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Low Voltage from Portable Generator

Posted by Loose_Screw (My Page) on
Mon, Aug 29, 11 at 11:42

During our recent hurricane here on the East Coast I had a problem with my portable generator. It has been very reliable for ten years, but this time it was working fine, then the engine cut out with a load on it. Then after restarting - the voltage slowly dropped until it was only producing almost nothing. The engine did not seem to rev under load like it used to do automatically. When I manually revved the engine the power did increase.

I don't have a voltmeter so I can't tell you the exact voltages, but it would barely spin my power drill when I used it to test the outlet. The oil and gas levels were okay. Is there some kind of sensor that tells the engine when to rev when there is an increased load? Any ideas what may be wrong?

Please don't tell me to take it in for service as this is a permanently mounted unit and there is no way I could transport it. It is a North Star 5500 watt unit. Thank you for any help!


Follow-Up Postings:

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RE: Low Voltage from Portable Generator

These things want to run at a constant speed, set at the factory. If it's not running at speed you need to figure out why not. Your two major causes of low voltage are overloading and slow engine speed. These NorthStars are junk to begin with, but your local lawnmower place can probably figure out what's wrong with it.


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RE: Low Voltage from Portable Generator

Thanks ronnatalie, I thought I remembered hearing it change RPM's when I put a load on it in the past, but I could be wrong. I thought North Star would be okay because it had a Honda engine, and I believe they are quite reliable.

It is definitely not overloading because all I plugged in was my hand drill and it would barely move. Any ideas why the engine speed would suddenly be slow? I can't really take it to a repair shop, like I said, it is permanently installed and we can't lift it.


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RE: Low Voltage from Portable Generator

You may have heard transient rpm changes in the past with sudden load changes. The sound will also change with power output. It is suppose to run at constant RPM. The speed control system is compromised.


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RE: Low Voltage from Portable Generator

Some of the smaller generators have inverter technology which does permit operation at variable engine speeds depending on the load. My generator depends on the simple engine governor to maintain speed. It runs 3600 RPM regardless of the load. Overloading it beyond the engine power would cause low speed or stalling. But the circuit breaker would probably trip. Neither of these conditions has happened with my usage.
I suggest checking every aspect of operation of the faulty generator. I suspect that an internal short in the generator unit could cause the described problem.


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RE: Low Voltage from Portable Generator

The generator may have lost its exciting current. If so, there's an easy fix.

Try plugging in the drill again and then spin it fwd. That may supply enough current to re-excite the coils and get everything running again. If it doesn't work, spinning the drill back wards will usually get it going.

Of course, if this works, the generator will start outputting immediately and you need to get your hand off the drill chuck quickly.

(It's an old trick when dealing with portable generators)


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RE: Low Voltage from Portable Generator

ionized, that is probably what I heard, transient changes.

bus driver, mine is connected to a transfer panel and neither the gen breakers or the panel breakers tripped. Could it be a problem with the governor? How would you check that?

enigma2, I've never heard of that. Sounds easy enough and harmless to try. So I just manually turn my drill while connected to the running gen? Are you pulling my leg?
(p.s., my wife says I've lost my 'exciting current' too, but you can't help me there)

Thanks everybody, great answers, but I was just hoping that it would be simple like changing the spark plug or something. But, nothing is ever easy. I guess I'll call the electrician if these ideas don't work.


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RE: Low Voltage from Portable Generator

Actually no. If you can spin the drill, it should create enough current to excite the field. (Won't hurt anything to try.)

Yea, my wife gave up trying to excite my current after I had a heart attack. So goes life :-)


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RE: Low Voltage from Portable Generator

Almost every 'generator ' is actually an alternator.
The difference is how the static magnetic field is developed.

In a generator the field is on the fixed windings around the rotor.
This means the output current must cross the slip rings from the rotating windings.

in an alternator the fixed field is on the rotating windings.
The output current is taken from the fixed windings.

Since ht exciting current is DC and only a few percent of the current output it is a much better solution.

In a typical car the battery supplies an easy reliable source of exciting current.

Only the largest systems have a battery to supply the exciting current.
Small generators rely in residual magnetism in the iron rotor to get the current started.

Once they have some output current thy can then supply the DC excitation.
If the residual field is lost, no output is available to start the process.
Spinning the alternator while sullying it with some current can allow then to start.

A universal motor (they have brushes and a commutator) typically has enough residual magnetism to generate some current if rotated by an outside force (you).


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