O.T. Help with portable generator
During this past week, most of my family members were without power (brother is still out). I have a couple of generators that were pressed into service. I used the one in the RV (Onan Microlite) with an extension cord to help out the brother. A smaller "carry around" is a Coleman Powermate Pulse 1850. This generator will put out 125vac @60hz while driving a 3.5amp Milwaukee drill. When I try to drive a 7amp SHop-Vac, the voltage falls off to around 93vac. The hz remains constant at 60, so the rpm is not the issue. The throttle just barely has to open to keep the rpm up, so it is not hp limited. I even ran a much bigger shop vac that pulls 11amps, with the same indicated voltage drop.
Coleman says they sold off the Powermate line over 10 years ago to "Powermate Inc." and that Powermate is in Bankruptcy. Visits to www.Powermate.com has lots of hyperlinks for manuals, parts, and pricing that are no longer functional.
My armchair analysis leads me to believe I have a defective voltage regulator, although I don't know for sure the generator even has one or what it looks like. Assuming it has one, are these things somewhat generic? Can I go to my friendly generator repair shop and ask for a "voltage regulator" for my 120v 15amp generator? Are voltage regulators adjustable? (thinking of the old voltage regulators from the 50's cars). Seems like the old car VR's had a bat, field, & current terminals or some such. Maybe today they are just some little solid-state block with 3 terminals or somesuch.
Thanks in advance. Hoping someone is here that has an intimate understanding of these little generators.
Usually those machines dont have any voltage regulators.
But open up the control panel and take a pic anyway.
Ok, here are some pics. The first shows the front end still connected to the "control panel". The generator supplies 120vac as well as 12vdc. The 2 pigtails off the windings are feeding the two differnt types of outlets (dc & ac). The second pic shows the front end without being connected to the panel. The closeups show the 4-way bridge rectifier for the 12vdc outlet and the device that I believe is used as a voltage regulator. The printed data suggests that it is some sort of capacitor made by Teapo with a value of 7.5uf and 350v. Can a capacitor act as a voltage regulator?
I MAY have found the answer. On a website that sells replacement parts, I found the following information:
Does your Coleman Powermate Generator have a voltage regulator?
Why do some generators have a voltage regulator and some generators do not?
Voltage Regulators have the ability to detect proper output voltage. If the proper voltage is not present, then the regulator will adjust the Rotor Excitation so that a higher voltage comes out of the Stator Power Winding. This component is typically put on more expensive units requiring closer output tolerances.
How is voltage regulated if my unit does not have a regulator?
Units without this type of control are regulated by Engine Governed RPM. A specified higher "no load" RPM setting is made subsequently producing a higher output level. When a load is placed on the engine, the engine rpm drops to a preferred rpm. This "loaded" engine rpm being maintained by the engine governor will produce voltage at a desireable output level.
from - http://www.arkansas-ope.com/COLEMAN_GENERATORS.html
Wouldn't running at a higher "loaded" rpm cause and increase in frequency? I can certainly adjust the govenor to maintain a higher speed, but I am concerned about the freq.
I also found this information:
"Because the capacitor acts like the AVR, it has to be close to the stated specification. If you notice on your generators, the value will be something like 33 uF (micro farad) with a small tolerance of about 5%. Over time, elecrolytic cap's dry out and the values change. This is typically seen by the voltage rolling off over time, but the revs are correct. (or the voltage is rolling off under a specified load)
The combination of the capacitor value, the impedence of the windings, 3000/3600 RPM forms to set the specified voltage. If one of these factors changes, the the voltage will be different to the specification.
eg: Low revs, right cap value = low voltage,
Low cap value, right revs = low voltage.
The accurate way to test a capacitor for the correct value is to use an "LCR bridge" to measure the capacitor value at a 3000/3600 Hz."
This guy is suggesting the capacitor is the "voltage regulator" and that they tend to lose capacity over time. The capacitor on my generator was made in 1993, so it could be at fault....
It may be the generator is rated in resistive load but can't take the start amps of an inductive type motor load. I have a welder/generator rated at 9KW on the power side. Thats 37amps @240 volts. It will weld 250 amps all day long, but when it comes to starting my 1hp motor on my air handler it will bog down the gas engine. Forget about the 3.5 hp motor on airconditioner compressor, doesn't even get it turning. If there was a battery pak or capacitor bank to boost for the starting load I think the amps are there to keep a motor going. From talking with other folks during power otages this seems to be characteristic of these small generators.
Sounds like a typical brushless setup. The capacitor feeds a portion of the generator output back to the exciter windings, which in turn induces power in a set of special windings in the rotor. These rotor windings are hooked up to rectifier diodes which feed power to the main field windings, and thus controls the output of the main stator windings.
If the capacitor degrades, or one or more of the rectifier diodes blows, generator output will be significantly affected.
