DC Machines: Generator and Motor, Back Emf

 DC Generators

A dc generator is an electrical machine which converts mechanical energy into direct current electricity. This energy conversion is based on the principle of production of dynamically induced emf. This article outlines basic construction and working of a DC generator.

Construction Of A DC Machine:

Theoretically, a DC generator can be used as a DC motor without any constructional changes and vice versa is also possible. Therefore, a DC generator or a DC motor can be broadly termed as a DC machine. These basic constructional details are also valid for both DC Generator and DC motor. Hence, let's call this point as construction of a DC machine instead of just 'construction of a dc generator'.

The above figure shows constructional details of a simple 4-pole DC machine. A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.
4. Armature core: Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed (fixed) to the shaft.
5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.
6. Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed (or fixed) to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Working Principle Of A DC Generator:

the working of DC Generator is based on Faraday’s law. It states that whenever a conductor cuts magnetic flux, an EMF (Electromotive Force) is induced across the conductor. The magnitude of this induced EMF is directly proportional to the rate of change of flux linkage.

To understand how EMF gets induced in a conductor, let us consider a single-turn rectangular loop ABCD rotating in a clockwise direction between the poles.

CASE 1:

At any instant of time, the conductor AB is close to the North Pole and CD to the South Pole as shown in the figure below.

For conductor AB, the magnetic field is from left to right while the force on it is acting upwards. Now, to find the direction of the induced current, we will use Fleming’s right-hand rule.

 

“If the thumb, forefinger, and middle finger of the right hand are stretched out and placed mutually perpendicular to each other in such a way that the thumb represents the direction of force, the forefinger represents the direction of the magnetic field, then the middle finger will give the direction of induced current.”

After applying the above rule to the conductor AB, the direction of the induced current is from A to B in the loop ABCD. This current flows externally from brush B2 to B1 powering the load on its way.

CASE 2:

After 180 degrees rotation of the coil, the conductor CD comes close to the North Pole while AB is near to the South Pole.

On applying Fleming’s right-hand rule to conductor CD, the direction of the induced current is from D to C. Although the direction of the current in the loop ABCD is reversed now, the external current still flows from brush B2 to B1.

 

So in both cases, the direction of the generated current is always from B2 to B1. Hence, a unidirectional current is obtained in the DC generator.

Applications

They are used in DC motors where speed control is necessary. They are used as portable generators where low power supply is required, in motorcycles as dynamos, in toys such as remote control cars and in appliances such as electric shavers.

Types Of DC Generators:

DC generators can be classified in two main categories, viz; (i) Separately excited and (ii) Self-excited.

(i) Separately excited: In this type, field coils are energized from an independent external DC source.

(ii) Self-excited: In this type, field coils are energized from the current produced by the generator itself. Initial emf generation is due to residual magnetism in field poles. The generated emf causes a part of current to flow in the field coils, thus strengthening the field flux and thereby increasing emf generation. Self excited dc generators can further be divided into three types -

    (a) Series wound - field winding in series with armature winding

    (b) Shunt wound - field winding in parallel with armature winding

    (c) Compound wound - combination of series and shunt winding

The EMF induced in a DC generator can be explained as follows

• When the loop is in position-1, the generated EMF is zero because, the movement of coil sides is parallel to the magnetic flux.
• When the loop is in position-2, the coil sides are moving at an angle to the magnetic flux and hence, a small EMF is generated.
• When the loop is in position-3, the coil sides are moving at right angle to the magnetic flux, therefore the generated EMF is maximum.
• When the loop is in position-4, the coil sides are cutting the magnetic flux at an angle, thus a reduced EMF is generated in the coil sides.
• When the loop is in position-5, no flux linkage with the coil side and are moving parallel to the magnetic flux. Therefore, no EMF is generated in the coil.
• At the position-6, the coil sides move under a pole of opposite polarity and hence the polarity of generated EMF is reversed. The maximum EMF will generate in this direction at position-7 and zero when at position-1. This cycle repeats with revolution of the coil.

It is clear that the generated EMF in the loop is alternating one. It is because any coil side (say AB) has EMF in one direction when under the influence of N-pole and in the other direction when under the influence of S-pole. Hence, when a load is connected across the terminals of the generator, an alternating current will flow through it. Now, by using a commutator, this alternating emf generated in the loop can be converted into direct voltage. We then have a DC generator.

Dynamically Induced EMF

When the conductor is moved in a stationary magnetic field so that the magnetic flux linking with it changes in magnitude, as the conductor is subjected to a changing magnetic, therefore an EMF will be induced in it. The EMF induced in this way is known as dynamically induced EMF (as in a DC or AC generator). It is so called because EMF is induced in a conductor which is moving (dynamic).

