Keeping your DC motor cool

[Jim]

I own a 2007 Zenn NEV with a stock 5 HP direct current motor made by Advanced DC. This motor, and others I have read about, tend to overheat after a few thousand miles. I know I could install a higher horsepower motor in my Zenn, but there are a lot of issues in doing so (the higher horsepower motor may overheat too, the mounting flange must be re-drilled to match the mounting holes in the Zenn tranaxle and this voids the motor warranty, the speedometer display may not display the correct speed, and the motor temperature  gage on the dash may not display the correct temperature).

What I am trying to find out is this: Is there a simple mod I could do to the motor to make it run cooler? Some people have suggested installing a cooling fan and/or cooling fins.

What do you folks suggest?

Thanks.

[leitmotif]

I believe if you look at the curves for the motor in short you are overloading it ie going into its peak power ratings.  If you reduce load the motor will run cooler. 
If you choose not to reduce load then you will need to install a cooling fan
OR
degrade the insulation and have a rewind to do in the future.

Dan Bentler

[madderscience]

"overheat after a few thousand miles" ?

I'm not sure why the motor would have more of a tendency to heat up after it has been used for a while.  At least with the brushes in a DC motor I understand overheating is more likely when it is new because the brushes aren't yet seated properly but in your case they should be broken in.   But that said, things can wear out and too much heat can certainly damage the motor.   Though a few thousand miles seems like it should be barely broken in unless something about the motor is very poorly done.

Can you add a fan or some other source of forced air cooling?   If it already has forced air cooling, is it working properly?  perhaps a simple cowl or something to direct some air onto the motor while you are driving would be enough to keep it cooler.   Is there any way to attach a heat sink?   That might help a little.  It makes no heat when you are stopped of course.

Final dumb question:   when is the last time you checked the air in your tires?  Perhaps you need to top them off (use maximum rated pressure, at least...) to bring the rolling resistance down a bit.  Has a big pile of extra baggage appeared in the back of the car?  if so, clean that out.  Might be enough to make the difference.

Good luck.

[Jim]

Thanks for your response.

The motor ran cool at first, but as time went on it began to run hotter. That's to be expected, but not to the extent that my motor was overheating. You could fry eggs on it. So I am in the process of installing a new motor - and am looking for suggestions on how to extend its life. Cooling fins (heat sinks) and/or a cooling fan may help.

[leitmotif]

Jim
IF motor hot enough to fry egg something is definitely wrong.  Rule of thumb if you can put your hand on it and hold it there for 3 to 5 seconds that is OK.

How do you know it is hot?  Dash temperature indication?  Check to see if it is telling you the truth.

In general hot motors are overloaded motors.

Not only check your tires but how much stuff you have in the car.  You just got to get rid of them 10 sacks of sand.

Is this the car that mounts the motor to a reduction box / differential unit?
Check you lubricant in the geaboxes.

Check for brake drag -- do you forget to release the parking brake??

Lot of junk around the motor ie is the motor cased in mud and straw??  are the vent holes clear?  Clean motors are happier motors.

Check your driving habits.  Very frequent stop and start will heat a motor.  Many motors especially large one 250 HP plus have factory specs for how often you can start.  You must allow time to let it cool itself between starts.  Lot of hill climbing??

After all those checks inspect the accessible parts.  Is the commutater a chocolate brown and uniform color - that is good.
Brush tension seem OK?  Commutator not scored?  Brushes worn?  Lot of carbon dust inside??
 
IF all above fails pull the motor.  Pull the brushes away from commutator and see if it spins freely.  If not check and or replace bearings.  Have motor shop check it out.  B and B on W valley is place I would go.  If you insist on new motor well OK but a rewind / repair mite save some bucks especially if the motor still runs.

PUT A FAN ON IT put a filter on the fan to keep bugs n dirt out of motor.
the cooler it is the better (to a limit - they dont like ice inside)

Dan Bentler

[Jim]

Hi Dan:

Thank you for your thoughtful suggestions.

I know it is hot because (1) the motor overheat light on the dash comes on, and (2) because I have felt the motor with my hand - VERY hot, can't touch it even for a couple of seconds without injuring myself.

The motor mounts directly to the transaxle. There is no transmission - just a 3-position electric switch: drive, neutral , and reverse.

My EV is much, much lighter and much, much less powerful than highway capable EVs. It has a 5 HP DC motor made by Advanced DC. The vehicle is a 2007 Zenn, which is a 1200 pound (including batteries) neighborhood electric vehicle (NEV). The Zenn has a 72 volt system (six 12 volt AGM batteries). It carries only 2 people, the driver and one passenger. The tires are inflated properly, the brakes do not drag, and there is an audible series of beeps if the parking brake is on.

As an NEV, it was designed for 25 mph or less. I reprogrammed the GE controller to allow for speeds up to 37 mph. I  turned off REGEN during the reprogramming process in order to put less demand on the motor. The faster speed allows me to travel our local streets safely. At 25 mph, I would be in danger every second.

