Genset Sizing

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 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 droop.
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 sized gensets.

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.

Motor
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
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 L.M.Photonics Ltd

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