parallel, and the required voltages vary by a factor of 2.
Regarding USA common voltages, I know of 110V, and some places it's 120V.
110, 115, 117, and 120V have all appeared on the nameplates of US appliances, light fixtures, etc. over the years. The current standard is 120V distribution voltage (meaning that is what the utility aims to give you) and 115V utilization voltage (meaning that your motor or whatever is expected to be able to deliver full performance at 115V). The difference is to allow for voltage drops in wiring and other such effects.
This difference between distribution and utilization ripples into the higher voltages as well. You'll see breaker panels labeled 240V (distribution) but most motor nameplates say 230V (utilization). Likewise with 480V panels and 460V motors. Older gear may be labeled 220V or 440V.
These come from a pole transformer with a centre-tapped winding, which is how you can have a 220V or 240V supply. It's not actually 3-phase
Correct. It is single-phase. Sometimes called split-phase or 240/120V.
US power distribution is a mess compared to EU/UK. Normal residential service in the US is from a center tapped single-phase transformer fed from two of the three phases of the high voltage distribution grid. The tap is grounded (or earthed in UK speak ;-) ) and called "neutral". We have 120V line-to-neutral for light loads and 240V line-to-line for larger loads like stoves and air conditioners. But both are single phase.
In modest commercial and office buildings where most of the load is lights and computers and such, the distribution transformer is three-phase, with a star (we usually say "Y" spelled wye) connected secondary. The center point is grounded and again called "neutral". Each winding produces 120V, so you have three individual 120V line-to-neutral phases, each of which supports 120V single phase loads. The line-to-line voltage is 208V (120V times sqrt(3) ). This is commonly referred to as 208Y/120V meaning 208V line-to-line from a wye (star) connected transformer with 120V from each of the three lines to neutral. The limited three phase loads in such buildings are almost always motors, and they are either designed specifically for 208V line-to-line, or they are dual-rated motors that can run on 230V or 208V without changing any connections at all.
A small industrial facility like a machine shop needs mostly three-phase for the machines, with just a little bit of 120V single phase for lights and maybe an office computer or something. They sometimes use an arrangement referred to as "high leg delta". This uses a 240V line-to-line delta transformer but grounds the middle of one side of the transformer. (Wikipedia has details and a drawing.) So you have 120V from two of the three phases to neutral. Those two phases and the neutral can be connected to an ordinary "split-phase" breaker panel and used just like residential power for the office computer and lights. The third phase is about 208 volts from neutral - it is the "high leg" in the name "high leg delta". No line-to-neutral loads are connected to this phase. All three phases can be sent to a three-phase panel to run 240V three-phase motors and other three-phase loads.
As buildings get bigger and need more power the distribution switches to pure three phase at either 240V or 480V line-to-line. Usually the distribution transformer is star (wye) connected with the star point grounded, but ungrounded delta is not uncommon. On a 480V system, the line-to-neutral voltage is 277V, and some lighting is designed to run on 277V to avoid the need for separate lighting panels. But in most buildings, a smaller transformer (or several, scattered through the facility) is used to step the 240V or 480V down to either 208Y/120V (using all three phases) or 240/120V split phase (using only two of the three phases). The resulting low-voltage power goes to "lighting panels" which supply lights and receptacles, while machinery is served from power distribution panels at either 240 or 480V line-to-line.
This mess is mostly the result of a desire for backwards compatibly and the hap-hazard growth of the original systems from the days of Edison and Tesla.
My understanding (please correct me if wrong) is that in the UK and EU, the majority of installations are fed by star connected transformers with 230V from line to neutral/ground and 400V from line-to-line. Residences get one phase from a utility owned transformer, and all loads are single phase 230V. Commercial and industrial buildings get all three phases. Lights and computers and such are connected line-to-neutral (and evenly distributed between the phases for balance reasons) while motors run on 400V line-to-line. The motors are star connected for this service, so the individual windings see 230V. This allows you to reconnect them in delta so they want 230V line-to-line. A VFD running on 230V single phase has a 325V DC bus and can create 230V line-to-line three phase, so the motors are happy. For really large loads, there is also 690V line-to-line which works out to 400V line-to-neutral. So a large motor with 400V windings can be connected in delta and run on the "normal" 400V, or reconnected in star to run on 690V with lower line currents to allow for smaller wire, breakers, etc. Overall it seems like a much better system.
Much as I do love the old vintage squirrel-cage induction motors on old kit, this is one place where I shrug, and I am happy to put a modern permanent magnet multi-pole brushless AC motor in there, and connect up a VFD. I am just seduced by electronic tricks that can deliver near full torque, even when hardly moving. Not all VFDs can do that. Many cheaper ones run out of power puff at low revs.
Full torque at zero speed comes from a vector drive, either with an encoder or "sensorless vector" (which doesn't usually work quite as well as true vector). However, we retrofitting an older machine, I like to keep any mechanical speed selection in place. Full torque from the motor at low speed does not mean full power. The gear or pulley mechanical reduction actually increases torque at low speeds, and no VFD can do that.
Sorry for the wall of words. Hopefully the info is useful to someone.
