Help with motor-transformer calculation.

[Frank White]

I have had this question for a while. Can anyone answer it?

If I wanted to run a 600V 50HP motor with a line current of approx 50A
on a 480V line. I would calculate that there is a 120V difference so
the autotransformer would be 120V X 1.73 X 50A or just over 10 KVA.

This is a small transformer, with small windings and small connection
lugs. It just does not seem right to be connect and run a motor of
this size. Something tells me that it is inadequate in size.

Is this a correct calculation?

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[Bud--]

I have had this question for a while. Can anyone answer it?

If I wanted to run a 600V 50HP motor with a line current of approx 50A
on a 480V line. I would calculate that there is a 120V difference so
the autotransformer would be 120V X 1.73 X 50A or just over 10 KVA.

This is a small transformer, with small windings and small connection
lugs. It just does not seem right to be connect and run a motor of
this size. Something tells me that it is inadequate in size.

Is this a correct calculation?

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It isn't right. It also wasn't right last time you posted:
http://groups.google.com/group/alt....electrical&rnum=3&hl=en&#doc_41457f12bf4e3f8c


bud--

 

[DaveC]

Thus spake Frank White:

Is this a correct calculation?

I don't know.

But here is a web site that is full of documents titled "Selection Steps" and
such to help you decide.

And there's a tech support phone number somewhere on that site that will
connect you to people who can give you a definitive answer to your question.
They make these transformers.

Good luck,

 

[Phil Scott]

--
Phil Scott
Ideas are bullet proof.

I have had this question for a while. Can anyone answer it?

If I wanted to run a 600V 50HP motor with a line current of
approx 50A
on a 480V line. I would calculate that there is a 120V
difference so
the autotransformer would be 120V X 1.73 X 50A or just over
10 KVA.

This is a small transformer, with small windings and small
connection
lugs. It just does not seem right to be connect and run a
motor of
this size. Something tells me that it is inadequate in size.

Is this a correct calculation?

No...and your 'theory' is loopy.. gots a cloo mon.

You figure the net Locked Rotor amps of the load in sizing a
transformer to service it...there is a little fudge factor
there... you can fudge more if its a blower motor, but not
much.

You can fudge even more if its a part winding start motor.
then you can get closer to sizing the transformer to meet the
running load.

and no, the transformer surge capacity is not the same ratio
as the motor start surge.

The motor start surge can be 300 to 400% of running amps...
the transformer will not take a surge nearly that high without
reducing its voltage output.. and that will stall the motor if
you stretch it much at all.

beyond that, you would respect yourself and the group if you
studied up first, not relying on magical thinking, hare
chrisna or the golden eye when trying to fathom what might
work or not.

Phil Scott

 

[Don Kelly]

--
Phil Scott
Ideas are bullet proof.


No...and your 'theory' is loopy.. gots a cloo mon.

You figure the net Locked Rotor amps of the load in sizing a transformer
to service it...there is a little fudge factor there... you can fudge more
if its a blower motor, but not much.

You can fudge even more if its a part winding start motor. then you can
get closer to sizing the transformer to meet the running load.

and no, the transformer surge capacity is not the same ratio as the motor
start surge.

The motor start surge can be 300 to 400% of running amps... the
transformer will not take a surge nearly that high without reducing its
voltage output.. and that will stall the motor if you stretch it much at
all.

beyond that, you would respect yourself and the group if you studied up
first, not relying on magical thinking, hare chrisna or the golden eye
when trying to fathom what might work or not.

Phil Scott

So, if the start load is small, start at 480V, switch to 600 for running.
The transformer is then not affected by the start inrush and is capable of
handling the running conditions. Go up a bit in size 480/600 Auto (3 phase)
at,say, 15KVA. Essentially the opposite of the usual autotransformer
reduced voltage scheme for motor starting.
It is feasible. Start current seen by the supply will be about 64% of normal
(motor 80%) -none of which is through the transformer, and start torque will
be about 64% of normal.

 

[DaveC]

Dang!

http://www.solaheviduty.com/products/transformers/BuckBoost/index.html

 

[Don Kelly]

----------------------------

In the transformer industry we use 1.1 kVA per HP as a rule of thumb.
That is why motor drive isolation transformers are rated at odd KVAs
such as 220, 275, 330, 440, 550, etc. These correspond respectively to
motor HP ratings of 200, 250, 300, 400, 500, etc.

Steve Lockridge, Manager
Alfa Transformer
www.alfatransformer.com

But the proposed transformer is an autotransformer- that makes a difference.
In this case transformer action accounts for 1/4 of the transferred power.
In this case using the 1.1 ratio the result is under 14KVA -say 15 as
suggested.

 

[Frank White]

Here is anothe example of this on a 600V system

3 PH 40HP 460V motor FLA = 52A wire size is #6 CU

600 to 480V autotransformer would be 13.5 KVA
(600V - 480V) x 1.73 x 52A = approx 11KVA

Use three 600V-120V 5 kva transformer in a buck configuration
for a 15KVA bank, you will be wiring a motor that requires a #6
copper wire to feed it, to a transformer that has # 12 GA wires
coming out of it. It doesnt seem right.

I bought a 30 KVA autotransformer that has #12 wires for
connecton.
The #6 wires that feed the motor get a bit warm during normal
use, I can imagine how warm the #12 GA wiring will be.

I have had this question for a while. Can anyone answer it?

If I wanted to run a 600V 50HP motor with a line current of approx 50A
on a 480V line. I would calculate that there is a 120V difference so
the autotransformer would be 120V X 1.73 X 50A or just over 10 KVA.

This is a small transformer, with small windings and small connection
lugs. It just does not seem right to be connect and run a motor of
this size. Something tells me that it is inadequate in size.

Is this a correct calculation?

Remove dashes "-----------" to e-mail.

[email protected]

Remove dashes "-----------" to e-mail.

[email protected]