DC MACHINES

Brushless DC (BLDC) Motor - Construction And Working

A Brushless DC motor or BLDC motor is the type which is most suitable for applications that require high reliability, high efficiency, more torque per weight etc.

How Does A Brushless DC Motor Work?

Commutator helps in achieving unidirectional torque in a typical DC motor Obviously, commutator and brush arrangement is eliminated in a brushless dc motor. And an integrated inverter / switching circuit is used to achieve unidirectional torque. That is why these motors are, sometimes, also referred as 'electronically commutated motors'.

Construction Of A BLDC Motor

Just like any other electric motor, a BLDC motor also has a stator and a rotor. Permanent magnets are mounted on the rotor of a BLDC motor, and stator is wound with specific number of poles. This is the basic constructional difference between a brushless motor and a typical dc motor.

There can be two types of BLDC motor on the basis of construction : (i) inner rotor design & (ii) outer rotor design. The following image shows an example of outer rotor BLDC motor.

Working Principle

Stator windings of a BLDC motor are connected to a control circuit (an integrated switching circuit). The control circuit energizes proper winding at proper time, in a pattern which rotates around the stator. The rotor magnet tries to align with the energized electromagnet of the stator, and as soon as it aligns, the next electromagnet is energized. Thus the rotor keeps running. The animation below will give you a clear idea of 'how a brushless DC motor works?'

Brushless Vs. Brushed DC Motor

• Brushes require frequent replacement due to mechanical wear, hence, a brushed DC motor requires periodic maintenance. Also, as brushes transfer current to the commutator, sparking occurs. Brushes limit the maximum speed and number of poles the armature can have. These all drawbacks are removed in a brushless DC motor. Electronic control circuit is required in a brushless DC motor for switching stator magnets to keep the motor running. This makes a BLDC motor potentially less rugged.
• Advantages of BLDC motor over brushed motors are: increased efficiency, reliability, longer lifetime, no sparking and less noise, more torque per weight etc.

Permanent Magnet DC (PMDC) Motors

Basic configuration of a permanent magnet DC motor is very similar to that of a normal DC motor. The working principle of any DC motor is same, i.e. when a current carrying conductor is placed in a magnetic field, it experiences a force. A permanent magnet DC motor also works on the same principle.

Construction

In a PMDC motor, permanent magnets (located in stator) provide magnetic field, instead of stator winding. The stator is usually made from steel in cylindrical form. Permanent magnets are usually made from rare earth materials or neodymium.

The rotor is slotted armature which carries armature winding. Rotor is made from layers of laminated silicon steel to reduce eddy current losses. Ends of armature windings are connected to commutator segments on which the brushes rest. Commutator is made from copper and brushes are usually made from carbon or graphite. DC supply is applied across these brushes. The commutator is in segmented form to achieve unidirectional torque. The reversal of direction can be easily achieved by reversing polarity of the applied voltage.

Characteristics

Characteristics of PMDC motors are similar to the characteristics of dc shunt motor in terms of torque, speed and armature current. However, speed-torque characteristics are more linear and predictable in PMDC motors.

Applications Of Permanent Magnet DC Motors

Permanent magnet dc motors are extensively used where smaller power ratings are required, e.g. in toys, small robots, computer disc drives etc. 

Advantages

1. For smaller ratings, use of permanent magnets reduces manufacturing cost.
2. No need of field excitation winding, hence construction is simpler.
3. No need of electrical supply for field excitation, hence PMDC motor is relatively more efficient.
4. Relatively smaller in size
5. Cheap in cost

Disadvantages

1. Since the stator in PMDC motor consists of permanent magnets, it is not possible to add extra ampere-turns to reduce armature reaction. Thus armature reaction is more in PMDC motors.
2. Stator side field control, for controlling speed of the motor, is not possible in pemanent magnet dc motors.

