What is a DC Shunt Generator & Its Working

We know that a self-excited motor is one of the types of DC generator which is available in three different types like compound generator, shunt generator, and series generator. The classification of DC generators can be done based on the way how the field windings are exited. In a DC shunt generator, the connection of two windings like field & armature can be done in parallel, where the armature winding supplies both the field & load current.

What is a DC Shunt Generator?

A DC shunt generator is one kind of DC generator where both the windings like armature & field are allied within the parallel connection. Here, the armature winding provides both the load & the field currents. For the excitation of a DC generator, it needs a DC field current. The DC field current may be excited separately through a DC source like a battery so that the DC generator also supplies the required energy for the field current.

Working Principle

The DC shunt generator working principle is similar to a normal generator like electromagnetic induction. In this generator, the connection of field winding can be shunt toward the armature. Once input is provided through the prime mover, then the conductor can be turned within the permanent magnetic field. Because of this reason, the flow of current will be induced within the conductors which are arranged under the magnetic field influence.

Based on the electromagnetic Faraday’s law, an electromagnetic force will be induced within the conductors once a conductor is turned within the magnetic field. So this induced emf can be utilized for generating energy and this energy can be used through other mechanical devices.

The flow of current-induced throughout the armature winding is irregular. So the armature winding output is irregular frequently. Here AC is changed into the DC due to the commutator. In this way, a DC output can be attained within a DC shunt generator.

DC Shunt Generator Diagram

The DC shunt generator working diagram is given below.

The field windings within a DC shunt generator are connected in parallel through the armature conductors. Here, the field winding includes a number of windings along with a thin wire including high resistance.

The connection of load can be done beyond the armature that is shown in the following diagram. A small current will supply throughout the field winding & a huge current supply will be there throughout the armature winding.

In the above generator diagram,

The shunt field current is ‘Ish’

Armature current is ‘Ia’

The load current is ‘I_L’

‘Ra’ Resistance of armature winding

‘V’ is a terminal voltage

‘Vbr’ is a contact drop of brush

Armature current can be provided through Ia = I_L + I_sh

Ish (Shunt field current) = V/Rsh, wherever Rsh is shunt field resistance

The equation for terminal voltage can be provided through V = Eg – Ia Ra – Vbr

Developed power within the DC generator is Eg Ia

Power transmitted toward the load = V*IL

Armature Current I_a = I_sh + I_L

Shunt Field Current Ish = V/R_sh

Terminal Voltage V = E_g -I_aR_a

Power Generated = P_g = E_g x I_a

Power transmitted to the load P_L = V x I_L

emf equation of dc shunt generator Eg = (PɸZN/60A) Volts

Features

The features of dc generator include the following.

• These are capable of generating a huge range of constant o/p.
• These generators include a huge terminal load.
• These are simple to design & construct.
• These are used to give changeable output power.
• DC generators are extremely consistent including 85-95%. of efficiency.
• They are solid & less weight.

Characteristics of DC Shunt Generator

The characteristics of DC shunt generator mainly include open circuit characteristics, internal characteristics & load or external characteristics which are discussed below.

Open Circuit Characteristics

The Open circuit characteristic of a shunt generator is shown below that is similar to a series generator.
The OA line signifies the resistance of the shunt field circuit. Once this dc generator runs at the usual speed, then it will build up an OM voltage.

At no-load conditions, the generator’s terminal voltage will be stable which is signified through the MC horizontal dotted line.

Internal Characteristics

Once the dc shunt generator is loaded, then flux for each pole can be decreased because of the armature reaction. Thus, e.m.f. can be produced on load is low as compared to the e.m.f. produced at no load. Consequently, the internal characteristics like E/Ia will fall down a little.

Load Characteristics

In the above characteristics diagram, the second curve shows the load characteristics of the dc shunt generator. It provides the main relationships among the terminal voltage like V & loads current like IL.

V = E – IaRa = > E – (IL + Ish)*Ra

Thus, exterior characteristics will recline under the curve of internal characteristic through the sum equivalent to fall in the armature circuit-like (IL + Ish)Ra

It may be observed from the exterior characteristic curve that the change within terminal voltage from no-load to full load is minute. The voltage at the terminal can be maintained constant by always regulating the R (field rheostat) routinely.

