I'm afraid Mike has made a slight slip,

At the risk of appearing a bit of a pedant I disagree, and I stand my ground.


An Induction genset, like an induction motor is asynchronous and does slip - it is just negativein that you spin it faster than synchronous and get energy out. It  has a monobloc shorted squirrel cage rotor, just like an induction motor, and no brushes. The revs have to change to keep the volts in tolerance, and the phase angle of the load is very critical to get interaction of slip rate  with V and F right. There is no AVR winding, Such machines can have a relatively good response to pulse resistive  loads == low "Zs" if you like, up to the point of total field collapse but are a sod to stabilise for changing reactance loads. All you can do is look at the output voltage and retrospectively wind the throttle to change  revs and flick capacitors in and out of circuit. I have sat astride too many of these with instruments to be unsure how they work?. (the smell of diesel in the morning and all that) The effect of one phase seeing a different capacitance to the others is interesting but I'll leave that for now. Actually in the very short pulse domain (tens of micro seconds now )the high surge current is propped up by the capacitors as well. Speaking only as a happy customer I can say that Fisher Panda make some very good asynchronous machines using this principle. Just do not expect it to be spot on-freq. ever. They can however be made short proof, due to the field collapse and that is sometimes a nice feature to have in the field.


However most generators that are made to be used with any arbitrary load or to run in parallel are certainly not that sort, as the V-F interaction with load phase angle is too hard to manage. Instead they are indeed the synchronous sort that Dave is thinking of, and  have an excited rotor that is in effect a slowly changing DC magnet spinning at exactly pole frequency, driven either by brushes and slips or by rotating transformer and on-armature diodes. Then the rotor speed and freq are in perfect lock and the rotor current varies to maintain volts, at least over a limited range. However, as noted the effective 'Zs' is less stiff. I'm aware of controllers having low frequency lock off, but I defer to Dave's greater knowledge as to what level you would set it to.


My original point was to be very careful with rules of thumb about short circuit current comparing extracts from different text books unless you know a lot about the particular genset under discussion. It may be more or less than you think, usually less.


M

I'm afraid Mike has made a slight slip,

At the risk of appearing a bit of a pedant I disagree, and I stand my ground.


An Induction genset, like an induction motor is asynchronous and does slip - it is just negativein that you spin it faster than synchronous and get energy out. It  has a monobloc shorted squirrel cage rotor, just like an induction motor, and no brushes. The revs have to change to keep the volts in tolerance, and the phase angle of the load is very critical to get interaction of slip rate  with V and F right. There is no AVR winding, Such machines can have a relatively good response to pulse resistive  loads == low "Zs" if you like, up to the point of total field collapse but are a sod to stabilise for changing reactance loads. All you can do is look at the output voltage and retrospectively wind the throttle to change  revs and flick capacitors in and out of circuit. I have sat astride too many of these with instruments to be unsure how they work?. (the smell of diesel in the morning and all that) The effect of one phase seeing a different capacitance to the others is interesting but I'll leave that for now. Actually in the very short pulse domain (tens of micro seconds now )the high surge current is propped up by the capacitors as well. Speaking only as a happy customer I can say that Fisher Panda make some very good asynchronous machines using this principle. Just do not expect it to be spot on-freq. ever. They can however be made short proof, due to the field collapse and that is sometimes a nice feature to have in the field.


However most generators that are made to be used with any arbitrary load or to run in parallel are certainly not that sort, as the V-F interaction with load phase angle is too hard to manage. Instead they are indeed the synchronous sort that Dave is thinking of, and  have an excited rotor that is in effect a slowly changing DC magnet spinning at exactly pole frequency, driven either by brushes and slips or by rotating transformer and on-armature diodes. Then the rotor speed and freq are in perfect lock and the rotor current varies to maintain volts, at least over a limited range. However, as noted the effective 'Zs' is less stiff. I'm aware of controllers having low frequency lock off, but I defer to Dave's greater knowledge as to what level you would set it to.


My original point was to be very careful with rules of thumb about short circuit current comparing extracts from different text books unless you know a lot about the particular genset under discussion. It may be more or less than you think, usually less.


M