Final answer:
Stepper motors have minimum torque when at standstill and may have slower starting characteristics compared to other motors. They can also be less efficient at low speeds or when not optimally driven, leading to overheating. Understanding motor physics, including back emf and current flow, helps explain these behaviors.
Step-by-step explanation:
The question pertains to the characteristics of stepper motors. Stepper motors are a type of electric motor commonly used for precise control of motion. When considering the options provided:
- (B) Minimum torque when at standstill is the correct characteristic. Stepper motors can hold their position at a standstill without rotating, which means they can maintain torque without motion.
- (D) Slow starting could be considered characteristic of a stepper motor when compared to some other motor types because of their precise control requirements.
- (E) Low efficiency may be observed in certain contexts, specifically when they operate at low speeds for extended periods or are not optimally driven, leading to excessive heat generation.
In terms of the physics of motor operation, when a motor such as a stepper motor is under mechanical load and slows down, the back electromotive force (emf) decreases, resulting in an increase in current flow. This allows the motor to do more work, such as pushing an electric wheelchair up a hill, despite the potential risk of overheating if the current remains too high. Without a mechanical load, the motor speeds up until the back emf nearly equals the driving emf, ultimately reducing the energy used to that which is sufficient to overcome friction.