The last two posts have some valuable information, if you still are having a hard time try this explanation...
1. Whenever a current passes through a wire, a magnetic field occurs around the wire. In a motor that wire is in turn wound around a metal structure that concentrates the magnetic field produced by the current flowing through the wire. A coil of wire resists the current flowing through it during the time that the magnetic field is changing to a steady state. In DC coils, this occurs when current is first applied and then reduces to the wire resistance. Input current is low when first applied and then builds to Ohm's Law values after some length of time. Likewise when the source is removed, the collapsing magnetic field will try to keep the current flowing. Coils that have many windings and strong magnetic fields will self generate kilovolt outputs while the field is collapsing. The ignition coil in cars operate on this principle. If the collapsing field can generate sparks in your car, the field in a motor can generate sparks across the brush assembly.
2. A loaded motor draws more average current than one with little or no load. (Remember the brush is in contact with a motor winding for a longer period of time.) Since the strength of the magnetic field in a DC coil is a function of the number of turns and the current in the wire, it follows that a loaded motor will have a much higher field generated by the winding. Since there is several windings in close proximity inside the motor all of them are within the field generated by the winding that has current flowing in it.
3. A coil of wire either moving inside a steady magnetic field or non-moving inside a changing magnetic field will have current induced in it. In most of the motors we use on the robot, the wire is moving inside a stationary magnetic field, so as the motor turns current is induced in the wire if the circuit is a closed loop.(coil is connected through brushes to the speed controller.)
4. The collapsing field causes a current to flow in the opposite direction of the current that created the field in all windings that are in the presence of the magnetic field. (i.e. all motor windings)
5. This phenomena was first studied in the early stages of motor development. Those engineers used the term EMF (Electro Motive Force) to describe the power supply (battery) in their experiments. Since the current induced by the collapsing field is opposite of the current that created it and in observation it appeared that another voltage source was opposing the applied power, the engineers called the phenomena "back EMF".
The changing magnetic fields in a motor creating high voltage arcing are not peculiar to DC motors as anyone with an AC drill can attest. Simply, any wire in a changing magnetic field will have current induced in it. The stronger the field, the more wire in the field, and the time it takes for the field to change all affect the level of voltage induced in the winding.
Commonly AC drill motors are "series wound", i.e. the field and armature windings are wired in series through the brushes. Other types of AC motors have no brushes and use the sine wave of the AC line to create the changing magnetic field. Brushless DC motors use drive circuitry to change the DC current into AC current and then operate as an AC motor.
This is a difficult principle to discuss in this forum. If anyone has any questions, please contact me.
askierkiewicz@wttw.com
Good Luck All