Senior Member
Joined: 03/05/2006
Location: AustraliaPosts: 119
| Posted: 08:17am 18 May 2006 |
I guess that KiwiJohn is referring to the use of induction motors as generators. The guts of this is that if you provide a rotating magnetic field to excite the machine and then drive it over synchronous speed, energy can come out. And its only a few % over speed. That is why some of the large machines can be changed over from direct drive to gearbox drive at critical wind speeds. So like with the very large induction wind machines, they are connected to the mains to provide the magnetisation and driven faster than synchronous speed, when they generate power. If the mains connection falls off, they stop generating and the brakes have to be applied to stop overspeeding. If you wanted to use an inverter to excite the machine I reckon you'd have to drive it up to synchronous speed before you connected the inverter, because when stationary, the machine would look like a short circuit. There may still be some surges when you connect it and the inverter would need to be capable of supplying almost purely inductive current, perhaps 25% of the nortmal full load current.
Another way is to rely on residual magnetism causing a small voltage to be generated and connect the right sized capacitor to the machine. This would enable generation to proceed at almost any speed after the machine was driven fast enough to supply current to the capacitor. Of course the output frequency would be variable depending on driving speed. This technique is possible but I don't think it is used much. I think everything is too critical and the ratings of the bits have to be exact. http://www.redrok.com/cimtext.pdf is a site that talks about it. DONT TRY THIS AT HOME! Why did I say that? Because we are talking about 240 or 415 volt systems and the capacitors operate at voltages much higher than that. I can tell that most members of the backshed are good at low voltages but in general are not ready for the dangerous stuff.
Megawatt Man