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What Is The Main Difference Between A DC Generator And An AC Generator

July 3, 2020
Difference between AC and DC generator

The main difference between a DC generator and an AC generator is that a DC generator will produce “Direct Current“, and an AC generator “Alternating Current“.

That’s the basic highschool answer for a homework assignment, but there’s much more behind it, and that response is not even precise…

I mean, you can just lightly modify the commutator in a DC generator and prove that a DC generator is a source of AC voltage … So, what’s the big deal?

Let’s get into context first.

Both AC and DC generators transform mechanical energy into electricity, depending on their work on Faraday’s law of electromagnetic induction.

Basically, they work as an inverse electric motor. Same concept behind, but quite some big feature differences.

The main differences consist of their structure, voltage, and usage.

What Are Two Major Structural Differences Between AC And DC Generators?

In contrast, Ac generator produces alternating current this current flows through a fixed coil, and a moving magnet, the north, and south poles of the magnet make the current flow in opposite directions, and here it produced.

Of course, Ac generator also has armature coil and commutators and here they rotate in a simultaneous manner.

The brushes are of carbon and they receive the current flows through the slip rings; the armature and the outside circuit are connected through only one brush.

For DC generator

As we illustrate before; Dc generator produces direct current (the current doesn’t change its direction) and this current flows through a coil rotates in a fixed field, and we attach the ends of the coil to a commutator which balances the charges flows into the generator.

It also has a commutator (slip rings) and armature coil and the current produced depends on the connection between the armature and the external circuit.

And of course we need a help to connect the commutator to the output circuit and that happens with the help of brushes.

1.- Armature

In Ac generator it’s constant it’s always the rotor.

In Dc generator, the armature may be the stator or the rotor.

2.- Commutator

Ac generator works with brushes and slip rings and it doesn’t contain commutator.

How Are AC And DC Generators Different
How Are AC And DC Generators Different

Dc generator works with one commutator additional to the brushes and slip rings.

 

Why Is The Produced Voltage Different Between Generators?

Ac generator produced a very high voltage which varies in amplitude and time, and it has a normal 60 or sometimes 50 HZ.

Produced voltage of AC generator
Produced voltage of AC generator

 

DC generator produces a relatively low voltage which is constant in amplitude and time, the output frequency equals zero.

Produced voltage of DC generator
Produced voltage of DC generator

Main Usage Differences Between A DC And An AC Generator

Ac generators used  for power generation in office and  home to power small and electrical appliances like:

  • Vacuum cleaners.
  • Food Mixers.
  • Dishwashers.
  • Refrigerators.
  • Juicers.
  • And electrical fixtures.

That’s because AC generation and transportation across long distances is very easy.

Dc generator is used to power very large electric motors such used for:

  • Subway systems.
  • Also, it can be used with battery charge banks for mobile and off-grid uses.
  • Also in the flashlight.
  • Hybrid and electric vehicles.
  • Flat-screen TVs.

That’s because DC generator provides reliable and efficient energy supply in this usage.

AC generator Vs DC generator | Pros And Cons

6.- advantages

Ac generator has the advantages of:
  • We can easily increase and decrease the AC current produced by transformers.
  • The energy losses in transmission are less than in DC generator.
  • It’s simple and cheap in construction costs.
  • The transformers also facilitate the distribution of the Ac voltage produced.

Dc generator has many advantages such as:

  • It has a simple design and construction.
  • It’s ideal for running big motors and big appliances which require direct current to provide power.
  • It’s able to charge batteries directly.
  • It reduces fluctuations described for some steady state applications by smoothing the output voltage by the regular arrangement of coils around the armature.

7.- disadvantages

As all other electric machines,  Ac generator has the disadvantages of:

  • They are high in cost.
  • High energy losses during transmission.
  • The output current produced  is 10 times more dangerous than the DC current

Unfortunately, DC generator has some disadvantages as:

  • Dc generator can’t be applied to a transformer so it’s difficult to distribute it.
  • There would be a voltage drop over long distances.
  • Dc generator has low efficiency because there are copper losses, eddy current losses, hysteresis losses and mechanical losses.
  • The construction is complex as it has a commutator and slip ring.

The waste of energy due to sparking occurred in air gaps.

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, to be fair we must tell you that Ac generators have the massive majority as we illustrated Dc generator brushes need periodic replacement at variance Ac generators need lower maintenance.