motor voltage question

I was wondering how critical the 10% variance is for motor voltage.
I have a 2 hp 550v mill and a 2.5 hp 550v lathe I would like to run.
I have built a 7.5 hp rotary phase converter based on plans found here and on the web and have balanced it to the FW specs.
it is a 240v converter with B being the wild leg.
AB=258v
BC=258v
AC=238v (line) so my generated legs are exactly line*1.08.
now for the rest of my question.
I bought a 6 kva autotransformer with legs to go from 240v-600v.
so I now have approxamatly 630v on the high legs and 598v on the line legs.
this is about 25v outside the 605v to stay within the 10%-will this do harm?
I did try drilling a hole with the mill to load it down but the voltages only slightly dropped.
amps are 1.2,.8,.7 for the mill at idle fla is 2a.
all opinions welcomed
thanx

You would be better off to run you motors on nameplate specifications.
7.5 Hp motors appear to be popular in this forum. People don't realize that they
are just wasting power. But having a nice looking gold painted motor is probably
too irresistible. Now to your question.
Have you ever considered to unwind turns on your transformer. I have rewound coils
and transformers on occasion. The formulas I use come in handy when dealing with
say a contactor that is not manufactured any more and is perfectly good. In a VFD
that I am designing I have changed 3 coils from 230v to 24v. My E.E. degree comes
in handy but there are formulas you can look up to help you. If your 3 phase transformer
has individual windings placed like 3 fingers in a row it would be very easy. And you will
not have to remove all that much. If you want more help just let me know with a email.

the three phase transformer I have is a very small box and I am not able to define each coil,there is only a variety of taps at the top for input volts and output volts.
it has taps for 208,240,400,460,600v so I chose 240,600 which is closest to my needs.
so as for unwinding it I am not able.My question is will I do harm being 25v over the 10% variance?

racer55 said:

the three phase transformer I have is a very small box and I am not able to define each coil,there is only a variety of taps at the top for input volts and output volts.
it has taps for 208,240,277,460,600v so I chose 240,600 which is closest to my needs.
so as for unwinding it I am not able.My question is will I do harm being 25v over the 10% variance?

Click to expand...

Ok, I understand. You could still do the re-wire and not worry about providing all
those taps. I like to put a few more turns on the secondary, measure, then remove
whatever it takes to get a voltage close to my requirement.

Your question:
Your input power changes value during the day. It might go as high as 10% or as low
as 10%. If you change your operating point from 550 to 598 and to 630 on the generated
phase you will have to compute +/- 10% around those numbers. If you run your motor that
way your stator windings will get hotter due to ohmic effects. The stator will get hotter.
The case will get hotter, the bearing cups and bearings will get hotter. The grease in the
bearings may not be able to take the extra heat over time and then you get a bearing
failure. What is worse is that before that happens you get a winding insulation shoot-through
or short, and maybe an electrical fire. Maybe a fuse protected motor will prevent the fire
but it's not worth the risk.

ok thanx for the reply

...I bought a 6 kva autotransformer with legs to go from 240v-600v.
so I now have approxamatly 630v on the high legs and 598v on the line legs.
this is about 25v outside the 605v to stay within the 10%

Click to expand...

Um, methinks you have started off with a Math error!

575V +10% = 632.5V maximum. What's the problem?

Jraef said:

Um, methinks you have started off with a Math error!

575V +10% = 632.5V maximum. What's the problem?

Click to expand...

thanx for the reply but methinks I thunk right about the math since if you read correctly the mill and lathe motors are 550v not 575v, I agree with your math but want to know if in my case is being outside the 10% variance harmfull?

racer55 said:

thanx for the reply but methinks I thunk right about the math since if you read correctly the mill and lathe motors are 550v not 575v,...

Click to expand...

D'oh!

Sorry. Mea culpa. I forget about the old 550V stuff out there, it's a non-standard voltage rating and I guess my brain went ahead and read it as 575V.

