DC Machine

A DC machine may be a DC generator or a DC motor. The DC generator converts mechanical energy into electrical energy, while a DC motor converts electrical energy into mechanical energy. 

A DC generator works on the principle that when a conductor moves in a magnetic field causes an emf induced in the conductor. A DC motor works on the principle that when a current carrying conductor is placed in a magnetic field, a mechanical force acts on conductor.

A DC machine has a coil winding wound on the poles, which gives a field stationary in the air gap due to DC supply in the field winding. The armature has commutator type winding with stationary brushes for connection to external circuit.

Construction of D.C. Machines

Basically DC machine consists of two main parts. One of them is the armature and other is field magnets. The amature is rotating part of the machine and field magnets is stationary part. In construction, there is no difference between DC generator and a DC motor.

When DC machine is working as a generator, its armature in rotated by external means and its field is connected directly to low DC supply voltage. Due to the rotation of armature conductors in the magnetic field, an emf is induced in the armature conductors according to the Faraday 's laws of electro-magnetic induction.

When DC machine is working as a motor its armature as well as magnetic field are fed direct with DC supply. The armature starts rotating because when a current carrying conductor is placed in a magnetic field, mechanical force is experienced on the conductor. They have the same parts.

A DC machine consists of the following main parts : 

1. Yoke : It is the outer cylindrical frame of the machine. Cast iron yokes are made for small machines because of cheapness but for large machines yoke is made of fabricated steel due to huge permeability. Yoke serves the following purpose :

(i) Poles are fixed on it

(ii) It provides mechanical protection to the inner parts of the machine.

(iii) It provides a passage for magnetic flux produced by poles.

2. Poles : Pole consists of two main parts :

(i) Pole Core : The Pole Core is made of cast iron of solid piece or cast steel. The pole core may be made of thin cast steel laminations riveted together. This type of pole is held in position with frame by means of bolt. The pole core is usually circular in section and field coil is wound over it to carry field current.

(ii) Pole Shoe : The Pole Shoe is laminated and is screwed to the pole face by means of counter sunk screws. The pole serves following purposes :

(a) It supports the field coils.

(b) It spreads out the magnetic flux in the air gap more uniformally.

(c) The Pole shoes have larger cross-section, hence reluctance or magnetic path is reduced.

Armature : The armature is made up of laminations and the armature winding is done on the armature core. The armature should be of magnetic material such as in order to provide a path of low reluctance to the magnetic flux.

Commutator : It is made up of hard drawn copper segments insulated from each other by insulation. The ends of the armature winding are connected to the commutator segments. The function of the commutator is to convert AC into DC.

Brushes :  The brushes are made of carbon. The brushes are used to give or get supply to the armature winding. The brushes are placed on the commutator segments.

Bearings : Bearings are fitted at the ends of the shaft and are supported in end plates.

Fan : Fans are fitted on the shaft in big machines to give air for cooling purpose. 

Generator Action : In case of generator the armature and commutator are rotated in the magnetic field. The alternating emf is induced in the armature conductors, but with the help of commutator we get unidirectional emf and current in the external circuit. 

Motor Action : In case of motor the DC supply is given to the armature conductors through commutator and DC supply is given to the field winding. With the help of commutator, we can change the direction of current in the armature conductors as the armature conductors pass from one pole to the next pole. So we get unidirectional electromagnetic torque produced.

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What is DC Generator? Working and it's types

What is DC Motor? Working and it's types

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 DC MOTOR

A machine which converts electrical power into mechanical power is called DC motor. From construction point of view there is no difference between DC Generator or DC Motor.

The DC Motors are very useful where wide range of speeds and good speed regulation is required such as electric traction.

WORKING PRINCIPLE OF DC MOTORS

The operation of DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, a mechanical force is experienced by it. The direction of this force is determined by Fleming's Left Hand rule and it's magnitude is given by the relation:

F = BI𝑙 Newton

For simplicity, consider only one coil of the armature placed in the magnetic field produced by a bipolar machine. When DC supply is connected to the coil, current flow through it which sets up its own field. By the interaction of the fields (i.e. field produced by the main poles and the coil). The tendency of this field is to come to its original position i.e. in straight line due to which force is exerted on the two coil sides and torque develops which rotates the coil.

Alternatively it can be said that the main poles produce a field F_m. When current is supplied to the coil (armature conductors), it produces its own field marked as F_r. This field tries to come in line with the main field and an electromagnetic torque develops in clockwise direction.

In actual machine, a large number of conductors are placed on the armature. All the conductors placed under the influence of one pole (say North pole) carry the current in one direction (outward). