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 DC machines: 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 motors 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 permanent magnet DC motors). PMDC (permanant magnet DC) motors are popularly used in small toys, e.g. a toy car.
• Self-excited DC machines: 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 a 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 dc machines – In this type, field winding is connected in series with the armature winding. Therefore, the field winding carries whole of the 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 dc machines – 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 dc machines – 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 generators as well as of DC motors can be achieved.

Read More...

EMF equation and Torque equation of a DC machine

EMF equation of a DC generator

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)

Now,

• Average emf generated per conductor is given by dΦ/dt (Volts) ... eq. 1
• Flux cut by one conductor in one revolution = dΦ = PΦ ….(Weber),
• Number of revolutions per second (speed in RPS) = N/60
• Therefore, time for one revolution = dt = 60/N (Seconds)
• From eq. 1, emf generated per conductor = dΦ/dt = PΦN/60 (Volts) …..(eq. 2)

Above equation-2 gives the emf generated in one conductor of the generator. The conductors are connected in series per parallel path, and the emf across the generator terminals is equal to the generated emf across any parallel path.

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 a 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)

Read More...

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.

[Also read: Armature reaction in DC machines]

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 carry emf in the same direction are connected in series. The following diagram shows a part of simplex wave winding.

Read More...

What is an electrical machine?

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.

Electric Generator:

An electric generator is an electrical machine which converts mechanical energy into electrical energy. A generator works on the principle of electromagnetic induction. It states that whenever a conductor moves in a magnetic field, an emf gets induced within the conductor. This phenomenon is called as generator action.

A generator basically consists of a stator and a rotor. Mechanical energy is provided to the rotor of a generator 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)

Electric 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 consist of two basic parts, stator and rotor. In many types of motors, electric supply needs to be provided for both stator and rotor winding. But in some types, like fixed magnet motors and induction motors, supply may be necessary for only one winding. Electromagnetic force between the two windings 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

Transformers:

Transformers do not actually make conversion between mechanical and electrical energy, but they transfer electric power from one circuit to another circuit. They can increase or decrease (step-up or step-down) the voltage while transferring the power without changing the frequency, but with the corresponding decrease or increase in the current. Input power and output power of an electrical transformer should ideally be the same.

Step up transformers increases the voltage level from primary to secondary but with the corresponding decrease in the current. Whereas, step-down transformer decrease the voltage level with the corresponding increase in the current so as to keep the power constant.

You can find articles related to electrical machines at the following link -

Index of Electrical Machines

Read More...