Parallel Operation Of Shunt Generators

Normally the generators are coupled in parallel at most of the power station through bus-bars. Bus-bars have positive and negative terminals and they must be dense thick copper bars. The positive and negative terminals of the bus-bars are connected to the positive and negative terminal of the generator respectively.

The above figure shows the shunt generator No.1 is connected to the bus-bars and delivering load. The shunt Generator No.2 is connected in parallel to the Generator No.1. When the load on the generator No.1 increases beyond its rated capacity, immediately the second shunt generator operate and wish the first generator to come across the raised load demand.
The paralleling of generator No.2 with the generator No.1 procedures are as follows:

• First the prime mover of second generator is taken up to the rated speed. Once starts running at its rated speed the field circuit of the second switch S2 is closed.
• Afterward close the second breaker (ie. CB-2) and the excitation of second generator is varied till it’s generates voltage identical to the bus-bars voltage. It is shown by voltmeter V2.
• Now the second generator is in parallel with the generator No.1. The generator no.2 will not supply any load when it is connected in parallel with generator No.1, since its generated EMF is same as that of bus-bars voltage. At this juncture the generator is assumed to be “floating” on the bus-bars.
• Once generator no.2 supply any current, then its generated voltage E thought to be more than the bus-bars voltage V. In such case, the current delivered by it is I = (E - V)/Ra. The generator No.2 can be made to supply appropriate load by raising the field current and so the induced EMF.
• By varying the field excitation, the load might be lifted from one shunt generator to another. In consequence if generator No.1 needs to be halt, then the entire load moved onto the generator No.2 if it has the ability to deliver that load. In such case, decrease the current delivered by generator NO.1 to zero. It is shown in ammeter A1. Finally open the circuit breaker No.1 and then S1 switch.

Salient Pole Rotor

In salient pole type of rotor consist of large number of projected poles (salient poles) mounted on a magnetic wheel. Construction of a salient pole rotor is as shown in the figure at left. The projected poles are made up from laminations of steel. The rotor winding is provided on these poles and it is supported by pole shoes.

• Salient pole rotors have large diameter and shorter axial length.
• They are generally used in lower speed electrical machines, say 100 RPM to 1500 RPM.
• As the rotor speed is lower, more number of poles are required to attain the required frequency. (Ns = 120f / P   therefore, f = Ns*p/120   i.e. frequency is proportional to number of poles). Typically number of salient poles is between 4 to 60.
• Flux distribution is relatively poor than non-salient pole rotor, hence the generated emf waveform is not as good as cylindrical rotor.
• Salient pole rotors generally need damper windings to prevent rotor oscillations during operation.
• Salient pole synchronous gerenator are mostly used in hydro power plants.

Non-Salient Pole (Cylindrical) Rotor

Non-salient pole rotors are cylindrical in shape having parallel slots on it to place rotor windings. It is made up of solid steel. The construction of non-salient pole rotor (cylindrical rotor) is as shown in figure above. Sometimes, they are also called as drum rotor.

• They are smaller in diameter but having longer axial length.
• Cylindrical rotors are used in high speed electrical machines, usually 1500 RPM to 3000 RPM.
• Windage loss as well as noise is less as compared to salient pole rotors.
• Their construction is robust as compared to salient pole rotors.
• Number of poles is usually 2 or 4.
• Damper windings are not needed in non-salient pole rotors.
• Flux distribution is sinusoidal and hence gives better emf waveform.
• Non-salient pole rotors are used in  nuclear, gas and thermal power plants.

Starting Methods Of A DC Motor

Basic operational voltage equation of a DC motor is given as
E = E_b + I_aR_a   and hence,     I_a = (E - E_b) / R_a
Now, when the motor is at rest, obviously, the back emf E_b = 0. Hence, armature current at the moment of starting can be given as I_a = E / R_a. In practical DC machine, armature resistance is basically very low, generally about 0.5 Ω. Therefore, a large current flows through the armature during starting. This current is large enough to damage the armature circuit.
Due to this excessive starting current -

1. the fuses may blow out and the armature winding and/or commutator brush arrangement may get damaged.
2. very high starting torque will be produced (as torque is directly proportional to the armature current), and this high starting torque may cause huge centrifugal force which may throw off the armature winding.
3. other loads connected to the same source may experience a dip in the terminal voltage.

