Difference Between AC Generator and DC Generator

Difference Between AC Generator and DC Generator - A Complete Guide

Edited By Vishal kumar | Updated on Jul 02, 2025 04:46 PM IST

Introduction
‘Generator’ comes from the word generate which means to produce anything itself, here Generator means an object which is used to produce electricity. In this chapter, we will study in detail about AC, DC Generator, its working and application. Later we will see the differences between AC and DC generators. First of all, before learning about AC/DC generators, we should learn about AC/DC.
AC (alternating current): In AC, direction of flow of current changes periodically, whereas
DC (direct current): In DC, there is no change of direction of flow of current.

This Story also Contains

What is AC/DC Generator?
Principle of AC/DC generator
Difference between AC and DC generator:

Difference Between AC Generator and DC Generator - A Complete Guide

NCERT Physics Notes :

What is AC/DC Generator?

A device that transforms mechanical energy to electrical energy as alternating current. It is called alternating current generator. A device that transforms mechanical energy to electrical energy as direct current. It is called direct current generator.

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Principle of AC/DC generator

On the basis of Faraday’s laws of electromagnetic induction, ac or dc generator work. It states: When a conductor is placed inside the magnetic field, EMF will be induced if the conductor cuts the magnetic lines. The magnitude of current depends upon the rate of change of magnetic flux inside the magnetic field.

Difference between AC and DC generator:

Difference between dc motor and dc generator is given by using the following aspects:

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Types of current produced:

AC generator converts mechanical energy into electrical energy and produces alternating current, while DC generator converts mechanical into electrical but it produces direct current.

Basic Construction

In AC Generator the coil through which current produces is fixed and magnet moves but in DC Generator coil rotates periodically. The AC generator does not have commuters but slip-rings, while the DC generator has commuters, which helps in producing direct current.

Output Voltage

AC generator Produces high voltage which varies with time but DC generator produces comparatively lower voltage and is time independent.

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Efficiency

AC generator has more efficiency as compared To DC generator as the energy losses in AC generator is very less.

Transmission

In the case of AC generators, the cost of transmission is less as compared to DC because there is no use of a transformer in DC current.

Production cost

It is said that the production cost of AC generators is less or AC generators are cheaper as compared to DC generators.

Application

Since AC generators are cheap and have higher efficiency it is used at home for producing electricity. Using the above explanations, we can distinguish between ac generator and dc generator.

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Frequently Asked Questions (FAQs)

1. How can an AC generator be converted into a DC generator?

As we have studied, the basic construction difference between the two is that the DC generator does not have a Commuter ring attached to the moving conductor, hence it produces electricity for positive half cycle. Hence by detaching the Commuter ring from the AC generator, we can change the AC generator to the DC generator.

2. In AC Generator, current reverses its direction A 10 times per second B 20 times per second C 40 times per second D 50 times per second

we have studied that current changes its direction periodically, so its frequency is 50 per second, which means it changes direction 50 times per second.

3. What are the types of ac generators?

AC generators are classified into two types. They are, induction generator, and synchronous generator.

4. In between ac and dc generator which has lower price?

Ac generator price is less than dc generator price.

5. What is the fundamental difference between AC and DC generators?

The fundamental difference lies in the type of current they produce. AC generators produce alternating current, where the direction of electron flow periodically reverses, while DC generators produce direct current, where electrons flow in a single direction.

6. How does the construction of an AC generator differ from a DC generator?

The main difference in construction is the method of current collection. AC generators use slip rings, which maintain continuous contact with the brushes, allowing alternating current to flow. DC generators use a split-ring commutator, which reverses the connection to the external circuit every half rotation, producing direct current.

7. Why do AC generators not require a commutator?

AC generators don't need a commutator because they are designed to produce alternating current. The slip rings allow the alternating current to flow directly from the rotating armature to the external circuit without needing to convert it to direct current.

8. What is the purpose of the commutator in a DC generator?

The commutator in a DC generator serves to reverse the direction of current in the armature windings relative to the external circuit every half rotation. This mechanical rectification converts the alternating current produced in the armature into direct current in the external circuit.

