Generators
Generators

Types of Dc generators with their advantages and disadvantages


Types of Dc generators

When we grant a space to talk about working principle of Dc generator and construction of Dc generator We touched on Types of Dc generators which need a high attention, as we illustrated Dc generator occupy a privileged position everywhere around us … in robotics, automobiles, small and also medium application, let’s start this thrilling subject.

Types of Dc generators:

types of DC generators

 

Dc generators can be classified according to the way in which the magnetic field produced in the stator ( the field excitation method) to:

  • Permanent magnet dc generators:

In permanent magnetic Dc generator, we don’t use external field excitation, why?  Because the flux produced by the permanent magnets situated around the armature.

So, we need this type of generator in low power applications like dynamos in motorcycles and in small toys.

Unfortunately, they generated low power you will found them in industrial applications.

Do you concentrate on me! If you; you will remember to ask me about the rotor, the permanent magnet Dc rotor is slotted armature made of layers of laminated silicon steel to reduce the eddy current losses.

 

permanent magnet dc generator

 

  • Separately excited dc generators:

In separately excited Dc generator field coils are energized from an independent external Dc source at the very least we can use batteries.

In this type, the generated EMF equals the sum of supply voltage and armature resistance drop and that’s mean … the output voltage depends on the armature rotation speed and the field current.

Bay attention here; those generators aren’t commonly used because they are expensive as a result of the requirement of additional power source or circuitry.

But if you look at them, they are used in:

  • Research work in laboratories,
  • Accurate speed control in Dc motors with Ward-Leonard system,
  • Few applications where self-excited types are unsatisfactory.

separately excited dc generator

You may ask me about the equations which explicate that; good:

The armature voltage drop=Ia*Ra.

if we guess that:

Ia=IL=I(for the output).

So, the load voltage will be:

V=I*Ra

And the generated power:

Pg=Eg*I

So, the power delivered to the load:

Pl=V*I

That’s very simple, just unleash your mind to understand and simplify everything.

  • Self-excited Dc generators:

In self-excited generator field coils are energized by the current produced by the generator,  the field winding is also connected to the armature winding in varying ways to achieve a wide range of performance characteristics.

 

self excited dc generator

To reach the rated required EMF we must start from the flux present the poles due to the residual flux which helps to induce some EMF when the armature rotated.

After that, some induced current produced then flow through the field coil and also through the load which strengthening the pole flux which as a result produced more armature EMF which also causes an increase in current flow through the field.

And by the way, the armature EMF raises and thus this process repeated until we reached the rated needed value, very perfect.

Use your mind, it’s the most used and existent  types of dc generators; so it is axiomatic to be classified into many types according to the relationship between the field winding and the external circuit to:

  • Series wound dc generator:

It is the best for:

  • Fluctuating loads because it has poor voltage regulation, it also has a lower terminal voltage than the ideal as a result of resistance losses and armature reaction.
  • Power supply because of their increasing characteristic of terminal voltage according to the increase in load current from no load to full load.
  • Dc locomotives for regenerative breaking as they supply field excitation current required.
  • And in series arc lightning.

And in the series generator, we connect the field winding series with the armature winding; we take care to design the series winding with few turns with thick wire and very low resistance to help the field winding carry whole load current.

series wound Dc generator

Here:

Ia=Isc(series field current)=IL=I.

So, the load voltage:

V=Eg-I(Ia*Ra).

The generated power:

Pg=Eg*I

 Finally, the load delivered power:

PL=V*I.

  • Shunt wound dc generator:

In this generator field winding is connected parallel with the armature winding so we apply the full voltage across the armature winding.

We take care to make the shunt winding with a large number of turns and a very high resistance; hence a smaller current less than 5% of the rated armature current is used.

 

 shunt wound Dc generator    

Here the armature current divided into two parts:

Ia=Ish+IL.

And we try to keep the shunt field current as small as possible to have maximum load current which gives us maximum effective power across the load.

while the shunt field current:

Ish=V/Rsh.

Load voltage:

V=Eg-Ia*Ra.

The generated power:

Pg=Eg*Ia.

So, the load delivered power:

PL=V*IL.

  • Compound wound Dc generator:

It is most widely used because of its compensating property in:

  • Power supply purpose and heavy power services.
  • Driving motors.
  • And also in small distance operation as a power supply for hotels, offices, homes, and lodges.

In this generator, there are two connections of the field winding one connected in series with the armature winding and the other connected in parallel with the armature winding. And it supplies a stable output voltage.

compound wound dc generator

And as a result of this importance of compound wound machine, we classify it into:

  • Short shunt Compound wound Dc generator:

Here we connect the field winding only in parallel with the armature winding, consequently, the series field current equals the current reached the load.

Short shunt Compound wound Dc generator

While the series field current:

Isc=IL.

and while the shunt field current:

Ish=(V+Isc*Rsc)/Rsh.

The armature current:

Ia=Isc+IL.

The load voltage:

V=Eg-Ia*Ra-Isc*Rsc.

The generated power:

Pg=Eg*Ia.

So, the load power:

PL=V*IL.

  • Long shunt Compound wound Dc generator:

Here we connect the field winding in parallel with the combination of series field and armature windings, consequently, the series field current equals the armature current.

 

Long shunt Compound wound Dc generator

while the shunt field current

Ish=V/Rsh.

The armature current equals the series field current:

Ia=Isc=IL+Ish.

So, the load voltage:

V=Eg-Ia*Ra-Isc*Rsc       it also equals

V=Eg-Ia*(Ra+Rsc).

The generated power:

Pg=Eg*Ia.

So, the load delivered power:

PL=V*IL.

 

finally, after we deepen in the different types of Dc generators we  should know that they are only used in industrial applications where there is a huge damage for Dc production, And that leads us to think in the difference between Ac and Dc generators