Consider a conductor of length l meters moving with a velocity of v m/s at right angles to a uniform stationary magnetic field of flux density B Wb/m2.Let the conductor moves through a small distance dx in time dt seconds. Then, 

Areasweptbyconductor,a=l×dxm2

∴Magneticfluxcutbyconductor,dψ=MagneticFluxDensity×AreaSwept

⟹dψ=BldxWb

Now, according to Faraday’s law of electromagnetic induction, the induced EMF will be,

e=Ndψdt=Bldxdt(∵N=1)

∵dxdt=Velocity V

∴e=BlvVolts

Above equation gives the dynamically induced EMF when the conductor moves at right angle to the magnetic field.

 

If the conductor moves at an angle &theta to the magnetic field, then the EMF induced due to only the perpendicular component of the velocity to the magnetic field.
e=Blvsinθ

Example:

A conductor of length of 0.8 m lies in and at right angle to a uniform magnetic field of flux density 2 Wb/m2$Wb/m^2$. The conductor moves with a velocity of 30 m/s. Calculate the EMF induced in the conductor. What will be the EMF induced if the conductor moves at an angle of 45° to the magnetic field?

sol:

Case 1 − When conductor moves at right angle to the magnetic field.

• e=Blv=2×(0.8)×(30)=48V

• Case 2 − When the conductor moves at 45° to the magnetic field.

• e=Blvsinθ=2×(0.8)×(30)×sin45=33.95V

Flemings right hand rule

Fleming's right-hand rule is used for electric generators. In an electric generator, the motion and magnetic field exist (causes), and they lead to the creation of the electric current (effect), and so the right hand rule is used.

•      First finger of the right hand is pointed in the directing magnetic flux.
•      Thumb is pointed in the direction of motion of the conductor.
•      Then the second finger will point in the direction of induced current.

Flemings left hand rule

Fleming's left-hand rule is used for electric motors. In an electric motor, the electric current and magnet field exist (which are the causes), and they lead to the force that creates the motion (which is the effect), and so the left hand rule is used.

DC Motors

A DC motor is an electromechanical energy conversion device, which converts electrical energy input into the mechanical energy output.

The operation of the DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, a mechanical force acts on the conductor. The magnitude of the force is given by,

                                                       F=BIL Newtons

Where,

B = magnetic flux density,

I = current and

L = length of the conductor within the magnetic field.

The direction of this is given by the Fleming’s left hand rule.

Construction of a DC Motor

Here is the schematic diagram of a DC Motor

A DC motor consists of six main parts, which are as follows

Yoke

The outer frame of a DC motor is a hollow cylinder made up of cast steel or rolled steel is known as yoke. The yoke serves following two purposes

• It supports the field pole core and acts as a protecting cover to the machine.
• It provides a path for the magnetic flux produced by the field winding.

Magnetic Field System

The magnetic field system of a DC motor is the stationary part of the machine. It produces the main magnetic flux in the motor. It consists of an even number of pole cores bolted to the yoke and field winding wound around the pole core. The field system of DC motor has salient poles i.e. the poles project inwards and each pole core has a pole shoe having a curved surface. The pole shoe serves two purposes

• It provides support to the field coils.
• It reduces the reluctance of magnetic circuit by increasing the cross-sectional area of it.

The pole cores are made of thin laminations of sheet steel which are insulated from each other to reduce the eddy current loss. The field coils are connected in series with one another such that when the current flows through the coils, alternate north and south poles are produced.

Armature Core

The armature core of DC motor is mounted on the shaft and rotates between the field poles. It has slots on its outer surface and the armature conductors are put in these slots. The armature core is a made up of soft steel laminations which are insulated from each other and tightly clamped together. In small machines, the laminations are keyed directly to the shaft, whereas in large machines, they are mounted on a spider. The laminated armature core is used to reduce the eddy current loss.

Armature Winding

The insulated conductors are put into the slots of the armature core. The conductors are suitably connected. This connected arrangement of conductors is known as armature winding. There are two types of armature windings are used – wave winding and lap winding.

Commutator

A commutator is a mechanical rectifier which converts the direct current input to the motor from the DC source into alternating current in the armature winding. The commutator is made of wedge-shaped copper segments insulated from each other and from the shaft by mica sheets. Each segment of commutator is connected to the ends of the armature coils.

Brushes

The brushes are mounted on the commutator and are used to inject the current from the DC source into the armature windings. The brushes are made of carbon and is supported by a metal box called brush holder. The pressure exerted by the brushes on the commutator is adjusted and maintained at constant value by means of springs. The current flows from the external DC source to the armature winding through the carbon brushes and commutator.

