×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

DETERMINE THE SIZE OF GENERATOR

DETERMINE THE SIZE OF GENERATOR

DETERMINE THE SIZE OF GENERATOR

(OP)
how can i get the amount of kva ( SIZE OF GENERATOR) to start a three electric motor of 4 kw.??
Kva = kw/ cos(fi)
  = 4 /0.85
= 4.70 kva is correct??

THANKS IN ADVANCE

GORKUS

RE: DETERMINE THE SIZE OF GENERATOR

Try about 3 x 4 kW = 12 kW to start a three phase motor.
Try (3 motors x 4 kW)+(2 x 4 kW) = 20 kW to start a 4 kW motor with two 4 kW motors running.
Try 3 motors x 4 kW x 3 = 36 kW to start three 4 kW motors together.
 

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

RE: DETERMINE THE SIZE OF GENERATOR

Can't guess with not much info! LRA of each motor, length of cables from panel to motors?

RE: DETERMINE THE SIZE OF GENERATOR

Listen to waross. He knows about generators and motors.

What you didn't include in your calculation is the high starting current that asynch motors have. It is more than three times rated current, six to eight times, actually. But as long as the motors do not need many seconds to start, the factor three is OK.

As I said. Do what Bill says.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: DETERMINE THE SIZE OF GENERATOR

Thank you Gunnar.
Rule of thumb #1 Allow three times motor kW or KVA for starting when the generator has other loads and voltage drop is an issue. This will usually be acceptable.
Rule #2 Allow 2.5 times motor capacity when the generator drives a single motor and voltage drop on starting is not an issue.
Rule #3 Don't use rules #1 and #2 blindly but take a look at the whole installation and apply some common sense.
These rules are based on experience with generators that would start the motor loads and generator installations that would not start the motor load.
I changed out quite a few generators that would not start the load, curtailed the load on a few others to avoid stalling the gen set and upgraded a couple more to accept increased loading.
Motor starting on a gen set involves quite a few factors, some of which interact.
The generator engine determines the kW available. The engine may be over sized depending on the rating (standby or prime). Almost all three phase to single phase conversions are over powered.
An over powered generator will start a motor easier and with less voltage drop than a marginally powered set.
If the motor starting load slows the generator more than about 3 cycles voltage drop will be unavoidable and is actually a good thing. It prevents the motor from drawing excess current due to magnetic saturation.
KVA determines the heating of the generator. Short term overloads for motor starting are allowed.
The load of a running loaded motor is mostly kW.
The load presented by a starting motor is more KVAR than kW.
The kW and KVAR combine to produce KVA which heats the generator and produces voltage drop. The AVR compensates somewhat for the voltage drop.
The Automatic Voltage Regulator and the governor have some effect. Does the AVR have UFRO? Is it self excited or PMG excited? These questions are considered when evaluating a marginally sized gen set, or considering adding load to an existing set.
Go back to rules #1, #2 and #3.
 

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources