Kind of hard to explain in words and without knowing whether you
have any exp with electriclal theory; maybe someone will come up
wiht a link.
: But of course that power goes *somewhere* right?
Sort of. Your assumptions will sort of work, but they're not
what's really happening.
In a resistor ckt, current and voltage are in phase. When the
ac sine wave is at its max point, so is current. Voltage drops,
current drops accordingly.
: In the interest of conservation of energy, even if that power
: no useful work in your electric motor, it's doing work
: I'm sure it's an obvious point but the answer isn't evident to
In an electric motor, the windings are a big coil. It sounds
like you understand that a little bit. Coils resist changing
currents. So, if the voltage jumps to its max, the current rises
slower than the voltage can rise because it has to create the
building magnetic field.
But in an ac motor, the voltage begins to fall (passes the
peak) before the current has made it all the way to the max it
would have reached if the voltage had stayed there. But the
voltage is falling toward zero now, and as the voltage falls, the
magnetic field begins to collapse. But, since it's a coil, it
cannot fall as fast as the voltage is falling. The voltage
passes zero now an continues on toward its negative peak, with
the current still trailing it, passes that peak, befoer the
current catches up, and starts toward zero again, and so on as
long as the power is applied.
P=IE but p does not= ie. (lower case means ac, upper DC). At
any point in time, where the voltage is max, the current is NOT
yet at max, and thus the power (p=ie) will be less than P=IE.
Current never gets to max, in fact for motors. So a straight
p=ie formula gives a lower wattage than if the current had
reached the max it COULD have reached, fi the voltage had stayed
there long enough.
Capacitors are just the opposite. The don't resist current
change, but they do resist voltage change. It takes time to
charge up to and discharge from a known voltage.
: If you have a PF 70% motor chewing up 700 watts, then 300 watts
: into heat loss of the inductive windings?
Sort of. The "lost" energy does create heating in the windings.
Perhaps the constant building
: up and tearing down of the magnetic flux is causing the
: via atomic realignments in the inductor itself?
Yup. It takes time for the flux field to build and time to
collapse, so it can't change as fast as the voltage does that's
being applied to it.
And similarly if you
: have a capacitive reactance device, the power loss goes into...
: Heat loss of the electrons rushing into and out of the
Capacitors store electrons. So, they spend time collecting
electrons while the voltage is applied, and then spend time
losing the electrons when the voltage is removed.
In both cases the speed of collection/loss of electrons depends
on the DC resistance components in the ckt. A resistor basically
passes current instantaneously since there is no reactive element
Capacitor stores electrons.
Inductor creates current flow from a collapsing field, resists
them during the building of hte field. Limited by the resistance
: If anyone has an understanding of this, I'd love to hear it...
: wondering about this one for a while. :)