180Hz Power using Over-Speed 60Hz Generator

[chris8410]

Greetings.
I am writing to ask the opinion on the following.

I am needing a horizontal 180Hz synchronous 3-phase 750kVA generator. This frequency is not commonly available in the used equipment market, and I do not have the funds to purchase new.

It has been suggested that a horizontal 60Hz 600RPM generator can be over-speed to 1800RPM (3x600RPM) to produce 180Hz power (3x60Hz). An 1800RPM motor would be used to over-drive the generator.

In theory, this idea is great. However, I have serious reservations about the ability of the field (rotor) to withstand the additional centrifical forces that would be generated as a result of the over-speed. These forces would be acting on the filed coils, pole piece assemblies, etc. In response to my concerns, the same individual who has propsed this idea has commented that rotor designs up to 1800RPM are often the same, thus the rotor "should" be able to withstand the additional forces.

Besides the forces acting on the rotor, I have concerns about the ability of the bearings to operate at a higher RPM. I have no specific information about the bearings, since the prospective generator has not be selected.

If it is determined that the rotor has issues with being over-speed, could the existing rotor be rewould for the higher speed (ie better coil bracing, stronger pole bolts, etc.)? Or, could a new rotor be machined along with new coils?

I would greatly appreciate thoughts and comments from others. I am an electric power engineer, but I am not a M&G specialist.

Thank you in advance.

 

[davidbeach]

How clean does your output power have to be? If you can stand some distortion, you might look into a VFD that has a 3 pu frequency output and not have to worry about spinning a generator apart. I don't know the answer to your question but suspect that you'll not find a generator that just happened to be designed for 3 pu speed.

 

[Marke]

Hello davidbeach

If you use a modern VFD, you will get a very chopped waveform which will no bear any resemblance of a three phase low frequency waveform. It will cause a sinusoidal current into a three phase inductive load however.

If you wanted to persue the VFD option, you would need to add a three phase output transformer, preferably a delta primary and star secondary and possibly some filtering also.
If you set the VFD up for a nominal output frequency of 180 Hz at the required nominal line voltage, you should get a reasonable sinewave out of the transformer. This could be cleaned up by additional filtering.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[chris8410]

Hello David,
Thank you for your response.

The power must be fairly clean, That is, I must have a clean waveform as well as a minimial amount of radiated noise. Therefore, a VFD may not be appropiate.

My application is performing induced voltage testing on large power transformers (up yo 600MVA). This test requires an application of up to 200% of rated voltage to the transformer under test (per IEEE C57.12.00 standards). So as to not saturate the transformer core, a frequency of 120Hz or above is used to supply the test voltage. In my experience, 180Hz is best. Simultaniously to applying test voltage, I must measure partial discharge (PD) on the terminals of the transformer. If the power supply is noisy (ie VFD), this radiated noise may be mistaken for partial discharge of the transformer under test.

I am not very familiar with VFD applications. Any more thoughts would be appreciated.

Thank you David.

Regards.

 

[ScottyUK]

Most large UPS modules can be configured for frequency conversion. Many do 50/60Hz conversion, and some can reach up to 400Hz. Speak to one of the UPS vendors and see what they have to say. There are many to choose from. I strongly suggest you discuss details of your requirement with them - especially load conditions seen by the source if the transformer under test breaks down.



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Sometimes I only open my mouth to swap feet...

 

[chris8410]

David, Marke and Scotty,
Thanks for the comments regarding VFD's. I will plan on contacting a reputable VFD manufacturer.

Regards,
Chris

 

[aolalde]

A standard 12 pole generator spinning at 1800 rpm will have severe stress due to centrifugal force, proportional to (w^2)*r or 9 times that at 600 rpm.The core losses at 180 HZ could overheat the stator. Reconstruction to withstand those negative factors could result in a higher expense as compared to a new equipment.
Airports use lots of M/G Sets to convert 60 HZ to 400-415HZ, could you test at higher frequency using one of those used M/G Sets?

 

[davidbeach]

Excellent idea, running a 400Hz generator at somewhat less than 0.5 pu would work far better than running a 60Hz machine at 3 pu speed.

 

[ScottyUK]

I'd thought of that but I've never seen a 400Hz set of roughly 1.5MVA rating to get 750kVA at 180Hz. Do they exist that large? Obviously it could be made, but a custom build is going to be pricey.


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Sometimes I only open my mouth to swap feet...