A large DC motor will pick up speed rather slowly due to its large rotor inertia. Hence, building up the back emf slowly causing the level of high starting current maintained for quite some time. This may cause severe damage. To avoid this, a suitable DC motor starter must be used. Very small dc motors, however, may be started directly by connecting them to the supply with the help of a contactor or a switch. It does not result in any harm because they gather speed quickly due to small rotor inertia. In this case, the large starting current will die down quickly because of the fast rise in the back emf.

DC Motor Starters

To avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. So, a DC motor is started by using a starter. There are various types of dc motor starters, such as 3 point starter, 4 point starter, no-load release coil starter, thyristor controller starter etc.
The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.
From the followings, 3 point starters and 4 point starters are used for starting shunt wound motors and compound wound motors.

3 Point Starter

The internal wiring of a 3 point starter is as shown in the figure.

When the connected dc motor is to be started, the lever is turnedgradually to the right. When the lever touches point 1, the field winding gets directly connected across the supply, and the armature winding gets connected with resistances R1 to R5 in series. During starting, full resistance is added in series with the armature winding. Then, as the lever is moved further, the resistance is gradually is cut out from the armature circuit. Now, as the lever reaches to position 6, all the resistance is cut out from the armature circuit and armature gets directly connected across the supply. The electromagnet 'E' (no voltage coil) holds the lever at this position. This electromagnet releases the lever when there is no (or low) supply voltage.

It can be seen that, when the arm is moved from the position 1 to the last position, the starter resistance gets added in series with the field winding. But, as the value of starter resistance is very small as compared to the shunt resistance, the decrease in shunt field current may be negligible. However, to overcome this drawback a brass or copper arc may be employed within a 3 point starter which makes a connection between the moving arm and the field winding, as shown in the figure of 4 point starter below.
When the motor is overloaded beyond a predefined value, 'overcurrent release electromagnet' D gets activated, which short-circuits electromagnet E and, hence, releases the lever and the motor is turned off.

4 Point Starter

The main difference between a 3 point starter and a 4 point starter is that the no voltage coil (electromagnet E) is not connected in series with the field coil. The field winding gets directly connected to the supply, as the lever moves touching the brass arc (the arc below the resistance studs). The no voltage coil (or Hold-on coil) is connected with a current limiting resistance Rh. This arrangement ensures that any change of current in the shunt field does not affect the current through hold-on coil at all. This means, electromagnetic pull of the hold-on coil will always be sufficient so that the spring does not unnecessarily restore the lever to the off position. A 4 point starter is used where field current is to be adjusted by means of a field rheostat for the purpose of operating the motor at rated speed by reducing the field current.

DC Series Motor Starter

Construction of DC series motor starters is very basic as shown in the figure. The start arm is simply moved towards right to start the motor. Thus, maximum resistance is connected in series with the armature during starting and then gradually decreased as the start arm moves towards right. This starter is sometimes also called as a 2 point starter.
The no load release coil holds the start arm to the run position and leaves it when the voltage is lost.

A running motor may be brought to rest quickly by either mechanical braking or electrical braking. The mechanical braking is applied by means of mechanical break shoes. Hence the smoothness of mechanical braking is dependent on the surface and physical condition of brakes. Smooth braking of a motor can be achieved by electric braking.

Electric Braking

The electric braking of a DC motor is of three types, (i) Rheostatic or dynamic braking, (ii) Plugging or reverse current braking  and (iii) Regenerative beaking.