9. Can a DC generator produce AC internally?

Yes, a DC generator actually produces AC internally in its armature. The commutator then mechanically rectifies this AC into DC before it reaches the external circuit.

10. How does the frequency of AC generated by an AC generator relate to its rotation speed?

The frequency of AC is directly proportional to the rotation speed of the generator. The relationship is given by the formula: f = NP/120, where f is the frequency in Hz, N is the rotation speed in RPM, and P is the number of pole pairs in the generator.

11. What determines the magnitude of the voltage produced by a generator?

The magnitude of the voltage produced by a generator depends on three main factors: the strength of the magnetic field, the number of turns in the armature coil, and the speed of rotation of the armature.

12. How does the principle of electromagnetic induction apply to both AC and DC generators?

Both AC and DC generators operate on the principle of electromagnetic induction. When a conductor moves through a magnetic field, or when a magnetic field changes around a conductor, an electromotive force (EMF) is induced in the conductor, generating electrical current.

13. How does the output voltage of an AC generator vary over time?

The output voltage of an AC generator varies sinusoidally over time. It starts at zero, increases to a positive peak, decreases back to zero, then increases to a negative peak before returning to zero, completing one cycle.

14. What is armature reaction, and how does it affect generator performance?

Armature reaction is the effect of the magnetic field produced by the current in the armature windings on the main magnetic field of the generator. It can distort the main field, leading to reduced efficiency and sparking at the brushes, particularly in DC generators.

15. Why are AC generators more commonly used for large-scale power generation?

AC generators are preferred for large-scale power generation because AC can be easily transformed to different voltage levels, allowing for efficient long-distance power transmission. AC also naturally reverses direction, which is useful for many applications and motors.

16. What is the role of brushes in AC and DC generators?

In both types of generators, brushes serve as the contact point between the rotating and stationary parts of the machine. In DC generators, brushes make contact with the commutator segments to collect the rectified current. In AC generators, brushes make contact with the slip rings to collect the alternating current.

17. How does the direction of current flow differ between AC and DC generators?

In a DC generator, the current always flows in the same direction in the external circuit due to the action of the commutator. In an AC generator, the current periodically reverses direction, flowing first in one direction and then in the opposite direction.

18. How do the magnetic fields in AC and DC generators differ?

In most AC generators, the magnetic field is stationary and the armature rotates. In most DC generators, the armature rotates within a stationary magnetic field. However, there are variations in both types where these roles can be reversed.

19. What is the significance of the number of poles in a generator?

The number of poles in a generator affects the frequency of the AC produced (in AC generators) and the number of commutator segments needed (in DC generators). More poles generally result in a higher frequency for a given rotation speed in AC generators.

20. Why is the output of a DC generator smoother than that of an AC generator?

The output of a DC generator appears smoother because the commutator acts as a mechanical rectifier, converting the internally generated AC to DC. This results in a more constant voltage and current flow, although there is still some ripple present.

21. How does the speed of rotation affect the output of AC and DC generators?

In both types of generators, increasing the speed of rotation increases the output voltage. In AC generators, it also increases the frequency of the alternating current. In DC generators, it increases the average DC voltage, but also increases the ripple in the output.

22. What is back EMF, and how does it affect generator operation?

Back EMF (electromotive force) is the voltage induced in the generator's windings that opposes the motion of the armature. It acts as a counterforce to the mechanical input, requiring more power to maintain the generator's speed as the electrical load increases.

23. How do AC and DC generators differ in terms of efficiency?

AC generators are generally more efficient than DC generators. This is partly because AC generators don't require a commutator, which can cause energy losses due to friction and sparking. Additionally, AC power can be more efficiently transmitted over long distances.

24. What is the purpose of field excitation in generators?

Field excitation provides the magnetic field necessary for the generator to operate. In both AC and DC generators, the field windings are energized to create a strong magnetic field, which is essential for the process of electromagnetic induction.

