Posted by Anonymous on .
a 12V , 3A motor takes 120 minutes to do a certain job. a 120V, 8A motor takes 5 minutes to do the same job. Which motor uses the least energy and by how much?

science 
bobpursley,
energy=voltage*current*timeinseconds
where energy is in Joules. 
science 
Anonymous,
a)
Power of the motor P = 3.8hp
= (3.8hp)(746W/hp)
= 2834.8W
Motor is 90% efficient.
Therefore Power absorbed by the motor is (2834.8W)/(0.9) =3150W
But power P = V I
then current in the motor I =(3150W)/(120V)
= 26.25A
(b)
Energy delivered to the motor E = P*t
= (3150W)(4h)(3600s/h)
= 4.536 *107J
(c) E = (3150W)(4h)
=(3.15*4)kWh
=12.6kWh
Therefore cost to run the motor for 4h is(12.6kWh)($0.16/kWh)
= $2.016