DC Generator

What is a DC Generator?

Machines that convert mechanical energy into electrical energy are called Electric Generators. The electrical energy generated is further transmitted and distributed through power lines for domestic, commercial use. There are two types of generators,

  1. AC Generator

  2. DC Generator

A DC generator is the type of electrical generator that converts mechanical energy into direct current electricity. Here, energy conversion is based on the principle of dynamically induced EMF production. These generators are most suitable for off-grid applications. DC generators supply continuous power to electric storage instruments and power grids (DC). 

Parts of DC Generator

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DC Generator consists of the following parts -

  1. Stator- A stator is a set of two magnets placed in such a way that opposite polarity faces each other. The purpose of the stator is to provide a magnetic field in the region where the coil spins.

  2. Rotor- A rotor is a cylindrical laminated armature core with slots. 

  3. Armature Core- The armature core is cylindrical in shape and has grooves on the outer surface. These slots accommodate armature winding in it. 

  4. Armature Winding- These are the insulated conductors placed in the armature core. Because of them, the actual conversion of power takes place.

  5. Field Coils- To produce the magnetic field, field coils are placed over the pole core. The field coils of all the poles are connected in series. When current flows through them, adjacent poles acquire opposite polarity. 

  6. Yoke- The outer hollow cylindrical structure is known as Yoke. It provides support to main poles and inter poles and gives a low reluctance path for the magnetic flux. 

  7. Poles- The main function of the poles is to support the field coils. It increases the cross-sectional area of the magnetic circuit, which results in a uniform spread of magnetic flux. 

  8. Pole Shoe- To protect the field coil from falling and to enhance the uniform spread of magnetic flux pole shoe is used. The pole shoe is fixed to Yoke.  

  9. Commutator- The commutator is cylindrical in shape. Several wedge-shaped, hard drawn copper segments form a commutator. The functions of a commutator:

  • To connect stationary external circuits to the rotating armature conductors through brushes and

  • To convert induced alternating current into direct current.

Working Principle of DC Generator

A DC generator operates on the principle of Faraday’s laws of electromagnetic induction. According to Faraday’s law, whenever a conductor is placed in a fluctuating magnetic field (or when a conductor is moved in a magnetic field) an EMF is induced in the conductor. If the conductor is guided with a closed path, the current will get induced.  The direction of the induced current (given by Fleming’s right-hand rule) changes as the direction of movement of the conductor changes.

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For example, consider the case, an armature rotating in clockwise direction and conductor at the left moving in an upward direction. As the armature completes its half rotation the direction of movement of the conductor will get reversed downward. The direction of the current will be alternating. As the connections of armature conductors get reversed, a current reversal takes place. Thus, we get unidirectional current at the terminals.

EMF Equation of DC Generator

The EMF equation for DC generator is expressed as,

Where,

Eg – Generated EMF across any parallel path

P- Total number of poles in the field

N- Rotational speed of armature(rpm)

Z- Total number of armature conductors in the field.

Ø- Magnetic flux produced per pole.

A- number of parallel paths in the armature.

Eg = (PØNZ)/60A

Losses in DC Generators

While converting the mechanical energy into electrical energy, there are losses of energy i.e. whole input isn’t converted into output. These losses are classified into mainly three types:

  1. Copper Loss- These losses occur while current flows through windings and are of three types: armature copper loss, field copper winding loss and losses because of brush resistances.

  2. Iron Losses- Due to the induction of current in armature, eddy current losses and hysteresis loss occur. These losses are also called Core losses or Magnetic losses.

  3. Mechanical Losses- Losses which occur because of friction between the parts of the generator are called mechanical losses.

Types of DC Generators

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Applications of DC Generator

  1. The separately excited type DC generator is used in power and lighting purpose using the field regulators.

  2. The series DC generator is used in arc lamps for stable current generator, lighting and booster.

  3. Level compound DC generators are used to supply power to hostels, offices, lodges.

  4. Compound DC generators are used for supplying power to DC welding machines.

  5. A DC generator is used to compensate for the voltage drop in the feeders.

FAQ (Frequently Asked Questions)

Q1. Derive the EMF Equation of the DC Generator for One Armature Conductor?

Ans-

1] induced EMF for one armature conductor

Let,

Eg – Generated EMF across any parallel path

P- Total number of poles in the field

N- Rotational speed of armature(rpm)

Z- Total number of armature conductors in the field.

Ø- Magnetic flux produced per pole.

A- number of parallel paths in the armature.

e- Rate of cutting flux.

According to Faraday’s law,

 e=dØ/dx

e=(total flux)/( time taken )

Therefore,

e= ØPN/60

Q2. Derive the EMF Equation of the DC Generator for Induced EMF for DC Generator?

Ans:

Induced EMF for DC generator

As we know, emf in each path is the same across the line

Hence,

Induced emf in DC generator

Eg = emf of one conductor × number of conductors connected in series.

Induced emf in DC generator is,

e= (ØP N/60) X (Z/A) 

For simple wave wound generator,

Total number of parallel paths are 2 = A

Therefore,

Induced emf for wave type of winding generator induced emf will be

(ØPN/60)  X  (Z/2)=  ØZPN/120

For simple lap-wound generator, the number of parallel paths is equal to the number of conductors in one path

i.e. P = A

Therefore,

Induced emf in DC generator is,

Eg = (PØNZ)/60A