# DC Motors Overheating On Long Trips



## esoneson (Sep 1, 2008)

Ask Jack.


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## Inframan (Jan 30, 2011)

Well my friend was down there not too long ago and he said this is what Jack said. I just wanted to know what everyone else thought.


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## brainzel (Jun 15, 2009)

From my experience, 60 miles at 55mph to 65mph with Helwig H49 brushes and without forced cooling is not a problem (car weight 3.000 lbs).
Nevertheless, if I had to rebuilt my conversion, I would probably take a blower.

The one or two motor decision would depend on your weight.

Michael


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## Tesseract (Sep 27, 2008)

Inframan said:


> I was told by my friend that ... warp 9 inch motor's brushes getting in some cases getting red hot while driving on the highway long distances say at a steady 65 for 100 miles sometimes even under that mph.


For carbon brushes to get red hot their temperature would have to be over 700C/1300F. Needless to say, a copper commutator isn't going to tolerate that for very long.

Can DC motors overheat when used to propel an EV at highway speeds for long distances? Yes. But so can any kind of motor, if it is undersized and/or the overall gear ratio is too low (ie - motor RPM is low and torque is high).

The part of a motor that gets the hottest will depend on the type of motor as well as the ratio of speed vs. torque it is delivering for a given output power. For example, the brushes/commutator in a series DC motor are usually the hottest when delivering high torque at low RPM, but the armature laminations will be hottest at high RPM and low torque (output power the same in both cases). In contrast, the rotor is usually the hottest part of an ACIM regardless of operating condition (except, of course, at no load) - the shorting bars in the rotor heat up the most at high torque while the rotor laminations heat up the most at high RPM.


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## major (Apr 4, 2008)

Inframan said:


> ....while driving on the highway long distances say at a steady 65 for 100 miles.....


You think that might have been a big heavy vehicle? Maybe an overload on the motor? Maybe they ignored the thermal sensor signal from the motor?

You can overload the motor and overheat it; sure. It does sound suspicious about the red hot brushes on a continuous duty run like described. Brushes typically overheat from excessive current on short time base events of minutes or seconds in duration. A continuous run is typically limited by the temperature of the windings or core. Either way, did they bother to list the actual load power versus the rated power of the motor? That might be more useful than badmouthing components.


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## gunnarhs (Apr 24, 2012)

Inframan said:


> I was told by my friend that Jack Rickard and Reverend Gadget have had experiences with warp 9 inch motor's brushes getting in some cases getting red hot while driving on the highway long distances say at a steady 65 for 100 miles sometimes even under that mph. Is there much truth to this or were they hotdogging it a little? Would a blower solve the problem? Is this really the largest downfall of DC motors preventing them from going even a 100 miles to a charge and up at highway speeds? My friend seems to think the only way to have a long range DC EV is to have dual motors. Is he right?


I am more an specialist in AC than DC-motors but from experience of Series and Sepex-DC (both brushed) from our Clean Car Association Members we have two cars driven over 30.000 km, one of them is short range (< 50 km range) and powerful (20KW - 40 KW), the other long range (200 km) and less powerful (11 KW, 20 KW max). I have also a good connection to a german converter who has converted over 20 cars with Series DC with WARP without problems.
The only thing experienced is that brushes ("coals" in our language) should be replaced after about 30.000 km (or when out of usage for long time) which is usually no problem.
In no case we have overheated a motor/brushes so it would be interesting to know what kind of controller and what kind of power (torque/speed) was applied during Jack's experience. Also if the motors used were new/old , had been out of usage for long time.
The controllers used in our cases where Curtis, Kelly, Sevcon and Peugeot and if something died it was usually the controller/battery/gearbox before motor was even getting hot. But in most cases when that happened it was a stupid configuration of the user rather than problems with the motor/controller 
(yes one was mine mistake  )


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## onegreenev (May 18, 2012)

Jack did not say RED hot. He just mentioned a few folks using lithium had DC motor failures and deduced it was an inherent flaw to run them long distances and to be careful. What I got out of the comments was a PR stunt to get folks turned around to have a look at AC motors because he has a pile of them to sell off. It has always been my understanding that you don't want to lug your DC motor but to keep the RPMs high to help keep the brushes cooler. 

I suspect failures were due to brushes wearing down and running the motors at low RPMs and high torque further complicating the crappy brush issue.


