A motor is normally rated for a given full load speed at a certain voltage. The emf will be more nearly given by its no load speed, which will always be higher.
How much effect this will make depends on the internal losses of the motor at full load. Large motors generally have a fairly low speed drop with load, some small ones may have a lot.
If you consider the motor in question and drive it from another motor, the current will fall as you speed it up and as you drive it slightly above its no load speed the emf will equal the supply volts. Drive it above this speed and it will start to generate. Roughly it will generate its rated current as a motor at about the same % speed above no load as the % below as a motor.
Typically the no load speed may be 10 to 20% above the rated speed so your 2100 rpm motor may generate 24v emf at perhaps 2300 rpm. The cut in speed at 12v would be half this or 1150 rpm.
Actual figures can only be determined from testing a particular motor, but the cut in speed as a generator will be above the nominal.
There is a secondary factor that comes into play that may also affect things. The brushes on a simple dc machine are shifted from the neutral axis to reduce sparking and the shift for a motor is the opposite way to a generator. This brush shift has some influence on the field but is not often very marked with permanent magnet devices that tend to have a strong field.
Ideally when you use it as a motor you should run it backwards and reverse the polarity of the leads for least sparking but I doubt that anyone does this.
Flux
