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#169070 - 09/24/07 05:40 PM generators
circuit man Offline
Member

Registered: 11/05/01
Posts: 287
Loc: saluda,s.c.
i have an old 1100 watt generator that has no circuit breakers, when i overload it the motor runs like it is wide open with no load.when i take the load off & drop it to a smaller load it acts like it's supposed too. it has not burnt out the windings.all is ok. what causes this?

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#169222 - 09/29/07 01:49 AM Re: generators [Re: circuit man]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Circuit Man:

Got a few questions to toss at you.

 Quote:

i have an old 1100 watt generator that has no circuit breakers,


It may be "Size Restricted" - meaning the Prime Mover (Gasoline Engine) can only produce enough Brake Horsepower to drive a load with 1100 Watts of True Power;
or the exciter may be "auto-regulated", so the Genny can only deliver 1100 Watts to a given load/load combination.

This would eliminate the need to protect the internal circuitry + receptacle from overcurrent issues, since there is no way to exceed their capacity by limiting the Genny's output power.

Very crude, yet effective technique, which keeps the manufacturing costs lower, + reduces maintenance issues
(also reduces the manufacturer's liability from fried wires emitting smoke + flames!)

 Quote:

when i overload it the motor runs like it is wide open with no load.


How are you overloading it?
If you are placing a bolted fault (solid short circuit) across the circuit, then the Prime Mover would attempt to run at full speed, trying to develop the highest Horsepower it may.

In that situation, the Governor will be run out to the extreme limits, and the Prime Mover should run at rated speed per the desired output frequency.

The Genny's windings will not fry, because the Prime Mover cannot develop any more Horsepower beyond this point.
After some time, the Genny's case will become warm, and the fuel should be consumed at the highest rate, since the Prime Mover is developing its rated Horsepower continuously.

This scenario would allow the full 1100 Watts to be driven into a load, with the Prime Mover's speed running steadily at top RPMs (according to the rated output frequency).

If what you are saying is the Prime Mover "Bogs down, then runs at a high speed" - which exceeds what would be normal for a governed frequency, then most likely the Exciter's field is automatically compensated (reduced) to limit the Genny's output.

Is the Generator's output AC or DC?

If you are intentionally overloading the Genny with an Electric Motor, here are a few questions:
  1. Is the Motor an AC Induction Motor?
  2. Is the Motor a Series-Wound Commutator Motor?
  3. Do you "load up" the Motor gradually, or start it with a large load?
  4. Is the Motor larger than 1/2 HP?
  5. Does the Electric Motor's speed reduce?
  6. What is the Voltage measured at the Motor / Genny output when loaded to capacity?


If you load up the Genny's output with Incandescent Lamps, do they dim down and stay dim when the Prime Mover's speed goes high?

Kind of sounds like an auto limit-compensated exciter.
If not, the Prime Mover is simply running at full speed and developing full HP.

 Quote:

when i take the load off & drop it to a smaller load it acts like it's supposed too.


Not sure what you mean here - do you remove the heavy load, then apply the smaller load instantly (with the Prime Mover running), or do you remove the large load, wait a few seconds, then apply the smaller load after the Prime Mover has re-governed its speed and output?

It could simply be that the Prime Mover is "tight", or the Generator is "tight" - or both, which results in a non-steady load applied to the Prime Mover when the "smaller load" is applied - causing it to "search' for a steady governor speed.

Some questions regarding the "small load" scenario:

Does the Generator / Prime Mover idle steadily at a low speed, when there is no load connected?

What type of load are you applying, when you connect the "small load"?

What is the overall reaction of the Prime Mover's speed (does it try to catch up to speed constantly, or just lag).

Does this behavior occur when applying various types of "small loads" across the circuit?

Is the smaller load below 1000 Watts?

Is it below 500 Watts?

Is it an Inductive load or Resistive load?

What is the Voltage at the load during this period of time?



 Quote:

it has not burnt out the windings. all is ok. what causes this?


Most likely, the windings will not lose smoke or flames, because the Prime Mover (the Gasoline Engine driving the Electric Generator) is limited to a certain Horsepower, and it can only transduce that level of Horsepower to the Generator.

