# Hertz trouble



## Farmer Andy (Jan 11, 2018)

I just picked up a used Generac 25kw PTO generator to replace one that finely died. I got it mounted and fired it up to do some testing. It produces 240v, but only 50 Hz at the recommended 540RPM input speed. At first it thought may be the HZ gauge was faulty, so I picked up a Kill-a-watt unite to check it out. This shows 50Hz as well. I've contacted Generac, and local Generac service centers, and no one seems to know what the problem is. Its not a speed problem. Ive checked the rpm on the alternator and id running at 1600 rpm. This should be right for a 4 pole unit. ( I think. I'm no expert, I just stayed at a Holiday Inn Express. )Does anyone have any Ideas? I do have a manual and picks of inside the unit to share with anyone who wants to tackle this mystery.


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## KRE (Nov 29, 2011)

4 pole requires 1800rpm for 60 hertz
4 pole requires 1500 for 50 hertz


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## Farmer Andy (Jan 11, 2018)

Oops. I guess I should have proof read. It should read 1800 rpm, but still producing only 50 Hz.


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## KRE (Nov 29, 2011)

Farmer Andy said:


> Oops. I guess I should have proof read. It should read 1800 rpm, but still producing only 50 Hz.


Fact, A 4 pole rotating field turning at 1800 rpm produces 60 hertz. Check your RPM with another device. If you have a photo tach you can check any florescent lamp(but not a C/P) At 60hertz input to the lamp you will read 7,200RPM (60hertz X 60 seconds X 2 (top an bottom of the sine-wave)


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## KRE (Nov 29, 2011)

The math for speed vs number of poles is easy. The target number is always 7200 for 60 hertz an 6000 for 50 hertz

1800 x 4 = 7200
1500 x 4 = 6000
1200 x 6 = 7200
900 x 8 = 7200 
3600 x 2 =7200 ect/ect


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