25. How does the process of self-excitation work in DC generators?

Self-excitation in DC generators occurs when residual magnetism in the field poles induces a small voltage in the armature. This voltage is fed back to the field windings, strengthening the magnetic field, which in turn increases the induced voltage. This process continues until the generator reaches its rated voltage.

26. Why are slip rings used in AC generators instead of a commutator?

Slip rings are used in AC generators because they allow for continuous, uninterrupted connection between the rotating armature and the stationary external circuit. This enables the alternating current produced in the armature to flow directly to the external circuit without rectification.

27. How does the waveform of the output differ between AC and DC generators?

The output waveform of an AC generator is typically a smooth sine wave, with voltage and current alternating between positive and negative values. The output of a DC generator, after commutation, is a unidirectional but pulsating waveform, which can be smoothed with additional components.

28. What is the function of interpoles in DC generators?

Interpoles, also known as commutating poles, are additional small poles placed between the main field poles in DC generators. They help to reduce sparking at the brushes by creating a magnetic field that counteracts the armature reaction, improving commutation and overall generator performance.

29. How do AC and DC generators differ in terms of maintenance requirements?

DC generators generally require more maintenance due to the wear and tear on the commutator and brushes. AC generators, with their simpler slip ring design, typically have lower maintenance needs. However, both types require regular inspection and maintenance of bearings, windings, and cooling systems.

30. What is the relationship between the number of armature conductors and the generated EMF in both types of generators?

In both AC and DC generators, increasing the number of armature conductors increases the generated EMF. This is because more conductors cutting through the magnetic field results in a greater total induced voltage. The relationship is generally linear, assuming all other factors remain constant.

31. How does load variation affect the output of AC and DC generators?

In both types of generators, increasing the load (current drawn) tends to decrease the output voltage due to internal voltage drops and armature reaction. However, voltage regulators are often used to maintain a constant output voltage under varying load conditions.

32. What is the significance of the air gap in generator design?

The air gap is the space between the rotor and stator in a generator. A smaller air gap generally results in a stronger magnetic field and thus a higher output voltage. However, the air gap must be large enough to allow for mechanical clearance and to minimize unwanted effects like cogging torque.

33. How do AC and DC generators handle reactive loads differently?

AC generators can directly supply both real and reactive power to loads, as the alternating current naturally accommodates phase shifts between voltage and current. DC generators, however, cannot directly supply reactive power. For DC systems, reactive elements like capacitors or inductors must be added separately to handle reactive loads.

34. What is the purpose of damper windings in AC generators?

Damper windings, also known as amortisseur windings, are used in AC generators to improve stability during sudden load changes and to prevent hunting (oscillation of rotor speed). They help to maintain synchronization and reduce harmonics in the output waveform.

35. How does the concept of synchronization apply differently to AC and DC generators?

Synchronization is crucial for AC generators when connecting to a power grid, requiring matching of frequency, phase, and voltage. DC generators don't need frequency or phase matching, but voltage matching is still important when connecting multiple DC generators in parallel.

36. What is the effect of changing the direction of rotation in AC and DC generators?

In an AC generator, reversing the direction of rotation will reverse the phase of the output AC. In a DC generator, reversing the rotation direction will reverse the polarity of the output DC unless the brush positions are also reversed.

37. How do AC and DC generators differ in their ability to handle varying loads?

AC generators generally handle varying loads more efficiently due to their simpler construction and the ease of transforming AC voltage. DC generators may require more complex voltage regulation systems to maintain stable output under varying loads.

38. What is the significance of the term "synchronous" in AC generators?

The term "synchronous" in AC generators refers to the fact that the rotor's mechanical rotation is synchronized with the electrical frequency of the generated AC. One complete mechanical rotation corresponds to one complete electrical cycle (or multiple cycles, depending on the number of pole pairs).

39. How does core loss differ between AC and DC generators?

Core losses, which include hysteresis and eddy current losses, are generally higher in AC generators due to the constantly changing magnetic field. In DC generators, the magnetic field in the stator is relatively constant, resulting in lower core losses.