Simply stated, the Prime Mover can only develop an equivalent Horsepower to the 1100 Watts rating of the Generator, so therefore the Prime Mover develops 1100 Watts and no more - after that point, it would stall.

The Exciter (Electromagnetic Field Windings, or "Rotor") which creates the Mag Field needed for Inducing the Stator windings, may be "Limiting" the output by dropping the Field strength to near nothing, if the output becomes excessive.
This would cause the Prime Mover to run in an "Unloaded State", as there is very little work being done at this time.

Please provide additional information, as I would like to help you understand what is going on with this Generator - and consequentially, explain other Generators too!

Scott
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#169276 - 09/30/07 05:33 PM Re: generators [Re: Scott35]
circuit man Offline
Member

Registered: 11/05/01
Posts: 287
Loc: saluda,s.c.
hi scott,thanks for teh info! this is an old WWII generator 120 volts a.c.60 cyc 1100 watts.it was made by MONGOMERY WARD.it doesn't matter what type of load i put on it lights or motor loads. they both act the same way. i put a 500 watt flood on it & you can here it load up. but if i exceed the wattage the prime mover just sounds like there is nothing attached or it's not loaded at all. i have a 4000 watt that acts the same way. i almost toasted the windings in it though. just ended up burning out the outlet.WOOOH!i belive your right the exciter just quits.no dead shorts, just loads. it has no idle just full speed.its ok when you plug something back in a few seconds later.the motor is just your standard induction type.i usually throw the full load on it,the motor usually runs slow aare even stops.THANKS FOR THE INFO!

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#169821 - 10/17/07 06:09 AM Re: generators [Re: circuit man]
jdevlin Offline
Member

Registered: 08/07/02
Posts: 402
Loc: welland ontario canada
Sounds like it might have some sort of internal breaker. What happens to your load when this happens? Does it go off?

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#169833 - 10/17/07 02:30 PM Re: generators [Re: jdevlin]
circuit man Offline
Member

Registered: 11/05/01
Posts: 287
Loc: saluda,s.c.
hi jdevlin. the load quits all together when it is overloaded.there is no internal breaker i can see.i pulled the plug out. this one was made around WWII.very old only a 3 h.p. motor pulling it. the big one is an 8 h.p. & it does have circuit breakers but they don't pop out. i thought i burnt out the stator in this one but only the connections too the 240 volt outlet went bad , because i used my ohm meter & a wiring diagram to trace the windings.also had to reconect the rotor windings to the comuntator because they some how came loose.think mayebe the wiring harness from the statoir caught them , so i tied them out of the way.as soon as i unplug it & plug a smaller load in it works fine.

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#169834 - 10/17/07 03:52 PM Re: generators [Re: circuit man]
jdevlin Offline
Member

Registered: 08/07/02
Posts: 402
Loc: welland ontario canada
Sure sounds like some sort of internal overload protection. Sorry I don't have enough knowledge to expand on what it might be.

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#174582 - 02/08/08 10:41 AM Re: generators [Re: circuit man]
hardwareguy Offline
Member

Registered: 06/15/07
Posts: 50
Loc: Texas
Hmmm.... maybe it's a bimetallic strip type thermal breaker?

That would cut all load and cause the engine to idle.

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#174629 - 02/09/08 08:33 PM Re: generators [Re: hardwareguy]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
The way that circuit man described things, leads me to believe the Exciter may be forced low in an overload situation.

However, an automatically resetting bimetallic strip would do the same thing from a prolonged overload.

The OP did mention the speed of the connected AC Induction Motor would drop with applied load, then stop altogether at maximum load.

I still am wondering if there is any measurable Voltage from the Generator, once the connected Motor stops, and the Prime Mover becomes unloaded?

If circuit man is still around, is there a measurable Voltage once the Induction Motor stops, and the Prime Mover runs high?

Try connecting a Volt Meter to the unused part of the Generator's Receptacle - or if only a Single Receptacle, connect the Motor to a multi-outlet cord, or plug strip, so you have a Receptacle to insert the Volt Meter's leads into safely and securely.

If there is NO VOLTAGE, then chances are there is an overload "link", which opens during excessive overload conditions (heat), then resets automatically once it cools down.

If there is SOME READABLE VOLTAGE, then chances are the exciter is pulled low somehow.

Scott
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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