Final answer:
The answer is Option D. A Shaded-pole motor is an AC motor that is recognized for its low starting torque suitable for applications such as refrigerator evaporator motors due to its simple and reliable design.
Step-by-step explanation:
The type of AC motor characterized by low starting torque and common in lightweight applications such as refrigerator evaporator motors is the Shaded-pole motor. This motor is suitable for these applications because it is inexpensive to produce and has a simple construction without the need for brushes or commutators. Shaded-pole motors use a shaded pole to provide the starting torque and do not use switchgear, making them highly reliable with a long service life, albeit with less efficiency compared to other motor types.
A Shaded-pole motor works on the principle that a small portion of each pole is wrapped by a short-circuited turn of wire, known as a shading coil. When AC power is applied, the magnetic field in the shaded portion lags the field in the unshaded part of the pole, creating a rotating magnetic field that is necessary to turn the motor's rotor. These motors are some of the simplest and most robust designs available, making them a favourite choice for many low-torque applications. Shaded-pole motors are single phase induction motors that have a rotor consisting of a squirrel cage winding and a stator with salient poles.
They are inefficient and have low starting torque due to the presence of a short-circuited copper ring or shading coil. This creates a small rotating magnetic field that lags behind the main magnetic field, causing the motor to start rotating.