Must stop drinking during the day...

The +- 10% voltage rating is fairly critical, but it also depends on loading. Torque changes at the square of the voltage change. So a 550V motor run at 630V will put out 130% torque which may end up being a problem for your mechanical components. It will also run in saturation so as rons said, the windings will run hot and that heat will transfer to everyhting else. Add to that the inherent unbalance in the voltage, which ALSO creates added heat in the motor, and I doubt they will last long.

If you use a 2:1 transformer however, i.e. a 240-480V, your 258V input will give you a 516V output, which will fit within the -10% range and provide you with about 88% torque. So as long as you don't over tax the machines, you would be better off that way.

You don't have to use the same tap for all of your legs. See what kind of voltage you get with using the 460 output tap for your manufactured legs and use the 600V output tap for your line voltage.

racer55 said:

ok thanx for the reply

Click to expand...

You have a non-standard voltage for those machines. What are they?
I looked up in my Micron catalog. No luck.
I looked up in my Acme Electric catalog and there are some transformers
for 220/240 - 440/480. They might even tell you you could use a one
spec'd as 208 - 480. Give them a call and see.

Acme Electric 800-334-5214.
www.acmepowerdist.com

thanx for the reply but my budget for this project is at the limit for now.

"I forget about the old 550V stuff out there, it's a non-standard voltage rating and I guess my brain went ahead and read it as 575V."

The various classes of voltages are as follows ...

"300 volt class" ...

110 = 115 = 120 (voltages are 1 * 110 or 1 * 115 or 1 * 120)

220 = 230 = 240 (voltages are 2 * 110 or 2 * 115 or 2 * 120)

"600 volt class" ...

440 = 460 = 480 (voltages are 4 * 110 or 4 * 115 or 4 * 120)

550 = 575 = 600 (voltages are 5 * 110 or 5 * 115 or 5 * 120)


Most modern transmission and distribution voltages are multiples of 115 (but a few are multiples of 110) ...

34,500 = 300 * 115 (very common for distribution in L.A., where this voltage is the highest distribution voltage anywhere in North America)

69,000 = 600 * 115 (uncommon, but used for subtransmission between L.A.'s system and the "independent cities municipals": Burbank, Glendale and Pasadena)

115,000 = 1000 * 115 (very common, but not common in L.A.)

138,000 = 1200 * 115 (very common; some of L.A.'s intracity transmission is still 138,000)

230,000 = 2000 * 115 (very common and becoming more so; most of L.A.'s intracity transmission is now 230,000, and all such new intracity transmission is 230,000)

287,500 = 2500 * 115 (used only for L.A.'s Hoover lines; one circuit was changed to 500,000 in about 1969, the other two circuits are still 287,500 AFAIK; for nearly 30 years 287,500 volts was the highest voltage anywhere in North America)

... yet some utilities are still using 110 as a basis, not 115 ...

33,000 = 300 * 110 (replaced by 66,000 for the most part)

66,000 = 600 * 110 (very common for subtransmission)

110,000 = 1000 * 110 (somewhat common for transmission)

132,000 = 1200 * 110 (rare)

220,000 = 2000 * 110 (very common for transmission)

275,000 = 2500 * 110 (Never seen in actual practice, but if there was a "2500 *" voltage in this series, it would be 275,000)

... and that leaves the transmission voltage which has no common multiplier, yet this transmission voltage is so ubiquitous in the West that it is in a class all by itself ...

500,000 (used by L.A. and many others for intercity transmission; also used by the so-called Western System, which my department within LADWP designed, for the North-South AC Intertie and also the East-West AC interconnections to that system; the North-South DC Intertie, which is LADWP-only is 1,000,000, as +/- 500,000 volts; the East-West DC Intertie, also called the Mountain Intertie, of which LADWP is a significant partner, is also 1,000,000, as +/- 500,000 volts)

Using the 240/600 volt taps for your utility leg would give you 595V output.
Using the 240/460 volt taps for your manufactured legs would give you 495V output.
All legs would be at or within the +/- 10% range.

