Author Topic: Generator output what changed???  (Read 6100 times)

0 Members and 1 Guest are viewing this topic.

christopher

  • Jr. Member
  • **
  • Posts: 52
Generator output what changed???
« on: December 11, 2014, 06:22:02 AM »
Hi all could sure use some help, I have been using a lister sr1 with a Brush gen head as a construction gen set.
It has run all the tools fine for about three weeks until a few days ago the air compressor at times would load up and not start, this
got progressively worse until it now will not start at all.
It will no longer run the table saw which it ran fine until this happened with the air compressor. All the other tools run fine including a second air compressor that is also capacitor start.
I have put my Kill a watt on it and the output is the same as before this happened 124 volts at 62.5 hz no load.
Could sure use any tips on what I could check, everything else it seems to run just fine with no loss of power.

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Generator output what changed???
« Reply #1 on: December 11, 2014, 10:33:01 AM »
Not enough information on the alternator to give any advice.  " with a Brush gen head "  Does this mean a generator manufactured by brush or does it mean brush as opposed to brushless ( and probably Chinese).

It seems almost certain the generator is not capable of supplying the reactive VA, it probably works fine on resistive load. What has changed I have no idea but various types of alternator have different ways of maintaining excitation with low lagging power factor during motor starts.

It is conceivably possible that slow starts with an inadequate alternator have killed the starting capacitors in the motors that now won't start.

The fact that one compressor still starts makes me suspect it may be a motor problem as well as possibly an alternator issue.

Starting single phase motors is very demanding on a small alternator. Only the best types of small alternator will cope adequately with motor starts. With compressors you will almost certainly need to completely unload the compressor during starting, your supply will not be stiff enough to start against any pressure.

Flux

ChrisOlson

  • SuperHero Member
  • ******
  • Posts: 3642
  • Country: us
Re: Generator output what changed???
« Reply #2 on: December 11, 2014, 04:55:34 PM »
A brush type gen head has an exciter winding with a diode, an AVR (Automatic Voltage Regulator), and a set of brushes on slip rings to supply the power to the field.  The best thing to do is put a voltmeter on the output and try to start one of these motors that won't start and see if you are getting a huge voltage drop.  If the voltage is dropping out of spec (anything below 110V) during what should be the starting surge either the exciter winding, the field winding or the brushes/slip rings are highly suspect.

I think the AVR itself is OK because you are getting normally regulated voltage at no load.  But the AVR can't supply the field with more current if the exciter has gone bad**, nor can it supply the field with more current if the slip rings are dirty or the brushes worn badly.  And if the field has a shorted winding in it due the insulation in the winding breaking down, the field will be weak and limit the generator's surge capacity.

**some gensets do not have a separate exciter winding, but instead take power from one of the stator windings and run it thru a diode for the AVR current.  If your gen head is this type, then you could have a bad stator winding also.

christopher

  • Jr. Member
  • **
  • Posts: 52
Re: Generator output what changed???
« Reply #3 on: December 12, 2014, 06:17:21 AM »
    Thanks guys sorry Flux no it is a Brush brand gen head . The table saw and compressor run fine on another small 3600 RPM screamer that I have I had also tried cleaning up the slip rings and brushes. 
  I will try to attach a diagram.

   
Sorry I could not get the diagram to attach but it is a simple aux field winding and a bridge rectifier, so by going by what ChrisOlson said I guess the aux field winding must be at fault.

   Thanks Chris

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Generator output what changed???
« Reply #4 on: December 12, 2014, 08:56:13 AM »
Yes , it will be a compounded alternator and built in an era where it mainly drove lights and predominately resistive loads. The motor starting capability will be poor. It will be fine with universal motors such as drills and grinders. Induction motors will be a challenge and any form of welder will be no go unless the alternator is way bigger than warranted for a SR1 engine.

Unless the alternator is 5kVA or bigger it will not do much single phase motor starting.

If the alternator has genuinely got worse rather than the motor capacitors suffering the only thing likely to change is the bridge rectifier in the compounding circuit. Some of the early ones used selenium rectifiers and none of them are likely to be much good now. If so replace with silicon. If it is already silicon you can check the individual diodes, one failing will virtually disable the compounding.

Do as Chris Olson suggests and check the voltage during a motor start, I am sure it will dip way below 110v and that may be the problem with a very long run up you may kill the motor starting capacitor.

If the alternator is working properly it will sustain nominal voltage with 120% full load resistive load as long as the engine speed holds up. If it droops with less than 100% full load resistive load and full engine speed the compounding rectifier is suspect. There are various big wire wound resistors in the machine, one will trim the no load voltage and that is fine, one may change the compounding and you may get an improvement playing with that one.

Try checking the voltage regulation at full kW resistive load, if that is wrong then find the fault and correct it. If that test works ok start suspecting motors. I suspect with a compounding machine you will struggle to get rated kVA below 0.8pf. During a start on a reasonable size induction motor you will be looking at 0.5pf or even worse.

I have also seen compounded machines fail to start a motor when a screaming little modern generator will walk away with it, such is progress. Change in technology to match modern demands.

Hope that gives you a starting point.

