Water cooling a 12HP air cooled motor

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liveaboard
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Water cooling a 12HP air cooled motor

Post by liveaboard » Sat Jun 22, 2019 2:48 am

I recently bought a 12 HP forklift motor, 48VDC and 240A.
To power my heavy workboat in the inner city Amsterdam canals, which has declared the city will be emissions free only for all transport in a few years.

This is not the right motor for the job, it's the motor I can pay for. There are lots of great motors for people with great budgets.

The motor is old and came with a newish [US made!] electronic switching control. Normally it will run at around 1/2 power, I need full power for stopping and turning.

Duty cycle at full power on a forklift is 25%

So my plan is to add a water jacket around the outside; this will keep the field coils cool and reduce the internal temperature, but 1/2 the heat will be in the armature. At full power, that's around 1.5Kw.

While keeping the original air cooling intact, I'd like to add water chambers to each end of the shaft. I'll weld stainless extensions on, machine them true, and pass them through chambers of water/glycol [the boat has an oversize heat exchanger for the diesel engine].

What I can't figure out is, how much heat will be transferred from the shaft to the water?
Let's say the wetted area is 2" long, with a 4" circumference, shaft temperature is 90C [200F] and the water temperature is 25C [77F].
Shaft rpm 3000-4000.
One online calculator seemed to suggest around 15W, another 2000W.
My gut feeling is around 500W.

The motor is at my workshop in Portugal; I'll ship it to Amsterdam with the [now rebuilt] main diesel engine in a couple of months.
I have to add some mechanical and electrical bits too.

Magicniner
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Re: Water cooling a 12HP air cooled motor

Post by Magicniner » Sat Jun 22, 2019 8:11 am

If you add in a heat pump you'll pull more heat energy out of the system than with a heat exchanger alone.

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Re: Water cooling a 12HP air cooled motor

Post by Bentworker » Sat Jun 22, 2019 10:17 am

You may not want to directly cool the motor with water for simplicity, and because a leak could be pretty catastrophic

On large hydroelectric generators the housing where the stator and rotor are is sealed, and inside there are large air to water heat exchangers that the air is drawn through as it circulates.

You could build some sort of ducting to direct cooled air from an water to air heat exchanger (old radiator) to the motor. Might be simpler, just as effective and cheaper
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Re: Water cooling a 12HP air cooled motor

Post by John Hasler » Sat Jun 22, 2019 11:01 am

A 65 K temperature difference will drive 190 watts through 10 cm of that shaft (assuming carbon steel: most stainless alloys have lower conductivity). That doesn't account for the temperature drop from the windings to the shaft and the (very poor) coupling from the smooth, round shaft surface to the water.

Not practical. Much better to enhance the air cooling with an external fan. If necessary you could do as Bentworker suggests and cool that air with water.

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Re: Water cooling a 12HP air cooled motor

Post by liveaboard » Sat Jun 22, 2019 6:27 pm

190W is pretty disappointing... Thanks for working that out for me.

The engine compartment is steel sitting in water; the whole thing is a radiator. Air temp in the engine compartment will be close to outside water temperature. But the small physical size of this motor means air cooling alone will never do it.
Leaks don't scare me at all. I've had a water cooled diesel in there for 20 years. Most multi-HP electric boat motors are water cooled; I don't know the details, maybe they only cool the case that way.

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Re: Water cooling a 12HP air cooled motor

Post by Magicniner » Mon Jun 24, 2019 4:01 am

Why not use an air/water intercooler and fan to provide large volumes of cool air through the motor's air cooling?

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Re: Water cooling a 12HP air cooled motor

Post by liveaboard » Wed Jun 26, 2019 3:34 am

Magicniner wrote:
Mon Jun 24, 2019 4:01 am
Why not use an air/water intercooler and fan to provide large volumes of cool air through the motor's air cooling?
Because the air temperature in the engine compartment is only a couple of degrees from the water temperature outside.
Such a setup would have no benefit.

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Re: Water cooling a 12HP air cooled motor

Post by Magicniner » Wed Jun 26, 2019 6:13 am

liveaboard wrote:
Wed Jun 26, 2019 3:34 am
Because the air temperature in the engine compartment is only a couple of degrees from the water temperature outside.
Such a setup would have no benefit.
If your starting conditions can be assumed not to change that's true :D

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Re: Water cooling a 12HP air cooled motor

Post by liveaboard » Wed Jun 26, 2019 7:09 am

As I said above, it's a steel hull; the whole thing is a heat exchanger. About 8 square meters of the engine compartment is in the water.
3 kw is a lot of heat in the football sized electric motor, but in this case, the air around it won't heat up much.
I'm not averse to adding complicated additional plumbing, but there has to be a significant advantage to it.

I'm working on a bigger fan, and water cooling the case. This will make the chunky field coil armatures into heat absorbers instead of heat emitters.
I hope.
With the additional airflow, I'm hoping for a 100% duty cycle at 1/2 power.

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Re: Water cooling a 12HP air cooled motor

Post by John Hasler » Thu Jun 27, 2019 7:13 am

Can you put a temperature sensor such as a thermistor or thermocouple in the windings? It could control your cooling and also warn you when to throttle back.

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Re: Water cooling a 12HP air cooled motor

Post by liveaboard » Thu Jun 27, 2019 12:45 pm

I thought about that a lot; the weak point is the armature and I can think of no possible way to monitor that temperature directly. The field windings are clamped to the [soon to be] water cooled case.
So I plan to have a temperature sensor in the outgoing air.
I haven't yet figured out how to place the gauges so I can see them easily underway; this boat has a big flat deck and a tiller.
I'll do that somehow, probably a panel that can be stowed below when the boat isn't in use [99.9% of the time].
There will be volts, amps, and AH used since reset. All from a $30 ebay gizmo [+ a $25 500A shunt].

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Re: Water cooling a 12HP air cooled motor

Post by liveaboard » Tue Jul 02, 2019 4:41 pm

So I decided to air cool the outside of the case, and increase air flow through the canter with a bigger fan.
I had a hard time finding the fan; then I realized that 1/2 HP blower that's been on the shelf for 10 years has one in it.
armature with origonal fan.jpg
case and feild coils.jpg
brush assembly.jpg
Made to be air cooled inside...
armature vent holes.jpg
Getting to it;
flame cut rings.jpg
The water jacket needs access holes to fit the field coil screws.
drilling water jacket.jpg

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