DC Machine: Construction, Components, and Armature Winding, Summaries of Electric Machines

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Lecture notes

OF

DC Machines

Department of Electrical engineering

MBM Engineering College, Jodhpur

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Figure 2: Pole Core and Poles Shoes representation

3. Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles. 4. Armature core: Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft.

Figure 3: Armature of DC machine

5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Figure 4: Armature Winding/coil of DC machine

6. Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Figure 5: Commutator of DC machine

Armature Winding Terminology:

Now we are going to discuss about armature winding in details. Before going through this section, we should understand some basic terms related to armature winding of DC generator.

Pole Pitch:

The pole pitch is defined as peripheral distance between centers of two adjacent poles in DC machine. This distance is measured in term of armature slots or armature conductor come between

emfs. If the coil span is less than the pole pitch, then the winding is referred as fractional pitched. In this coil, there will be a phase difference between induced emf in two sides, less than 180o. Hence resultant terminal voltage of the coil is vector sum of these two emf’s and it is less than that of full-pitched coil.

Figure: full pitched and half pitched coils In practice, coil pitch (or Span) as low as eight tenth of a Pole Pitch, is employed without much serious reduction in emf. Fractional pitched windings are purposely used to effect substantial saving in copper of the end connection and for improving commutation.

Pitch of Armature Winding

Back Pitch (YB)

A coil advances on the back of the armature. This advancement is measured in terms of armature conductors and is called back pitch. It is equal to the number difference of the conductor connected to a given segment of the commutator.

Front Pitch (YF)

The number of armature conductors or elements spanned by a coil on the front is called front pitch. Alternatively, we define the front-pitch as the distance between the second conductor of the next coil which connects the front, i.e., commutator end of the armature. In other words, it is the number difference of the conductors connected together at the back end of the armature. We are showing both front and back pitches for a lap, and a wave windings in the figure below.

Resultant Pitch (YR)

It is the distance between the beginning of one coil and the beginning of the next coil to which it is connected. As a matter of precautions, we should keep in mind that all these pitches, though normally stated concerning armature conductors, are also times of armature slots or commutator bars.

Commutator Pitch (YC)

Commutator pitch is defined as the distance between two commutator segments which two ends of same armature coil are connected. We measure commutator pitch in term of commutator bars or segment.

Single Layer Armature Winding

We place armature coil sides in the armature slots differently. In some arrangement, each one side of an armature coil occupies a single slot. In other words, we place one coil side in each armature slot. We refer this arrangement as single layer winding.

Two Layer Armature Winding

In other types of armature winding, arrangement two coil sides occupy every armature slot; one occupies upper half, and another one occupies the lower half of the slot. We so place the coils in two layers winding that if one side occupies upper half, then another side occupies the lower half of some other slot at a distance of one coil pitch away.

Armature Winding of A DC Machine

Based on type of winding connections we classified armature winding of a dc machine into two types. These winding connections are same for DC generator & DC motor. Types of Windings in DC Machine, 1. Lap winding.

2. Wave winding.

Lap winding of a DC Machine

In this type of winding the completing end of one coil is connected to a commutator segment and to the start end of adjacent coil located under the same pole and similarly all coils are connected. This type of winding is known as lap because the sides of successive coils overlap each other.

6. Number of parallel paths = mP. Where, m = multiplicity.

Example: For instance, the number of parallel paths for a 6-pole duplex lap winding is given by 6 x 2 = 12 paths.

7. The total number of poles are equal to the total number of brushes.
8. If Ia is the total armature current, then current per parallel path is Ia /P.
9. Lap winding is used for low voltage and high current machines.

Wave winding of a DC Machine

In wave winding the coils which are carrying current in one direction are connected in series circuit and the carrying current in opposite direction are connected in another series circuit. A wave winding is shown in figure.

If after passing once around the armature the winding falls in a slot to the left of its starting point then winding is said to be retrogressive. If it fails one slot to the right it is progressive.

Notes on Wave winding

The following are the important points to be remembered pertaining to wave winding, 1. Both pitches YB and YF are odd and of same sign.

2. Back and front pitches may be equal or differ by 2 and are merely equal to pole pitch.

3. Resultant pitch, YR = YF + YB = (Z ± 2)/

P = Number of poles Z = Total number of conductors.

4. Commutator pitch, YC = YA (Average pitch)

YC =(Number of commutator bars ± 1)/(Number of pair of poles).

5. Number of parallel paths are equal to 2m,where m is the multiplicity.
6. The number of brushes required are two irrespective of the number of poles.
7. If Ia is the total armature current then current carried by each path or conductor is Ia/2.

8. Since a wave winding is a series winding, it is used for high voltage and low current machine. Emf

Equation of a DC Generator

As the armature rotates, a voltage is generated in its coils. In the case of a generator, the emf of rotation is called the Generated emf or Armature emf and is denoted as Er = Eg. In the case of a motor, the emf of rotation is known as Back emf or Counter emf and represented as Er = Eb. The expression for emf is same for both the operations. I.e., for Generator as well as for Motor