# %Vreg during DOL vs. SS Starting

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### #1 Suerbatica

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Posted 18 May 2006 - 11:12 PM

Hi everyone, I'm an Electrical Technologist in Training (but I'm not getting much training ), I'm running a motor simulation at work and I'm having trouble getting my head around it.
I have to do a model in ETAP to confirm the Utility company's claim that we need a soft start on our 1100 and 1500hp compressor motors to avoid voltage drop during D.O.L.
My problem is, how do I do this comparison by hand to verify what I am getting out of the computer simulation?
(1) I know the locked rotor current from the specification table of the motor, (2) from that I can get the locked rotor torque.
What I don't know is how do I calculate how much voltage will drop from the Utility side? When this motor starts, it is on the secondary side of a transformer. The only information I have about the Utility is it's S/C MVA, and the Utility supply voltage at the primary winding of the transformer. I know how to find the power etc for the motor but I don't understand how it relates to a voltage drop in the Utility service. Or how to solve for it.
I know that I probably haven't expressed myself in the best way, so please ask questions and I will do my best to answer in better detail, and technical term.
Thanks in advance for any assistance you can provide. Cheers!

Suer

### #2 GGOSS

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Posted 19 May 2006 - 06:02 AM

The more current you draw from the supply, the greater the voltage drop. Through the addition of a soft starter, you would expect reduction in starting current and hence a reduction in voltage drop.

In order to do the calculations you will need to collect information regarding supply capacity, supply impedance, cable impedance, minimum starting current for the motor/load combination etc etc.

If you can get all of that together, then I would suggest a google search for "voltage drop caculations' will yield some good information including several calculation tools that are available as free downloads. Alternatively if you do the google search first, the calculator you download will highlight the information you require to do the calcs.

Hope that helps.

Regards,
GGOSS

### #3 Suerbatica

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Posted 24 May 2006 - 07:32 PM

Cheers! I appreciate the time you took for the response. Thanks very much

Suer

### #4 Suerbatica

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Posted 29 May 2006 - 12:09 AM

This calculator looks like it will be great, I'm waiting for our IT guy to install it. But I noticed that this software doesn't SHOW me how its done. I'm looking for actual formulae so that I can do this by hand. The software is a great tool but I want the knowledge of how it's done. If someone could give me a hand with this I would greatly appreciate it.

Suer

### #5 marke

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Posted 29 May 2006 - 01:31 AM

Which calculator??

### #6 jraef

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Posted 29 May 2006 - 07:41 PM

The formula is extremely complex, hence the need for computerized programs such as ETAP. I use SKM (a competitor of ETAP) and it takes a lot more information to be accurate, such as creating a motor model (torque-speed curve); a load model (WK^2, current-speed-torque profile, thermal limits etc.), a transformer model (size, impedance, X/R etc.), distribution model (i.e. bus or cable lengths, resistance etc.) and of course a starter model. All those variables makes it really difficult to do by "hand".

I should add though that Marke's program available on the sister site to this forum, LMPhotonics.com is a great simple program if you don't know all the details. Since he sells it though, I doubt he will give away his secrets!

http://www.lmphotoni...m/busbarexe.htm
"He's not dead, he's just pinin' for the fjords!"

### #7 Suerbatica

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Posted 30 May 2006 - 02:10 AM

Well I appreciate your concern, but complexity does not bother me. Motor Voltage Drop has been around for over 60 years, if those old coots could do it, so can I

Again, if anyone can proivide any assistance in figuring out voltage drop of motors during startup by going over the formulae with me, I would greatly appreciate it. Feel free to be technical and complex.

So far all I have is simplified impedance models: Vmotor = Zmotor / (Zmotor + Xmotor) * 100 But I question it's accuracy even though they claim it's within 10%. 10% however, is a substantial amount of error in my world.

Thanks if you can lend a hand,

Suer

### #8 Suerbatica

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Posted 02 June 2006 - 03:03 PM

I managed to track down an Excel Spreadsheet from G.E. that calculates Volt Drop during Start. I'm working my way through it now. Some of it is complex like the system base impedance calc, but for the most part it looks pretty straight forward.
I won't trouble you gents with anymore questions, I just wanted to post that I do not need any more assistance with this topic. I'll manage from here

Cheers,

Suer

### #9 marke

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Posted 02 June 2006 - 09:33 PM

Hi Suer