Flux

ChrisOlson

  • SuperHero Member
  • ******
  • Posts: 3642
  • Country: us
Re: Generator output what changed???
« Reply #5 on: December 12, 2014, 11:25:12 AM »
Sorry I could not get the diagram to attach but it is a simple aux field winding and a bridge rectifier, so by going by what ChrisOlson said I guess the aux field winding must be at fault.

I could be but I didn't realize it is an ancient shunted field unit.  More than likely you are getting a huge lagging power factor when it tries to start an inductive load due to aged components in the unit.  The junction in those old selenium diodes in the bridge is probably broken down with age so the field circuit is "leaking" and it won't respond to inductive loading with large lagging power factor.

In the long run, if you intend to keep running induction motors with it, you should look at replacing the gen head with a modern brushless, capacitor regulated unit like a ST5.  For starting induction motors the brushless capacitor regulated heads produce instantaneous field current that no brush-type AVR head can match.  I got one on a Robin diesel and that thing will start a 10hp single phase motor with no problem, while a similar AVR regulated head has the same problem as your Brush head where the lagging power factor causes the motor to stall and refuse to start because the voltage is lagging the current by more than 90 degrees.

christopher

  • Jr. Member
  • **
  • Posts: 52
Re: Generator output what changed???
« Reply #6 on: December 12, 2014, 11:31:06 AM »
   Thanks flux it is a silicon rectifier I will try to change this if that does not work I will play with the resistors I just do not understand why it worked fine for about 3 weeks and then slowly failed to work unless it is the rectifier or the windings shorting.
  Thanks for all the ideas.

   Chris

ChrisOlson

  • SuperHero Member
  • ******
  • Posts: 3642
  • Country: us
Re: Generator output what changed???
« Reply #7 on: December 12, 2014, 11:45:51 AM »
   Thanks flux it is a silicon rectifier I will try to change this if that does not work I will play with the resistors I just do not understand why it worked fine for about 3 weeks and then slowly failed to work unless it is the rectifier or the windings shorting.

If that's one of those old Brush generator heads that sort of looks like a bullet because of the rounded end on it, it seems to me I've seen some capacitors in there on the brush leads too when I had one apart several years ago.  I'm not sure what the purpose of those is (I would assume to maybe reduce RF noise from the brushes), but one of those could be bad and shorting the field to ground too.

christopher

  • Jr. Member
  • **
  • Posts: 52
Re: Generator output what changed???
« Reply #8 on: December 12, 2014, 11:52:36 AM »
  Thanks Chris it is not one of the bullet shaped ones but does have the noise suppression caps another thing to check can I just eliminate these? as I have no handy replacement and am not worried about noise.

 thanks Chris

ChrisOlson

  • SuperHero Member
  • ******
  • Posts: 3642
  • Country: us
Re: Generator output what changed???
« Reply #9 on: December 12, 2014, 11:54:36 AM »
If one of those is bad and leaking thru the can, you should be able to just take it out and it will still work fine.  I'm pretty sure all they do is smooth the current the brush set to reduce radio noise.

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Generator output what changed???
« Reply #10 on: December 12, 2014, 12:58:45 PM »
You can remove the RFI suppressor capacitors but failure is rare,

Similarly a partial short in the series winding is not impossible but very unlikely as the voltage is low.

Some Brush machines used 2 rectifiers and 2 series coils so look around in case it is one of those but the 2 rectifiers are usually obvious and often inside the brushgear covers.

Machines supplied to Lister as Start-O -Matic sometimes had a current transformer in the compensating circuit feeding the rectifier which again could be a source of failure. A failed rectifier could stress the CT secondary.

You should be able to check the compounding with a resistive load such as a bank of halogen lamps or an electric heater, if the engine doesn't droop the volts ought to hold within 3% at full load resistive, any droop much beyond this suggests trouble.

Flux

OperaHouse

  • Hero Member
  • *****
  • Posts: 1308
  • Country: us
Re: Generator output what changed???
« Reply #11 on: December 12, 2014, 04:07:41 PM »
A friend wanted me to fix his light tower generator.  I couldn't find anything wrong with it, but it put out low voltage.  Had a low voltage  high current AC supply so I flashed it on the AC side of the diode. It has been working for two years.   On his portable generators, three had melted brush holders.  Other than that the brushes an slip rings looked fine.  Found it interesting that the published life expectancy of replacement brushes was only 500 hours.

christopher

  • Jr. Member
  • **
  • Posts: 52
Re: Generator output what changed???
« Reply #12 on: December 17, 2014, 04:39:15 AM »
Thanks guys I finally got a chance to check the gen out, It was the bridge rectifier all is back to running fine. :)

    Chris

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Generator output what changed???
« Reply #13 on: December 17, 2014, 10:14:49 AM »
Thanks for the follow up, nice to have the occasional success story.

Flux

equiluxe

  • Jr. Member
  • **
  • Posts: 54
Re: Generator output what changed???
« Reply #14 on: March 07, 2015, 11:11:19 AM »
Back emf from the motors could cause the field rectifiers to blow, I have seen that happen many times. It is best to replace the bridge with a higher voltage rated one 400 or 500 volts.