Showing posts with label DC Machines. Show all posts
Showing posts with label DC Machines. Show all posts

## Differences Between Lap Winding & Wave Winding

Let us discuss on Comparison Between Lap Winding and Wave Winding. All the major differences between lap winding & wave winding provided here.

### Differences Between Lap Winding And Wave Winding

Lap Winding Wave Winding
1. In this winding all the pole groups of the coils generating
e.m.f in the same direction at any instant of time are
connected in parallel by the brushes.
1. In this winding all the coils carrying current in the
same direction are connected in series i.e., coils
carrying current in one direction are connected in one
series circuit and coils carrying current in opposite
direction are connected in other series circuit.
2. Lap winding is also known as parallel windings. 2. Wave winding is also known as series winding.
3. The number of parallel path is equal to the number
of poles i.e., A = P.
3. The number of parallel paths is always equal to 2
i.e., A = 2.
4. The number of brush required by this windingis always
equal to the number of poles.
4. The number of brushes required by this winding
is always equal to 2.
5. The machine using lap winding requires equalizer rings
for obtaining better commutation.
5. The machine using wave winding does require dummy
coils to provide the mechanical balance for the armature.
6. Lap windings are used for low voltage and high current
machines.
6. Wave windings are used for high voltage and low
current machines.
7. Lap windings are generally used for machines of ratings
above 500 kW.
7. Wave windings are generally used for machines of
ratings below 560 kW

## Armature Winding of A DC Machine [ Lap & Wave Winding ]

### 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.

Lap winding may be simplex (single) or multiplex (doublex or triplex) winding. In simplex lap winding the connection of the winding is that there are as many parallel paths as there are number of poles.

Whereas for duplex, the number of parallel paths are equal to twice that of the number of poles and for triplex it is thrice. For this reason, the lap winding is called multiple or parallel winding. The sole purposes of such type of windings are,

(a) To increase the number of parallel paths enabling the armature current to increase i.e., for high current output.

(b) To improve commutation as the current per conductor decreases.

Notes on Lap winding

1. The coil or back pitch YB must be approximately equal to pole pitch i.e., YB = Z/P.

2. The back pitch and front pitch are odd and are of opposite sign. They differ from each other by 2m, where m = 1,2,3 for simplex, duplex, and triplex respectively.

i.e., YB = Y± 2m

When YB > YF i.e., YF + 2m then the winding progresses from left to right and such a winding is known as progressive winding. If YB < YF i.e., YB = YF - 2m
then the winding progresses from right to left and such a winding is known as retrogressive winding.

3. The average pitch,YAVE=YB + YF )/2.

4.Resultant pitch, YR is always even as difference between two odd numbers is even and is equal to 2m.

5.Commutator pitch, YC = m i.e., , 2, 3, 4 etc. for simplex, duplex, triplex, quadruplex etc.

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)/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.

Tags:armature winding of dc machine wikipedia,lap winding and wave winding animation.

## Why the Stator [Yoke] is Not Laminated in DC Machines?

Usually laminations are provided to reduce the eddy current losses taking place in the magnetic materials used to provide path for the flux. Whenever the flux linkages with the magnetic materials changes, e.m.f will be induced in the armature conductors. Apart from this, some e.m.f will also be induced in yoke and core of the machine. As there are closed paths in the yoke and core because of the induced e.m.f’s circulating current flow in them. These circulating currents are called eddy currents and the resultant I²R losses are called eddy current losses.

Both the armature core and yoke are magnetic materials used to provide path for the flux and eddy current losses taking place in both of them. But, eddy current losses in yoke are comparatively less because the induced e.m.f’s in the yoke are comparatively less. because of less variations in the flux linkages with respect to time when compared to armature core i.e., dφ/dt  is less in case of stator. Hence, the stator [yoke] is not laminated whereas the armature core is laminated.

## Commutator | Commutation In DC Machine [Generator & Motor ]

### Commutation in DC Machine or Commutation in DC Generator or Motor

The commutator is a split ring of larger size with large number of splits (commutator segments). It is called a mechanical, rectifier in generator and an inverter in motor. The connections to the commutator depends upon the type of armature windings. These are made of hard copper so as to withstand the brush forces which are placed upon the commutator segments. The position of the brushes is generally based on the winding. Commutator consists of number of segments or bars insulated from each other and are combined together tightly to form a cylinder as shown in figure, and fitted on the insulated Shaft of the armature. This is known as commutator.

To insulate the segments of commutator thin layers of (0.5 to 1 mm) mica is used. The ends of the coils wound on the armature are soldered on the segment of commutator. The insulating mica sheet is usually M or V shaped so as to prevent the segments from flying out due to centrifugal forces.

