Heat

[Ian]

I grid tie the output from my 7.5 KVA ST alternator. I have tried two 7.5 KVA ST alternators on otherwise the same equipment. As I am based in UK, the alternator outputs are nominal 230 volts single phase 50Hz.

I full wave rectify the outputs and feed the resultant DC into the grid tie inverter. The inverter squirts about 4 kW into the grid. This is quite a demanding load for the alternators as power is only drawn at the peaks of the waveform. Whilst the rated current output is 32 amps, I suspect that the instantaneous output at the peaks will be maybe 100 amps or more due to the capacitors looking almost like a short circuit at this point.

Both alternators manage to happily supply this power all day but one of the alternators runs significantly hotter than the other under the same conditions. The cool running one has a case temperature of about 10 degrees C (maybe 20 degrees F) above ambient but the hotter one probably has a case temperature of about 30 degrees C (55F?) above ambient; it feels much hotter.

Both alternators have the same stator winding cross sectional areas and they are both equally clean with nothing to impede airflow.

Does anyone have any idea why one alternator should run significantly hotter than a similar one under these conditions ?

Regards,
Ian

[Doug]

At least 1/2 of the heat in a small machine like this is caused by eddy currents in the iron.
The fastest and sneakiest way to cut corners on a machine is reduce the amount of iron and raise the core density. Sometimes bad iron itself is the problem, but itsvery hard to say whats going onand why without completely stripping the machine a checking all the numbers on iron, air and copper,

Doug

[rcavictim]

Maybe one is all copper wound and the hotter one was compromised at the factory by being would with aluminum wire?  Or even worse with aluminium wire?   

[Ian]

rcavictim - I have had both in pieces at some time or other and I can confirm that all the windings on the stator and rotor are copper.

Regards,
Ian

[Doug]

You would be amazed by how much difference eddy currents can make in the laminations.....

Generaly relay steel has a light coat of oxides on them that seems to be enough to keep things in check and combined with a dip in the varnish tank generaly things aren't a probelm. However if there has been somehting done to disturb the laminations and get shiny metal against shiny metal you can generate a lot of localized heat.

I Used to have to run a die ginder in the slots of motors that have blown windings to remove the copper that might short circut the laminations. One of the tests I would do to check iron was several wraps heavy cable threw a stator and connected to a AC power suply. As It warmed up I would feel for hot spots and grind away material where needed or beat the crap out of the core with an hard wood block and sledge hammer to try and shift things/ break the connections between the shorting laminations.

You had to check these things if you didn't and went ahead and rewound a machine without checking sometimes you got a core with bad iron that would run hot and die fast....

[Jim Mc]

Perhaps the output impedances of the two machines differ enough to affect the current waveform.  What's the RMS current measure in each?  Any difference?  Maybe one has a more effective cooling fan? 

[Doug]

Syncronous machines all have a high output impedance ( Syncronous reactance )

There are several reactive forces at play inside, inductance or the windings armature reaction ect.

This is why the load regualtion is so much easier on a Dynamo VS a Sycrounous generator, and also why we have such large voltage swings with changes in power factor.

If I was realy going to test this and find out why first step would be a bridge measurement of the winding resistance, surge comparison of the coils ( look for internal shorts and other defects ) and an open core excitation test ( energize the open core at low voltage and check the PF of one against the other ).

Then of course there is the obvious and simple things to check as Jim pointed out cooling fan lol....

Doug

Doug

[Ian]

Great, more good ideas, many thanks.

I do not have a clue how to measure the impedance - is it easy to do ? What can it tell ? What can I do to adjust it ?

The output current from both machines is 17 amps (at about 263 volts ACrms and about 60 Hz).

I have measured the static resistance of these generators whilst I previously had them apart...

Rotor (field) coil approx 17.1 ohms
Stator coil < 1 ohm
Harmonic winding <5 ohms

Amazingly, both generators had exactly the same winding resistances within the repeatability of measurement with my digital multimeter.

Both generators have the same diameter cast aluminium alloy fans with the same number of fins. The castings are poor quality with a lot of roughness to the fins. It is possible that one fan is better at shifting air than the other. Any ideas on how to measure this ? Both the fans "feel" to be shifting around the same amount of air. The slip-ring end screen and the fan-end finger screens are identical in free air flow area. The generator that runs hotter has slightly less opening area into the doghouse and the doghouse sits closer to the generator body - which gives less free open air path than the cooler runninmg generator (which has a larger doghouse which also sits about an inch higher over the generator).

The eddy current view is interesting. I thought that the individual laminations of the stator and the rotor were pre-lacquered before assembly and then the whole thing dip lacquered again once assembled. If there has not been an obvious fault, is it likely that this lacquer system might have broken down ?

Would there be any benefit in me applying a spray acrylic lacquer to the rotor and stator coils and laminations in a bid to "shore-up" the existing lacquer system or would this be counterproductive ?

Regards,
Ian

[Doug]

Some high end machines probably sprayt something on the steel but most don't anymore, the natural oxides on the iron surface after heat treating and what everr varnish wicks in between is enough.

Yes coating the entire machine iner workings is a good idea, I feel the windings on all the machines I have seen pitcures of and my own included seems very thinned and only dipped and baked once.

http://www.putfile.com/pic.php?img=5312668

The product I used is an electrical finnish developed by GE called Glyptal 1201A.
Don't use any product on a winding not specificaly made for it. They are generaly known as electrical finnish or air dry varnish and should have a minimum class F insulation rating.

A more expensive alternative is to have the machine hot dipped and rebaked in a motor repair/winding shop.