Generator and counter electro-motive force

[Njguy]

I was hoping someone could confirm that I have this concept correct. If you have a generator made up of permanent magnets on the rotor and coils on the stator when the rotor rotates, it produces a voltage in the coils of the stator. This means that the stator coils produce their own magnetic field of opposite polarity. This is why the generator slows down when you put a load on it because the coils now have more current flowing through them thus producing a stronger opposing field. All of this is called Back EMF correct? Because the rotor slows down under load the voltage also drops off, which is why a governor will then supply more torque to maintain the rpm and voltage thus the required generated wattage...?

Any flaws here?

 

[JimB]

Basically correct.
Rather that referring to back EMF, it may be more correct to refer to back MMF.

MMF = Magneto Motive Force.

The current in the stator coil produces a magnetic field (MMF) which opposes the magnetic field of the rotor, requiring more torque to maintain rotor speed.

If anyone has a better explaination, I am prepared to be corrected.

JimB

 

[crutschow]

Back EMF typically refers to the voltage generated when the current in an inductor is changed.

 

[Mike odom]

More correctly, a current is induced in the stator, not a voltage (the voltage develops as a current flowing through a resistance)... this current flowing, produces its own magnetic field, which is opposed to the primary field that generated the current.

 

[kinarfi]

Also, the rotating magnet is not a permenent magnet, but an electro magnet, thus the current which creates the magnetic field can be controlled and is applied via slip ring, just like your alternator in you car. Some have a permenent magnet just to get things started. If no load is applied there will be no current, but as you apply load, you draw energy and the removal of energy comes from the mover or stored energy of rotating, thus draw energy, rotor slows down or engine works harder.

Mike, I disagree that current is induced in the stator, it is voltage and current is determined by the load.

 

[kinarfi]

Also, when I think of CEMF, I think of it as the voltage generated in the motor which is what restricts the current in the motor, created by the rotating magnet field induced in the rotor by the stator.

 

[KeepItSimpleStupid]

Counter EMF seems over-rated. When I think of counter EMF, I think Vw = Vm-I*Rw

Or simply, Vm (The voltage measured at the motor terminals) is Vw (The effective voltage across the motor) minus (I * Rw) where I is the current flowing and Rw is the winding resistance.

The counter EMF is simply I * Rw.

 

[crutschow]

Also, the rotating magnet is not a permenent magnet, but an electro magnet, thus the current which creates the magnetic field can be controlled and is applied via slip ring, just like your alternator in you car. Some have a permenent magnet just to get things started.
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Contrary to cars, most motorcycles have alternators with permanent magnet rotors. They regulate the voltage by shunting away the excess current (which, of course, is rather inefficient) causing the shunt regulator circuit to get quite hot. I've had two regulator circuits burn out because they weren't sufficiently cooled so now I've added a muffin fan with a shroud to help keep it cool.