The Difference Between Rotating Field And Rotating Armature in Generator Systems

in AC Machines, DC Machines, Electrical Machines

Introduction

Generator is an electrical machine that converts mechanical energy to electrical energy. The mechanical energy came from different sources both renewable and non-renewable.
Renewable energy came from the natural forces of nature such as the flow of water in water falls and river banks, wind and ocean current.
Non-renewable is produced by burning of fossil fuels thru boiler systems and internal combustion engines.

Generator Systems

In the power plant the last stage of power production is the generator system before the power is dispatch in the substation and switch yards.

The generator operates by the principle of electromagnetic induction and simply speaking electromagnetic induction work with these three key components, namely:

Conductor
Magnetic Field
Relative Motion

Thus,

Courtesy: www.electronic-tutorials.com

By means of electromagnetic induction a voltage is produced in the conductor when the conductor is moved in a magnetic field. When a load is connected to the terminals of the conductor, a current will flow.

To make it more clear, we can relate the above statements to the parts of a practical generator, viz:

Conductor - is the armature coil where the voltage is being produced.
Magnetic Field - is the field coil where the magnetic field is being produced. The magnetic field is being produced when the field coil is supplied with external DC power source such as DC generator or a battery (exciter) thru brush contacts.
Relative Motion - is the mechanical action from the prime mover.

Rotating Armature Generator

Typical Rotating Field Generator Diagram

Rotating armature type of generator place the field coils in the stator and place the armature coils in the rotor. In this setup, the magnetic field is stationary and the coils that induces a voltage is rotating.
This type of generator is usually used in DC generator systems since it is impossible to have a DC output from the stator.

Commutator mounted in the rotor shaft

Why it is impossible to have DC output in the stator side?

DC generator is actually an AC generator where a commutator is mounted in the rotor shaft.
Commutator converts AC to DC as the shaft rotates.
Commutator will not work if it is kept stationary.

Why we need DC generator?

DC generator is important when steady and large DC power is needed and battery cannot sustain that requirement.

Rotating Field Generator

From the word itself that means that the field coil that produced magnet (magnetic field) is located in the rotor and being moved by a prime mover action.

This is completely opposite to the rotating field type of a generator but this is the most common type of AC generator used in the industry.

Typical Rotating Field Generator Diagram

As describe in the diagram the excitation system is supplying DC power source to the rotor thru the brush contacts.
The frequency of the alternating voltage generated in the stator windings is proportional to the number of poles in the rotor field and the speed of shaft rotation.

What are the advantages of rotating field (stationary armature) over rotating armature?

For large power plant that has a generator voltage level up to 11 kV, rotating field is advantageous since the stator can accommodate large volume of conductors to attain this voltage level.
High voltage armature should be kept stationary and it can be accomplished by placing the armature coils in the stator. Rotating coils with high voltage at the terminals level is risky due to sparking in the slip rings.
Low voltage level in the rotor has low inertia and the required starting effort to move this is also less.
The ventilation arrangement for high voltages can be easily designed if it is stationary.

Summary:

There are two main types of generator, namely rotating field and rotating armature.
Rotating field has field winding on the rotor and armature in the stator.
Rotating armature has armature in the rotor and field winding in the stator.
Rotating armature is used only for DC generator since it is impossible to get DC output when the armature is stationary.
Rotating field is advantageous for AC generator specially in the plant with high voltage level like 11kV and up.

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