What is winding and constructions?

Alternating Current (AC) winding is generally of the 3-phase type due to the inherent advantages of the 3-phase machine. Armature coils must be connected to the yield balance (equivalent in magnitude and 2/3 of the series phase difference of the rod) 3-phase electromotive forces (emf). The armature should be divided into step-bands to start with slots around the perimeter.

In the construction of AC machines, the frame is used as an external protective shield to protect it from environmental conditions. The frame also serves as the outer perimeter, allowing the interior to be easily placed. The static state section of the devices is the stator winding attached stator.

The first AC motor in the world of Italian physicist Galileo Ferraris.
CC0 1.0 | Image credits: https://commons.wikimedia.org | Silvanus Phillips Thompson

A rotor is a moving part that moves clockwise or counterclockwise depending on the thrust applied to it. The bearings provide optimal friction for the rotor to run smoothly. The fan is installed to eliminate unwanted heat received while the rotor is running. It is taken out by the ventilation provided in the back of the machine. The shaft is provided to give a mechanical output when the rotor rotates. Slip rings are used for a typical AC machine that uses a rotating armature stationary field winding. In this case, the slip rings allow the input alternating current to flow continuously through the coils.

Phase grouping

Consider a case where the Slot per phase per pole (SPP) is an integral number. Such AC winding is called integral-slot winding. For instance, assume 12 slots for a pole pair for a 3-phase armature, then the Slot angle becomes360°12=30°. Let the one coil-pitch has full six slots, so there will be a total of two pitches, 12 Slots from left to right; the six slots have a pole effect with the specific direction of the electromotive force (emf) on the coil-side, and the other six slots have the opposite pole effect in the opposite direction of emf.
Single-layer or double-layer AC winding uses the 60° phase group. It is also possible to use the 120° phase group, in which the slots used should be under poles and divided into three phase-bands. For instance, each phase band has four slots. The Slots for the return coil-side for this phase-grouping are not available on single-layer winding. It is used only for double-layer winding. The 120° phase group is rarely approved.

Types of winding

Single layer winding

Single-layer windings are generally not used in practice except on certain kilowatt machines due to the aforementioned errors. Single-layer winding diagrams can be centered, lap, or wave type. Only the concentrated type winding is described here, while the lap type is described in the two-layer winding‌. Wave winding is not used in AC winding machines due to some problems with the end connection.

Concentrated winding

Concentrated windings can be classified into two main categories, half-coiled and whole-coiled. In the first type, coils with a phase group are centered on adjacent pole pitches. The duration of individual coils may be more or less than the pole pitch but the average coil span is equal to one pole pitch. Such an arrangement is made to prevent the crossing of two coils under one phase group. In a split AC winding, each coil group is divided into two sets of centered coils, and the return coil-side is shared with the other group. It is clear from the data that such an arrangement is possible only when the SPP is equal.

Double layer winding

Double layer windings are of two important classes depending on the slot per phase per pole (SPP) winding value. Either SPP is an integer or SPP is a fractional number. To meet the symmetry requirement between phases, the number of slots divided by phase must be an integer.

The first phase-group coils spreading just under the pole pair are connected in series (the end of the first coil is connected to the start of the next coil to the right of the first coil). The second coil-group of the phase is below the South-North poles and must therefore be connected opposite the first coil-group for the additive emf. It can be seen that the alternating coil-groups are connected in reverse. The winding looks like a partition. It can also be observed that the coil-sides in a given slot belong to the same phase. All coil-groups of phase can be connected in series or series-parallel.

Integral slot winding

So this type of winding has the SPP as an integer. SPP is the fractional number of windings called fractional slot windings. The partial slot winding is easily adapted with a double-layer arrangement.

Advantages of AC winding

Although the twisting of a fractional slot may seem somewhat complicated, there are some technical advantages and it can be done easily. The number of spaces for selected weapons does not need to be a significant multiplication of the number of poles. So one may choose a certain number of spaces where there is notching gear. This results in savings on machine tools. This flexibility can be used effectively when the number of poles varies widely as in the case of synchronization equipment.

Construction of AC machines

However, the stator and rotor are the main components of an induction machine. The stator consists of three main parts: the outer frame, the stator core, and the stator winding. The outer frame is the outer case of the motor. Its function is to support the stator core and protect the internal components of the machine from mechanical impacts. The stator core is made of thin soft-iron laminate with a thickness of 0.3 to 0.5 millimeters to reduce hysteresis and eddy current losses. In addition, each lamination is insulated from the other with a thin layer of varnish on both sides for better insulation. The slots are punched on the inner edge of the stamping. The stator winding carries the three-phase winding to the stator core, where the six terminals of the winding are connected to the terminal box of the machine. Moreover, the three-phase closure is connected to the star or delta.

The rotor is that part of the motor which moves or rotates. It has cooling wings. At the rear are another bearing and a cooling fan connected to the rotor. There is a fan casing around the fan. There are two kinds of rotors: phase wound rotor and squirrel cage rotor. Most induction motors used in the industry have a squirrel cage rotor‌ due to their simple and rigid structure. It has a laminated cylindrical core with semi-closed circular slots on the outer perimeter. Copper or aluminum rings are placed at each end of Copper or Aluminum bar conductors in these slots. Moreover, the rotor winding is permanently short-circuited and no external resistance is added to the rotor circuit. The slots are inclined compared to the shaft to achieve smooth and sufficient torque and reduce the magnetic locking of the stator and rotor and increase the rotor resistance as the rotor bar conductor length increases.

On the other hand, phase injury involves stampings such as the rotor, stator core. Many semi-closed slots have holes on their outer perimeter. The 3-phase insulated winding is installed in these slots. The rotor should have the same number of poles as the stator. The rotor winding star is connected and its other three terminals are connected to the slip ring. In this case, any external resistance is added to the rotor circuit. The rotor is also curved as in a squirrel cage rotor. Finally, the shaft is fixed to the rotor to transfer mechanical energy.

Context and Applications

This topic is important in both undergraduate and postgraduate studies in:

  • Bachelors of Technology in Electrical Engineering
  • Masters of Technology in Electrical Engineering

Practice Problems

Q1. Which of the following is same in all the coils of distributed windings?

a) Length
b) Pitch
c) Width
d) Thickness

Answer - a

Explanation - In distributed winding all the cores have the same lengths.

Q2. In which of the following one core is placed in each slot?

a) Double layer winding
b) Single layer winding
c) Concentric winding
d) None of the above

Answer- b

Explanation - One core is placed in each slot in Single layer winding.

Q3. In which of the winding number of coils per pole per phase is equal to 1?

a) Double layer winding
b) Single layer winding
c) Concentrated winding
d) All of the above

Answer - c

Explanation - In concentrated winding, the number of coils per pole per phase is equal to 1.

Q4. Which of the following are the main components in the construction of AC induction motors?

a) Stator
b) Rotor
c) Both of the above
d) None of the above

Answer - c

Explanation - Stator and rotor are the main components of the AC induction motor.

Q5. What is the important benefit of distributing the winding in slots?

a) It reduces the size of the machines.
b) It reduces the harmonics in produced electromotive force.
c) It adds mechanical strength to the winding.
d) None of the above.

Answer - b

Explanation - If the winding is distributed in slots, It then reduces the harmonics in generated emf.

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