Final answer:
To operate a DC motor at its base speed, ensure the balance of back emf with driving emf, and manage current and resistive power to prevent overheating. Calculate back emf using the difference between driving voltage and the product of current and resistance.
Step-by-step explanation:
To operate a DC motor at its base speed, you must ensure the motor encounters a balanced state where the back electromotive force (emf) is nearly equal to the driving emf. The back emf increases proportionally with the motor's angular velocity and is opposed by the driving emf.
When a normal load is present, the DC motor draws a balanced current to perform its work, but if the mechanical load causes the motor to slow down too much, the current increases significantly due to a lower back emf, and this can lead to overheating and potential damage to the motor from resistive power losses (P = I² R).
For a motor operating normally, you can calculate the back emf using the formula: back emf = V - I * R, where V is the driving voltage, I is the current, and R is the total resistance of the motor. For example, the back emf when a small series-wound DC motor from a 12-V car battery draws 4.0 A under a normal load can be calculated by knowing the resistance in the circuit.
The resistance of the armature and field coils, as well as the back emf at full speed and different speeds, can be determined given specific current draws and the known resistance of part of the motor's circuitry.