Voltage Drop Due To Heavy Load

Dear Sir,

Please give some more details (1) Comp.Motor Voltage as in the name

plate (2) The frequency of Start / Stop say per hour (3) Will the Comp be

running fully loaded or % of loading (4) What is the present method of starting ?

(5) What do you mean by "which is about 480 A"- Starting Current Or running

current ? (6) Is it a HT motor , because 48 A @ 3300 V will give 274 Kws.

which is nearest to 250 Kws taking the current as given by you.

Please think about soft starting.

Manroop.Chennai.

Manroop,

1/ read question first

2/ if there's insufficient information, say so

3/ don't make assumptions

4/ think before answering

Dear Guest,

Thanks for your advice.Also do not hide as Guest !. Any how I am glad to note the original question is a bit ambigous as noticed by Mr.JReaf . Furthur it shows you did not read the words fully and advice others & please avoid this sort of comments in future. Atleast I was able to assume something which you did not try at all. Incidentally Mr. JReaf has the highest ratio 11.25 % ( Good answers / posts).

Excuse me if I am a bit out-spoken on this and it is a good intention to have some decorum in CR4.

Manroop.Chennai

1) Motor Voltage is 400V.

2) Frequency of operation shall be once in 4 to 5 hours, with a running time of about 1 hour.

3) Compressor is expected to run at about 85% load, but for calculation purposes, let us consider it runs on Full load.

4) Star/Delta starting, it comes with its own panel with star delta starter. We have to lay cable from Substation to this panel.

5) About FLA, I assumed it to be 0.85 x 250kw, at 400V 3phase power.

Thank, Dilip.

For this size compressor you need to get a good starter and also unload the compressor by closing induction valve. Is it delta/Y????... The resistance of stator wiring is probably comparable to the resistance of power line so the voltage drop could be up to 240 V.

Is this a new project or you deal with existing compressor? If the installation exists then you just measure voltage drop if it is new, you better think to get a good frequency conversion with automatic start up. Going to the next size would be waste of energy that costs much more! Do it right.

Is the motor of the compressor a three phase squirrel cage or wound rotor?

You could also use old style that employs ... got yourself a book Handbook of Electrical Engineer" that would provide you with reaqdy to go systems.

I would suggest you to see this thread first: http://cr4.globalspec.com/thread/57342/Cable-Selection-as-per-Theory-Calculation. It has much detail about the cable selection.

I understood that it doesn't have the information about the permissible voltage drop during motor starting. You need to calculate the voltage drop during motor starting and select the cable so that the motor terminal voltage doesn't drop below 80% of the motor rated voltage (it is better to verify it with the motor manufacturer). If you select the cable so that the voltage drop at running condition is below 2%, it will satisfy the starting condition voltage drop (because the starting current is 10 times of running current).

If the compressor load is supplied by local generator, you also need to make sure that the generator capacity is well enough for the motor starting (see http://cr4.globalspec.com/thread/58246/Power-Generator-Sizing)

Hope it helps.

- MS

Dilip, you may find this cable calculator useful http://www.aeicables.co.uk/csdisclaim.asp it is to BS7671, I don't know what reg's you work to.

With the compressor being star/delta starting the loading won't be as bad as you think. Also it's not usual to rate the cable for starting load but for running load.

"Starting current of the compressor goes to about 10times the running current, which is about 480A."

Is this something you have measured or is this theoretical on your part? If it is measured, what kind of motor do you have here? Starting current should not be more than 6 times (600%) of motor nameplate Full Load Current when started DOL! So assuming the FLC was 480A (the way you word things is a little ambiguous), your starting current should not have been more than 2880A.

Or maybe you are confusing "Inrush Current" which is a very very brief spike of current that occurs when the motor windings are magnetizing, with "Starting Current" which is the peak current after the inrush that the motor will pull during acceleration. If you made this mistake, you do not need to consider the "Inrush" current in your voltage drop calculations and cable selection.

I would just pick you up on one point. 6 x FLC was for older motors as I found out by bitter experience.
Strange as it may seem it was a 250KW compressor, the motor had burnt out. So following company policy the old motor was scrapped and a new high efficiency motor was ordered.
Came the day for it's first test run. Radio the control room to tell them to start the compressor. Clunk, the contactor closed, bang, the MCCB opened and Tony's sh^ting himself wondering how he could get 3 wires messed up.
Modern high efficiency motors can be up to 9 x FLC. The new MCCB I had to fit was 7000A as against the original 4000A inrush withstand. Thermal characteristics were the same.

Fitting the new MCCB is a different story, "it's the same footprint so a direct replacement", NOT! 2 hours to fit turned in to 10!

Dilip, the cable for the compressor I altered was 150mm with DOL starting.

Hi,

Thanks to all of you who took interest to answer my query... Its great to find that there are people out there to help. Now, I find that the cable need not be sized as per starting current, but as per the running current only.. Anyway, to be safe, I am now going for next size, ie, 2 Nos x 4Cx240 sqmm Copper cable, which solves the problem..

It has been a great experience...

Regards, Dilip.

Dilip, that is twice the size needed. Added to which you only need 3 core.
It's a waste of money!

Did you look at the cable calculator?

Hi, Thanks.. This is a job which I am negotiaing the price with the client - the original offer I sent them with 2 nos 4x185 sqmm cable shall be now revised with 4x240 sqmm.. There is a common Power/control panel in built for the compressor, which needs neutral, hence 4core cable...

Regards, Dilip.

If you're only wanting the neutral for control use a 3 & ½ core

Hi , You are right, But here in Dubai they use either 3 or 4 cores... its a waste of money, which is not realized by Gulf countries.... In all GCC, its the situation...

Regards, Dilip.

Geez, if I was doing it, I'd use British Standard 7671. You know, this is about the most frequently asked question in CR4, Mate, and the fact that you 'aven't done yer research and 'aven't the foggiest how to tackle the problem other than ask some other nana to do it over the interweb does rather worry me. I mean, WTF are they paying you for? <Wheeze>

Don't these people know how to do this stuff for themselves any more? <Splutter, cough>

GET THE MOTOR AND CABLE DATA SHEET.

1.STARTING CURRENT .

2.STARTING PF -Φ

3.CABLE -R& Xlmtr

Calculate the voltage drop by √3 I(Start)(RcosΦ+Xsin Φ).

Refer the system design criteria whether this voltage drop is within designed limit or not.

Hi not sure what kind of starter you are using, for starting the compressor, but i would presume that the starter is DOL, if it is going to draw that much current, I would size the cable as to the motor rated load as this would give you a bit of a soft start.

But 10 x motor rated current? I do find this very high, how did you come up with this current calculation?

I have found on previous projects DOL is only about 6 x full motor current, older soft starters draw 3 x full load current and star delta starters to be about 2 x full load current if set correctly.

Cheers

Joe