DC motor as generator, overheating. 1

[guydewdney]

Hi - 1st post - be gentle

I have an old water mill, with a large 8 tonne iron water wheel on the side.

This drives a chain / belt / gearbox setup that drives a generator.

Due to the fun of waterwheels, I have decided to go down the route of DC->Inverter->grid setup - but thats not the problem.

I have approx 2kw worth of water flow (variable) - and the generator is producing 800 odd watts (output) - but the casing is at 90+ degrees centigrade. This is half a ton of 50Hp reliance / Baldor motor....

Motor spec on the plate:-
50Hp, 1100 RPM, 300vDC Field, 500V armature.
Running at:-
1Hp output(yes, one), 1100rpm, 280VDC field, 410VDC arm

Field is created by taking 240V AC, rectifying, and smoothing with some random big capacitors I have kicking about (6, motor start jobbies - 30uF each)

why oh why oh why is it making more heat than electricity?

 

[waross]

If the generator speeds up that much when the load is removed (by de-energizing the field) then it is probably generating much more than your instruments indicate.
Does the generator speed up when you disconnect the inverter?
You may have internal faults in the generator which are loading the water wheel down and heating the generator internally. If the generator speeds up when you disconnect the field but doesn't speed up when you disconnect the inverter with the field still energized you probably have internal problems.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

It is good that this is off-hours. Site rules do not allow us to discuss non-professional issues. But on week-ends, I think it is OK.

As Waross says. If it speeds up when unloaded, it is probably producing a lot more power than you think. The problem may be in your measuring equipment. Get yourself a decent DC clamp-on and a voltmeter. I am not at all sure that the 'eco' meter is good at DC.

And, please, do not mention power factor when discussing DC. It simply does not sxist, has no meaning at all, and makes the yhread look "dangerously unprofessional" to site administrators.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

I think my typing reveals how upset I was. Read "exist" and "thread".

Your place sounds quite interesting, Guydewdeny. I would love to see it. I can bring all the instrumentation you need. All I ask is a cool beer. Where in the world is it?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[guydewdney]

Skogsgurra - power factor does exist with AC systems. The field (was) powered by the AC 240v 50Hz UK mains. This went through a rectifier (bridge). The eco meter (something similar here

http://cgi.ebay.co.uk/Ecosaver-Elec...66:2|39:1|72:1683|240:1308|301:1|293:1|294:50

)is plugged into the AC mains - not DC.

Anyway - powering the field from the armature appears to have worked - it uses the residual magnetism to generate a small voltage, which excites the field, which creates a larger armature voltage, and over a few minutes (I was surprised at how long it took...) it reaches a steady state of 180-odd volts DC, freewheeling.

Loading the generator using the PC configurable grid tie inverter, which naturally slows the genny, I now have 260 odd watts from somthing like 20 litres per second, over 4.5 metre head (theoretical 9.81*4.5*20 = 880 watts, so a total efficiency of 30% water to exported electricity)

Better than nothing. I now have V belt slippage to fight with! (the torque is incredible - somthing like 38,000ftLbs when all the buckets are full.)

As for professional / non professional discussion - wheres the grey line? I have an honours degree in Mechanical Engineering... but I was useless at the motors bit And people have asked me to help 'fix' their waterwheels for money - which makes me a professional water wheel electricity generation installer...

Guy

 

[waross]

Where's the grey line?? It's quite wide and it moves around from time to time.
I suspect that a lot of your lost energy (poor efficiency) may be in the drive train.
I would love to see your site also, but at the moment I can't afford to visit anyplace that fit's the description "The field (was) powered by the AC 240v 50Hz UK mains."
Can you post some pictures?
And I agree with Gunnar, even disregarding AC/DC/power factor issues, look for better instrumentation.
You may be able to use the interpole winding as a current shunt.
Even though there is not a lead from the interpole internal connection, the interpole winding will be connected to one set of brushes. If you are able to accurately measure the resistance of the interpole, you may determine the current by measuring the voltage drop across the interpole. Voltage/Resistance will give you current. The accuracy is temperature dependent so check the resistance of the interpole winding when the machine is at operating temperature. You can probably do this with a medium quality digital multi-meter.
If Gunnar pays a visit we want even more pictures.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

UK! That's a lot closer than USA. And, if you are in the Lake District, I will probably be there to visit a paper mill soon. Pud and Scotty live there, too. Plan to see them as well. You may have to buy more than one beer...

About professional vs non-professional. I thought I should mention that issue. The site is rather enormous and there is a problem keeping it useful. So, "homework" (both student's and dabbling in fields of non-expertise) are usually red flagged. Anyhow, this thread has brought us an interesting insight in hydroelectric plants. And a new friend. So, I think it is safe.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[guydewdney]

I'm afraid I am at the other end of England - sunny Somerset.

I am interested in this statement

You may be able to use the interpole winding as a current shunt.
Even though there is not a lead from the interpole internal connection, the interpole winding will be connected to one set of brushes. If you are able to accurately measure the resistance of the interpole, you may determine the current by measuring the voltage drop across the interpole.

I see no reference to an interpole winding - is this another name for the 'intercoil' referred to in the documentation?

