NCEES #119 Power

[thewalt33]

NCEES problem #119 is asking for starting torque given that it is 150% of full load torque. The first line reads "An induction motor is rated at 1,000 hp, 1,165 rpm, 2.4 KV, 201 A, 3 phase, 60 Hz, 0.94 lagging power factor at full load."

Is there something I should be looking for to figure out what to use for Power? The answer uses the 1,000 hp to calculate full-load torque, but I seem to remember other places where you calculate full load power as P = sqrt(3) * V * I * PF even when given the full load hp.

The supplied answer insinuates that since it is torque, we only look at the mechanical properties which I assume means we look only at HP. Is that correct? And if so, does this only apply to torque, or other applications as well? I appreciate any clarification!

 

[Flyer_PE]

NCEES problem #119 is asking for starting torque given that it is 150% of full load torque. The first line reads "An induction motor is rated at 1,000 hp, 1,165 rpm, 2.4 KV, 201 A, 3 phase, 60 Hz, 0.94 lagging power factor at full load."
Is there something I should be looking for to figure out what to use for Power? The answer uses the 1,000 hp to calculate full-load torque, but I seem to remember other places where you calculate full load power as P = sqrt(3) * V * I * PF even when given the full load hp.

The supplied answer insinuates that since it is torque, we only look at the mechanical properties which I assume means we look only at HP. Is that correct? And if so, does this only apply to torque, or other applications as well? I appreciate any clarification!

Which values you use depends on what information you are looking for. In this problem, you are determining a value dependent upon output power (POut). In this problem, the output power of the motor is given so it can be used directly.

Real power supplied to the motor (PIn) is determined by P = sqrt(3) * V * I * PF.

 

[cableguy]

Yup, full load torque is dependent on the 1000 hp power output and the RPM, and starting torque is 1.5x that number.

They seem to like to give you too much information, to see what you'll do with it. I guarantee that the answer based on input power will be one of the choices as well.

 

[thewalt33]

Thank you both for the help.

One more question for clarity. If this stated that it was a generator, would it change from 150% output power to 150% input power and make the calculation essentially the same?

 

[Jonjo]

NCEES problem #119 is asking for starting torque given that it is 150% of full load torque. The first line reads "An induction motor is rated at 1,000 hp, 1,165 rpm, 2.4 KV, 201 A, 3 phase, 60 Hz, 0.94 lagging power factor at full load."
Is there something I should be looking for to figure out what to use for Power? The answer uses the 1,000 hp to calculate full-load torque, but I seem to remember other places where you calculate full load power as P = sqrt(3) * V * I * PF even when given the full load hp.

The supplied answer insinuates that since it is torque, we only look at the mechanical properties which I assume means we look only at HP. Is that correct? And if so, does this only apply to torque, or other applications as well? I appreciate any clarification!

If I remember correct they asking for Torque , the Mechanical Power depend of the torque look at this equation : TxW = Power / where W is the angular velocity (rad /sec) , you have to convert the RPM to rad/sec , and also dealing with a few convertions , well let me know if you need , I will solve for you .

Then T=Power/(W) , then everything is about convertions units , I remember I did it , and that was all about.

 

[Guillaume]

Thank you both for the help.
One more question for clarity. If this stated that it was a generator, would it change from 150% output power to 150% input power and make the calculation essentially the same?

Any induction motor can be a generator if it turns slightly above its synchronous speed (in this case, 1200rpm); then your mechanical power Pm becomes your input power. If it was stated induction "generator" with the same values, the calculation would have been the same but it would have been a different induction motor (since 1165 rpm would have been the value above the synchronous speed of 1080rpm)

A good formula related to the torque is : Pm=n*T/9.55,

where Pm is the mechanical power in Watts, T in N.m and n the rated speed in rpm.

Hope it will help. Good luck in your studies.