40. What is the purpose of using laminated cores in both AC and DC generators?

Laminated cores are used in both types of generators to reduce eddy current losses. By stacking thin, insulated sheets of magnetic material, the path for eddy currents is restricted, reducing overall power loss and improving efficiency.

41. How does the concept of power factor apply differently to AC and DC generators?

Power factor is a concept specific to AC systems, representing the phase difference between voltage and current. It's crucial for AC generators in determining the real power output. DC generators don't have a power factor as voltage and current are in phase by nature.

42. What is armature skewing, and why is it more common in DC generators?

Armature skewing involves slightly twisting the armature slots along the axis of rotation. It's more common in DC generators to reduce torque ripple and minimize harmonics in the output voltage. This technique helps to produce a smoother DC output.

43. How do AC and DC generators differ in their ability to change output voltage?

AC generators can easily change output voltage using transformers without affecting the generator itself. DC generators typically require changes to the field excitation or the use of electronic voltage regulators to alter the output voltage.

44. What is the significance of the term "exciter" in generator systems?

An exciter is a smaller generator used to provide the DC current needed for the main generator's field windings. It's crucial in both AC and DC systems for controlling the strength of the magnetic field and thus the output voltage of the main generator.

45. How does the concept of "hunting" apply to AC and DC generators?

Hunting refers to oscillations in generator speed and power output. It's more significant in AC generators, especially when synchronizing with a grid. In DC generators, hunting can occur but is generally less problematic due to the absence of the need for frequency synchronization.

46. What is the role of capacitors in AC and DC generator systems?

In AC generator systems, capacitors are often used for power factor correction and voltage regulation. In DC generator systems, capacitors are primarily used for smoothing the output voltage, reducing ripple in the DC output.

47. How do AC and DC generators differ in their response to short circuits?

AC generators can typically withstand short circuits for longer periods due to the zero-crossing points in the AC waveform, which help to extinguish arcs. DC generators are more susceptible to damage from sustained short circuits due to the constant current flow.

48. What is the significance of the term "commutation" in DC generators?

Commutation in DC generators refers to the process of reversing the current in the armature coils as they pass from one magnetic pole to another. This process, facilitated by the commutator and brushes, is crucial for producing a unidirectional current in the external circuit.

49. How do AC and DC generators differ in terms of their ability to parallel multiple units?

Paralleling AC generators requires precise synchronization of frequency, phase, and voltage. DC generators are generally easier to parallel, requiring only voltage matching and proper load sharing mechanisms.

50. What is the purpose of interpoles in DC generators, and why are they not typically used in AC generators?

Interpoles in DC generators help to reduce sparking at the brushes by creating a magnetic field that counteracts armature reaction. They are not typically used in AC generators because the alternating nature of the current naturally helps to minimize the effects of armature reaction.

51. How does the concept of "field weakening" apply differently to AC and DC generators?

Field weakening in DC generators is used to increase speed above the base speed by reducing the field current. In AC generators, field weakening can be used to control reactive power output, but it doesn't directly affect the speed due to the fixed relationship between speed and frequency.

52. What is the significance of the "neutral plane" in DC generators?

The neutral plane in a DC generator is the position where the armature coil is not cutting any magnetic flux, and thus no EMF is induced. Proper brush positioning relative to this plane is crucial for efficient commutation and minimizing sparking.

53. How do AC and DC generators differ in their ability to supply power over long distances?

AC generators are far superior for long-distance power transmission because AC voltage can be easily stepped up or down using transformers, allowing for high-voltage, low-current transmission that minimizes power losses. DC generators are less suitable for long-distance transmission without additional conversion equipment.

54. What is the role of brushless excitation systems in modern generators?

Brushless excitation systems, used in both AC and DC generators, eliminate the need for brushes and slip rings in the excitation circuit. This reduces maintenance requirements and increases reliability. In AC generators, it often involves a small AC exciter and rotating rectifiers, while in DC generators, it may use permanent magnets or other brushless designs for field excitation.