Re: Transformers 3 phase power calculations and fuse size

--- In CAD_CAM_EDM_DRO@yahoogroups.com, gary <garyclick1@c...> wrote:

> Wally,
>
> Our Flash Butt Welder secondary transformer is 140,000Amps at 4

volts

> or 560KVA. In the weldering process this transformer is placed in a

dead

> short across the work piece (hence the term flash). The incoming

service

> is 480VAC /3P @ 800 Amps. This transformer is controlled and

switched by

> by 12 Water Cooled SCR's and is connected to a 30 Ton Water Chiller

to

> prevent a meltdown..
>
> Two other Heat Treating machines have 1300 KVA Transformer Tanks

and 100

> Ton Water Chillers.
>
> gary

Darn it, you made me think! It has been so many years since I had to
do a 3 phase power calculation I had to look it up.

Since we talk about 3 phase power transformers here from time to time
I will do a few calculations here. For those of you that already
know this please check my work.

In 3 phase we can not just multiply amps x volts like single phase we
have to use the square root of 3 in our formula.

Power in VA = Amps x volts x 1.732

Using the numbers above.

800A x 480V x 1.732 = 665088 VA or 665KVA

Lets assume 90% efficiency of transformer.

.9 x 665KVA = 598KVA, this should power our 560KVA welder nicely.


Let's try one practical example for the types of things we are doing
here in CCED. There have been many discussions about power supplies
for Bridgeport size mills here. The power supplies talked about have
ranged in size from 750VA to 1500VA. Let's pick 1000VA or 1KW and
make a few calculations.

We have 1KVA transformer wired for 220V primary and it has a 56V
secondary.
We want to know the following.
1) Primary current.
2) Secondary current.
3) Fusing recommendations.

Our original formula. Power in VA = Amps x volts x 1.732.

Rearranging we get. Amps = VA/Volts x 1.732.

Primary current = 1000VA / (220V x 1.732) = 2.62A
Secondary current = 1000VA / (56V x 1.732) = 10.3A

Let's add our fudge factor here to make these numbers a little more
realistic. Once again we will assume a 90% efficiency factor. This
number is only applied to the primary.

2.62A primary x 1.1 = 2.88A corrected primary current.

One important point to remember here is we have calculated the
current for each winding. A 3 phase transformer has 3 input windings
and 3 output windings.

Now we need to calculate the fusing requirements for the transformer.

Electrical Code Article 450-3(b) states that each transformer 600
volts, nominal or less, shall be protected by an individual
overcurrent device on the primary side, rated or set at not more than
125% of the rated primary current of the transformer. Where the
primary current of a transformer is 9 amps or more and 125% of this
current does not correspond to a standard rating of a fuse or
nonadjustable circuit breaker, the next higher standard rating shall
be permitted. Where the primary current is less than 9 amps, an
overcurrent device rated or set at not more than 167% of the primary
current shall be permitted. Where the primary current is less than 2
amps, an overcurrent device rated or set at not more than 300% shall
be permitted.
Electrical Code Article 450-3(b)(2) states if a transformer 600 v,
nominal, or less, having a an overcurrent device on the secondary
side rated or set at not more than 125% of the rated secondary
current of the transformer shall not be required to have an
individual overcurrent device on the primary side if the primary
feeder overcurrent device is rated or set at a current value not more
than 250% of the rated primary current of the transformer.
Electrical Code Article 450-3(b)(2) states that a transformer 600 v,
nominal, or less, shall be protected by an individual overcurrent
device on the secondary side, rated or set at not more than 125% of
the rated secondary current of the transformer. Where the secondary
current of a transformer is 9 amps or more and 125% of this current
does not correspond to a standard rating of a fuse or nonadjustable
circuit breaker, the next higher standard rating shall be permitted.
Where the secondary current is less than 9 amps, an overcurrent
device rated or set at not more than 167% of the secondary current
shall be permitted.
What I get out of these codes is that for our example we have to fuse
the primary and do not have to fuse the secondary. Furthermore our
primary fusing can be as high as 167% of primary current ratting.
For our example 167% of 2.88A = 4.8A fuse maximum per winding. I
would say our minimum fuse size should be 125% to give us a little
safety margin. 125% of 2.88A = 3.6A.

