DETERMINE THE SIZE OF GENERATOR 1

[gorkus]

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

 

[waross]

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
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"Why not the best?"
Jimmy Carter

 

[burnt2x]

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

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

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