Final answer:
The stability of a DC series motor refers to its ability to maintain constant speed even when the load or operating conditions change. A series motor is considered to be unstable because as the load increases, the motor's speed decreases due to the increased current drawn and decrease in back emf.
Step-by-step explanation:
The stability of a DC series motor refers to its ability to maintain a constant speed even when the load or operating conditions change. A series motor is considered to be unstable because as the load increases, the motor's speed decreases. This is due to the fact that the motor draws more current to produce the necessary torque to overcome the increased load. As a result, the back emf (electromotive force) of the motor decreases, causing the motor to slow down.
One way to think about the stability of a series motor is to compare it to a car going uphill. When the car goes uphill, it loses speed because it needs more power to overcome the force of gravity. Similarly, a series motor loses speed as the load increases because it needs more power to produce the necessary torque.
In summary, a series motor is considered to be unstable because its speed decreases as the load increases. This is due to the fact that the motor draws more current to produce the necessary torque, leading to a decrease in the back emf of the motor.