Looking in a Bussman fuse catalog it looks like a FRN-R fuse is
suitable. It is rated for 250V and has moderate inrush capability to
allow the capacitor bank to charge up. The FRN-R has the following
sizes in the range we want. 4A and 4.5A, and once again each leg of
the 3 phase transformer should be fused, for a total of 3 fuses.

Picking the proper fuse for the proper job can be tricky. A 4A fuse
of one style will not behave the same as another style 4A fuse. I
would like to see a good write up on proper fuse selection if anyone
knows of one.




References.

Website were I found 3 phase power calculations and NEC electrical
code info.
http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

Bussman fuse catalog.
http://www.bussmann.com/shared/library/catalogs/FullLine_Cat.pdf


Wally

> cnc_4_me wrote:
>
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Stevenson"

<john@s...>

> >wrote:
> >
> >
> >>Transformer shouldn't be a problem as we can get custom ones

wound

> >>
> >>
> >here with not a lot of problem.
> >
> >
> >>I have a nice one here that outputs 33,000 volts at 5 amps. Is
> >>
> >>
> >California looking for a new electric chair ?
> >
> >165,000 watts or 165Kw. What in the word do you do with it and

were

> >would you get enough power to use it!!!
> >
> >Our houses in the USA typically have a 220v at 200 amp service.
> >
> >44,000 watts or 44Kw.
> >
> >Wally
> >
> >
> >
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Stevenson"

<john@s...>

> >wrote:
> >
> >
> >>>Hi John
> >>>
> >>>Just a couple thoughts...
> >>>
> >>>If 20 amps is your maximum peak current per motor you would
> >>>
> >>>
> >certainly
> >
> >
> >>>want to have that capability to prevent nuisance tripping in full
> >>>accel 3-d diagonal moves.
> >>>
> >>>However, depending on the machine, average current requirements
> >>>
> >>>
> >will
> >
> >
> >>>probably
> >>>be much less. In my large low friction router avearage cureent

is

> >>>
> >>>
> >less than
> >
> >
> >>>one quarter
> >>>the peak capability.
> >>>
> >>>So I rate the transformer accordingly. The supply capacitor can
> >>>
> >>>
> >deliver the
> >
> >
> >>>full current for short periods just fine...like a fast ramp up

or

> >>>
> >>>
> >ramp down.
> >
> >
> >>>I can calculate the cap size requirements but I must know a bit
> >>>
> >>>
> >about the
> >
> >
> >>>machine dynamics.
> >>>
> >>>One other thought I would offer...if individual motors are fused
> >>>
> >>>
> >and only
> >
> >
> >>>one
> >>>fuse blows, the others will perhaps keep going resulting in an
> >>>
> >>>
> >unforseen
> >
> >
> >>>path
> >>>and a possible crash scenario. The estop system must be such

that

> >>>
> >>>
> >if any one
> >
> >
> >>>axis
> >>>Malfunctions all axes stop. A stop on following error or similar
> >>>
> >>>
> >system can
> >
> >
> >>>do this.
> >>>
> >>>As far as transformers for UK use I am not sure. Custom toroid
> >>>
> >>>
> >kits with a
> >
> >
> >>>pre-wound primary
> >>>and customer wound secondary (about 1 volt per turn) are

commonly

> >>>
> >>>
> >available.
> >
> >
> >>>Les
> >>>
> >>>
> >>>
> >>Les,
> >>This is for a Bridgy Series l conversion using 110volt treadmill
> >>
> >>
> >motors as 100 volt servos.
> >
> >
> >>There will be no 3D moves only two axis will be cutting at any

one

> >>
> >>
> >time, all my work is 2 - 1/2D.
> >
> >
> >>Good point about the fuses, This can be accomplished by power on
> >>
> >>
> >relays wired into the stop circuit.
> >
> >
> >>Transformer shouldn't be a problem as we can get custom ones

wound

> >>
> >>
> >here with not a lot of problem.
> >
> >
> >>I have a nice one here that outputs 33,000 volts at 5 amps. Is
> >>
> >>
> >California looking for a new electric chair ?
> >
> >
> >>
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> >>
> >21/04/2005
> >
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> >
> >
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