Most purely mechanical generators have a pretty good sine wave output. The issue as pointed out is that voltage, especially on the cheaper ones bounces all over the place, typically as the load changes. Purely mechanical generators attempt to run at a fixed RPM in order to maintain a constant voltage output.
Generators with built in inverters generate an intermediate voltage that is then converted to a fixed AC output. The voltage is stable and the generator is able to run at a variable speed as output voltage is no longer tied to generator speed. That said, there is no guarantee this is a pure sine wave output and for low cost generators, I don't have confidence that is what they would use. It is more costly to implement a high efficiency sine wave inverter.
It is impossible to say what damage is any will be caused by a non sine wave inverter. Most things will be driven by a switch mode supply and most of them will be happy with a non-sine input, but depending on the design, they may not turn on assuming the output voltage is too low as the peak voltage is lower than the peak of a true-sine normally. The inrush currents are higher in a power supply when driven by a non-sine inverter and that will place higher strain on the input capacitors and other components leading to earlier failure. Instantaneous .... unlikely, but reduced life, possibly.
Semiman
Hi, a guy from the 50Hz world. First thing to be aware of when getting an alternator generator (I know that sounds weird but stick with me), is that many cheap 50Hz 240V generator sets are 60Hz 230V knockoffs. In short the alternator in these sets is rated for 230V (this is two 115V phases in series). Normally these generators have two outlets, each 115V 60Hz. You can check the polarity of these two outlets by bridging a Voltmeter across the the active pin of one outlet to the outlet pin of the second outlet. If you see 0 volts a.c. then the two outlets are connected in parallel, but if you see 230V a.c. then they are connected in anti-phase. This makes it simpler for the manufacturer to change the output to 230V a.c. Now for a 60Hz set the engine revolves at 3600 rpm, but for a 50Hz output the engine rpm is reduced to 3000 rpm. Now it is the way of things that if an alternator produces 230V a.c. at 3600 rpm then all things being equal, it will generate 3000/3600x230V or 191.7V a.c at 50Hz.
Needless to say this is a long way from the Australian standard of 240V (actually 230V -6%/+10%).
The way they get it back to 230V at 50Hz is to increase the excitation current through the field coil of the alternator to increase the magnetic field strength within the gap between the rotor (the revolving centre) and stator (the stationary body) of the alternator. This then increases the output voltage back to 230V. Only trouble is that this is a 20% increase in the field current, and hence magnetic field strength, puts the stator/rotor combination close to a limiting process called magnetic saturation. This is where an increase in excitation current produces only a little increase in magnetic field and hence only a small increase in output voltage. In short, the alternator has reached its maximum output power under a normal load at 50Hz, with little in reserve to counteract reduction of output voltage under a fluctuating maximum load.
To add to this deficiency, we now have a reciprocating motor, designed to provide maximum mechanical power output at 3600 rpm, now being expected to output the same mechanical power at 16.7% less rpm. Well reciprocating motors don't work like that and for a simple reciprocating engine reducing rpm by 16.7% means, to a first approximation, power output drops by 16.7% also. Why, because with a fixed displacement per stroke, then a fixed amount of gasoline / air mixture is ignited per compression stroke and so a fixed amount of mechanical energy is generated per revolution of the engine. This means mechanical output power is directly related to the rpm of the engine. Hence the lower the rpm speed of the engine the lower the maximum mechanical power it can generate and hence the lower the maximum electrical output can be obtained from the generating set.
Finally one last thing to note on the engine side. The mechanical energy that is stored in the rotating crankshaft, pistons and flywheel is proportional to the square of the rpm of the engine. This means that at 3600 rpm the motor has (3600/3000 squared) or nearly 44% more stored mechanical energy to keep the generating set close to 60Hz output under sudden load increase / decrease.
At 50Hz output the situation is reversed and compared to a 60Hz setup,
the same generating set has 30.5% less mechanical energy stored to counteract load fluctuations. This is an important issue as it means that a load increase from zero to full will cause the 60Hz setup to grunt a little but recover both voltage and frequency within a second or so of the load increase. But with the same generating set reduced to a 50Hz setup, the output voltage may well collapse with insufficient field current to counter the load increase and the output frequency drop as the motor staggers with insufficient stored energy to handle the sudden load increase and barely enough mechanical power to restore frequency to normal. Hence this droop may well last several seconds before stabilizing to normal values.
In summary then, is it any wonder one hears stories of how bad an alternator generating set is compared with an inverter generator when supplying delicate electronic equipment. The caveat here is that one should ensure that both the alternator and motor are rated for the frequency of operation found in your local area before purchasing,
especially if your local area has 50Hz mains.