550V +/- 10% is 495-605V

Torque1st said:

Using the 240/600 volt taps for your utility leg would give you 595V output.
Using the 240/460 volt taps for your manufactured legs would give you 495V output.
All legs would be at or within the +/- 10% range.

550V +/- 10% is 495-605V

Click to expand...

I see your point about being within the 10% range but with my limited knowlege of this stuff I would expect that the 100 volt imbalance would do more harm than being 25 volts outside the 10% range.

peterh5322 said:

"I forget about the old 550V stuff out there, it's a non-standard voltage rating and I guess my brain went ahead and read it as 575V."

The various classes of voltages are as follows ...

"300 volt class" ...

110 = 115 = 120 (voltages are 1 * 110 or 1 * 115 or 1 * 120)

220 = 230 = 240 (voltages are 2 * 110 or 2 * 115 or 2 * 120)

"600 volt class" ...

440 = 460 = 480 (voltages are 4 * 110 or 4 * 115 or 4 * 120)

550 = 575 = 600 (voltages are 5 * 110 or 5 * 115 or 5 * 120)


Most modern transmission and distribution voltages are multiples of 115 (but a few are multiples of 110) ...

34,500 = 300 * 115 (very common for distribution in L.A., where this voltage is the highest distribution voltage anywhere in North America)

69,000 = 600 * 115 (uncommon, but used for subtransmission between L.A.'s system and the "independent cities municipals": Burbank, Glendale and Pasadena)

115,000 = 1000 * 115 (very common, but not common in L.A.)

138,000 = 1200 * 115 (very common; some of L.A.'s intracity transmission is still 138,000)

230,000 = 2000 * 115 (very common and becoming more so; most of L.A.'s intracity transmission is now 230,000, and all such new intracity transmission is 230,000)

287,500 = 2500 * 115 (used only for L.A.'s Hoover lines; one circuit was changed to 500,000 in about 1969, the other two circuits are still 287,500 AFAIK; for nearly 30 years 287,500 volts was the highest voltage anywhere in North America)

... yet some utilities are still using 110 as a basis, not 115 ...

33,000 = 300 * 110 (replaced by 66,000 for the most part)

66,000 = 600 * 110 (very common for subtransmission)

110,000 = 1000 * 110 (somewhat common for transmission)

132,000 = 1200 * 110 (rare)

220,000 = 2000 * 110 (very common for transmission)

275,000 = 2500 * 110 (Never seen in actual practice, but if there was a "2500 *" voltage in this series, it would be 275,000)

... and that leaves the transmission voltage which has no common multiplier, yet this transmission voltage is so ubiquitous in the West that it is in a class all by itself ...

500,000 (used by L.A. and many others for intercity transmission; also used by the so-called Western System, which my department within LADWP designed, for the North-South AC Intertie and also the East-West AC interconnections to that system; the North-South DC Intertie, which is LADWP-only is 1,000,000, as +/- 500,000 volts; the East-West DC Intertie, also called the Mountain Intertie, of which LADWP is a significant partner, is also 1,000,000, as +/- 500,000 volts)

Click to expand...

So am I correct in reading this information to conclude that since my motors fall within the 600v class,my manufactured voltages are within limits and I should be fine?

racer55 said:

I see your point about being within the 10% range but with my limited knowlege of this stuff I would expect that the 100 volt imbalance would do more harm than being 25 volts outside the 10% range.

Click to expand...

peterh5322 would probably know if it would be a problem. 3 phase line voltages are always unbalanced but 100V difference sounds like a lot. It does get you within tolerance and may be worth trying since it is within your budget and I doubt if anything would burn up.

With a "Fitch type" converter (named for Fitch Williams, who, I believe, first thoroughly characterized and popularized this type of converter) it is usually possible to obtain a +/- 10 percent balance, FOR ONE LOAD POINT, only.

