Using the motor performance curves shown in the off standard-frequency graph of the illustration, what would be the impact of operating a motor significantly below its rated frequency? See illustration EL-0006.
• Use the off standard-frequency graph (right-hand chart) only, not the voltage chart. • Look at how each performance curve (RPM, starting current, full load current, starting & breakdown torque) moves when frequency goes below 0% (to the left). • Remember that a negative change in frequency on the x‑axis corresponds to operating below rated frequency.
• On the frequency chart, when you move several percent left of 0 on the x-axis, which curve clearly shows a decrease in percent performance on the y-axis? • For each answer choice, find its matching curve on the graph and ask: does that curve go up, down, or stay about the same as frequency decreases? • Which option describes the quantity whose curve trends downward the most when frequency is reduced well below normal?
• Be sure you are reading the frequency (right) chart, not the voltage (left) chart. • Confirm you are following the correct labeled curve for each choice (RPM, starting current, full load current, starting & breakdown torque). • Check the sign: a line sloping down into negative values means the performance quantity is lower than normal at reduced frequency.
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