In the design process of an alternator, it is desired that the number of poles be four-pole, three-phase and two-layer winding. Which of the following number of slots is NOT desirable for the design?

If the number of slots(s) per pole(p) per phase(ϕ)  is an integer, then the winding is called an integral-slot winding i.e. \(q = \frac{s}{{p\phi }}\) = integer.

We have different slot values and pole (p) = 4 and phase (ϕ) = 3

1. For s = 12, \(q = \frac{{12}}{{3 \times 4}} = 1\;\)= integer
2. For s = 24, \(q = \;\frac{{24}}{{3\; \times 4}} = 2\) = integer
3. For s = 48, \(q = \;\frac{{48}}{{3 \times 4}} = 4\) = integer
4. For s = 32, \(q = \;\frac{{32}}{{3 \times 4}} = 2.66\) ≠ integer

So correct answer is s = 32.

Important Points 

Types of  winding:

Concentrated Windings:

• All winding turns are wound together in series to form one multi-turn coil.
• All the turns have the same magnetic axis.

Examples

• field winding for salient-pole synchronous machine.
• D.C. machine
• Primary and secondary winding of a transformer.

Distributed Windings:

• All winding turns are arranged in several full-pitch or fractional-pitch coils.
• These coils are then housed in the slots spread around the air-gap periphery to form phase or commutator winding.

Examples

• Stator and rotor of induction machines.
• The armatures of both synchronous and D.C. machines.

Single-layer winding:

• One coil-side occupies the total slot area.
• Used only in small ac machines.

Double-layer winding:

• Slot contains 2 layer winding.
• Used nearly for 5kW machines.

Similarly multi-layer winding

Used above for 5kW machines.