Originally Posted by

**philly**
The motor I am referencing in this thread is the same motor I have referenced in another thread regarding the thermal capacity of the motor.

This motor information is: 2300HP, 1140rpm, 4.16kV, 312FLA

We have recently been running this motor at around 265A as mentioned in my previous thread. I had someone state to me today that weather we run this motor at 265A or at its full load of 312A we are outputing the same amount of HP on the motor shaft. I am no sure if I believe this statement and my understanding is as follows:

Ouput HP is a function of HP= Torque x Speed. For this mill grinding motor the speed of the motor is kept constant at 60Hz and 1140rpm.

You are right here, at 312 A the Output HP have to increase, unless the speed had decreased somehow. But this is not what you are saying. You say the Speed stays constant. Right?

Originally Posted by

**philly**
The torque on the motor is then dependent on how loaded the motor is. The current that goes along with this toruqe is the current that would align with any given torque on a motor speed vs torque curve and speed vs current curve. So for example if we are running at a given speed, there would be a given torque and current value for that speed. The given torque x the motor speed would give the motor HP output.

So if we are running with a given load on this motor at 265A then this current value equates to a given toruqe value on the motor curve. At this torque value we can multiply it by the speed to get the motor output hp. Now lets say we start loading the mill more. As we load the mill more the torque requirement on the motor increases. As the torque requirement increases we move left on the motor motor speed vs torque curve. At the new required torque value we will have an increased current value since we also move to the left on the motor speed vs current curve. So with a higher current value we have a higher torque value. Since torque increased and speed is kept constant I would expect the motor output hp to also increase, and prove that as current increases then torque and therefore HP increases.

The only part that I'm unsure about is how slip effects my explanatin above. As we are moving to the left on the motor torque curve we are also increasing the slip with is slowing the motor. So as we increase torque and slow the motor speed is this essentially a wash with the motor output HP staying the same? Or is the slip speed and speed decrease small compared to the torque and current increase and therefore the overall output HP is increased.

Can anyone help me out with grasping this concept?

The slip, also takes part in this issue. But since the slip is relation between the frecuence of the system (60Hz), and the speed of the machine, and you have stated that they both stay the same. Then theres no reason for the Output Hp to stay similar after an increase of the current.

From my notes, I have that for induction motors, this the Output Power (p.u):

P={3*Vth^2*Rr*[(1-s)/s]}/{[(Rth+Rr/s)^2]+Xth^2} (1)

I = Vth/[(Rth + Rr/s)^2 + Xth^2] (2)

where,

P = output power

Vth = Thevenin Voltage from the rotor perspective.

Rr = rotor resistance

s = slip

Rth = thevenin resistence from the rotor perspective.

Xth = thevenin inductance from the rotor perspective.

So, if you change the current value, automatically you change the thevenin value Vth (see (2)). And then the P value change as well due to that change.

Also if you change the slip value, by changing the speed for example, the P value will change as well.

Last edited by Mayimbe; 10-20-09 at 01:03 PM.
Reason: adding stuff

"I am the son and heir... Of nothing in particular"

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