Calculating Cost of Running Motors 2

[RJB32482]

Hello,
We have a 250 HP 3 Phase Motor. I want to calculate the cost per day or hour for running the motor.

Can someone help me with this?

Thanks

 

[CJCPE]

I think this method is worth posting as an FAQ.

Some additional observations:

If the horizontal line intersects the vertical line above the upper end of the capacitor current, the motor must have a leading power factor -- must be operating as synchronous condenser. I don't think that this method would indicate direction of power flow.

If there is significant harmonic current, the vectors will not all lie in the same plane and this method will not work.

 

[waross]

Hello CJCPE thanks for the encouragement.
I've been thinking all day about your last post. I'm pretty sure of my concept but not sure how to explain it.
I was refering to uncorrected induction motors.
The motor current line, or the line from the top of the vertical line, represents the current that is a component of KVA. The angle of the line is related to the power factor.
When you think about a motor with a leading power factor, let's start with an uncorrected motor, and connect a small capacitor and use this method to complete the triangle.
The motor is still running with a lagging power factor so we add another capacitor. The original uncorrected motor current is still the same at the same phase angle. The vertical line doubles in length because we have doubled the capacitors. The new line current is the vector sum of the motor current and the capacitor current. At unity power factor the vector representing line current will be horizontal. At a leading power factor the line current vector will be angling upwards from the intersection with the botto
This is based on measuring the uncorrected motor current.
However, if you are measuring the current at the MCC and one of the motors has capacitors connected to it in the field so that its power factor is leading, and you add more capacitors at the MCC for testing, yes, you are correct. If you are testing a synchronous motor then you are also correct.
I was assuming that the test would be made on induction motors with no field-connected capacitors. The comment about perpetual motion was meant in jest, but the point is/was if you're testing similar motors and the line goes the wrong way on one of them, you’ve probably got your numbers mixed. I guess if an attempt at humor results in a misunderstanding of the main point then the humor was inappropriate. I apologize, and accept your point.

Your comment on harmonics is interesting. I'm going to think about this for a while before I respond. My thinking will be along the lines of, Will it make a difference if the ammeter responds to average current or RMS current? Will harmonics make a difference to the old style of revenue meters with the phase shifting coils? Will the harmonics have the same effect on the power factor meter as they do on the ammeter?
I'm also trying to visualize the effect of a distorted voltage waveform on motor current and capacitor current. (I find the 3rd harmonic in phase distortion the easiest to visualize.)
respectfully