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#177321 - 04/29/08 09:21 AM Hertz problem on 3ph gen  
sparky25yr  Offline
New Member
Joined: Apr 2008
Posts: 6
This is a Kohler 6500W generotor 3 ph 208/120 output auto start NG. Very old model.
The voltage is good at 122V
The hertz were very high.

No load test
(leg A to C) hertz high
(leg A to B) hertz 50 to 90 all over the place
(leg C to B) hertz 50 to 90 all over the place
(All legs to neutral) all over the place
I then connected a load of 15 amps
1 three phase motor 3HP
and 3 resistive heaters (electric stove large burners)
Adjusted the speed of the motor to 58-60 hertz from leg to leg voltage is good.
That B leg still has fluxuation.
All legs to neutral have fluxuation.
All voltages are good 208/120
Generator is 208 delta with transformer in gen panel for 208/120Y

Does anyone have a idea for a fix on the hertz

Work Gear for Electricians and the Trades

#177323 - 04/29/08 10:28 AM Re: Hertz problem on 3ph gen [Re: sparky25yr]  
LarryC  Offline
Joined: Jul 2004
Posts: 783
Winchester, NH, US
If the engine is not surging, I suspect the the meter being used to measure the frequency is seeing noise and interpriting it as varying frequency. I seriously doubt a little 6.5 KW generator has a transformer to convert a delta wound alternator to a wye output. Did this unit come off of a reefer truck?


#177324 - 04/29/08 11:23 AM Re: Hertz problem on 3ph gen [Re: LarryC]  
sparky25yr  Offline
New Member
Joined: Apr 2008
Posts: 6
The engine is not surging and you may be right on the meter seing noise.
This unit does have a transformer in it.
Like I said old unit.
This is the backup for my home.
I pulled it out of a nursing home in 1999 when they upgraded to a 200A model.
The Kohler salesman (old guy) that sold the new unit said he sold them this one when they built the place in the 60s
hour meter shows 1050 hrs.

#177326 - 04/29/08 11:31 AM Re: Hertz problem on 3ph gen [Re: sparky25yr]  
sparkyinak  Offline
Joined: Jul 2007
Posts: 1,327
If you have access to an ocilliscope plug 'er in and see what you get.

"Live Awesome!" - Kevin Carosa

#177330 - 04/29/08 12:02 PM Re: Hertz problem on 3ph gen [Re: sparkyinak]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,121
Is this a pure alternator or is it an inverter design?

Greg Fretwell

#177331 - 04/29/08 12:21 PM Re: Hertz problem on 3ph gen [Re: gfretwell]  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Is the neutral bonded to ground, or floating? If it was permanantly installed, they may have removed the bonding jumper- good for connection to an 3-pole ATS, but bad if trying to use it as a portable. Might be a loose/dirty connection on phase B, too.

The frequency is directly proportional to the speed. If the speed isn't varying (you'd hear it for sure), then I'd suspect the meter is giving you a bad frequency reading. I'd check the neutral first, as that might be what's throwing off the meter.

#177334 - 04/29/08 02:05 PM Re: Hertz problem on 3ph gen [Re: SteveFehr]  
sparky25yr  Offline
New Member
Joined: Apr 2008
Posts: 6
The neutral is not bonded to ground at the generator. this is a permanent installation and it does go through a 3 pole ATS.
The neutral does bond to ground by:
N from gen through ATS to (generator breaker panel) neutral buss bar then to main panel neutral buss which is bonded.

#177336 - 04/29/08 02:53 PM Re: Hertz problem on 3ph gen [Re: sparky25yr]  
LarryC  Offline
Joined: Jul 2004
Posts: 783
Winchester, NH, US
A 3 phase generator going thru a 3 pole transfer switch as a backup for a single phase residence?

The alternator is not going to be happy due unequal loading on the phases.


#177347 - 04/29/08 07:19 PM Re: Hertz problem on 3ph gen [Re: LarryC]  
sparky25yr  Offline
New Member
Joined: Apr 2008
Posts: 6
Yep your right the way you think un equal loading but that was an easy fix.
Three pole breaker in main panel feeding a 3 phase ATS to a 3 phase panel. no 240 volt loads or watch how you plug the breakers in and make sure it will run on 208V
Three legs in balance for the 3 phase generator!

oh yes modify the ATS to monitor only two legs for 240v
Used the junk laying in the back room

Last edited by sparky25yr; 04/29/08 07:25 PM.

#177352 - 04/29/08 10:23 PM Re: Hertz problem on 3ph gen [Re: sparky25yr]  
frenchelectrican  Offline
Joined: Feb 2003
Posts: 939
Wi/ Paris France { France for ...
is this unit is gas or diesel unit ??

first thing you have to set the prime mover to proper HZ speed useally either 1800 or 3600 rpm even 1200rpm on some models. the 1800 and 3600 is most common one.

get the prime mover speed set up first before you mess around with voltage otherwise you will have heckva of time with it.

once you set up the speed correct the no load should be at the excat or a hair over the base speed.

now to set the voltage regulator that get pretty tricky depending on the type of connection and winding itself.

if you are running in combation delta/ zigzag set up that will raise some hair up a bit due oddball set up you have to find the netrual referance [ you will have to check the nameplate connetion diagram for this ] and with floating delta system they are hard to get correct reading if the load is not balanced at all.

the only way you can get run 208v proply is reconnect it to wye format

let us know what lead numbers i may have a trick or two with it


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

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