PMG Excitation

Report
EXCITATION SYSTEM
SHUNT Excitation

AVR power supply & reference
voltages.

shunted on the alternator output
terminals.

AVR generates and regulates
the excitation current as a
function of the alternator output
voltage.

Extremely simple design and
ideal for basic applications.

Cannot tolerate high overloads.

Does not offer a short-circuit
capability.
PMG Excitation

Alternator same design as SHUNT.

AVR powered by a permanent magnet
generator (PMG).

PMG delivers constant voltage
independent of the main winding.

The voltage reference is shunted on
the alternator output terminals.

Suitable for demanding applications.

PMG system has a high overload
capacity

Improved performance over SHUNT
system.
AREP Excitation

The AVR power supply voltage
comes from 2 independent auxiliary
windings.

AVR power supply is independent
of the voltage sensing measured on
the alternator output.

Excitation current to the alternator
exciter is independent of voltage
distortions (harmonics) due to the
load.

AREP system gives high overload
capacity & short-circuit capability.

Particularly suitable for demanding
applications
EXCITATION SYSTEMS
PMG
ADVANTAGES
DISADVANTAGES
AREP
SE
High starting
capacity
High starting
capacity
Short-circuit
capability
Short-circuit
capability
Self protected
against short circuit
Intrinsic build-up
No extra length
No extra length
Extra
length
Specific winding
No short- circuit
capability
High number
of components
Small added cost
Low starting
capacity
Added cost
Sensitive to
distorting loads if
thyristor controlled
VOLTAGE (V)
SE CHARACTERISTIC
( or auxiliary winding H1/ AREP)
AREP/ PMG CHARACTERISTIC
AREP SERIES
( Auxiliary winding H3/ AREP)
SUSTAINED
VOLTAGE DIP
LOAD %
100%
LOW OVERLOAD
200%
300%
HIGH OVERLOAD - SHORT CIRCUIT
3-Phase PMG Excited, 3-Phase Sensing
3-Phase Self Excited, 3-Phase Sensing
3-Phase AREP Excited, 3-Phase Sensing
EFFECT OF DRILLING RIG
LOAD ON ALTERNATOR
DESIGN
DRILLING RIG:

A drilling rig creates a borehole, or well, where oil and natural gas can be
extracted for the production of fuels and other petroleum-based products.

Drilling rigs may be described as mechanical or electric. These terms
refer to the method in which power is supplied to the larger equipment on
the rig.
On mechanical rigs, power from the engines drives the rig equipment
either directly, through a clutch or through a torque converter.


Electric rigs use engine power to drive one or more generators. The
generated electricity is then used to operate motors for the larger
equipment on the rig. There are three types of electric rigs:

DC (Direct Current)
 SCR (Silicon Control Rectifier)
 VFD (Variable frequency Drive)
ELECTRIC RIG:

Electric motors power the draw works, rotary table, pumps and other
systems with electricity from the generator. These applications are called
electric drilling rigs or electric rigs.

The electric motors used for hoisting, drilling and pumping require high
torque at zero rpm and variable speed characteristics for efficient operation.
These characteristics are possible using the following methods:


Direct Current (DC) Generators
Alternating Current (AC) Generators with Silicon Controlled Rectifiers
(SCR)
 AC Generators with Variable Frequency Drive (VFD) Motors
GENERATOR SIZING

A rig operating with the power limiter light on does not mean the engines are
being efficiently operated. Larger kVA generators (or other remedial action)
may be needed because generators may be at kVA limit and engines at only
30 to 50% load. A difficulty in efficiently sizing and operating an SCR or DC
rig is the common assumption that “x” amperes represents “y” power; this is
not true.

If power factor is 1.0, then “x” amperes represents “y” power. At power
factors below 1.0, power is less than the amperes indicate.

A power factor from 0.3 to 0.9 on an SCR rig, under steady-state conditions,
is evidence that generator sizing is important. During hoisting, power factor
varies from 0.0 to 0.95.

There may be cases where the minimum number of engines cannot be
operated because of a high generator kVA requirement. Before examining
these variables, it is first necessary to review some characteristics of DC
motors
DC Motor Characteristics



The rpm of DC motors is
primarily controlled by the
voltage
to
the
motor
(recognizing that motor type
series, shunt and control system
field weakening, etc. is related
factors). Ampere draw of the
motor controls torque output of
the motor.
This leads to the realization that
a DC motor can work hard at
low rpm (draw high amperage
and produce high torque) and
not load the engine (but load the
generator) when operating at
low DC voltage/low rpm.
A method to calculate AC
generator power factor due to
current draw of a DC motor
powered through an SCR
system.

Mentioned graph the effect of
motor rpm (or DC voltage) on the
power factor of the driving AC
generators. For a constant rpm
(DC voltage), power factor is the
same from no load to full load.

The best way to improve system
power factor is to ensure that DC
motors are run at as high an rpm
as possible. Every DC ampere
presents a 0.85 kVA load on the
generator, regardless of DC
power. Operating a DC motor at
high rpm reduces ampere load,
therefore kVA

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