Structural Differences Between DC And AC Generators

July 12, 2020
difference between ac and dc generator

When we are new in the world of generators, many questions can arise. One of the principal differences between AC and DC generators is the structural difference between them.

“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 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 electromagnetic induction law.

Basically, they work as an inverse electric motor. Similarly, there is the same concept behind, but quite some big feature differences.

Differences

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

Structural Differences

What are the two major structural differences between AC And DC Generators?

AC Generators

The AC generator produces an 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.

Of course, the AC generator also has an armature coil and commutators, and they rotate simultaneously.

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

In Ac generator, the armature is constant, and it is always the rotor. However, the AC generator works with brushes and slip rings, and it doesn’t contain a commutator.

DC Generators

The DC generator produces a direct current (the current doesn’t change its direction), and this current flows through a coil that rotates in a fixed field. We attach the coil’s ends to a commutator, which balances the charge 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 help to connect the commutator to the output circuit, which happens with brushes.

In the DC generator, the armature may be the stator or the rotor. Also, the DC generator works with one additional commutator to the brushes and slip rings.

Voltage Differences

Why Is The Produced Voltage Different Between Generators?

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

Nonetheless, the DC generator produces a relatively low voltage, constant amplitude, and time; the output frequency equals zero.

Usage Differences

AC Generators

They are used as 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 straightforward.

DC Generators

They are used to power huge electric motors, for example:

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

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

AC Generator Vs DC Generator / Pros And Cons

Pros

AC Advantages

They have the following advantages:

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

DC Advantages

They have the following advantages:

  • 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.

Cons

AC disadvantages

Like some other electric machines, AC generator has some disadvantages:

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

DC disadvantages

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 of 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.

Types of DC Generators

DC generators occupy a privileged position everywhere, such as in robotics, automobiles, and small and medium applications. Thus, it is essential to discuss the types of DC generators.

DC generators can be classified according to how the magnetic field is produced in the stator (the field excitation method).

1. Permanent Magnet DC Generators

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

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

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

The permanent magnet DC rotor is a slotted armature made of layers of laminated silicon steel to reduce the eddy current losses.

2. Separately Excited DC Generators

In separately excited DC generators, 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. That means the output voltage depends on the armature rotation speed and the field current.

Attention here; those generators aren’t commonly used because they are expensive due to 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

To be clear, we will explain it with equations.
Considering the following variables:

This image contains the meaning of each variable that will be used in the equations in the post

The image represents graphically the armature voltage drop equation

If we guess that

The image represents graphically the equalitys of Ia, IL and I

So, the load voltage will be

The image represents graphically the load voltage equation

And the generated power

The image represents graphically the generated power equation

So, the power delivered to the load

The image represents graphically the equation of power delivered to the load

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

3. Self-excited DC Generators

In self-excited generator field coils are energized by the generator’s current. The field winding connects with the armature winding in varying ways to achieve a wide range of performance characteristics.

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

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

And by the way, the armature EMF rises, which is repeated until we reach the rated needed value.

It is the most used and existent type 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.

4. 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 due to resistance losses and armature reaction.
  • Power supply because of their increasing characteristic of the terminal voltage, according to the increase in load current from no load to full load.
  • For regenerative braking in DC locomotives, since the current is supplied to the field excitation.
  • In series arc lightning.

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 the whole load current.

Considering the following variables:

This image contains the meaning of each variable that will be used in the equations in the post

Here:

The image represents graphically the equality of Ia, Isc, IL and I

So, the load voltage

The image represents graphically the load voltage equation

The generated power

The image represents graphically the generated power equation

Finally, the load delivered power

The image represents graphically the equation of load delivered power

5. Shunt-wound DC Generator

In this generator field, the 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 many turns and a very high resistance; hence we use a smaller current less than 5% of the rated armature current.

Considering the following variables:

This image contains the meaning of each variable that will be used in the equations in the post

Here the armature current divided into two parts

The image represents graphically the armature current equation

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

The image represents graphically the shunt field current equation

Load voltage

The image represents graphically the load voltage equation

The generated power

The image represents graphically the generated power equation

So, the load delivered power

The image represents graphically the load delivered power equation

6. Compound Wound DC Generator

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

  • Power supply purpose and heavy power services.
  • Driving motors.
  • Also, small distance operations are a power supply for hotels, offices, homes, and lodges.

In this generator, there are two connections to 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.

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 by the load.

Considering the following variables:

This image contains the meaning of each variable that will be used in the equations in the post

While the series field current is

The image represents graphically the series field current equation

and while the shunt field current

The image represents graphically the shunt field current equation

The armature current

The image represents graphically the armature current equation

The load voltage

The image represents graphically the load voltage equation

The generated power

The image represents graphically the generated power equation

So, the load power

The image represents graphically the load power equation

  • 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.

While the shunt field current

The image represents graphically the shunt field current equation

The armature current equals the series field current

The image represents graphically the armature current equals the series field current

So, the load voltage

The image represents graphically the load voltage equation

It also equals

The image represents graphically the load voltage equation

The generated power

The image represents graphically the generated power equation

So, the load delivered power

The image represents graphically the load delivered power equation

Finally, we must tell you that AC generators have a massive majority. As illustrated, DC generator brushes need periodic replacement at variance, while AC generators need lower maintenance.