Magnetization Characteristics of DC Shunt Generator

For a DC shunt generator action, the magnetic field is required that is provided through permanent magnets, electromagnets which gets exciting current through an exterior source & excited electromagnets from the obtained current of the DC generator itself.

The main purpose of permanent magnets is limited to extremely small generators. In a compound generator, the fields like series & shunt may be linked so as to assist each other. When the total flux generates then it produces a high emf, so this connection is called cumulative.

However, the series & shunt windings are connected then the flux arrangement through one resists the other, after that the induced emf will be lesser. So this kind of connection can be called the differential.

The normal magnetization characteristic curve for a DC shunt generator is shown below. The produced voltage like ‘Ea’ is linked toward the field winding current. This generator generates a voltage Ea even in the absence of a current ‘If’.

The tiny voltage on zero excitation is mainly because of the residual magnetism within the material pole.
The magnetization curve increases suddenly whereas the magnetic circuit is unsaturated. When the magnetic circuit saturates then the curve flattens out.

There is an Rc (critical field resistance) that lets a generator be self-exciting. In the generator, the voltage can be increased, once the whole resistance within the field should be low as compared to the critical resistance or Rc.

For the rated machine speed, the critical resistance can be verified from the curve of magnetization. To achieve this, a tangent line can be drawn toward the magnetization curve beginning from the source. The tangent slope line mainly signifies the Rc or critical field resistance.

Load Test on DC Shunt Generator

Aim: To perform a load test on a DC shunt generator, here is the experiment. So that we can illustrate its internal & external load characteristics. The required components of this experiment includes M.C type Ammeter (0 to 20mA & 0 to 2mA, M.C type Voltmeter (0 – 300 Volts), Wire Wound Rheostats Wire (0 to 370 ohms/1.7 A) & Digital tachometer (0 to 3000 rpm).

Name-plate details for motor and generator includes voltage, speed, current & o/p

Working Procedure

As per the circuit diagram above shown, give the connections and maintain the motor field rheostat within the least position & the generator field rheostat in the highest position at the beginning.

Begin the MG set & get it to the generator’s rated speed by adjusting the motor field rheostat.
Maintain the speed stable at this rate during the DC load test because the generated emf mainly depends on motor speed.

Change the voltage of the terminal to the rated value through the generator field rheostat. Maintain the rheostat within this location throughout the test because s its difference changes the circuit field resistance & thus the generated emf.

Apply the electric load to note the load current (IL) values; terminal voltage (V) & field current. If at dissimilar values of the electric load until complete load current is attained. Analyze the armature current within every case like Ia = IL + If

Give the measurement connections for the armature resistance. Make a note of the voltage fall like Va beyond the armature for various current values flowing through it. In every case, armature resistance can be measured

Ra = Va /I, Ra (Hot) = 1.25 Ra. Note down the values which are close together like the armature resistance ‘Ra’

Measure the produced e.m.f. at every load current value E = V + IaRa

Illustrate external characteristics versus V & IL and also internal characteristics like E versus IL

Applications

The applications of DC shunt generators include the following.

• This motor is used for electroplating
• These motors are used for battery charging
• Used in stable voltage applications
• They provide lighting & excitation to alternators.
• These are used to offer field excitation current within DC locomotives for regenerative braking.
• These are used where speed control is required like in DC motors.
• These are used as portable generators wherever less power supply is necessary
• DC generators are used as dynamos within motorcycles, in toys & in appliances like electric shavers.
• These DC generators are used within arc welding where high voltage drop & stable current are necessary.

Thus, this is all about an overview of the DC shunt generator. A DC shunt generator is one kind of electric generator where both the windings like field & armature are allied within parallel. So the armature provides both the field & load currents. A DC generator without a permanent magnet needs a DC field for excitation. Here the field is excited separately through a DC source like a battery so that the DC shunt generator offers the required energy for the field current. Here is a question for you, what are the different types of DC motor available?