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?

 

[davidbeach]

Is the field in the right direction?

 

[guydewdney]

yes - its bi-directional, and so works the same both ways...

 

[Skogsgurra]

The power used for excitation is not to be ignored. Some motors used to have up to 5 % excitation power compared to rated power. That may not be the case here, but is a possibility.

Even with 2 % excitation power, you will have the same nominal excitation power as the useful output from the generator.

Friction losses and windage losses can be rather large in old machines. Bearing conditions may have a part in the low efficiency. If you run 50 HP, the losses will not be large in comparison. But at 1 Hp, that's quite another story.

Also, if there is a compound (series) winding, make sure it is correctly connected.


Gunnar Englund

http://www.gke.org

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

 

[guydewdney]

The field is 60 to 100 watts (as measured with a plugin type meter)....

Bearings arnt good - Ill accept that - its a little noisy, and a lot less when I greased it..

heres a similar motor to what I have:-

http://www.reliance.com/cgi-bin/dcexplode.pl?D5150R-BV

its not 'old' particuarly.

Its STABSHUNT not series... but you would be right

 

[waross]

According to the spec sheet in the link you gave, the field dissipates 933 watts when hot. More when cold.
You may not need the filter capacitors. The inductance of the field is a pretty good filter.

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

 

[MikeHalloran]

Is the motor's fan going in the correct direction?



Mike Halloran
Pembroke Pines, FL, USA

 

[edison123]

Regardless of the generator load, the shunt field is fully energized and will be dissipating heat continously. What is the field current you are seeing at 285 V DC ?

I would remove the series field (STABSHUNT is a series field winding) from the circuit for the generator operation.

 

[Skogsgurra]

"field is 60 to 100 watts"

How do you mean? Does it change that much? It ought to be rather stable. And about ten times as much as that. Around 1000 W, I would think.

Are you sure that your data are right?

Gunnar Englund

http://www.gke.org

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

 

[waross]

Is it possible that the capacitors and the inductance of the field are creating a tuned circuit and the resulting higher frequency AC is heating the field poles with eddy currents? Remember the field poles may be solid rather than laminated and so be susceptible to AC induced eddy currents.

The field is 60 to 100 watts (as measured with a plugin type meter)....

Based on the information that you have provided, this does not make sense. It should be 600 W to 1000 W.

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

 

[Skogsgurra]

No Bill. That doesn't happen. But the capacitors could/should be removed. Peak rectification adds abt 40 % to voltage.

Gunnar Englund

http://www.gke.org

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

 

[guydewdney]

sorry - this is a little closer to what I have. The actual version isn't listed, as its a special order item.

http://www.reliance.com/cgi-bin/dcexplode.pl?D5050R

I have connections for:-
field (F1 and F2)
Armature (A1 and A2)
And series? stab? field (S1 and S2)

I believe that S1 and S2 are large cable - a similar size to the armature cables - and thus a bit irrelevant for me as they are the stabilising field - I don't need a precise speed control, which is what I believe this is for.....

The field cables are tiny, and powered by my dodgy rectified mains setup. The 60 to 100 watts range is dependant upon speed, temp, and output current / voltage - I have not examined the relationship. At any given moment, the power consumption is steady.

Please bear with me - I'm working from home, on a limited budget, on a 100+ year old waterwheel that takes 45 minutes to stop, and half an hour to start again....

I cannot see a way of removing the series coil, if indeed I have one. Ill check again (the casing is boiling hot, so its not easy...)

Thanks again people.

 

[guydewdney]

<scurries off and opens case to look at connection diagram>

OK - its connection version 406770-1 found here

http://www.reliance.com/pdf/pdf/dccd/406770-1.pdf

I do NOT have the optional AX1 connection to remove the 'inter-coil'

I have NOT connected A2 to S1 for series field - S1 and S2 are open circuit.

