Size of Carbon Brush lead

[maypot]

Hi,

I am replacing one 250 k W 400 V slip ring motor with a spare motor . During an inspection on the spare motor, I noted that there were only two carbon brushes per rotor phase. The size of each carbon lead is 10 sq mm ( two leads for each carbon brush ) which means that only 40 sq mm is available to carry the rotor current on each phase. The nominal rotor current is 238 A and rotor voltage is 638 V. I suspect that carbon brushes have been removed on this motor .
The motor type is RS 400M10 and the motor number is 109600355.
Any help to confirm the exact number of carbon brushes.

Bob

 

[aolalde]

Maypot

Please verify the reported brush dimension, something is wrong, 40 mm^2 is ridiculous.

Metal-Graphite brushes having the higher current capacity (Amps/ sqcm) are capable to handle 15.5 to 24.8 Amperes per square centimeter.

If we get an average, lets say 20.15 A/cm^2; for 238 amperes, the required area is:

total brush area=238/20.15 = 11.81 cm^2 = 11.81*100 mm^2 = 1,181 mm^2

the reported 40 mm^2 total area per phase is only 3.3% of an average requirement!!

Note that other types of brushes handle lower current densities.

Electro-Graphite - 11.6 to 14 A/cm^2
Graphite - 8.5 to 10.9 A/cm^2
Carbon-Graphite – 6.2 to 8.5 A/cm^2

 

[UKpete]

maypot/aolalde, do you mean that the braided copper brush pigtails are 40mm² total per phase? At 6A/mm² that sounds ok to me, assuming they are in some sort of airflow.

So what are the actual brush sizes?

 

[maypot]

Aolade/UKpete,

The 40 mm^2 I am referring is the total size of the copper brushes pigtails.I am not worried at all about the dimension of the carbon brush which is 50 mm by 30 mm i.e 1500 mm^2 cross section.
I presume that the 40 mm^2 is just enough for the rated rotor current.

Bob

 

[UKpete]

If the copper area is truly 40mm² then there should be no problem. Did you calculate this area from measuring the diameter? You should note that the true copper area is less than pi*d²/4 by a factor of about 0.6, because the pigtail is made up from fine strands typically max 0.12mm diameter.

As an example, I worked on a modern dc traction motor with a current of 179A continuous (360A max) that has 2 brushes each of area 630mm². The brush drawing shows 4 pigtails per brush each of nominal diameter 2.5mm and true copper area 2.92mm².
On the continuous current this gives a brush current density of 0.14A/mm² (the brushes are electro-graphite hence this agrees with aolalde's figures) and a current density in the flexible connection of 7.7A/mm² - much higher values are experienced on the short-term ratings. The pigtails were silicone sleeved but were in the armature cooling air flow. I never saw any sign of overheating.

I can't find any reference to pigtail/flexibles current density in my Morganite and National Carbon books, but your pigtails will have been adequately sized for the carbon brush.