Meaning of "Max Unbalanced Load Capacity" On Small Generator Nameplate

[JohnMcNutt]

I have seen many small 1ph standby gensets with a max unbalanced load capacity figure on the nameplate.

Usually this number is given as 50%.

Some larger ones have a rating of 25% or even 10%.

I am not sure how exactly to apply this number. 10% at what point in the calculation?

 

[waross]

Acceptable voltage limits.
A generator may have a 50% voltage drop under load. That's why we use Automatic Voltage Regulators. The AVR increases the excitation current to compensate for the voltage drop under load. The AVR sees the total voltage (240 Volts). If a heavy load on one 120 Volt winding causes that voltage to drop to 60 Volts, the AVR will see, initially, 180 Volts. It will increase the excitation about 33%.
Now the voltages across the 120 Volt windings will be 60 V x 133% = 80 Volts, and 120 Volts x 133% = 160 Volts.
Unacceptable.
In the real world, non-linearities including saturation will probably affect these simple calculations, but you get the point.

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

 

[JohnMcNutt]

What I mean is, what would a 25% load imbalance look like? 25% of the more heavily or more lightly loaded leg?

 

[itsmoked]

They are meaning a single phase 120/240V application and that you only toss 120 volt loads on the generator and pile them all on only only one leg and the neutral. If you don't spread them evenly over both legs or are using a 240V load you will get badly regulated voltage.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wayne440]

I have assumed that unbalance spec is referenced to the rated line current, IOW if a 120/240V single phase set is rated for 50A per line and 50% imbalance, the lines should be within 25A of each other. Here is a spec sheet that refers to the rating as a percentage of "rated standby current".

http://www.kraftpower.com/wp-content/uploads/2012/01/G50U.pdf