Thanks

guy

 

[ScottyUK]

Skogs,

I'm not in the Lake District but you'll pass not far from me on your way there. Harry is a bit further north in the civilised bit of the country - I guess you will be using the ferry to Newcastle? Would love to meet up for a beer, I think you can find my contact details elsewhere.




----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

Yes I was referring to the intercoil winding. The resistance of the armature circuit is less than 0.5 Ohms, the intercoil winding will be just a part of this. The voltage will be quite low, possibly less than one volt at light loading. Even if you can not determine the resistance with adequate precision you will be able to get a voltage proportional to the current or loading by measuring across this coil. This may be useful.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[starkopete]

Hi,
If this is a motor now being driven as a generator as stated, your interpoles will be the wrong polarity in relation to the direction of rotation and the field polarity. You will have to reverse the interpole polarity in relation to the armature polarity (this can usually be done on the brush holder connections). I don't know if this is the source of your heat problem but it should be corrected.

Thanks

 

[edison123]

starkopete

you might want to rethink that.

 

[waross]

I agree with edison123.
The series poles are not in use. We are discussing the commutating poles.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Yes, changing polarity of the interpoles will produce heavy sparking and shorten brush and commutator life. Leave them as is.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[guydewdney]

Well - that was fun. It was a friends birthday last night, so we all went to the pub. Much Cider was consumed.

At 3AM the wheel starts 'making strange noises' according to my painfully sober wife.

Still not really safe to operate heavy machinery, I attempted to investigate, but failed.

This morning reveals worn out brushes, and a slight hangover.

The series field winding IS now in circuit - the C3088.pdf file linked to above shows to wire it differently according to shaft rotation direction, and wether it is a generator or motor. It is rotating CCW when looked at from commutator end, and is wired A1 to S1, A2 is +ve and S2 is -Ve

As for pics - we have a general website -

http://www.gotopia.co.uk

heres a pic of the wheel spinning for the first time is 40 odd years

http://files.engineering.com/getfil...eb3-4bdc-a14d-7e36a43ebccc&file=spinning1.JPG

 

[Skogsgurra]

Hey! That looks like a lot more than 1 HP! No wonder your machine is running hot.

If you make this properly, I guess that you can make good money out of it.

Keep us posted once your hangover is over.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Here is a check that may be made with just a voltmeter.
With your machine disconnected, apply 120 VAC to the shunt field. Measure the AC voltage at the brushes. It should be zero volts. If it is not zero volts, rotate the brush holders until you do read zero volts across the brushes.
This is your initial brush position.
Now rotate the machine slowly while watching the voltage across the brushes. It should remain at zero. If the voltage at the brushes varies when the armature is rotated, you have a strong indication of possible shorted or grounded turns in the armature.
Now remove the 120 VAC from the field. If you have sparking at one edge of the brushes when running normally, shift the brushes slightly to cover the sparks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[guydewdney]

Thank you waruss

I take it 240 VAC will be just as suitable? The brushholder carrier ring (plastic) is held by 4 bolts, and is easy to swing round for brush replacement (so you can get to the bottom ones etc) - I take it this is what you refer to?

There are marks on the edge of it - much like timing marks on a car / bike crankshaft - but nothing to align it to! I had to move the carrier to access the brushes, and put it back in the same place, but I shall do your test at the next opportunity, subject to confirmation that what I think you mean is correct.


On another note - I have now put in series a 500 watt halogen security light to reduce the field voltage, which is about the best I can cobble together at home with all the shops shut...

Guy

 

[waross]

Because of the inductive reactance of the field it will draw much less current with AC than with DC. 240 VAC should be OK on a field rated for 300 VDC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

guydewdney, I believe that your idea of why the water wheel/generator speeds up when the field is disconnected is correct. Specifically, the power produced by the generator necessarily reduces the speed of the wheel. You are taking power from the wheel. Once the field is removed, you stop taking power from the wheel and it speeds up.

For a shunt wound generator (which you have), the power output of the generator would be reduced to almost nothing once the field is turned off. The residual magnetism of the field poles will produce a voltage that is proportional to speed, but that voltage should be small and, without excitation (field), should decay to nothing if an actual load is present. In other words, when you turn off the field, the load presented to the water wheel should turn of as well.

With respect to the temperature of the DC generator (or a DC motor), the fields will generate heat continually regardless of load. The temperature rating that your motor/generator can sustain continuously without damage is on the data sheet you provided and is Class F = 155C. The outside temperature will often be 20C or more than the inside temperature (which the rating is based on), so take this into consideration.

Next, the power output from your generator is obviously in question. I do not know how your inverter operates. Most commercial inverters are designed to convert AC to DC and may include circuitry to restict power flow in the other direction. Assuming that your inverter is designed for regeneration (DC to AC), then the amount of power that you pass to the line will be proportional to the difference between the rectified voltage and the line voltage. Higher rectified voltage means higher power output to the line.

Finally, I am curious to know; are your power output readings based on the net power produced at the utility meter, meaning total generator power output minus your internal consumption, or is the total output 800W gross? If your readings are net power to the utility minus your consumtion, then your actual generator output is obviously higher than you think.

Sorry for the long post but I got carried away. I hope this helps you.

 

[itsmoked]

The outside temperature will often be 20C or more than the inside temperature

The other way round..

Keith Cress
kcress -

http://www.flaminsystems.com