Transforming down from 600V 2

[itsmoked]

Need to run 120HP of motors.

Any rational way to inexpensively drop down 600V on a portable system to 440 3Ph like with a buck boost or autotransformer?

Or must I go with a regular dry type isolated transformer?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith;
We used to do a lot of work for a plant that had an interesting mix of 575 V(600 V) motors and 460 V (480 V) motors.
We had a power distribution center custom built.
The center had;
1> A metering section,
2> A main switch;
3> A 600 V power distribution section, a fused disconnect switch to supply each motor. The motors had local starters and controls. We could have used an MCC section.
4> Two 480:120 V dry type transformers. The transformers were buck boost rated. They were connected in open delta to drop the 600 V incoming to 480 V.
5> Last, a 480 V power distribution section to feed the 480 volt motors.
The next step was to supply the motors that had been identified as the wrong voltage, and replacement motors or equipment at the wrong voltage. The underground conduits to the various motor locations entered either the 480 volt section or the 600 volt section and it was not feasible to relocate a fed from one section to the other.
We had pairs of small dry type transformers behind control consoles all over the plant, running 480 volt motors on 600 volt feeds and vice versa.
A quick example of sizing:
Assume a 100 hp motor drawing 120 amps at 480 volts.
The auto transformer will be running 24 amps in the 480 volt winding. 24 A x 480 V /1000 = 11.52 KVA
As a double check, the auto transformer will be running 96 amps x 120 volts in the 120 volt winding. 96 A x 120 V = 11.52 KVA.
I would use a pair of 15 KVA transformers for this example.
That is the magic of auto transformers; you can adjust the voltage on a 100 hp. motor with 30 KVA of transformer capacity.
You can use one large pair of transformers, or you can use a air of small transformers at each motor.
I guess there is a good reason for not using 600 volt motors.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill; Thanks for the info.

The motors exist already. They want to run the system on the already available 600V.

Check my math here if you would please.

2 x 60HP = 120HP

(460V x 154A x 1.73)/1000 = 122.5kVA

Heavy duty starts more than once an hour so

122.5kVA x 1.2 = 147kVA

If I use three transformers (not open delta)
147kVA / 3 = 49kVA per isolation transformer.

But we shall use autotransformers so:

The ratio is 480 and 120 so 4/5 and 1/5

The transformer core has to carry 1/5 of the VA since it is only transforming(bucking) the 120.

So the transformer only needs to be:
49kVA x 1/5 = 9.8kVA or 10kVA.

Have I missed anything?





Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. With your configuration, you have a hard time finding the right ratios for the voltage you need. I have never considered that configuration, so I have never worked out the actual voltages. I think you may find that your delta connection give approximately 90 V drop rather than the 120 V you are looking for.
A star configured auto transformer gives a symetrical system, but:
It may be difficult to find the correct voltage ratings. You would be looking for a 277:70 V transformer. You could use 480:120 V transformers but you would take a capacity hit. Your KVA ratings would be 277/480 or 58%.
I prefer an open delta auto transformer connection, similar to the connection used in autotransformer starters. There is no phase shift and no reduction in the KVA rating. If you must do arc flash calculations, one line is direct and the other two are limited by the transformers.
If you are using sensitive ground leakage detection, your resistive and capacitive leakages will be higher on two phases. If you mount the transformer close to the motors this difference will be minimized.
In open delta, each transformer must carry full phase current. The boost windings must carry the motor current plus the primary current.
I would use the current (154 A) times the effective ratio (5/4) times the voltage (120 V)/1000 = 23 KVA. I would use a pair of 25 KVA transformers. A set of 15 KVAs at each motor would give a greater starting margin and a more dependable system.
Tip; I don't factor extra duty for frequent starting into my original calcs. I work out exactly what I need and then see how much extra capacity I gain going up to the next standard size transformer. I often find that I have a good margin with the smallest suitable standard transformer size.
In regards to star versus open delta, how much do you want to pay for symetry?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

To get 480V from the diagram as drawn, you need 516 and 84V Remember you need to vectorially add the two voltages. With 480 and 120 the resultant voltage to the motor is 433V.

Three autotransformers in wye or two in open delta as suggested by Bill would work much better. Or a 600V VFD set with a maximum output voltage of 480V.

 

[waross]

Thank you for the correction David. Now that I take a second look, your figure of 433 volts looks better than my estimate.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Yow. I missed that.

See this is why I ask you guys.

For this retrofit the original motors are actually nameplated,(my contact sez), at 440V not the more modern 460 or 480. So I might still be okay on the voltage ratios. I will mull that over some more.

This is all just a budgetary issue for someone, to go to someone, with a proposal, to do an refurb verse a new unit.

But I'm learning.

Keith Cress
kcress -

http://www.flaminsystems.com