DC Generator

What is a DC Generator?

Device which converts mechanical energy to electrical energy is known as a Generator. DC generator being a type of generator produces DC power. It uses the principle of electromagnetic induction to do the conversion process.

Principle of Generator

According to the Faradays law of electromagnetic induction, a conductor moving in a magnetic field cuts the magnetic lines of force. This causes an emf to be induced in the conductor. This induced emf will cause a current to flow if the circuit of the conductor is in closed condition.

                                             

From the principle of DC generator, the most important parts of the DC generator are:

• Magnetic Field
• Conductor moving inside a magnetic field.

Parts of a DC Generator

Important parts of a DC generator are:

1. Stator

It is the fixed and main part of the DC generator. The function of the stator is to supply the magnetic fields were the coils rotate. Stator consists of permanent magnets (two of them with opposite poles facing) which is placed to fit around the rotor.

2. Rotor (Armature Core)

Rotor is the next main part of the DC generator. It consists of slotted iron laminations which are stacked so as to form a cylindrical armature core. The laminations are usually provided to reduce the loss due to eddy current.

3. Armature Windings:

The slots of the armature core are used to hold the armature windings. It is in the form of a closed circuit winding connected in series –parallel to increase the amount of current generated.

4. Yoke

It is the outer frame of the DC generator made using steel or cast iron. It provides the required mechanical strength and helps in carrying the magnetic flux given by the poles.

5. Poles

Poles are used to support the field windings. Generally, the field windings are wound on the poles and they are parallel or serially connected with the armature windings. Also, the poles are given joint to the yoke using welding process or by using screws.

6. Pole shoe

The main function of the pole shoe is to spread the magnetic flux and to prevent the field coil from slipping.

7. Commutator

Commutator acts as a rectifier converting AC voltage in the armature winding to the DC voltage across the brushes. It is made of segments of Cu. Each Cu segment is insulated from each other using sheets of mica and is placed on the shaft of the machine.

8. Brushes

It ensures electrical connections between the commutator and the external load circuit.

Single Loop Generator

Consider a loop, say, ABCD. Assume it is rotating in clockwise direction in a uniform magnetic field with a constant speed. When the loop ABCD starts rotating, flux linked to the coil to the sides CD and AB will start changing. Now the emf induced to the sides of the coil also starts changing. In actual, the emf induced on one of the coil side will add to the emf induced to the other coil.

                                                 

Case1:  At position 1

• Here the emf is zero.
• This is because the magnetic flux lines are moving parallel.

Case2: At position 2

• Here a low emf is induced.
• The coil sides are at an angle w.r.t the flux.

Case3: At position 3

• Here the emf generated is maximum.
• The coil sides are at right angles with the flux.

Case4: At position 4

• The emf generated is low.
• The coil sides are at an angle w.r.t the flux.

Case5: At position 5

• No magnetic lines are cut.
• The emf induced is zero.

Case 6: At position 6

• The direction of emf generated is reversed.
• Coil sides move under a pole of opposite polarity.

Case 7: At position 7

• Maximum emf is generated here.

The cycle will then repeat. Here the emf generated is an alternating one. This alternating voltage is then converted into dc voltage by a device known as the commutator. After the conversion of ac to dc by the commutator, the dc generator is obtained.

Consider the figure3 below, the sides of the coil AB and CD is under North Pole and South Pole respectively. Segment c1 of the commutator connects the coil side AB to the point R as shown in the figure below. Also segment c2 connects the coil side CD to the point T. c1 is in contact with the negative brush and c2 is in contact with the positive brush. Current flows from T to R through the load. Thus we can say, the coil gets short circuited for a short time by the brushes. This period is known as Commutation Period.

Now after the first rotation of 180 degree, AB is under South Pole and CD is under North Pole. c1 is in contact with positive brush and c2 is in contact with negative brush. Here now AB is connected with point T by segment c1. CD is connected with point R by segment c2. Current flows through the load from T to R. After this, we get the DC current from the brush terminals.

                                   

Types of DC Generator

DC generator can be classified as:

1. Separately Excited

In this case, the coils are energized by the help of an external DC source. In the figure, armature current is Ia, load current I_l, V is the voltage and E is the emf generated.

2. Self- Excited

Here, the coils are energized by the current produced from the DC generator itself. Self-excited generator is divided into:

• Series Wound: Here the field winding is series with the armature winding. In the figure, Rsc is the series winding resistance, Isc is the current, Ra is the armature resistance, Ia represents the Armature Current, I_l is the load current, E is the generated emf and V is the voltage.

 

• Shunt Wound: Here the field winding is parallel with the armature winding. In the figure, Rsh is the shunt winding resistance, Ish is the current, Ra is the armature resistance, Ia represents the Armature Current, I_l is the load current, E is the generated emf and V is the voltage.

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• Compound Wound: It is the combination of the seires and shunt wound generators.