Hello testingpump
Sizing a generator for a single motor application is not too difficult provided that you understand the generator and the motor and starter and how they operate.
I have some information on my site at
Firstly, the genset is made up of four important components, the Engine, the Alternator, The Governor and the AVR.
The Engine produces KW and has a short term overload margin that can vary from as low as 120% up to 300%. The governor regulates the speed of the engine to allow for variations in load and minimise variations in output frequency. The engine must produce enough KW to cover the shaft load on the motor, the losses in the motor, the losses in the starter and the losses in the cables.
The Alternator produces KVA and it must produce sufficient current to enable the motor to start. The minimum start current required is dependant on the minimum start torque required by the pump at all speeds to full speed, the aility of the motor to convert amps into torque at all speeds to full speed and the ability of the starter to control the current applied to the motor. The AVR controls the excitation applied to the motor to regulate the voltage and cope with changes in load without variations in voltage.
Alternators have anshort term overload capacity, typically between 120% and 300%.
Some alternators are "self excited" That is, the excitation is taken from the output voltage generated. The initial flux is residual flux in the rotor which generates a small voltage which is fed back to increase the excitation and produce more flux and more output voltage etc. The problem with the self excited system, is that when the alternator is overloaded, the voltage drops due to the impedance of the alternator. The falling voltage reduces the excitation and the voltage falls even further. - The output voltage very quickly collapses for a small overload. Better systems use a permanent magnet generator to provide the excitaton and this results in a far superior short term overload performance.
AVRs also come in a number of flavours, at one end or the scale, there is the single phase half wave peak rectifier type which effectively samples once per cycle, and at the other end of the scale, there is the three phase full wave averaging detection system that in effect is continuosly sampling. The single phase peak reading system is very sensitive to transient loading and harmonics and does not tend to perform well on motor starters. The three phase full wave averaging AVRs usually give the best performance for motor starting applications.
So, to determine the size, calculate the start curent required and he start time. Then find the overload capacity of the alternator and size accordingly. In the case of a submersible pump, you will probably need somewhere in the order of 250% - 300% current to start. If you use an Alternator with a three phase averaging AVR and a PMG exciter, you could have an averload capacity as high as 300% in which case, the Alternator rating could be equal to the KVA rating of the motor.
The engine will need to provide sufficient KW to start the motor. Determine the peak shaft load during start, and on a pump, this will be at full speed and will be 100% load. Then determine the losses in the motor during start. If the motor has a full load efficiency of say 90%, then at full load, there will be a copper loss in the order of 5% - 7% of the motor rating. During start, the copper loss will be much higher. If the loss was say 7% at full load, the during start with a start current of 300%, the copper losses in the motor will be 7 x 3 x 3 = 63%. Determine the losses in the alternator the same way, probably similar in magnitude to the motor losses. The cable losses must also be considered. If the cable has been sized for a 5% voltage drop, then there is 5% loss at full load. During start at 300% current, the cable losses will be 5 x 3 x 3 = 45%, so we have a total KW requirement of 100% (shaft power) + 63% (motor copper loss) plus 63% (alternator copper loss) plus 45% (cable losses) plus say 20% (iron loss in motor and alternator etc) giving a total load on the motor of 281% of the motor rating. If the engine is capable of a short term overload capacity of 300%, then you could rate the engine at close to the input power rating of the motor plus starter plus cable loss (= motor rating / efficiency plus say 10% for motor loss plus 10% for alternator loss plus 5% for cable) If the engine has a 120% overload capacity, then you would need to rate the engine at around 2.4 times the motor power rating.
So, there is no easy rule of thumb, you need to do some engineering. Often, I find that people go for the cheapest option on a price per KW basis, and end up with a large machine to start the pump, but a machine with a higher cost per KW will offer a much better overload capacity and require a much smaller machine to do the job with a lower installed cost.
It is best to try to operate the engine at around 70 - 80% load if possible. Oversizing the engine (selecting a machine with a low overload capacity) will result in more frequent maintainance and higher running costs.
Best regards,
Mark Empson
