NCEES Power Practice Question # 524

[ndekens]

So I understand the answer givin but I do not understand the reasoning as to the solutions explination of why choice B is incorrect.

Choice be states "By starting a motor unloaded until it reaches operating speed then connect the load via a clutch in order to reduce starting current requirements."

There explaination for not choosing B is "Locked Rotor current is not a function of load; however starting a motor under loaded conditions can increase the time it takes it to reach its operating speed and cause its start-up temp to rise."

Does this make sense to you guys? Can you explain in more depth?

 

[Dark Knight]

So I understand the answer givin but I do not understand the reasoning as to the solutions explination of why choice B is incorrect.
Choice be states "By starting a motor unloaded until it reaches operating speed then connect the load via a clutch in order to reduce starting current requirements."

There explaination for not choosing B is "Locked Rotor current is not a function of load; however starting a motor under loaded conditions can increase the time it takes it to reach its operating speed and cause its start-up temp to rise."

Does this make sense to you guys? Can you explain in more depth?

NDE,

I am going to try to answer. When a motor starts the current it needs is high because it has to create a magnetic field to beat the mechanic forces and start moving it. Once it starts and the magnetic field is created it comes down but it is still flowing. Under loaded conditions the demand for the motor is going to be higher than starting from zero load so you can say the start up current is going to be much higher. That will surely increase the start-up temp.

Hope that helps. If not let me know. I cannot remember the problem and the conditions. Looks like I better bring the Manuals and the Sample test with me to the office. Sorry about that.

 

[Cicero_Kid]

For an induction motor, starting current = locked rotor current. Therefore, the starting current will be the same, whether the motor is unloaded or loaded.

Kid