I want to build a diesel engine driven generator. The type of gen I have is asynchron, 400/690 V, 7,5 kW, 9,4 kVA and I was told that I need to stimulate it with capacitors (the unit is supposed to work without connection to a public netstand-aloneisland-service). Who can give me instructions how to do the el. part, size/type of capacitors etc?
Because it lacks a separate field circuit, an induction generator cannot produce reactive power. In fact, it consumes reactive power, and an external source of reactive power must be connected to it at all times [1]. So you are right that you will need a three-phase capacitor bank at the output terminals (delta-connected). How to size the capacitors? That depends on a few things. Namely what kind of loads are going to be attached to it. I presume this is going to be an emergency generator operating alone (most induction generators are used this way nowadays). In that case, your capacitor bank will need to account for the generator consuming reactive power and the loads consuming reactive power. So, if you know what you are going to connect to the generator, figure up an estimate of the total reactive power of your loads. Then estimate how much reactive power your generator consumes (not sure how to do this; maybe estimate from your 7.5kW and 9.4kVA ratings?). Finally, to size your capacitors equate the total power consumed by gen and loads to this formula: estimated_3phase_power_consumed 3*(line-to-line_volt)^2/X where X 1/(2*pi*freq*C) Solve for C and there you go. My source indicates that the output voltage for an induction generator can vary quite a bit with varying inductive load, so bear that in mind and maybe oversize the caps a bit. Also, the types of capacitors should be able to withstand AC obviously and be rated within your expected operating temp, etc, etc. Good luck.
