D.C. windings are

sometimes 2-layer type
never 2-layer type
windings are A. sometimes 2-layer type B. never 2-layer type C. always 2-layer type
none of the above

The correct answer is: D. none of the above.

DC windings can be either 1-layer or 2-layer type, depending on the design of the motor. 1-layer windings are simpler and less expensive to manufacture, but they have lower efficiency than 2-layer windings. 2-layer windings are more complex and expensive to manufacture, but they have higher efficiency.

The type of winding used in a DC motor is determined by the motor’s application. For example, a motor that is used in a high-performance application, such as a power tool, will typically have a 2-layer winding. A motor that is used in a low-performance application, such as a fan, will typically have a 1-layer winding.

Here is a brief explanation of each option:

  • Option A: Sometimes 2-layer type. This is not always the case. DC windings can be either 1-layer or 2-layer type, depending on the design of the motor.
  • Option B: Never 2-layer type. This is also not always the case. DC windings can be either 1-layer or 2-layer type, depending on the design of the motor.
  • Option C: Always 2-layer type. This is not always the case. DC windings can be either 1-layer or 2-layer type, depending on the design of the motor.
  • Option D: None of the above. This is the correct answer. DC windings can be either 1-layer or 2-layer type, depending on the design of the motor.