As you mentioned, hot motors are overloaded motors. I think that when I reprogrammed the controller for faster speeds, the motor was taxed beyond its capabilities. I considered going to a stronger 7.5 HP motor, but some who have tried that tell me the results were not good - still overheating. In addition, there are other issues in making such a conversion - the mounting flange on the new motor must be redrilled to match the hole pattern of the Zenn transaxle (thus voiding the warranty), the speedometer may read incorrectly because of incompatible motor speedometer magnets, and the temperature sensor on the motor may not work with the Zenn's existing overheat gage.

So I am looking for some other way to allow me to continue using my existing motor at the faster speed of 37 mph without shortening its life span. Recent reports I have read on the internet indicate some success with the installation of a high flow squirrel cage fan mounted directly on the motor in such a way as to force lots of air through the INSIDE of the motor. The fan that was selected is the Dayton 12VDC 2C646. It sells for about $65.

Regards,

Jim

[leitmotif]

I would - off top of head - say a squirrel cage fan (best) blowing air THRU the motor at 100 cfm or better would suffice.  Make sure the air is filtered to keep bugs dirt etc out.  One of those small engine foam carburator cleaners should suffice.  I myself would go with a good paper type auto filter.  The cleaner the motor internals the happier the motor will be

The other thing I would do is dial the controller back to factory settings.

Dan Bentler

[Jim]

Thanks for your comments, Dan.

This blower is rated at 176 CFM at 0.0 in. SP and 148 CFM at 0.5 in. SP (I don't know what "SP" means. I expected to see something like inches of mercury or pounds per square inch). Your figure of 100 CFM certainly is encouraging, and - coming from you - makes that fan seem like a good choice.

Now if I can just get it installed without messing something up! ;>)

Jim

[leitmotif]

Jim

SP stands for static pressure ie discharge pressure of fan in terms of inches water.  As static pressure (resistance / friction) go up flow (CFM) goes down.  Looked in Grainger that fan should work and maybe then some.  Use a flexible tubing with SMOOTH interior (least friction).  No Seattle Comm Coll has a Zenn in their electrical lab torn down to the frame.  It had a small blower attached to the motor with about 1" tubing.  Yours will definitely keep that motor cool.  Ideally get some tin and adapt from square outlet to inside dia of tubing say 1 1/2".  Sheet metal shop can fix you up with this if you are not a tin bender.  Dont remember if there are two "holes at either end of motor which would be ideal.  Air goes in one end and out the brush rigging commutator end to keep motor clear of carbon dust.  Mount blower high up in chassis to prevent sucking up water and filter blower intake.

Dan Bentler

[Jim]

Thanks again, Dan. You've been a big help. Much appreciated.

Jim

[leitmotif]

Jim

Do NOT forget that the blower is just a band aid on gangrene.  It does NOT fix the existing problem.  Keep a close eye on that motor reduce / keep load to a minimum (climb hills slower, no jack rabbit out of stoplights, reduce stops starts, etc), do a brush commutator inspection.

Another thought one end of that is basically a flat plate - if you drill 5 or 6  holes (1/2" or so) in end you may just be able to duct that blower right on to the end of the motor opposite the brush rig.

Wire a relay into the red light to start and stop blower maybe or better yet just let it run at least for the first go around.

Dan

[Jim]

Good thoughts. Thanks Dan.

Jim

Do NOT forget that the blower is just a band aid on gangrene.  It does NOT fix the existing problem.  Keep a close eye on that motor reduce / keep load to a minimum (climb hills slower, no jack rabbit out of stoplights, reduce stops starts, etc), do a brush commutator inspection.

Another thought one end of that is basically a flat plate - if you drill 5 or 6  holes (1/2" or so) in end you may just be able to duct that blower right on to the end of the motor opposite the brush rig.

Wire a relay into the red light to start and stop blower maybe or better yet just let it run at least for the first go around.

Dan

[Stew-2]

Another cooling solution, drawing from my R/C days again, would be to do a water cool setup. Downside is it's more complicated than a simple fan, but if you need increased cooling because you're pushing the performance envelope of your motor, this may be the answer.

Tightly wrap the motor with mild copper tubing available at the hardware store, don't cover any openings for normal airflow to the inside of the motor. Get small transmission cooler with fan, like pictured below, get 12v water pump, something for an RV would be ideal, a couple lengths of hose and hose clamps, wire it all up and you're done.



the pump may not need to operate continuously, same with the cooling fan, there would be many options on how to control it, manualy, thermostatically, etc.

[Monermaje]

A DC machine can operate at any rpm from zero to top speed, with full current applied. This is what is meant by 'constant torque' operation.

To successfully accomplish this, the cooling mechanism must be independent of machine shaft speed.

Usually, the way to size the cooling fan is to determine spatial constraints (how much room do I have for installing the blower motor and fan assembly?), the volume of air required, and the permissible acoustic noise.

A high-speed (2 pole) fan and motor combination will be physically smaller, but will have difficulty delivering high pressures and/or high volumes. Additionally, the acoustic noise will be significant, in part due to the windage of the fan motor.

A slower speed (4 pole or 6 pole) fan and motor combination will occupy more real estate, but will run quieter and deliver more pressure and volume for about the same price.

To determine volume requirements - the best method is to assume 2.8 cubic meters per minute required per kilowatt of loss when the DC machine is at full load. If you don't know the losses exactly, a reasonable approximation would be to assume LOSS = (0.08 * RATED KW) ... in other words, the machine is 92 percent efficient.