I have turned off the water - and am waiting for it to slow enough so I can turn off the field winding power (it speeds up dangerously when I do this)

Guy

 

[guydewdney]

Sorry to double post - cant see the edit button...

Right - have now disconnected the capacitors, and the field power meter is now reading 500 watts - I guess it has a very poor power factor correction system..

 

[edison123]

You should not remove the interpole coil AX2-A2. Do not connect A2 to either of the series field (S1-S2 or S3-S4), Both the series fields should not be in circuit.

Give Shunt field supply to F1 & F2. Take the generator voltage from A1 & A2.

If the output is connected to a dc battery system, you have to isolate the generator from the battery system and then only switch off the field power. If you remove the field power before you isolate the generator from the dc battery system, the machine will work as a motor with no field current and will race to destruction.

 

[itsmoked]

guydewdney; You just HAVE TO provide us with a picture or 5 at some point. I am dying to see this.

Water works running a generator. Like a little bit of heaven.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

"have now disconnected the capacitors, and the field power meter is now reading 500 watts"

Good! That is more likely. Your 60 - 100 W were probably due to a misplaced decimal point. And, remember, this is DC. No power factor involved.

Another thing that seems very peculiar: You say that "it speeds up dangerously" when you remove field winding power. Again, I do not understand. Is it a motor or a generator? A motor speeds up if you remove the shunt field and have series field connected. You have a generator and the series field is not connected. Have you really experienced that "speeding up dangerously"?

(For all that want to tell me about it: Yes, I know that DC motors and DC generators are the same thing )

Gunnar Englund

http://www.gke.org

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

 

[flexoprinting]

"Bearings arnt good - Ill accept that - its a little noisy, and a lot less when I greased it.."



Hold up here! Bearings should not be noisy at all, Look here first.

Regards

 

[waross]

I hope that you have a diode between the generator and the battery bank. If you are connected to the battery bank without a diode and the generated voltage is low the batteries will try to run the generator as a motor and it will get hot.
Further to Gunnar's comment re Motors and generators being the same, I consider the difference between a DC motor and DC generator to be only a few volts or a few RPM. If a DC motor is driven over speed by the load, a small percentage increase in the speed will cause it to generate and push power back into the supply. Likewise, if a DC machine is driven at a constant speed, the voltage (which may be controlled by the field strength) only has to change a few volts to go from generating to motoring.
With a proper diode in the circuit the output current will be zero until the generated voltage exceeds the battery voltage and the machine will not waste power by trying to motor.
BTW, field voltage at 280 V rather than 300V will but the speed at which the machine changes from a motor into a generator about 300/280 or 7% higher. That would push your minimum speed up to 1100 RPM x 300/280 or 1179 RPM.


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

 

[guydewdney]

Theres no battery bank - it runs, as in the first post - to a grid tie inverter. It cannot 'motor' and as such, there are no diodes.

Yes - it speeds up when the field power is disconnected - I have seen over 1500 rpm (rated is 1100). This is due to it unloading the driver (water wheel) and allowing it to free spin.

I have found

http://www.reliance.com/pdf/instruction_manuals/dc/C3088.pdf

this document, which relates to DC motors in this range. It suggests connecting A1 to S2 and using A2 and S1 as the power connections. I am trying this next, along with a self exciting field winding, as I appear to be generating minus 400 watts (taking more to run the field winding than it generates.... doh...)

Im not going to replace the slightly rumbly bearings until I have proven the principle - this thing is in my house, down steps, and is incredibly heavy. If I can get some sensible power from it, then I will replace the bearings. If I can't, then I have saved myself a few hundred quid on some massive bearings and hours of time.

60-100 watts is measured using an AC plug-in houshold energy meter, the sort of thing sold by 'eco' companies. It is a cheapo version, and thats why the PF was so far out. With no caps in the circuit, it may be behaving itself, and reading roughly correct.

This isn't precision engineering - and theres no manuals for water wheels or Victorian layshaft bearing losses - so theres a lot of guesswork.....

Guy