AC 3 phase voltage stabilizer ( using servomotors ) versus auto transformer

[Elec777]

Dears,
The problem is: compressor motor nameplate reads 400/415 volts while supply voltage at site is 380volts, what is the best selection for increasing the supply voltage from 380 to 400 volts AC, a 3 phase voltage stabilizer ( using servomotors ) or autotransformer, please consider that supply voltage is unstable (fluctuating), the motor nominal current is 65 amps.

thanks and best regards

 

[waross]

You have two problems:
1. The voltage fluctuates.
2. The voltage is too low.

How much does the voltage fluctuate?
Electric motors will run accept a reasonable mount of supply voltage fluctuation.
When the nominal supply voltage is below the rated motor voltage, a low voltage fluctuation becomes serious.
When the supply voltage is corrected, the fluctuation may no longer be a problem.

I prefer passive solutions.
As a first step I would install two auto-transformers in open delta to boost the nominal supply voltage to 400 Volts.
Depending on the range of voltage fluctuation I may correct the nominal voltage to a little higher than 400 Volts. But, at this time I have no idea as to the range of voltage fluctuation.

The issue with low voltage is reduced starting torque and higher motor current for a given load. This leads to over heating of the motor.
The problem is not low voltage by itself, it is a combination of rated load and low voltage.
Either the load must be reduced or the voltage must be corrected to avoid overheating the motor.
One passive solution is to over-size the motor. This will allow the motor to drive the load at reduced voltage.
Some loads may be safely driven with lower supply voltage by reducing the drive ratio. That is change the vee belts so that the load turns slower. That is not a good idea for reciprocating compressors. These compressors depend on the flywheel effect and reduced flywheel effect may result in unacceptable current surges or, in extreme cases, stalling.


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

 

[itsmoked]

install two auto-transformers in open delta to boost the nominal supply voltage

Bingo.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Elec777]

Dear Waross,
1- voltage fluctuates between 373 to 378 volts
2- you have suggested installing two autotransformers in open Delta.

grateful if you could explain to me why two transformers, not one.

best regards

 

[xnuke]

Your voltage fluctuations are negligible.

You have a three-phase load, so you need, at a minimum, two auto transformers in open delta to create a boosted three-phase voltage. You could use three auto transformers, but that would cost more. One is not enough. You could boost a single phase with one, but not all three phases.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

If you use a single transformer it must be a three phase transformer. The capacity must be greater than 65A x 400 V x √3 = 34641 VA. You may find a 40 KVA transformer.
With an auto-transformer the minimum capacity of each transformer must be 65 Amps x the voltage rating of the low voltage winding.
If we add 32 Volts boost we will be at about 405 Volts. Right in the correct range of supply voltage.
65 Amps x 32 Volts = 2080 VA. I would use 3 KVA transformers.
So, Two transformers at 3 KVA each or one transformer at 40 KVA.
Your choice.
However the choice may not always be straightforward. I worked for years in a culture where the suppliers paid a kick-back to any installer bringing in an order for a transformer. In that culture I would not be surprised to see a 100 KVA or even a 250 KVA transformer used.


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

 

[jraef]

Are you sure this is even necessary? If motors are involved, voltage ratings are typically +-10%, so a 400V rated motor should be fine on a supply down to 360V. 400V was the "harmonized" voltage level intended to accommodate existing 380V through 415V systems in different countries.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

I'd use the hand test. If after the compressor has run for extended period you can put your hand on the motor you have nothing to worry about. Do nothing!

If you can't put your hand on the motor then break out a clamp-on ammeter. Check each of the three phases while the compressor is running near to the shut-off or unload pressure. Compare the amp readings with the name plate. If they're below the FLA value on the motor plate, you're done. Do nothing!

Otherwise boost the voltage as suggested by waross.

However! If you have other machines that are in the same situation then perhaps you should get a three phase transformer big enough to run them all. You can frequently get them on ebay for about 1/4 the new price.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Or bigger auto-transformers. We had a customer who had an entire MCC feeding about half of his motors fed by two large auto-transformers in open delta.

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

 

[zlatkodo]

Hi, Elec777,
You don't have to do anything .
AC Windings

 

[Elec777]

Dears,
I really need to do something, I have a problem with big compressor fails to start sometimes, I tend to go with Waross

suggestion for open delta autotransforme .

Dear Waross,
could you please advise more information regarding your suggestion of installing two open delta 3phase autotransformers.
best regards for all colleagues
appreciate your comments

 

[waross]

It looks as if two of 5 KVA transformers will be required.
Look at this web site:

https://www.hammondpowersolutions.com/

And look at their buck-Boost Calculator:

http://equotes.hammondpowersolutions.com/eCalc/HPSBBCalculator.php#top

HPS Buck-Boost Selector

Data Information
Results

Transformer Information
Part Number: Q005DTCF
Qty: 2

Buck-Boost Configuration
Phase: 3
Supply Connection: Delta
Measured Supply Voltage: 370V
Desired Load Voltage: 407V
Load Rating: 73.43 kVA (104.17A)
Connection Diagram #: 7
Connection Diagram Picture:
Buck-Boost Specification
Universal LV Transformer: BK 1PH 5000VA 240/480-24/48 CU
Wiring Diagram: 7
Material: Aluminium
Frequency: 50/60Hz
Ambient Temp Rating: 40°C
Temperature Rise: °C
Insulation Class: °C
It looks like two of 5KVA transformers will do the job
Efficiency Regulation: No Efficiency Specification
Sound Level: NEMA ST-20
Approval: CSA,UL
Enclosure PN(npl): NQ5-N3R
Enclosure Dimensions(H/W/D): 17.25in(H), 10.00in(W), 7.75in(D)
Total Weight (NPL): 105.0Lb
Enclosure PN (Image):
Enclosure CAD Drawing (DXF): NQ5-N3R
Engineering Drawing (PDF): Q005DTCF
Installation Manual: IOMGE1PH
Connection Diagram: #7
UPC Code: 80342304375
*
Save as PDF Print

Link


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

 

[Elec777]

thanks a lot,
Waross, its great you are on the forum
thanks to everybody in the forum