Generator poupouri. Ratings. kW vs KVA Standby vs prime

This Generator Tutorial/discussion is most suited to three phase sets above about 15 KVA.

  • What is the difference between KW and KVA?
  • How are they determined?
  • What can you expect to find?

First, there are exceptions.
(And below about 10 or 15 KVA, there are a lot of exceptions…)

KVA (or Kilo Volt Amps) - This is the product of the rated Amps and the rated Volts, divided by 1000.
Amps. This is determined by the temperature rise of the windings under load.
Usually the maximum rated Amps remains constant, but a prime power set will be rated at a lower temperature than a standby set. The voltage may change. A set may be rated for 120:208 Volts star, or 120:240 Volts four wire delta, or 480 Volts star.

Example: A standby set may have windings rated for 100 Amps maximum.
If this set is to be used at 120:208V the maximum KVA will be 120V x 100A x 3 / 1000 = 36 KVA.
Generator sets rated at or above 15 KVA are typically 12 lead machines, suitable for use at 480 Volts.
If this set is to be used at 277:480V, the windings will be in series rather than in parallel.
The voltage across each winding will be 277V / 2 = 139 Volts.
277V x 50 Amps x 3 = 41.6 KVA.
However the set will still be rated at 36 KVA due to prime mover concerns. The same set may be rated at 50 Hz and used at common 50 Hz voltages. The KVA rating will change as the utilization voltage is changed.

The bottom line:
Do not exceed the rated current per winding.

:kW, KW or Kilo Watts.
The maximum kW is dependent on the power of the prime mover.
The prime mover or engine must supply enough mechanical kW to support the electrical kW output of the set.Typically, and more typically as the size of the set and the load increase, loads tend to be somewhat inductive.
With this in mind, it is common practice to rate sets at 80% Power Factor (PF). When sizing a set, you must know the nature of the expected load. If the load power factor will be below 80% you may size the set by the KVA rating. If the PF of the load will be above 80% PF you must size the set by the kW rating.
While three phase sets are rated at 80% PF, single phase conversions often result in the kW capability of the prime mover being greater than the KVA rating of the set.

Standby versus Prime ratings.
To understand the differences, it may help to look at the expected lifetime of typical sets.
A standby set may be exercised for two hours per month, and support 4, five hour power failures.
A total of 44 hours per year.
A prime set will run 24/7. There are typically at least two prime sets, alternating the load.
There are about 8700 hours in a year, so our prime set may be expected to run about 4400 hours in a year. A prime power set may run as much in a year as a standby set runs in 100 years. The build and ratings reflect this disparity in expected run times.

Example: Starting with a basic set, the standby set is rated at the maximum capability.
The standby ratings allows occasional overloads of 10%. The rating of the basic set is reduced to allow occasional loading up to the standby rating. The 10% buffer also allows the set to lose some power with age and still produce rated power after 4,000 or 8,000 hours of run time.

Mechanical:
A prime set will often have added features that may or may not be present on the basic standby rated sets.
These may be:

  • A larger oil pan to allow longer running between oil changes.
  • An oil cooler.
  • Provisions for a remote radiator.
  • Any other accessories that the manufacturer feels will enhance the lifetime of the set.
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