### Importance of Commutator in a D.C Machine

1. As the voltage build up in the armature conductors is A.C form, to convert it into D.C voltage commutator is used in the external circuit in generator operation, whereas in D.C motor it produces unidirectional torque.

2. It facilitates the collection of current from armature.

3. It helps in connecting the armature with the external circuit.

4. It converts  alternating quantity into a direct quantity.(i.e., voltage or current) and vice—versa.

5. It keeps rotor or armature M.M.F stationary in space even though the armature is rotating.

### Commutator Working | Commutation in DC Machine

The induced e.m.f in the armature conductors of a DC generator can be made unidirectional with the help of commutator or slip rings and brushes.

The principle behind this can be understood with the help of following explanation,

What is the function of a commutator?

The function of commutator is to collect alternating current induced in armature conductor and convert it into unidirectional current.

Constructionally it is cylindrical in shape built with hard drawn high conductivity copper segments. In figure two segments E and F are shown. They are separated by a thin layer of mica insulation usually of V—shape. Two brushes X and Y are mounted on the segments.

In figure (Case 1) the first half rotation i.e., from 0 to 180° the coil position, with the segments are shown. The direction of current flow is from point A to B.

In the next half rotation as in figure (Case 2) the segments are interchanged. Thus, the direction of current in this case is from B to A, similarly though the segments. i.e., 0 to 180°. is from A to B and 180 to 360° is from B to A.

By considering the total revolution 0 to 180° is taken as positive and 180 to 360° as negative. This current is passed through the load resistance R in the external circuit and develops the unidirectional or DC voltage.

## Construction of DC Machines (Motor&Generator) With Diagrams

### Construction of DC Machine

A Direct Current (DC) machine is an energy conversion device. It converts electrical energy to mechanical while working as a motor and mechanical energy to electrical energy while working as generator. Hence, it is known as electromechanical energy conversion device. It consists of stator and rotor. Stator is the assembly of main parts like yoke, main poles, pole shoe, inter pole windings etc., and the rotor is the assembly of armature, armature winding, commutator fan etc.
 Fig.1

### Major Parts in DC Machine Construction

Yoke

Yoke is the outer cover of the machine supporting and protecting the internal parts. It is made of low reluctance ’ material like silicon steel or cast iron, Since, it has to carry the magnetic flux i.e., to provide the closed path for the flux produced through the poles.

Pole Core and Pole Shoe

Pole core is generally a solid material and pole shoe is a laminated one in small machines, but the pole shoe and pole core both are laminated made of annealed steel in modern days. The purpose of the pole core is to provide flux and to support the field windings, Whereas the pole shoe is stretched so as to provide uniform air gap along the armature core and also to provide uniform flux distribution in the air gap.

Brush and Brush Holders

Brushes are the structures placed on the rotating commutator through which the unidirectional current is to be collected. Generally it is made of carbon, which can give smooth surface at the contacts so as to reduce the spark and wear and tear of the commutator bars. These are fixed to the stator core (yoke) by means of brush holders.

Field Windings

The field windings are wound initially on a wooden former and then installed into the pole cOre. These are generally made of low resistivity materials like copper or aluminum. There are two ways of connecting the field winding to the armature in case of self-excited machine. They can be connected in series or shunt. If it is connected in series less numbered turns with larger cross-sectional conductors are used. If it is connected in shunt, the winding would be of large turns, whose cross-section is less, so as to withstand for whole supply voltage.

 Fig.2

Interpoles

These are the pole structures generally smaller than main poles and is placed in between the main poles. These windings of the inter poles are of less turns since it is connected in series with‘armature windings. The main purpose of these inter poles is to reduce the armature reaction, thereby reducing the sparks at the brush contacts. The polarity of the inter pole is made same as that of the main pole ahead of it in the direction of rotation.

Rotor

The rotor is generally rotating part which carries the armature, armature windings and the commutator on the same shaft. The armature core is made of laminated silicon steel. The main purpose is to hold the armature windings and to provide the low reluctance path for the flux.

 DC Motor Animated Image
Commutator Construction In DC Machines

The commutator helps in converting this alternating emf into direct current emf. This is, therefore, also known as mechanical converter.

Armature Windings In DC Machines

The windings placed in the armature is called the armature windings and is generally connected in two ways,
(i) Lap windings
(ii) Wave windings.
Lap windings are preferred for higher currents and low voltage and wave windings are preferred for higher voltages and lower currents.

Tags:-DC machine construction,dc motor,generator construction and working,dc machine construction and working principle