(i) Rheostatic or dynamic braking:

In case of DC shunt motor, armature is disconnected from the supply and a rheostat (variable resistor) is connected across it. The field winding is left connected across the supply. Obviously, now armature is driven by the inertia and hence machine starts acting as a generator. Thus the machine will now feed the current to the connected rheostat and heat will dissipate at the rate of I²R. Braking effect is controlled by varying the resistance connected across the armature.
In case of DC series motor, motor is disconnected from the supply and field connections are reversed and a rheostat is connected in series. The field connections are reversed to make sure that the current through field winding will flow in the same direction as before.

(ii) Plugging or Reverse current braking:
In this method, armature connections are reversed and hence motor tends to run in opposite direction. Due to reversal of the armature terminals, applied voltage V and back emf Eb starts acting in the same direction and hence the total armature current exceeds. To limit this armature current a variable resistor is connected across the armature. This is similar for both series and shunt wound methods.
Plugging gives greater braking torque as compared to rheostatic braking. This method is generally used in controlling elevators, machine tools, printing presses etc.

(iii) Regenerative braking:
Regenerative braking is used where, load on the motor has very high inertia (e.g in electric trains). When applied voltage to the motor is reduced to less than back emf Eb, obviously armature current Ia will get reversed, and hence armature torque is reversed. Thus speed falls. As generated emf is greater than applied voltage (machine is acting as a DC generator), power will be returned to the line, this action is called as regeneration. Speed keeps falling, back emf Eb also falls until it becomes lower than applied voltage and direction of armature current again becomes opposite to Eb.

Speed Control Methods Of DC Motor

Speed Control Of Shunt Motor

1. Flux Control Method

It is already explained above that the speed of a dc motor is inversely proportional to the flux per pole. Thus by decreasing the flux, speed can be increased and vice versa.

To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram. Adding more resistance in series with the field winding will increase the speed as it decreases the flux. In shunt motors, as field current is relatively very small, I_sh²R loss is small and, hence, this method is quite efficient. Though speed can be increased above the rated value by reducing flux with this method, it puts a limit to maximum speed as weakening of flux beyond the limit will adversely affect the commutation.

2. Armature Control Method

Speed of a dc motor is directly proportional to the back emf E_b and E_b = V - I_aR_a. That means, when the supply voltage V and the armature resistance R_a are kept constant, speed is directly proportional to the armature current I_a. Thus, if we add a resistance in series with the armature, I_a decreases and, hence, the speed also decreases. Greater the resistance in series with the armature, greater the decrease in speed.

3. Voltage Control Method

a) Multiple voltage control:
In this method, the shunt field is connected to a fixed exciting voltage and armature is supplied with different voltages. Voltage across armature is changed with the help of a suitable switchgear. The speed is approximately proportional to the voltage across the armature.

b) Ward-Leonard System:

This system is used where very sensitive speed control of motor is required (e.g electric excavators, elevators etc.). The arrangement of this system is as shown in the figure at right.
M₂ is the motor whose speed control is required.
M₁ may be any AC motor or DC motor with constant speed.
G is a generator directly coupled to M₁.
In this method, the output from the generator G is fed to the armature of the motor M₂ whose speed is to be controlled. The output voltage of the generator G can be varied from zero to its maximum value by means of its field regulator and, hence, the armature voltage of the motor M₂ is varied very smoothly. Hence, very smooth speed control of the dc motor can be obtained by this method.

Speed Control Of Series Motor

1. Flux Control Method

• Field divertor: A veritable resistance is connected parallel to the series field as shown in fig (a). This variable resistor is called as divertor, as the desired amount of current can be diverted through this resistor and hence current through field coil can be decreased. Hence, flux can be decreased to the desired amount and speed can be increased.
• Armature divertor: Divertor is connected across the armature as in fig (b).
  For a given constant load torque, if armature current is reduced then flux must increase. As, Ta ∝ ØIa
  This will result in an increase in current taken from the supply and hence flux Ø will increase and subsequently speed of the motor will decrease.
• Tapped field control: As shown in fig (c) field coil is tapped dividing number of turns. Thus we can select different value of Ø by selecting different number of turns.
• Paralleling field coils: In this method, several speeds can be obtained by regrouping coils as shown in fig (d).

