Armature Winding in DC Machine

Armature Winding in DC Machine

Armature winding is defined as an arrangement of conductors to produce EMF by relative motion in a magnetic field.

In a DC machine, the armature is a rotating part. And it provides housing to the armature winding.

There is a number of slots available in the armature core and these slots in which conductors are housed.

As we have seen in the construction of a DC machine, the armature is placed between the magnetic poles and it is free to rotate.

The conductors are connected in parallel to increase the voltage rating and the conductors are connected in series to increase the current rating.

Here you can find the various terms that are related to the armature winding;

Terms Related to Armature Winding

1) Conductor

The active length of wire in which EMF is induced in armature winding is known as a conductor. It is also known as the length of wire embedded in the slot of an armature.

2) Turn

When two conductors placed in different slots connected together, it forms a close path is called a turn.

3) Overhang

The portion of a coil that is not participating in the energy conversion process. But it is used to provide a connection between two active conductors known as an overhang.

4) Coil

The two active conductors along with the overhang create a coil. The coil may be single turn or multi-turn.

5) Coil side

A coil consists of two coil sides placed in different slots.

6) Coil span (coil pitch)

The distance between two coil sides of a coil is known as coil span or coil pitch. It is measured in terms of armature conductors.

7) Pole pitch

The total number of armature conductors divided by the number of poles is known as pole pitch. It is denoted as τp.

It is also defined as the periphery of the armature divided by the number of poles.

    \[ \tau_p = \frac{\pi D}{p} \]

In simple words, the pole pitch is the distance between the two adjacent poles.

  • If the coil span is equal to pole pitch, the winding is known as full-pitched winding.
  • If the coil span is less than the pole pitch, the winding is known as short pitch winding.

8) Back pitch

The back pitch is defined as the distance between the top and bottom coil sides of a coil (measured from the back of the armature). It is denoted as Yb.

The back pith is measured in terms of armature conductor or coil sides. It is always an odd number and nearly equal to pole pitch.

9) Front pitch

The front pitch is defined as the distance between the two coil sides connected to the same commutator segment. It is denoted as Yf.

The front pitch is measured in terms of armature conductors or coil sides. It is always an odd number.

10) Winding pitch (Y)

The winding pitch is defined as the distance between starting ends of two nearby coils. It is denoted by Y and it is always an even number.

For lap winding: Y = Yb -Yf

For wave winding: Y = Yb+Yf

11) Commutator pitch

The distance between two commutator segments where the starting and finishing end of a coil are connected is known as the commutator pitch. It is measured in terms of commutator segments.

Types of Armature Winding

According to the connection of a conductor, the armature winding of a DC machine is classified into two types;

  • Lap Winding
  • Wave Winding

Lap Winding

In lap winding, there are many parallel paths and the number of parallel paths is equal to the number of poles in a DC machine.

Hence, this winding is known as multiple winding or parallel winding.

It is necessary to use lap winding when the machine is used for an application where a large current is required.

Therefore, the lap winding is suitable for comparatively low voltage and high current DC machines.

Lap Winding

In lap winding, the coils overlap each other. An equalizer connection is required in case of lap winding.

In lap winding, the starting and finished ends of a coil are connected to the adjacent commutator segments.

Hence, the finishing end of the first coil connects to the same commutator segment as the starting end of the second coil. And so on.

Conditions for simplex lap winding:

  • The back pitch and front pitch must be nearly equal to the pole pitch.
  • The back pitch and front pitch must be odd numbers. And both should not be equal but the difference between them should be equal to 2.
  • Commutator pitch should be ±1.
  • Winding pitch should be even number.

    \[ Y_b = \frac{2C}{P} ± K \, and \, Y_b = Y_f ± 2 \]

Where,

C = Number of coils
P = Number of poles
K = Number for making back pitch odd integer

Wave Winding

In wave winding, the number of parallel paths through the armature winding is equal to two (A=2). The wave winding is also known as series winding.

Generally, the wave winding is used in high voltage low current DC machines.

For the same number of pole and armature conductors, the wave winding gives more EMF than lap winding.

In wave winding, an equalizer connection is not necessary. To provide mechanical balance in the armature winding, sometimes, dummy coils are used.

In wave winding, the finishing end of the first coil is connected to the starting end of another coil that is located under a similar pole to the one under which the first coil started. It means the starting of two consecutive coils are nearly equal to twice the pole pitch apart.

The front-end connection will not overlap each other. And it looks like a wave. Therefore, this type of winding is known as wave winding.

Wave Winding

Conditions for simplex wave winding:

  • The back pitch and front pitch must be an odd integer. They may be equal or differ by 2.
  • The winding pitch must be even number.
  • Commutator pitch Y_c = \frac{Y}{2} and it must be integer.

    \[ Y = \frac{2C \pm 2}{\frac{P}{2}}  \, and \, Y_c = \frac{Y}{2} \]

Positive sign for progressive winding and Negative sign for retrogressive winding.

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