That is a film type capacitor, which as a rule is very stable and long-lived, unlike the electrolytic type mentioned in your quote.
However, since it is so small, it probably is a "metallized film" type. Those can degrade if they get hit with higher voltage spikes, since their "self-healing" mode is to actually blast some of the metal coating off the plastic film. That usually just loses a small spot, but it is possible for several "hits" to cause a cut right across the film, cutting off some of the coating, and reducing the capacity.
Many digital meters have a capacitor checking function, use it if yours does and see what it says (you must disconnect one lead of the wiring). if OK, cap is not the problem, if NOT OK you know what to do.
You are correct that a higher rpm would throw off the frequency.
Your problem is that the "series impedance" is such as to drop too much voltage.
I am not clear on the higher power shop vac...... it also had 93V? I don't regard a shop vac as calibrated test equipment, so it is not very possible to determine what is up via that method. Inrush currents etc throw it off.
My suggestion is to get a bunch of 100W bulbs, and connect them one at a time, checking voltage once they have come up to brightness.
See if the 7A worth (about 8 bulbs) is still at 93 V. if so, you have some regulation problem.
If NOT, the genset does not "like" shop-vacs.......
Some gensets are "tuned" by a capacitor and if overloaded they WILL drop voltage. Motor starting is an overload, but a small motor may be OK.
If they can't get the motor going fast enough to develop good back EMF, they may NEVER crawl up to rated voltage. The fact that "apparently" your genset has the same droop with what seems to be two different loads suggests the tuning may be the culprit. Either bad parts, OR that system simply does not like heavy start loads.
Check the cap, and if it is OK, and no wiring issues are around, that genset may simply not run those motors well.
If you KNOW it used to run them OK, all bets are off, and you simply have not found the problem.
The HP issue suggests that you may not have found the problem. The exciter diodes can be an issue, wiring can be an issue. much of that stuff is on the rotor and hard to get into on many gensets.
The fact that you have good voltage under some loads is a 'counter suggestion" that there are no bad diodes etc, since bad (low) exciter ampere turns generally results in low voltage. If the feedback system were defective, one might expect bad exciter MMF and low volts even with lighter loads.
The original design considerations for this are still a bit opaque to me, but I do know
that you can test each component directly and if one is off spec, that can be fixed
and you can check to see if it improves matters.
So I would test:
1) all windings for continuity and isolatation from the frame.
2) test the diodes in the bridge to be sure that each one conducts in the
forward direction and is open in the reverse direction.
3) measure the capacitor with a meter (many of the newer DVMs have capacitor
check function) or replace with a known good unit.
I'm not so sure I agree. If you had a bad exciter diode in the bridge, there might still be sufficient field current to generate correct output voltage at no/low load, but insufficient muscle to keep the voltage regulated as demand increased. I'd imagine you'd probably see a reasonable output voltage until you hit maybe 50% rated capacity, then rapid dropoff after that.
Originally Posted by JST
The voltage is related to the rpm of the generator, AND the magnetic field established by the wound field.
Originally Posted by gnorbury
I presume that if a diode is bad (open) the current in the field windings will not be as much as with it in place and working. So, the magnetic field would have to be less with a bad diode.
How MUCH less is not known, but it will be LOTS less for a shorted diode, and "somewhat" less for an open.
That being the case, the unloaded voltage would have to be less with a bad diode, even if only a fairly small amount less (proportional to the reduced field).
Since he has 125VAC, that does not sound like "less" and argues against a bad diode.
I suppose that if it for some reason still works just as well with a bad diode, then it might do somewhat as you say. But then it seems they might have an "extra" diode, and be able to lower costs even more by removing it (and maybe it's winding too).
voltage is not directly related to rpm in all generators. frequency is. capacitors are often used for voltage regulation. it could be that your cap is puking out
often brushless units have a diode glued right into the rotor
Since we are talking about a closed loop feedback system, the "regulator" (capacitor) will attempt to balance the power back to the field windings to maintain stable output voltage. The output voltage remains in spec until there is no more power available to drive the field any harder. Blowing a diode on the rotor would effectively cut the available field power in half (assuming it was a bridge rectifier configuration), so I would anticipate seeing adequate voltage regulation only when the genset was lightly loaded. A shorted diode would cause even more problems than an open one due to the inability to maintain proper field polarity.
Originally Posted by JST
Having said all that, a capacitor problem is far more likely.
As long as the regulation is not calling for full excitation (lightly loaded unit) it
will probably maintain voltage.
But why guess on this? Like I said, test the bridge and the capacitor for specific
With a given magnetic field, it surely IS related to rpm....... that is basic physics, you cannot escape it.
Originally Posted by gpkull
With some feedback, the relation may be affected by feedback....... because the feedback affects the magnetic field. The issue is dependent on the "range" of feedback possible. if it can correct for a bad diode, at least partially, then voltage might drop to some intermediate voltage with 'some" load, and drastically with "more" load.