2. Variable Resistance In Series With Armature

By introducing a resistance in series with the armature, voltage across the armature can be reduced. And, hence, speed reduces in proportion with it.

3. Series-Parallel Control

This system is widely used in electric traction, where two or more mechanically coupled series motors are employed. For low speeds, the motors are connected in series, and for higher speeds the motors are connected in parallel.
When in series, the motors have the same current passing through them, although voltage across each motor is divided. When in parallel, the voltage across each motor is same although the current gets divided.

How A DC Motor Works?

Working Principle Of A DC Motor

A motor is an electrical machine which converts electrical energy into mechanical energy. The principle of working of a DC motor is that "whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force". The direction of this force is given by Fleming's left hand rule and it's magnitude is given by F = BIL. Where, B = magnetic flux density, I = current and L = length of the conductor within the magnetic field.

Fleming's left hand rule: If we stretch the first finger, second finger and thumb of our left hand to be perpendicular to each other AND direction of magnetic field is represented by the first finger, direction of the current is represented by second finger then the thumb represents the direction of the force experienced by the current carrying conductor.

Above animation helps in understanding the working principle of a DC motor. When armature winding are connected to a DC supply, current sets up in the winding. Magnetic field may be provided by field winding (electromagnetism) or by using permanent magnets. In this case, current carrying armature conductors experience force due to the magnetic field, according to the principle stated above.

Commutator is made segmented to achieve unidirectional torque. Otherwise, the direction of force would have reversed every time when the direction of movement of conductor is reversed the magnetic field.

This is how a DC motor works!

Back EMF

According to fundamental laws of nature, no energy conversion is possible until there is something to oppose the conversion. In case of generators this opposition is provided by magnetic drag, but in case of dc motors there is back emf.

When the armature of the 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 (I_a) . The circuit diagram below illustrates the direction of the back emf and armature current. Magnitude of Back emf can be given by the emf equation of dc generator.

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 allowing more armature current. Increased armature current will increase the torque to satisfy the load requirement. Hence, presence of the back emf makes a dc motor ‘self-regulating’.

Types Of DC Motors 

DC motors are usually classified of the basis of their excitation configuration, as follows -

• Separately excited (field winding is fed by external source)
• Self excited -
• Series wound (field winding is connected in series with the armature)
• Shunt wound (field winding is connected in parallel with the armature)
• Compound wound - 
• Long shunt
• Short shunt

Classifications Of DC Machines : (DC Motors And DC Generators)

Each DC machine can act as a generator or a motor. Hence, this classification is valid for both: DC generators and DC motors. DC machines are usually classified on the basis of their field excitation method. This makes two broad categories of dc machines; (i) Separately excited and (ii) Self-excited.

• Separately excited: In separately excited dc machines, the field winding is supplied from a separate power source. That means the field winding is electrically separated from the armature circuit. Separately excited DC generators are not commonly used because they are relatively expensive due to the requirement of an additional power source or circuitry. They are used in laboratories for research work, for accurate speed control of dc motor with Ward-Leonard system and in few other applications where self-excited DC generators are unsatisfactory. In this type, the stator field flux may also be provided with the help of permanent magnets (such as in the case of a permanent magnet DC motors). A PMDC motor may be used in a small toy car.
• Self-excited: In this type, field winding and armature winding are interconnected in various ways to achieve a wide range of performance characteristics (for example, field winding in series or parallel with the armature winding).
  In self-excited type of DC generator, the field winding is energized by the current produced by themselves. A small amount of flux is always present in the poles due to the residual magnetism. So, initially, current induces in the armature conductors of a dc generator only due to the residual magnetism. The field flux gradually increases as the induced current starts flowing through the field winding.
  
