Using the motor performance curves shown in the off standard-voltage graph of the illustration, what would be the full load current of a 450 VAC motor rated at 75 amperes of current at full load if the motor supply voltage is 405 VAC? See illustration EL-0006.
• Use the off standard-voltage graph on the left side of EL-0006 (percent change in voltage vs. percent change in motor performance). • Compute the percent change in voltage going from 450 VAC (rated) to 405 VAC (actual). • From the graph, find the percent change in full load current for that voltage change, then apply that percentage to the rated 75 A full-load current.
• What is the percent voltage change when dropping from 450 VAC to 405 VAC, and is it positive or negative on the horizontal axis? • On the off standard-voltage graph, where does the full load current line intersect that percent voltage change, and is the current increasing or decreasing? • After you find the percent change in current from the graph, how do you convert that percentage into actual amperes starting from 75 A? Write out the math.
• Be sure you are using the voltage graph (left) and the full load current line, not starting current or torque. • Double-check your percent voltage change calculation: (ΔV / rated V) × 100%. • After applying the percent change to 75 A, verify that your final value is reasonable: does current go up or down when voltage is reduced, according to the graph?
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