- #36
SK97
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Yes so after doing the method i posted earlier, I am still obtaining an answer of 20 ohms.cnh1995 said:Yes, isn't that mentioned in the problem?
So something I'm doing must not be right, but i can't understand what?
Yes so after doing the method i posted earlier, I am still obtaining an answer of 20 ohms.cnh1995 said:Yes, isn't that mentioned in the problem?
Post your working. Maybe there is some arithmatic mistake.SK97 said:Yes so after doing the method i posted earlier, I am still obtaining an answer of 20 ohms.
So something I'm doing must not be right, but i can't understand what?
ok here it is:cnh1995 said:Post your working. Maybe there is some arithmatic mistake.
Your calculations look correct to me.SK97 said:ok here it is:
Ea for 260V: Vdc - IaRa
260 - 30(0.25) = 252.5V
Ea for 300V as calculated before is 292.5V
T (260V) = 252.5 (30) / (1276.36) (2pi/60) = 65.65 Nm
T (300V) = 292.5 (30) / (1276.36) (2pi/ 60) = 56.67 Nm
now : T (300V) / T (260) = I (f) 1 / I(f) 2
65.65 / 56.67 = 5/ I(f)2
1.158 = 5 / I (f)2
I (f) 2 = 5 / 1.158
I ( f) 2 = 4.32 A
then I (f) = Vdc / Rf + Radj
so: 4.32 = 260 / 40 + Radj
Radj = 20.24 ohms
So it's just the nature of the question that made both values close to each other?cnh1995 said:Your calculations look correct to me.
When you used terminal voltages (300V and 260V) instead of back emfs, you got Radj=20 ohms and the new value for Radj=20.24 ohms, which is very close to the previous value. This is because the ratio of terminal voltages (260/300) is almost equal to the ratio of back emfs (252.5/292.5).
I think so.SK97 said:So it's just the nature of the question that made both values close to each other?
Yeah that seems to make sense.cnh1995 said:I think so.
To summarize,
the motor develops 65 Nm torque with 300V supply voltage and 30A armature current, and the speed is 1276 rpm. If you reduce the terminal voltage to 260V with the load unchanged, the field current will reduce and the speed will also decrease in order to allow for more current to flow in the armature, so as to maintain the torque at 65Nm. If you want to bring the speed back to 1276 rpm and armature current to 30A, you'll have to reduce the load torque from 65 Nm to 56 Nm and increase Radj from 20 ohm to 20.24 ohm.