Standby Generator Starting kW

[CuriousElectron]

Greetings All,
What is typical ratio between skW and rated kW of a gas generator? I know that gensets are capable of providing higher starting kVA for motor starting, but do they also provide higher skW for limited time? What is the limiting factor here.. is it the temperature rise on alternator windings(thermal)?
Say a gas generator is rated for 100kW, would it be capable to provide 150skW for short duration to start a motor load?
Thank you,
EE

 

[Gr8blu]

NOTE: I am making an assumption that the load (motor) to be started is a Squirrel Cage Induction Machine (SCIM).

In the case of a SCIM, the bulk of the transient current required during a start sequence is reactive. This is because the machine is operating at a very low power factor (around 20%, or 0.2 per unit). That means the ratio of real power (kW) to reactive power (kVar) is roughly 1 to 5. You'd need to look at both the KW and KVA rating of the generating unit. Typical designs have kW = 0.8 * kVA (a power factor of 80%).

For reliable starting, the kVA and kW of the load machine must be no more than the capability of the generator. In some cases, that will be the generating unit values (kVA and kW) MINUS any other loads already being powered.

 

[waross]

Small generators below about 10 KVA are generally rated for unity power factor. kW = KVA.
Above that size, a rated PF 0.8 or 80% is common. kW = KVA x 0.8
The kW of a generator is determined by the available power of the prime mover.
The KVA is determined by the operating voltage and by the rated current.
As an illustration, if a 100 KVA, 120/240 Volt center tapped delta generator is reconnected in star, for use at 120/208 Volts, the rated KVA drops to 86.7 KVA.
While this would technically raise the PF to 86.7KVA/80kW, or 0.92, by convention it will still be rated at 0.8 PF.
Motor starting;
Generally the available set capacity in Amps should be 3 times the rated Amps of the motor being started.
Use this with judgment.
An unloaded set rated for 60 Amps will start a motor rated at 20 Amps.
A set rated at 120 Amps with an existing load of 60 Amps will start the same motor easier.
Most customers will be satisfied with the voltage drop, especially when they are aware of the extra cost of a larger set to reduce the voltage drop from motor starting.
When a large motor starts, the mechanical load will slow the set down and the frequency will drop.
This is partly because of the governor response to a large block load and partly because the prime mover is overloaded.
Now UFRO comes into play.
Under Frequency Roll Off.
When the frequency drops 3 Hz below the set frequency, UFRO starts to drop the voltage proportionately.
This is to maintain the V/Hz ratio.
Without UFRO, a dropping frequency would result in a rising V/Hz ratio and possible saturation of inductive components.
In the event that the only load on the set is the motor to be started, a 2.5:1 ratio will generally start the motor but the resulting voltage drop will often be more than the customer will accept.
Other equipment online may malfunction due to the low voltage.

There will be times when a job specifications mandate a minimum motor starting voltage drop.

In that case, check the websites of the major Gen-Set manufacturers.
They have software available for sizing gen-sets.
You will be able to specify any voltage drop that you wish and the program will size that set for you.
The default setting will suggest a set larger than the 3:1 ratio but they are in the business of selling sets and the larger the better.

I have over 20 satisfied customers starting air conditioning units with sets sized at 3:1. The Cat software says that every one is undersized.