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?

 

[ScottyUK]

Bear in mind that even if there is no external fan there is usually an internal one to cool the rotor and move air through the stator. If you were to apply the field continuously to a stationary machine then the windings might well overheat without the internal air movement to help cool them.


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

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

 

[rhatcher]

itsmoked is right, my post should have read: The outside temperature will often be 20C or more less than the inside temperature (which the rating is based on), so take this into consideration.

Thanks for the correction itsmoked. BTW, I like the name.

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[guydewdney]

My power readings are from the Grid Tie Inverter (GTI).

It has power in, power out, voltage in / out etc etc readings. Its made by Power-one / Magnatek, and is called a PVI-3600-UK (its based on a photovoltaic product, 3.6kw, and configured for the UK - obv an engineer came up with the name )

As the genny is now self excited, the power reading is total out.

I have discovered a document detailing the current / efficiency curve of the motor, and it suggests that at the 5amp level that I am at (and cannot go any higher - the GTI has 6A fuses...) the efficiency is somthing like 30% I think its just toooo big for the application. If I could get the armature amps up to somthing like 30A, then efficiency is suddenly 80+% -its almost a vertical line at the low end, then horizontal from 30A to max.

Anyone got a 15Hp / 10Kw 500V DC motor kicking about?

Next thing Im going to try - feeding the F1F2 field with rectified and adjustable mains via a Variac (ah - the joys of eBay..). I had a long chat with the engineers at Cummins, and they seemed to think that an AC generator would be a Bad Idea (TM) as the 'load' is highly capacitive due to the smoothing caps in the inverter. So Im going to try approx 120V from the Variac and slowly increase it until somthing interesting happens

We shall see.....

 

[guydewdney]

Waross - I have performed the check you suggested - I read 0.2VAc on the brushes - I assume this is within tolerance (or I might well be missing the point, and its the last 0.001V that matters

Varistors arrived today to protect the end user(s) from large back emf's due to grid shutdown / fuse blowing...

 

[waross]

You should be able to get zero volts by moving the brushes slightly. This is the initial position for the brushes. If the brushes spark slightly in service they are then moved slightly so as to "cover" the sparks.
The second part of the test is to rotate the armature.The voltage should remain constant at zero or a low value. Slight voltage variation as commutator segments pass the brushes may occur, but voltage swings once or twice a revolution indicate armature problems.

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

 

[guydewdney]

OK - I have re-done the first part - I was surprised at how much I had to move the brush holder - approx 1/2" anti-clockwise from 'upright' to achieve zero volts on the brushes.

I have a slight sparking on the trailing edge of one brush I can see - I cannot confirm the same from other brushes until I remove covers. I have just had to replace a link in the main drive chain (1 1/2" pitch chain - similar to an ANSI chain - but 100 years old) and that has knackered me out - plus it takes over an hour to do a re-start - so I'll leave that for another day (I don't have a 1000V insulated crowbar to move the brush holder....)

 

[guydewdney]

Silly question - if half the brushes were in use, then the current in each of the armature coils would be doubled (assuming constant total output current) thereby increasing the overall efficiency....

or is this madness? ISTR that the 12 o clock pole (N) is directly connected to the 9 o clock (W) pole, and thus the 3 and 6 are the 'same'

 

[guydewdney]

Update - I have been informed of a new possible method of creating my power:- inverter drive running in full regen mode.

Mods - would you permit me to start another thread asking some really really dumb questions about this setup, or are you sick of thickos like me?

 

[itsmoked]

Are you saying replace your DC motor/generator with something else?

You could possibly run a gridless inverter from from your DC source.

Are contemplating something else than your existing monster motor?

In a completely gridless situation like yours you might consider just using a rack of automotive alternators running in their normal mode charging a battery bank. Then you have a large number of options to get from 12VDC to your standard house power. Also a dead alternator becomes something you can carry in two hands instead of a logistical nightmare. And, any failure of a single alternator would be a graceful degradation rather than 'lights out'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[guydewdney]

Sort of - more along the lines of runnign a 5.5kw 3 phase induction motor, using a inverter as a 'brake'.

total cost, new, 1600 pounds.

How 'efficient' is the brake?

 

[Sparweb]

Guy,

You NEED to visit this site:

http://www.fieldlines.com

I contribute regularly there, as I'm running an off-grid wind system among out-buildings around my acreage. I have converted a 7.5 HP motor into a DC generator using permanent magnets. On bench tests get about 2 kW out of it at 500 RPM. My current windmill generator(s) were created from 3 HP motors, and they produce 6-700W at 700 RPM.

Since your hydro energy source is limited to about 2kW, I think you could equip yourself with a more manageable generator.

Anyone got a 15Hp / 10Kw 500V DC motor kicking about?

uh... how about a 25HP...


Steven Fahey, CET

 

[itsmoked]

Very efficient. However you would have several other issues to deal with moving the entire thing back towards Weird Ville.

1) Before those drives will "regen" they generally need to see a fixed frequency steady bus to feed onto. They don't want to feed onto an dead bus.

2) You need to "excite" the induction motor to have it generate anything in the first place. How would this occur without a live bus to power the VFD in the first place so it can then excite the motor.

See? Two tickets to a freak show.

With, say, auto alternators, they would have their fields excited by the batteries in the first place.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[guydewdney]

Please forget anything to do with auto alternators - speed is nowhere near right.

The system will regen back to the house 3 phase supply. It is supplying the inverter, which supplies the motor.

there are NO BATTERIES involved.

 

[itsmoked]

That's fine.

I thought you were off grid. If that's the case why would you have power from the house in the first place?

Or is that a wrong assumption on my part..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[guydewdney]

Sorry - maybe I should be clearer:-

I am ON grid. I currently have no method of being off grid. The DC motor currently feeds a Grid Tied inverter which requires the grid (single phase) to work.

The suggested inverter drive system will require the presence of a good 3 phase supply. In the UK, we have to comply with certain regulations that prevent connection to the grid by generators unless certain conditions are met (mostly relating to quality of supply, and a 'settling in' period). I have three phase.

To this end, I have a certified 'box-o-tricks' which prevents me from connecting unless these conditions are met. The conditions are laid out in a document called G83/1 - which is not freely available to the public (yet its a 'public', rather than industrial, set of guidelines or rules).

Once I have finally sorted out my 99.99% of the time on-grid and exporting electricity, I shall look at the off grid problem for when the power goes down. To this end, I have an old 1950's Lister CS diesel engine which runs on just about anything - but thats not the problem here.

My question to the Gods of Motors that reside here is this:-
When using an inverter drive in regen mode, driving a (say) 5.5kw 6 pole or 8 pole motor at (say) 500 rpm, with a 1kw 'brake' effect - how efficient is it? Will the motor overheat with no external cooling (assuming normal operating conditions)? This would be the worst case - the opposite mode would be running the motor at say 900 rpm and 'braking' at 5kw - which I understand would be quite efficient, as everything is running at its rated loads, its just the motor is leading not lagging