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Why does a 120V-240V dual voltage single phase table saw motor produce 4 times more torque at 240V than at 120V?

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2008-11-18 12:41:19
2008-11-18 12:41:19

I believe it has to do with the windings in the motor. When wired to 120V, half the motor has a positive polarity and the other half has a negative polarity. Then as the current alternates (60 hertz AC) the polarity flips, the magnetic poles oppose each other and cause the motor to spin. I think in a 240v winding the motor is in quadrants instead of halves. So it would be like have 4 magnets opposing each other instead of two and you have double the voltage. The calculation for electrical power (which is converted to torque) is voltage-squared divided by the resistance. If the voltage is doubled, the power is increase by 2-squared, or quadrupled. In reality, the torque produced will be slightly less than 4 times more (because some of the power is lost as heat), but it should be pretty close. -- The second answer is the best answer. The first answer could suggest several things, depending on how you read it. Nonetheless, they are all incorrect. The power calculation (in Watts) is really all you need to do. Voltage squared, divided by resistance. Resistance stays the same, so power quadruples when voltage doubles. If the motor spins at the same speed, then torque should effectively be linear with power, aside from the extra heat created in the components from the higher current.

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It doesn't. A 3-phase motor will provides constant torque because the three alternating fields produce exactly the same effect as a single rotating field.


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