# Finding the Armature Resistance from winding/coil resistance

• Engineering
• Retcon_1
In summary, the conversation discusses finding the armature resistance using the back e.m.f and armature current, but lacking enough information to work out the back e.m.f. The simplest way to do this is using the coil resistance and number of poles, with the first instinct being to use 16 coils with 0.3 ohm resistance each. However, as it is a lap wound machine, the 16 coils would be in parallel across 4 poles, resulting in 4 parallel resistances. After calculations, the armature resistance is determined to be 0.3. It is also mentioned that in a lap wound generator, the number of parallel paths is equal to the number of poles.
Retcon_1
Homework Statement
The Blades of a 20kW, 400V machine rotate at 2 revs per second at rated wind speed.

This four pole machine has a lap wound armature consisting of 16 coils each with a resistance of 0.3ohms. calculate the resistance of the armature.
Relevant Equations
V=E - Ia*Ra
I'm struggling to find the exact way to do this and think that I am overthinking the equation,

all of my lesson notes tend to explain how to find the armature resistance using the back e.m.f and the armature current, we can figure out the armature current but do not have enough information to work out the back e.m.f. so that means that the simplest way to do this is using the coil resistance and number of poles.

So the first instinct would be to do 16 coils with 0.3 ohm resistance each so 16*0.3 = 4.8 - which seems high for the armature resistance.

that being said as it is a lap wound machine the 16 coils would be in parallel across the 4 poles so 16/4 = 4 which gives the 4 parallel resistances

so 4*0.3 = 1.2

1/R = (1/1.2)*4

R=1/3.334 = 0.3

In a lap wound generator number of parallel paths = number of poles.
So, your solution, seems to me, is correct.

Retcon_1

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