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DC Machine motor constant equation

  • Engineering
  • Thread starter slain4ever
  • Start date
  • #1
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Homework Statement



Given the no-load test results of a DC motor connected in shunt to be:
Speed: 3000rpm
Applied voltage: 239V
Field current: 0.25A
Calculate the motor constant K_m, where K_m = K


Homework Equations


not really sure about the equations to use
i have these:
Km = T/ SQRT(P)
Km = Kt / SQRT(Rt)
but not really sure how I can use these to help solve the problem.

The Attempt at a Solution



I'm sure it's just a matter of finding the right formula than just plugging the values in. So if someone can give me the correct equations it would be very helpful, thank you
 

Answers and Replies

  • #2
63
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ok I found this site which helped a lot, something about a conversion factor for the torque so i managed to get a bit further but im still confused about those two differing formulae

T Nm * 3000RPM * 0.1047 (conversion factor) = P
T * 3000 * 0.1047 = 239 * 0.25
T = 59.75 / 314.1
T = 0.190226 Nm

Km = T/ SQRT(P)
Km = 0.190226 / SQRT(239 * 0.25)
Km = 0.02461

OR
Km = Kt / SQRT(Rt)
Kt = T/I
R = v/i ???
Km = (0.190226/0.25) / SQRT(1436)
km = 0.0200795

these two values are very similar
just wondering if the methods are correct
 
  • #3
CWatters
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I'm a bit rusty but I'd look at the speed equation.

N = K × Eb ÷ f

Not sure if/how you calculate the flux f but perhaps it's enough to say

N = K_m x Ev

where k_m = K/f ???

I think the flux f is fairly constant due to high resistance of the windings.
 
  • #4
CWatters
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Deleted.
 
  • #5
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not sure about those formulas

is this working right?

E_a = (k_m)(W_m)
V_T = E_a + (I_a)(R_a)
No load test, therefore I_a = 0
Therefore E_a = V_T
V_T/Wm = k_m
k_m = 239/(3000*(1/60)*2pi = 0.76
 
  • #6
rude man
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What is your definition of "the motor constant"?
 

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