There are two major components to a genset, the Engine
and the Alternator.
The Engine supplies power, rated in KW or HP and the Alternator provides
voltage and current and is usually rated in KVA, volts and Power Factor.
For the best performance, it is important to select the correct engine
and alternator and couple them together rather than assume a standard
set. This can result in the most commercial and best performing result.
The Engine must
supply all the power required by the installation, this includes
work power and loss power. If the engine is not large enough to
supply all the power demanded, it will slow and the frequency will
During start, the motor will draw up to its rated KW (particularly as
it approaches full speed) plus a high copper loss in the stator. If the copper loss is 5% at full load, and the motor is
started with a DOL (Full voltage) starter, it will draw Locked Rotor Current
In sizing the engine for an installation, it is necessary to determine
the maximum KW demand and the continuous KW demand and ensure that
the engine is suitably rated.
The engine has a continuous output rating and has a short term maximum
power rating. The short term rating can be used to provide the energy
for starting motors, but often the overload capacity is not sufficient
to provide the full start requirement without over sizing the engine.
The Locked Rotor Current could be in the order of 700%
of the rated current of the motor, so the copper loss will be 7 x 7 x
5% of the rated power of the motor, or just under 250% of the motor rating!!
The same applies to the cable losses. If the cable loss is 5%, then under
full voltage starting, the power demanded from the engine could be another
250%. Additionally, the copper loss of the alternator could add another
250% power demand on the engine. Now we have a power demand of around
850% of the motor rating. Reduced voltage starting will reduce the start
current and thereby the power demand on the engine. With very low inertia
loads, the inertia of the engine and alternator may be sufficient to supply
the power to start the load, but there will still be a significant frequency
The engine is fitted with a governor which is a means of speed regulation.
The governor will adjust the throttle on the engine to keep the speed
and output frequency constant. Severe overloads will often result in a
droop in speed during start, and a surge in speed as the load comes off.
It is best to have a relatively slow load application to allow the governor
to track the load.
If the engine is a diesel engine, it is preferable to try to size the
engine so that the continuous operating power is reasonable high. Continuous
operation at light load will increase the required maintenance on the
engine plus increase the fuel consumption.
The Alternator supplies the current to
the load. The Alternator has a finite internal impedance and the voltage
is regulated by an AVR (Automatic Voltage Regulator) which controls the
excitation applied to the alternator. There is a finite maximum excitation
that can be applied and this limits the maximum current that the alternator
can supply. When the alternator is fully excited, the excitation is saturated,
additional load will cause the voltage to drop quickly. The alternator
tends towards current limiting.
The AVR monitors the output voltage either by single phase, half wave,
peak reading or by three phase full wave averaging detection systems.
The single phase method is usually connected across two phases but is
only measuring on the peak of one half cycle per cycle. The three phase
averaging method has six times the effective sample rate and is able to
respond much quicker to any variations and provide a more stable output
in response to step and transient loads.
Where a single phase AVR is used, it is best to avoid getting too close
to saturation of the excitation system, as there can be hunting of the
AVR as it tries to regulate the output voltage as the load drops off.
Apply a larger "safety margin" in alternator sizing when using
a single phase AVR.
Alternators have a rated short term overload capacity and this can supply
the start current to motors. Some alternators can be fitted with excitation
boost kits to further increase the short term overload capacity. Typically,
the short term overload capacity of an alternator is in the region of
130% to 200%. It is important to determine the maximum that can be achieved
reliably. If this information is not available, use 120%.
Soft Starters are one of the better ways
of minimising the start current without any steps or transients. This
can lead to the best reduction in voltage disturbance on the genset, but
there are some issues. The peak reading AVR can become more confused with
the use of a soft starter due to the harmonic currents during start. Some
peak reading AVRs may be unstable when used with soft starters.
The Altenator has a very finite impedance and because of this, the output
voltage waveform will be distorted by the discontinuous current waveform
drawn by the SCRs. This is not a problem except that the effect can be
a level of phase modulation of the output waveform. The soft starter can
become confused and try to track the phase modulation, making it worse
etc and the net result can be instability or imbalance in the start current.
Some soft starters will loose control when used on a marginally sized
genset and cause excess current and KW for little or no gain in start
torque. When this happens, the generator will commonly go unstable and
shut down. This is not a problem will all soft starters. It is most likely
to occur in starters using fast feedback algorithms internally, such as
"true torque control". Changing the start mode, or start settings
may overcome the problem, or changing the soft starter type may also overcome
the problem. Ensure that the soft starter is suitable for use on marginally
The Electrical Calculations software provides
for engine and alternator sizing for installations using one or two induction
motors only. There is no allowance for residual load. The assumption is
that motor 1 will always start first. If there is only one motor, leave
all the parameters for motor 2 as zero. Installations with more than two
motors, or with significant other residual load, will not be as dependant
on overload ratings for the engine and alternator sizing.
Rated Current = Rated full load current of the motor
Rated Voltage = Rated voltage of the motor
Rated Power = Rated shaft power of the motor.
Start Current = Current required to start the machine. This current is
a function of the motor, the driven load and the starting method used.
For Full voltage starting (DOL) the start current is equal to the locked
rotor current of the motor, irrespective of the load being started.
Cable Voltage Drop. = Total voltage drop between the Alternator and the
motor. For genset applications, this should be less than 5% in order to
minimise the power dissipated during start. This will have a major bearing
on the Engine rating.
Motor Efficiency = Rated full load efficiency of the motor.
Alternator Voltage = Output voltage of the alternator
Alternator Efficiency = rated full load efficiency of the alternator (excluding
the excitation energy)
Engine Overload Capacity = Rated short term load capacity of the Engine.
Typically 130% to 200% of the continuous rating.
Alternator Overload Capacity = Rated short term load capacity of the alternator.
Typically 130% to 200% of the continuous rating.
The Electrical calculations software is a suite of calculations
of value to electrical engineers and electricians, particularly those
involved in the control of motors.
The suite can be customised for distribution to show any
name and logo. This can be a valuable advertising tool for electrical
distributors, wholesalers and manufacturers. For more information, email
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