Onan UF Series Instruction Manual And Parts Catalog page 64

6 I N S T A L L A T I O N
2. Engine Pulley Speed. - The engine may have a variable speed gov-
ernor which regulates engine speed at about
8-1/2 to 12%. If so, governor operation is best at the maximum rated
speed of the engine. When the generator is operating at or near its
capacity, the tendency of the engine is to increase speed when the load
is suddenly removed. The engine governor does not react fast enough
at low speed to prevent momentary acceleration and high voltage. This
may possibly cause serious damage to any electrical equipment left
connected. Governor action is much better when the engine is operat-
ing at its rated speed, and the engine cannot increase its speed too
much when the load is removed. If the engine has a constant speed gov-
ernor, the speed regulation is 5% or less and the above effects are not
present.
Low generator speed causes low voltage and frequency. If, for ex-
ample, an 1800 rpm generator is slowed to 1500 rpm, the frequency of
the current produced will be 50 cycles instead of 60 cycles. The com-
bination of low voltage and frequency will almost certainly result in
burned out windings of any motors connected to the generator.
3. Pulley Selection. - The rated speed of the engine or electric motor
will determine the size of pulley to use on the
generator. To determine the correct generator pulley size to use, pro-
ceed as follows, or refer to the PULLEY SELECTION TABLE.
A. Multiply the DIAMETER of the driving unit pulley by its SPEED
in rpm (revolutions per minute).
B. Divide the above result by the nameplate speed of the generator
(1800 rpm).
Example: A driving pulley 15 inches in diameter operates at 1200
rpm. Multiply 15 x 1200 getting a result of 18, 000.
Divide the 18, 000 by 1800 rpm (generator pulley speed
desired) and the final result is 10, which expresses the
size of the pulley (in inches) required. If this same
driving unit has an 8 inch pulley operating at 1200 rpm
the generator pulley would then be 5.33" or approximately
5-3/8" (8 x 1200, divided by 1800 = 5. 33).