  Self-excited machines can be further classified as –
  • Series wound – In this type, field winding is connected in series with the armature winding. Therefore, the field winding carries whole load current (armature current). That is why series winding is designed with few turns of thick wire and the resistance is kept very low (about 0.5 Ohm).
  • Shunt wound – Here, field winding is connected in parallel with the armature winding. Hence, the full voltage is applied across the field winding. Shunt winding is made with a large number of turns and the resistance is kept very high (about 100 Ohm). It takes only small current which is less than 5% of the rated armature current.
  • Compound wound – In this type, there are two sets of field winding. One is connected in series and the other is connected in parallel with the armature winding. Compound wound machines are further divided as -
    • Short shunt – field winding is connected in parallel with only the armature winding
    • Long shunt – field winding is connected in parallel with the combination of series field winding and armature winding

According to this classification, a wide range of characteristics of DC generator as well as of DC motor can be achieved.

EMF Equation And Torque Equation Of A DC Machine

Consider a DC generator with the following parameters,

P = number of field poles
Ø = flux produced per pole in Wb (weber)
Z = total no. of armature conductors
A = no. of parallel paths in armature
N = rotational speed of armature in revolutions per min. (rpm)

Therefore, Eg = PΦNZ / 60A

For simplex lap winding, number of parallel paths is equal to the number of poles (i.e. A=P),
Therefore, for simplex lap wound dc generator, Eg = PΦNZ / 60P

For simplex wave winding, number of parallel paths is equal to 2 (i.e P=2),
Therefore, for simplex wave wound dc generator, Eg = PΦNZ / 120 

Torque Equation Of A DC Motor

When armature conductors of a DC motor carry current in the presence of stator field flux, a mechanical torque is developed between the armature and the stator. Torque is given by the product of the force and the radius at which this force acts.

• Torque T = F × r (N-m) …where, F = force and r = radius of the armature
• Work done by this force in once revolution = Force × distance = F × 2πr    (where, 2πr = circumference of the armature)
• Net power developed in the armature = word done / time
  = (force × circumference × no. of revolutions) / time
  = (F × 2πr × N) / 60 (Joules per second) .... eq. 2.1

But, F × r = T and 2πN/60 = angular velocity ω in radians per second. Putting these in the above equation 2.1
Net power developed in the armature = P = T × ω (Joules per second)

Armature Torque (Ta)

• The power developed in the armature can be given as, Pa = Ta × ω = Ta × 2πN/60
• The mechanical power developed in the armature is converted from the electrical power,
  Therefore, mechanical power = electrical power
  That means, Ta × 2πN/60 = Eb.Ia
• We know, Eb = PΦNZ / 60A
• Therefore, Ta × 2πN/60 = (PΦNZ / 60A) × Ia
• Rearranging the above equation,
  Ta = (PZ / 2πA) × Φ.Ia (N-m)

The term (PZ / 2πA) is practically constant for a DC machine. Thus, armature torque is directly proportional to the product of the flux and the armature current i.e. Ta ∝ Φ.Ia

Shaft Torque (Tsh)

Due to iron and friction losses in dc machine, the total developed armature torque is not available at the shaft of the machine. Some torque is lost, and therefore, shaft torque is always less than the armature torque.

Shaft torque of a DC motor is given as,
Tsh = output in watts / (2πN/60) ....(where, N is speed in RPM)

Armature Winding Of A DC Machine

Basically armature winding of a DC machine is wound by one of the two methods, lap winding or wave winding. The difference between these two is merely due to the end connections and commutator connections of the conductor. To know how armature winding is done, it is essential to know the following terminologies -

1. Pole pitch: It is defined as number of armature slots per pole. For example, if there are 36 conductors and 4 poles, then the pole pitch is 36/4=9.
2. Coil span or coil pitch (Ys): It is the distance between the two sides of a coil measured in terms of armature slots.
3. Front pitch (Yf): It is the distance, in terms of  armature conductors, between  the second conductor of one coil and the first conductor of the next coil. OR it is the distance between two coil sides that are connected to the same commutator segment. 
4. Back pitch (Yb): The distance by which a coil advances on the back of the armature is called as back pitch of the coil. It is measured in terms of armature conductors.
5. Resultant pitch (Yr): The distance, in terms of armature conductor, between the beginning of one coil and the beginning of the next coil is called as resultant pitch of the coil.

