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Ya gotta love it....students who have never been on the website before, suddenly pop up the day of their first posting, and need complicated final questions, or entire projects done for them...then get really pissy when you call them on it.
On a DC motor assuming the 200 ft# is dead stall torque so under that condition you can treat the motor as a purely resistive load. The standard voltage Vs current formulas would tell you the total power drop and thusly the torque decrease based on the voltage and theoretical ohms load the motor would equate to.
An AC motor in practical life terms would not show any real performance drop from a 10% voltage decrease. The complex induction and reactance in an AC motor will make it just increase the amps to compensate for the less incoming voltage.
AC motors typically have a sweet band instead of a sweet spot. That is they have a window of voltage and current that will give the same output capacity.
Most common AC motors have a plus 10% to minus 15% nominal voltage input. At any point in that range the rated output power is achieved and the running efficiency will typically only vary a few percent.
However that sweet band can also vary greatly when dealing with specialty motors or just cheaply made motors too.