I have three 10HP frequency inverters on my fan application for greenhouse, they all work well during the last two years, but one of them is noise running currently, i don't know what's the problems, what's i have to do? does the inverter needs routine maintenance?

Maintenance is very important for frequency inverter. i am daily dealing such kind of problems. cleaning required once in a month or depends on Application and environment of working place.

Preventive maintenance will definitively extend the lifespan of the frequency inverter. As said here before, negative impacts on the drive will first depend on the environment (heavy industry like casting for example). We recommend complete cleaning of the electronic cards and ventilator replacement.

Most of inverter users intends to never touch the inverter if it is working.they are considering cost of preventive maintenance as some unnecessary expense which is feeding by paranoia.when faced with some invisible costs such as production line stops, high repairment or replacement cost they gonna aware importance of the preventive maintenance.Preventive maintenance is essential for long term working without any defects.the cheapest way of maintenance issues is protect your systems from defects before they occurs by make preventive maintenance periodically.

First of all, you have to find the diagram circuit. That is the key.

Forget about honest manufacturers - they are hard to find. Just go to any non-authorized (they will not lie to you) repair service before buying anything and talk to engineers there - what and why to buy, and how to obtain the diagram circuits.

After you have these diagram circuits you will become free - I've heard Americans love this so much. You can hire any good engineer on your choice, you can fix any manufacturers' mistakes and even improve your equipment.

The next step in buying the equipment. The guy you had met in repair service will help you for a reasonable price, but be sure he is honest or at least is not friend of the vender. I would buy the used equipment if its price is low enough - at least 3-5 times less than the new one's.

After the equipment is bought and installed, you must check it for possible problems. Better of all to have thermovision or at least nightvision (but then you will wait till a night) to detect exact points of overheating.

After these points are detected, you must find ways to fix these problems and this is why you need to have diagram circuits before buying anything. Some problems are solvable with additional fans installing (do not buy "unserviceable"), but some require changes in schematics...

What about capacitors - misuse of them is a common "mistake" of manufacturers. For example, one of worldwide known ones had 600uF after 220 volt 50 Hz bridge rectifier and load was 4000 Watt - good engineers will tell you what it means. These poor capacitors were on the edge of bursting, and note again - that was a Worldwide Known Manufacturer... By the way - it did not changes despite it knows my opinion.

And a bit about fans - I still had no seen unserviceable ones. At least you can make holes ans grease through them."

The same about inverters. Of course it is a good idea to clean all inside.
What to me, I just wash most of equipment with a water jet.

Look at the Prairie Turbines site again. The price of the micro-controller including the hall dvceie is $255. The $45 book provides a complete list of material and instructions to build everything else.The micro-controller monitors the speed sensor and provides control signals to two solid-state relays. One relay connects the motor to the grid when the speed reaches the motor's 1800 RPM synchronous speed and disconnects the motor if the speed drops below 1800 RPM. The other relay is energized to release the brake when the controller is energized and de-energized by the controller when the motor speed exceeds the maximum safe operating speed. If this occurs, the controller apparently must be manually reset.I did not find the maximum safe operating speed stated on the web site. If the full load speed of the motor is 1750 RPM, the full load slip is 1800 minus 1750 or 50 RPM. The speed at which the motor would generate full load current and present full load torque to the turbine is 1800 RPM plus the full load slip or 1850 RPM. With the design described, a three-phase motor is connected to a single phase source with capacitors connected between the source and the third motor phase. That is likely to increase the motor slip somewhat.I am not sure what happens if the motor is driven above the speed at which it generates full load current. A three-phase motor operating as an induction generator connected to a three-phase supply has a torque vs. speed curve that is something like the mirror image of the motoring torque vs. speed curve. The curve is likely different with the single-phase connection used in this case. I don’t know what the speed vs. current curve looks like in either case, but I will see if I can find it. I am also curious to know if the Alan Plunkett listed on the web site as an author is the Alan Plunkett that is well known author of technical papers on inverter and variable frequency drive subjects.Added information:I found information indicating that, for a 3-phase motor connected to a 3-phase supply, the speed vs. current curve for induction generator operation is like a mirror image of the speed vs. current curve for motor operation. That would mean that the maximum speed would need to be limited to the synchronous speed plus the full-load slip. The web site indicates that the full-load speed for the motor that they used is 1725 RPM for 3-phase operation. That would make the maximum speed 1875 RPM plus the additional slip due to the single phase with capacitor arrangement.