My rule of thumb has worked well for many smaller sets.
The spare capacity in Amps should be three times the motor full load current in Amps.
If you are starting a motor alone, you may use 2 1/2 times, but the voltage drop and frequency drop will be too great for other loads.
My application was multiple independent A/C units on residential standby sets.
Over 20 installations and trial and error at each site to determine which and how many A/C units could be started on the generator.
But if you need to defend your choice, Cat used to have free sizing software for generator sets.
A couple of notes:
The Cat software default voltage drop is quite conservative.
Residential customers were happy with a greater voltage drop when the A/C started than the Cat default voltage drop.
After a number of years of successful operation, I ran a number of my installations through the Cat software and all failed.
Some notes:
If you are designing a new installation and want accountability, use the Cat software (If it is still available).
If you are adding a large motor to an existing generator, you may use 3:1.
Prime versus Standby sets. A standby set is typically derated for prime applications.
Prime x 1.1 equals standby rating. The 3:1 ratio is based on Standby ratings. For a prime rated set, multiply the rating by 1.1 before applying the 3:1 ratio to the spare capacity.
If the required capacity is slightly above the capacity of a standard sized set, the Cat software will recommend the next larger standard size set.
Memory fades, but I believe that the software lets you change the default voltage drop.
Changing the voltage drop slightly may allow one size smaller set.
Add a note to your plans;
"Calculated for a maximum X% voltage drop under motor starting.
Motor starting is highly reactive.
The 6 times motor starting current is typically 3 times kW and enough reactive current to bring the total according to Pythagoras to 6 times.
So, the prime mover will have enough torque to supply the kW demand at 3:1.
That is why my 3:1 ratio has been successful.
The apparent current during motor starting may be greater than the rated capacity of the set.
The AVR will go a long way toward holding the voltage up during motor starting.
If voltage drop rather than frequency drop becomes an issue, you may consider a current boost circuit.
This circuit is typically used to avoid voltage collapse and provide enough fault current to ensure the operation of the protection system.
Basically a CT on an outgoing phase with the output dropped across a suitably sized resistor.
The voltage drop across the resistor is bridge rectified and connected in series with the AVR output.
These circuits fell out of fashion with the introduction of Permanent Magnet Generators to supply excitation power.
If your set is equipped with a PMG, you probably will not need a current boost.
I should probably mention UFRO, Under Frequency Roll Off.
When starting a motor that overloads the prime mover, the prime mover will tend to slow down.
This drops the frequency, and with a drop in frequency, the impedance of the motor drops.
Lower impedance means more current.
Lower impedance also may lead to saturation of the motor core.
More current is not a good thing for an already overloaded gen-set especially the disproportionate current increase resulting from magnetic saturation of the motor being started.
To mitigate this, UFRO was developed.
If the frequency drops more than 3Hz, UFRO cuts in and reduces the voltage in proportion to the reduction in frequency.
Starting a motor with reserve capacity of 2 1/2:1 will probably result in a large voltage drop, by itself.
Add to this the additional voltage drop introduced by UFRO, and the combined voltage drop and frequency drop may be unacceptable for other loads.
I am not saying that you cannot start a motor at 2 1/2:1 ratio, but if you do, I want you to be aware of the possible consequences.
In some cases, the lights may dim and UPS units may beep a few times.
In other cases important machines may trip off.
I have tried to combine several years of field experience with many hours of research into one post.
I hope this is helpful.
Good luck.
PS: If the generator spare capacity is 60% or 70% of the generator capacity, expect to see the generator leaning into it's mounts and a lot of black smoke.
If the spare capacity is 10% or 20% of the generator capacity, starting the motor will be mostly a non-event. The percentage overloads on the set will be much less. You have more room for a fudge factor.
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Ohm's law
Not just a good idea;
It's the LAW!
