posted on Feb, 28 2010 @ 05:38 AM
Originally posted by ATS4dummies
With a conventional stator generator (magnets and wires) as you consume electricity, it becomes harder to turn. This is also called back emf.
Well not exactly as you say there.
The reason the generator becomes harder to turn as you load it up is simply that you must input, as mechanical energy, the power that it's delivering
as electrical energy plus the losses in the overall system. Simply put, the more load applied to the system the greater the effort required to supply
it.
It's a popular misconception that 'back-EMF' is a loss but that couldn't be further from the truth. Back-EMF is the voltage generated by a
conductor (generator windings) moving through a magnetic flux IE the output of the generator IS the 'back-EMF' which is 100% useful.
In the case of a motor, the motion of the windings through the magnetic flux generates a voltage in opposition to the applied voltage which is
proportional to the speed of the motor but again, it is not opposing the motion in any way unless it can establish a current in opposition to the
applied current which, of course, it can't because it's always less than the applied voltage in the case of a motor. When the motor is at
standstill there's 0 back-EMF so at that time it draws 'locked rotor current'. As it speeds up the back-EMF rises, reducing the effective voltage
on the winding until it the motor reaches a speed where the difference between applied and back-EMF applied to the effective impedance of the motor
represents the actual load on the system. Back-EMF drops as the motor slows due to a load increase allowing the input power (I^2.Z) to increase to
match the load. So, in brief, back-EMF is how a motor actually regulates its input power in relation to the load applied and without back-EMF a motor
would draw locked rotor current at all loads which would be absolutely useless.
Homopolar generators are simple and largely impractical except for a few monsters that have been built for the purpose of providing high DC current
sources for laboratory use. What makes them impractical is the extremely low voltage they generate per disc.