Armature winding can be done as single layer or double layer. It may be simplex, duplex or multiplex, and this multiplicity increases the number of parallel paths.

Lap Winding And Wave Winding

In lap winding, the successive coils overlap each other. In a simplex lap winding, the two ends of a coil are connected to adjacent commutator segments. The winding may be progressive or retrogressive. A progressive winding progresses in the direction in which the coil is wound. The opposite way is retrogressive. The following image shows progressive simplex lap winding.

In wave winding, a conductor under one pole is connected at the back to a conductor which occupies an almost corresponding position under the next pole which is of opposite polarity. In other words, all the coils which carryy emf in the same direction are connected in series. The following diagram shows a part of simplex wave winding.

Basic Construction And Working Of A DC Generator

DC Generator

A dc generator is anelectrical 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:

Note: A DC generator can be used as a DC motor without any constructional changes and vice versa is also possible. Thus, a DC generator or a DC motor can be broadly termed as a DC machine. These basic constructional details are also valid for the construction of a 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 the constructional details of a simple 4-pole DC machine. A DC machine consists 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.



Armature core (rotor)

4. Armature core: Armature core is the rotor of the 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 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 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.

Commutator

Working Principle Of A DC Generator:

According to Faraday's law of electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the conductor. The magnitude of induced emf can be calculated from the emf equation of DC generator. If the conductor is provided with the closed path, the induced current will circulate within the path. In a DC generator, field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus, an electromagnetically induced emf is generated in the armature conductors. The direction of induced current is given by Fleming's right hand rule.

Need of a Split ring commutator: 

According to Fleming’s right hand rule, the direction of induced current changes whenever the direction of motion of the conductor changes. Let’s consider an armature rotating clockwise and a conductor at the left is moving upward. When the armature completes a half rotation, the direction of motion of that particular conductor will be reversed to downward. Hence, the direction of current in every armature conductor will be alternating. If you look at the above figure, you will know how the direction of the induced current is alternating in an armature conductor. But with a split ring commutator, connections of the armature conductors also gets reversed when the current reversal occurs. And therefore, we get unidirectional current at the terminals.

Types Of A DC Generator:

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) Selfexcited: 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

What Is An Electrical Machine?

Definition: An electrical machine is a device which converts mechanical energy into electrical energy or vice versa. Electrical machines also include transformers, which do not actually make conversion between mechanical and electrical form but they convert AC current from one voltage level to another voltage level.

Generator:

A generator is an electrical machine which converts mechanical form of energy into electrical form. Generator works on the principle that whenever a conductor moves in a magnetic field, an emf gets induced in the conductor. This principle is called as generator action.
Generators have generally two basic parts named "Stator" and "Rotor". Mechanical energy is provided to the rotor of a generaotor by means of a prime mover (i.e. a turbine). Turbines are of different types like steam turbine, water turbine, wind turbine etc. Mechanical energy can also be provided by IC engines or similar other sources.
To learn more about how generators work, read the following articles.

• AC generator (converts mechanical energy into Alternating Current (AC) electricity)
• DC generator (converts mechanical energy into Direct Current (DC) electricity)

Motor:

A motor is an electrical machine which converts electrical energy into mechanical energy. When a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force and this is the principle behind motoring action.
Just like generators, motors also consists two basic parts, stator and rotor. In a motor, we give electric supply to both the stator and rotor windings which causes a mechanical force between the stator and rotor. This force causes the rotor to rotate.
To learn more about electric motors, read the following articles.

• AC motors: (i) Induction motors and (ii) Synchronous motor
• DC motors: (i) Brushed DC motor and (ii) Brushless DC motor