Working of DC Motor

Consider a two pole DC motor as shown in the figure. When the DC motor is connected to an external source of DC supply, the field coils are excited developing alternate N and S poles and a current flows through the armature windings.

 

All the armature conductors under N pole carry current in one direction (say into the plane of the paper), whereas all the conductors under S pole carry current in the opposite direction (say out of the plane of the paper). As each conductor carrying a current and is placed in a magnetic field, hence a mechanical force acts on it.

By applying Fleming’s left hand rule, it can be seen that the force on each conductor is tending to move the armature in anticlockwise direction. The force on all the conductors add together to exert a torque which make the armature rotating. When the conductor moves from one side of a brush to the other, the current in the conductor is reversed and at the same time it comes under the influence of next pole of opposite polarity. As a result of this, the direction of force on the conductor remains the same. Therefore, the motor being rotating in the same direction. 

Applications

DC motors are important in many applications. In portable applications using battery power, DC motors are a natural choice. DC machines are also used in applications where high starting torque and accurate speed control over a wide range are important. Major applications for DC motors are: elevators, steel mills, rolling mills, locomotives, and excavators.

Types of DC Motor

There are 4 major types of DC motor and they are,

• Series DC Motor
• Permanent Magnet DC Motor
• Shunt/Parallel DC Motor
• Compound DC Motors

Back- EMF of DC motor

According to the fundamental law of nature, no energy conversion is possible until there is something to oppose the conversion. In case of generators, magnetic drag provides this opposition, but in the case of dc motors, there is back emf. Presence of the back emf makes a dc motor ‘self-regulating’.

When the armature of a motor is rotating, the conductors are also cutting the magnetic flux lines and hence according to the Faraday's law of electromagnetic induction, an emf induces in the armature conductors.

The direction of this induced emf is such that it opposes the armature current (Ia). The circuit diagram below illustrates the direction of the back emf and armature current. 

Significance of Back-EMF

Magnitude of back emf is directly proportional to speed of the motor. Consider the load on a dc motor is suddenly reduced. In this case, required torque will be small as compared to the current torque. Speed of the motor will start increasing due to the excess torque. Hence, being proportional to the speed, magnitude of the back emf will also increase. With increasing back emf armature current will start decreasing. Torque being proportional to the armature current, it will also decrease until it becomes sufficient for the load. Thus, speed of the motor will regulate.

On the other hand, if a dc motor is suddenly loaded, the load will cause decrease in the speed. Due to decrease in speed, back emf will also decrease which allows more armature current. Due to increase in armature current  the torque will increase to fulfil the load requirement.  

OCC (Open Circuit Characteristics)

The open circuit characteristics (O.C.C) or magnetization characteristics is the curve that shows the relationship between the generated EMF at no-load (E0) and the field current (If) at constant speed. It is also known as no-load saturation curve. Its shape practically the same for all types of DC generator whether separately excited or self-excited.

In order to determine the open circuit characteristics of a DC generator, the field winding is disconnected from the machine and is excited by an external DC source. The generator is run at its normal speed. The field current (If) is increased gradually from zero and the corresponding values of generated EMF at no-load (E0) is noted from a voltmeter connected across the armature terminals.

 

On plotting the relation between the E0 and If, we obtain the open circuit characteristics as shown in the figure.

Following points can be observed from the open circuit characteristics curve −

• Initially the field current is zero, there being some generated emf OA, which is due to the residual magnetism.
• From point A to B, the curve is linear. It is because in this range, the reluctance of iron core is negligible as compared to the air gap. The reluctance of air gap is constant and hence it has linear relationship.
• After point B, the reluctance of iron also comes into picture. Since, at higher flux densities, the relative permeability (μr) of the iron decreases and hence the reluctance of iron is no longer negligible. This results in the deviation of the curve from the linear relationship.
• After point C, the pole cores begin to saturate magnetically and the generated emf (E0) at no-load tends to level off.

Derivation for Induced EMF for DC Generator

Let us suppose there are Z total numbers of conductor in a generator, and arranged in such a manner that all parallel paths are always in series.
Here,
Z = total numbers of conductor
A = number of parallel paths
Then,
Z/A = number of conductors connected in series
We know that induced emf in each path is same across the line
Therefore,
Induced emf of DC generator
E = emf of one conductor × number of conductor connected in series.

Induced emf of DC generator is

Simple wave wound generator
Numbers of parallel paths are only 2 = A
Therefore,

Induced emf for wave type of winding generator is

Simple lap-wound generator
Here, number of parallel paths is equal to number of conductors in one path
i.e. P = A
Therefore,
Induced emf for lap-wound generator is