I'm not convinced it could compensate for a bad diode 100% even at lighter loads..... But I don't think the total power capacity has been mentioned, so we don't know what a 7A load is in the scale of things. Presumably the genset is at least 1200 or 1800 watts, maybe more. in thatc case, 7A could be anything from a substantial part of full power to maybe 20% of full power.
Since we are already suggesting testing the cap, which appears to be the only feedback regulator, the whole feedback issue is separate....
it is possible that the feedback is correcting for light loads and not enough for heavier. I already suggested a light bulb load test, which will definitely determine whether there is a basic problem of not enough juice, be that from regulation or something else, vs an issue of a type of load it cannot handle.
Generators which have the field dependent on the output voltage are susceptible to "breaking down" of voltage on a short or low load......... the short reduces output, which reduces the field, which reduces output..... and it drops down, to a lower voltage, or to zero. It is the reverse of the "build-up"that occurs at startup, and is why a separate exciter is used on most generators.
Some generators with a capacitor feedback system use a fancy "parametric" control system. I am not familiar with all the problems that can occur with that type.
Others have a "base field" and a system which does an adjustment to that, up or down, dependent on load. In that case, if feedback fails, the base field will provide a pretty constant voltage that is typically lower than full output voltage. The two different loads that BOTH gave 93V is a point in favor of that system.
Do the tests, that is the way to find the problem. We could type suggestions until the wee hours, but we are speculating, since we have no access to the machine.
I have ordered a new capacitor since, according to the generator-type websites, these capacitors do degrade with time. It is about $20 with shipping and should arrive this week. These Coleman generators are looked at with some incredulity, since everyone seems to be amazed they work as well as they do with so few components.
If they do degrade, it is because they were not designed-in well.... Presumably they carry too much current, or are under-rated for voltage, or over-heated in the unit. Otherwise they would work for a long time.
I am somewhat amazed that there would be a problem with a film cap in that application...... capacitors work well for many years in power systems when properly selected and used.
I suspect that unit in question is extra-cheaply made...... I missed the "Coleman" brand name before, that may say it all.
Most likely the rectifier. There is a permanent magnet that generates low current on startup. This current is then used to feed the main field that will generate the the full load current. IF I remember correctlly, the rectifier converts the AC to DC to drive the field.
So basically, you are running off the voltage and AC current from the permanent magnets which are a low percentage of max output.
I had a similar issue on an old wards 8HP 4KW. Swapped the rectifier and fixed.
Get the part number off the rectifier and look it up at www.nteinc.com then find a local dealer, maybe Grainger. Should be much cheaper than a generator parts dealer. They are somewhat standard, since they are very simple and just differ for current rating and packaging.
Well, I replaced the capacitor when it arrived a couple of days ago - NO CHANGE! Today I swapped out the 4-way bridge rectifier - NO CHANGE! I changed my load to an resistance heater that has a 750w & 1500w switch. When driving the 750w load, the generator puts out close to 120vac; when sswitched to the 1500w load, it drops to around 80vac. Indictive loads from the 2 shop vacs produce the same result as a week ago. The DC output for charging batteries shows an open voltage of around 17vdc. I'm ready to throw this POS out and buy something else, but I hate to be beat by something like this. It starts, it runs, it puts out acceptable voltage up to around 7amps then the volatge drop renders it useless. It is supposed to give me about 15amps, and I want it to do what it was designed to do!
Making progress. The 4 way bridge rectifier seems to be only for the auxillary charging circuit. The field diodes were buried inside. I removed them completely to check them out. First, the windings do not appear to be shorted, open, or shorted to the case in any way. The diodes both behave in opposite, but unexpected ways. They both have a resistor tied across the diode. I would expect little or no resistance when running the diode forward, and I think I should see the value of the resistor when running backward. Now the results: one diode reads open both ways (maybe blew the resistor as well), the other diode reads about 4.4ohms both ways.
The resistors appear to be 21k ohms since the bands look like Red, Brown, Orange, & Gold and the diodes are Motorola MR760 (6amps 1000v)
Since no readings match the "resistor expected" reading, I think I have one shorted diode and one open diode, with a bad resistor as well.
Any comments on my reasoning?
Last edited by Jim Caudill; 09-28-2008 at 08:29 PM.
Your logic seems OK to me. I have been thinking about this "DC" energising stuff and its not very clear how it works. If the genny runs at 60 Hz then the speed of rotation is fixed so the output voltage then depends on the DC energising current. My problem is that if the output voltage is coupled back to the diodes via a capacitor ( impedance about 330 at 60Hz), then an increase in the output voltage results in more current through the capacitor and one would think more DC current, which would lead to more output voltage. . . !
The above makes me think that either the DC derived field is opposing some permanent magnetic field or that the added DC is making the armature core saturate, so its magnetic efficiency goes down or possiblely there is some otherway which I have not thought of