motor test question

[jcgjupiter]

A question asking for total demand in VA on a service for 7 motors. This is from memory but 3 motors were like 480v 3 phase 10 hp, 10 hp, 5 hp,

Then it listed the following motors fed from a transformer.
3 motors were 240v don't remember hp with one of them 15 min duty cycle.
Then a 120v 30 min duty cycle.
It listed nameplate amps for all the motors.

I don't know what duty cycle has to do with demand on the service, but I calculated VA for all the motors, using VA = E time I times 1.732 for the 3 phase motors and volts time amps for the single phase motors. Total them up. But nothing I came up with matched any answer. I tried everything i could think of.
I have gone back over all my books, Mike Holts, Tom Henry's, Stallcup's, can't find anything like this.
Can anyone give me some tips.
Thanks

 

[peter]

Re: motor test question

This is just a guess. But the highest rated motor is multiplied by 1.25.
Would that help things fit in?
Offhand, I don't know what the duty cycle has to do with something like this. Naybe someone else can chip in.
~Peter

 

[peter]

Re: motor test question

"It listed nameplate amps for all the motors."
This might be another problem. You have to use the tables 430-147, 148 and 150 to get what the Code considers to be the amperages for the various horsepower motors. Not the nameplate amperages except for overloads.
It would be easier to deal with this if you had a photographic memory and could tell us what the actual choices were.
~Peter

 

[jcgjupiter]

Re: motor test question

Thanks,
No it didn't say nameplate it just listed motors like:
230V 1 phase 7.5 hp 35 amps
something like that.

But i think since it was asking for total service demand, i forgot the 25 percent extra for largest motor, like a previous reply said.
Thanks to all for the help.

 

[wrestless]

Re: motor test question

Dont forget it 125% of the largest plus the FLA
of others on that phase were they all 3 phase?

 

[bob]

Re: motor test question

JC
I you are looking for the demand and all of the motors are running at the same time, it is the total va of the sum of the motors. Your method of caculating gives watts and not va. Use the tables in section 430 for the amps for each mtor.

 

[wannawaterski]

Re: motor test question

Hey jupiter, I saw your question, and I was actually checking here for the exact same question. I noticed that you didn't have the specifics of all the motors, but guess what...I did! So I posted another question. Maybe check back and see if anyone can help. I've taken two test with this on it, and am getting rather pissed. Just curious, is this for the State of Wa. 01 Administrator Exam?

Good, luck, and hopefully we'll find the help we need here!