GENERATOR SELECTION CONTENTS - Diesel Generator

STANDBY RATINGS TO NEMA MG1-22
SECTION 4302
January, 1990
In response to a market requirement especially on those areas where American engines are used as the prime
movers Newage offer continuous operation standby ratings for their range of brushless a.c. generators. The graph on
the next page indicates the principles involved.
These ratings are in accordance with the United States specification NEMA MG1-22.
Extracts from NEMA Standards - Part 22 - June, 1972.
MG1 - 22.40 Temperature Rise.
Machine Part Method Temp. rise 40°C ambient
of temperature
determination Class of insulation system
Note IV
A B F H
Armature windings Resistance 60 80 105 125
Field windings Resistance 60 80 105 125
Temperature rises in the above table are based upon generators rated on CONTINUOUS DUTY basis. Synchronous
generators may be rated on a STANDBY DUTY basis (See MG 1-22, 84). In such cases it is recommended that
temperature rises not exceed those in the foregoing table by more than 25°C under continuous operation at the
STANDBY RATING.
MG1-22.84 Standby Generator.
Synchronous generators are at times assigned a STANDBY rating where the application is an emergency back-up
power source and is not the prime power supply. Under such conditions, temperature rises up to 25°C above those
for CONTINUOUS-DUTY operation may occur in accordance with Note IV of
MG1-22.40.
Operation at these STANDBY temperature rise values causes the generator insulation to age thermally at about four
to eight times the rate that occurs at the CONTINUOUS-DUTY temperature rises values, i.e., operating 1 hour at
STANDBY temperature rise values is approximately equivalent to operating 4 o 8 hours at CONTINUOUS-DUTY
temperature rise values.
Stamford generators are generally insulated with a Class H system throughout. Where prime power continuous duty
ratings are used to the full Class H temperature rise no standby rating is offered.
Prime power continuous duty ratings are often quoted to a Class F temperature rise EVEN THOUGH the insulation
system is Class H. This may be to comply with certain standards or to meet other operational parameters. In this case
a standby rating can be offered.
In general the Class F standby ratings and Class H prime power ratings on Stamford generators are identical. The
user then has the choice which standard to quote.