|
Industrial Electric Motors and Generators Menu
Electric Generators Suppliers
Generators operational characteristics are well understood. The following are the principle operational considerations: Voltage - Generated voltage is the emf denoting
the electric pressure between phases in the armature.
The magnetic flux linking each armature coil
changes as the machine rotates. The change in flux
per turn occurs at the conductors in the armature
slots. Each conductor is regarded separately as it
cuts the flux. At a specific rotating speed, instantaneous
volts per conductor are proportional to air
gap flux density at the conductor.
Current - Current is the rate of transfer (flow)
of electricity, expressed in amperes. Field current
required for a particular load condition, i s determined
by the magnetic circuit, in conjunction with
armature and field windings. Load current is equal
to the generated voltage divided by the impedance
of the load.
Speed - Normally, a generator operates at a constant
speed corresponding to the frequency and
number of poles. Variations may occur due to
changes in driving torque, load, field excitation, or
terminal voltage.
Frequency - AC frequency is determined by the
rotating speed and number of poles of the generator.
Frequency is usually expressed in Hertz, the frequency used most is 60 Hertz. A two-pole generator
must operate at 3600 rpm to maintain 60 Hertz.
Four-pole and six-pole units must operate at 1800
rpm and 1200 rpm, respectively, to maintain 60
Hertz. Frequency at 60 Hertz is expressed in the
following equation:
Frequency (60 Hertz) = (Speed in rpm)(Pair of Poles)/60
Power - power is the term used to describe the
rate at which electric energy is delivered by a generator
and it is usually expressed in watts or kilowatts
(103 watts).
Watts - Watts are units of active or working
power, computed as follows: volts x measured or
apparent amperes x power factor.
Volt amperes reactance (Vars). Vars are
units of reactive or nonworking power (1 var = 1
reactive volt-ampere).
Power factor - power factor is the ratio of
active or working power divided by apparent power.
The relationship of apparent power, active power,
and reactive power is shown in below. The
hypotenuse represents apparent power, the base
represents active power, and the altitude
power triangle represents reactive power.
of the
Power
factor (the cosine of angle 0) is a unitless number
which can be expressed in per unit or in percentage.
For convenience, kilo (103) is often used with the
terms volt- amperes, watts and vars in order to
reduce the number of significant digits.
% Power Factor = kW/kVA * 100
Where:
kW = Kilo Watts
kVA = Kil0 Volt-Amps
References: “Joint Departments of the Army and the
Navy, Operation Maintenance and Repair
of Auxiliary Generators, 26 August 1996”
© Copyright 2000 - 2006, by Engineers Edge, All rights reserved. Disclaimer |
