# [Series DC] Heat generation question



## Siwastaja (Aug 1, 2012)

Probably most heat is generated in the resistive losses in the windings. There is no good thermal contact from the winding copper to the iron, so you would need to use the weight of the _windings_ only in your calculation for the peak heat.

The enamel on the wires can withstand something like 160...220 deg C depending on the wire. At that temperature, resistance has gone up quite a bit, increasing losses further. If this limit is exceeded only a bit, the life expectancy of the wiring goes down, so you can do that every now and then. If you exceed it too much, the enamel will burn and the windings can short out.

Another problem is at the brushes. There is probably less heat there than in windings, but OTOH there is less surface area there, too.

For a brushed DC motor, 90% efficiency sounds like a peak efficiency measured at the optimum point. Rating it at very high currents probably lowers the efficiency.

Active cooling by pushing a lot of air through the inside of the motor (including brushes) is very good for increasing average power, but not that effective for the highest peaks.


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## major (Apr 4, 2008)

Frank said:


> Theoretical question: if a 100# motor is 90% efficient and is subjected to 1000A at 150V (150KW) for 1 minute,......


The commutator and brushes will fail


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## Frank (Dec 6, 2008)

I know that's pushing it hard but Metric ran two WarP 9's (in parallel) from a Z2K for about 35 seconds at the Texas Mile last year. I know the H49 brushes are reputed to work better for racing. I don't know why though. Do the brushes "burn up"? My 1 minute time was for convenience. How about 30 seconds? Any thoughts Major?


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## major (Apr 4, 2008)

Frank said:


> I know that's pushing it hard but Metric ran two WarP 9's (in parallel) from a Z2K for about 35 seconds at the Texas Mile last year. I know the H49 brushes are reputed to work better for racing. I don't know why though. Do the brushes "burn up"? My 1 minute time was for convenience. How about 30 seconds? Any thoughts Major?


According to Tom Brunka of Helwig last week at EVCCON, the race grade brush is electrographitic with no additives so can run extremely hot but does not have long life and needs a rather high minimum current to seat and film properly. In other words, won't survive easy street duty. Go figure 

On the other hand, the "street grades" have additives which can only tolerate 400°F and above that will seep the additive and seize in the holders.....not good. However those grades will seat and film with proper break-in and work well at low current densities.

I can't tell you how many seconds it takes the brush to heat to 200°C at 454 A/in² and 1500 ft/min. But it is likely just a few seconds, not minutes 

And the failure most often seen on the drag strip with these crazy overloaded motors is the zorch. Shop talk for arc flash or plasma event. You are aware that plasma is used in a controlled manner to cut metal, right? Well in an uncontrolled plasma event, or zorch, the commutator, brush rigging, conductors in the area and even sometimes castings are sliced and diced and melted and spit out. It ends the day at the track real quick


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