Course work help

If your mind works that way try  to do this without memorising some magic formula you cannot remember the units for.

Just rebuild the formulae from the basics.

Power is force times distance moved in one second

Power is volts times amps across a resistor (voltage sources have no internal resistance.)

generator voltage is more or less proportional to revs. for small changes. (depends a bit what you assume about the field - series wound versus permanent magnet differ in that way, as  in one case the magnet changes as well as the armature current.)

What do we know

Well we know the torque (that's a force and a radius (- to convert to force times distance you need to realise that there s a a factor of 2pi between radius and the circumference of the circle. )

So we know the force, and as we know the distance that force moves in a second (n revs of the circumference)  we can get to watts.

Do not be tripped there are 60 seconds in a minute, we need revs per second....


If we know supply voltage and watts of work being done, we know current for the whole thing, if it was lossless.


So we know that spinning at that speed the as a genset the motor generates 115V, in opposition to the 120V supply. The extra 5V is what is pushing the current we just calculated, and we can get back to an effective load resistance from that and the current we have just calculated.


Now we want half as much power again, but the load resistance does not change so the voltage across it must rise, and therefore the back EMF must fall, and as this generator voltage is reduces ,  pro-rate do the revs.


Give it a go and come back. I have the rough numbers in my head but I do not want to poison your thinking too early.

M

If your mind works that way try  to do this without memorising some magic formula you cannot remember the units for.

Just rebuild the formulae from the basics.

Power is force times distance moved in one second

Power is volts times amps across a resistor (voltage sources have no internal resistance.)

generator voltage is more or less proportional to revs. for small changes. (depends a bit what you assume about the field - series wound versus permanent magnet differ in that way, as  in one case the magnet changes as well as the armature current.)

What do we know

Well we know the torque (that's a force and a radius (- to convert to force times distance you need to realise that there s a a factor of 2pi between radius and the circumference of the circle. )

So we know the force, and as we know the distance that force moves in a second (n revs of the circumference)  we can get to watts.

Do not be tripped there are 60 seconds in a minute, we need revs per second....


If we know supply voltage and watts of work being done, we know current for the whole thing, if it was lossless.


So we know that spinning at that speed the as a genset the motor generates 115V, in opposition to the 120V supply. The extra 5V is what is pushing the current we just calculated, and we can get back to an effective load resistance from that and the current we have just calculated.


Now we want half as much power again, but the load resistance does not change so the voltage across it must rise, and therefore the back EMF must fall, and as this generator voltage is reduces ,  pro-rate do the revs.


Give it a go and come back. I have the rough numbers in my head but I do not want to poison your thinking too early.

M