With a lot more work, it is usually possible to obtain what has been called a "CNC" type converter, which has a +/- 5 percent balance, also FOR ONE LOAD POINT, only.

Recall that the source feeder is usually 120/240 volts single-phase.

Only L1 and L2 are used, and N is strictly avoided.

Usually, L1-L2 is around 230 to 240 volts. It could be as high as 264 (1.1 * 240), or it could be as low as 207 (0.9 * 230).

264 could be seen on a very lightly loaded distribution line at its very end point (voltage tends to rise towards the end of a lightly-loaded line).

207 could be seen on a very heavily loaded distribution line at its very end point (voltage tends to fall towards the end of a heavily-loaded line).

For practical reasons, let's take the feeder voltage as somewhere between 230 and 240, as that is the nominal voltage for today's customer feeder. It averaged 234 right after the conclusion of WW-II, and it has risen, somewhat, thereafter, but it has fallen, somewhat, after the second world-wide energy crisis which began about a decade ago (the first such crisis was in the 1970s, orchestrated largely by the Saudis and their many friends in the Third World).

At every measurement, you should take the L1-L2 voltage measurement, and then "normalize" the other measurements to that L1-L2 measurement.

While I do not know, with certainty, which motor type Fitch used for his designs, and for the measurements which resulted, the results will vary quite a bit, depending upon the motor type.

Motors, as used for RPC idlers, can fall into four basic classes:

1) pre-NEMA, which is pre-1952, and for which the Richardson curves (which see ... they are in the archives) apply,

2) early NEMA, which is post-1952, but probably pre-1980s,

3) late NEMA, which is probably post-1980s, and for which I believe Fitch did his remarkable work, and

4) very high efficiency, which is post-1999 (round it off to 2000, if you will).

Motors within classes (2) and (3) can usually be addressed by what I call my "1-2-3-30" rule-of-thumb.

Meaning, 1 µF per idler quarter horsepower for the Cac power factor correcting capacitor (run-type), continuously connected, 2 µF per idler quarter horsepower for the Cbc capacitor (run-type), continuously connected, 3 µF per idler quarter horsepower for the Cab capacitor (run-type), continuously connected, and 30 µF per idler quarter horsepower for the Cab paralleled capacitor (start-type), intermittently connected during starting, only.

Motors in the first class, (1), may require less capacitance, whereas motors in the fourth class, (4), may require more capacitance.

A motor in the first class is likely to be much easier to turn into an RPC than a motor in the fourth class.

Bear in mind that it is completely impossible to obtain a balanced three-phase system for any other than one load point.

That is a law of physics, and very possibly also a law of Nature.

thanx for the reply peterh5322
while there has been a lot of information in your posts in this thread,I am not nearly as well versed in electrical theory as you are so I am having a bit of trouble finding the answers to my question in your posts.

Why did you buy the transformer?
Who is the manufacturer and can you take a picture of it and post it up.

I bought the transformer to step up the voltage from my 240v RPC to run my 550v mill and lathe.

the link for my transformer is here:

http://www.hammondpowersolutions.com/upload_files/htp-08_sec9.pdf
I hope live links are allowed here?
I have the 6kva 600, 480, 400, 240, 208 Volts
the first picture of the wall hung transformer is what mine looks like.

on a side note I tried hooking the load wild leg to the 480v tap and the load line to to the 600v taps and the voltages all fell within about 10v of 575v.
I don't know if this is ok to do but volts and amps seem fine. I have only used it like this to check voltage/amp draw so far on my 2hp 550v mill light load.
not sure if this is the right terminology but I guess this would be :
X1=A 240v H1=600v
X2=B 258v H5=480v (wild leg)
X3=C 240v H3=600v

results were:
H1,H5=575v
H5,H3=575v
H1,H3=581v
no idea why H1,H3 droped from 598v?Line might have been a bit below 240v that day I forget?