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## major (Apr 4, 2008)

onegreenev said:


> Jack *did not say RED hot*. He just mentioned a few folks using lithium had *DC motor failures* and deduced it was an *inherent flaw *to run them long distances and to be careful. What I got out of the comments was a *PR stunt* to get folks turned around to have a look at AC motors because he has a pile of them to sell off. It has always been my understanding that you don't want to lug your DC motor but to keep the RPMs high to help keep the brushes cooler.
> 
> I suspect* failures *were due to *brushes wearing down* and running the motors at low RPMs and high torque further complicating *the crappy brush issue*.


How many rumors can you guys start in a thread


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## onegreenev (May 18, 2012)

No rumors. Just comments. And my take on Jacks past comments.


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## PZigouras (Jun 5, 2010)

So, let's say it's true and there is indeed excessive heat build-up at the commutator… will [forced induction] cooling directly to the comm eliminate this problem altogether?

I've never lost a DC motor due to heat... but I don't want to start now. I have three of them...


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## Hollie Maea (Dec 9, 2009)

Jack Rickard says many things, some of which are true. While I would agree that AC motors have some advantages, keep in mind that you can literally deduce the contents of his store's inventory by what he's telling people about technology. I would guess that his pile of Siemens motors aren't flying off the shelf as fast as he would like. Not that they are bad motors--my motor is essentially a crummier version and I'd gladly switch if they were cheaper.


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## dragonsgate (May 19, 2012)

Could the fact that high amp hour lithiums capable of giving higher C's for longer times have anything to do with heating up DC brushes. Is there any way to monitor brush and motor temps in the cockpit while driving? I am not talking about a piezo buzzer that goes off after everything has over heated but actual gage similar to an ICE temp gage.


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## Tesseract (Sep 27, 2008)

dragonsgate said:


> Could the fact that high amp hour lithiums capable of giving higher C's for longer times have anything to do with heating up DC brushes. Is there any way to monitor brush and motor temps in the cockpit while driving?...


Yes, lithium batteries mean you get a higher average power for longer time, and is what makes overheating possible.

I have recommended using an optical thermometer to monitor brush temperature here and elsewhere before. More specifically, the 0-400F model of the Omega OS136. Of course, even this "low cost" model is $185.

Otherwise, using a blower and belly band to force air through the motor can have a dramatic effect on allowed power output and brush temperature. Somewhere in the annals of my post history I describe the reduction in brush temperature (monitored with an OS136) on the dual WarP-9 Volvo station wagon Rebirth Auto converted.


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## Russco (Dec 23, 2008)

Tesseract said:


> Yes, lithium batteries mean you get a higher average power for longer time, and is what makes overheating possible.


Electric vehicle motors are only rated for one hour duty.

Why is this?

Generally, in the old days with lead, the batteries would crap out after one hour. By rating the motor temperature rise to reach the limit in one hour, the motor can be rated at a much higher horse power. 

A continuous rating, such as used for residential and industrial AC motors would result in a MUCH lower horse power rating.

Sort of sales BS to promote a higher HP rating by using a one hour rating.

Selecting too small a motor will increase heating. So will driving in too high a gear. 

Adding a couple hundred CFM via a squirrel cage blower into the brush/commutator end of the motor will help motor cooling drastically.

Remember, heat kills electronics. Be cool.


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## major (Apr 4, 2008)

Russco said:


> Electric vehicle motors are only rated for one hour duty.
> 
> Why is this?


The one hour rating is a carry over from the forklift industry from when the EV conversions started using forklift traction motors or modifications of those. It really had nothing to do with the battery. Forklifts used lead acid batteries large enough to run 8 hours. 

Depending on the design, size and ventilation of the DC traction motor, a continuous thermal rating could range from slightly under one hour to as long as 6 or 7 hours to stabilize motor temperatures. The industry needed a standard which could be verifiable and more indicative of the use profile so a one hour load was established for the traction motors. 10 or 15 minute ratings were used for the pump motors as well as a percentage on-time duty cycles. 

A well ventilated fan cooled or forced air cooled motor will likely have equal hp ratings for continuous and one hour periods because the temperature at rated load reaches equilibrium in less than one hour. But a poorly vented or totally enclosed large motor may have a one hour rating several times its continuous power rating.


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