700 hp motor

[rbalex]

Re: 700 hp motor

Prior to the Tables being expanded beyond 200hp, I always used 1.2A/hp for 480V (460V motors) for a design first estimate and ratioed it for other voltages. I never looked back. At worst, it rendered a slightly conservative value that I still never regretted using.

 

[charlie b]

Re: 700 hp motor

Originally posted by iwire: Hi all, for my own education I am interested in how each of the respondents came up with their figures.

Originally posted by charlie b: 790 amps.

By "interpolation," which is the engineering term for SWAG. I noticed that for large 3 phase, 460V motors, if you double the HP, you don't quite double the current. The current for a 350 HP is shown as 414. Double that and you get 818. Ratio it down to 480 volts, from the tabulated 460 volts, and you get 793. I rounded that down to 790. Then I put in a qualifier (i.e., "on the order of") to prevent me from looking bad, just in case I was wrong. The engineering term for that is "estimate."

 

[jasonet]

Re: 700 hp motor

Jim,
After coming in and reading,

That's kiddy talk. Let's talk like a adult. We're here at this site to learn.
Bye now,
Jim

I went back and I read my msg. again

Member # 24315 posted August 14, 2005 08:58 PM
--------------------------------------------------------------------------------
Howdy Jim, how was your day? I'm sure you meant Horsepower= 746 watts and not 759 watts, just be careful next time. I know your point was the equation HP(watts)(VAC)/PF = it will give you apretty good idea of what your going to have for FLA. Not a homerun, but just shy of fence.

Ya got something to say, then say it, sarcasm doesn't arouse
Get off your high horse jim, no one is above making mistakes. Just a little notice, I will admit my mistake, I gave the wrong value,

You are absolutely right! I did say HP=759watts. You pointed out and corrected my mistake, as anyone should have, My reaction to it was both childish and very unprofessional! As you said, we are here to learn.

I would like to offer my apologies and say that I am SORRY, first to you Jim, for my being rude and disrespectful, it will not happen again.
I also apoligize, and say I'm Sorry to all forum members, moderators and guest that read my momentary lapse of judgement.

Thank You,
Jasonet

 

[james wuebker]

Re: 700 hp motor

Jasonet
Glad your aboard on this site. Maybe I can learn sometime from you. I see that your a Industrial Electrican. I do a lot of that and there's so much to learn. Hope to see you again.
Thanks!
Jim

 

[rcwilson]

Re: 700 hp motor

My motor vendor's perfomance data book shows a 700 HP WPII, 460 V motor with full load amps (FLA) of

786A for 3560 rpm, (94.7% eff & 88.0% pf)
793A for 1780 rpm, (94.7% eff & 87,4% pf)
795A for 1185 rpm, (94.3% eff & 87.1% pf)
822A for 890 rpm, (94.4% eff & 81.5% pf)

Values for Open Drip Proof (ODP) motors are similar.

Note that everyone's estimates are in line with these numbers, close enough for initial equipment and cable sizing.

Jasonet's useful equation used power factor =0.75. That number actually repesents the power factor and the motor efficiency together. The power factor for this particular design is 81 - 88% and efficiency is 94.4%. The equation still works because 0.94 x 0.81 is awful close to 0.75.

 

[hochwire]

Re: 700 hp motor

Thanks for the input!!!

 

[friebel]

Re: 700 hp motor

To: Jasonet, I had to comment on the very positive message that I read from Jasonet. It was very refreshing to read his apology comments. It certainly helps to keep this Forum on a Professional level. Thank-You Jason, for your apology and positive comments.

 

[stew]

Re: 700 hp motor

the formula I use is 746 watts per hp X hp divided by 1.76x voltage x eff x pf. For this motor using 480 volts and 92 % efficiency and 95% pf I come up with 803 amps. I believe this is the correct formula is it not?

 

[rcwilson]

Re: 700 hp motor

Stew - That is the correct formula. Your guess at power factor for this size of motor may be low.

Most of the 500-1500 HP motors have lower power factors, 80%-90%.

One approximation for this equation: if we divide 746W/HP by an assumed 92% efficiency and 81% power factor we get 1 KVA per HP. So 1 KVA per HP is a reasonable aproximation.

At 480 V, 1 KVA is 1.202 amps. So 1.2 amps per HP is also a good rule of thumb, for 480 V motors.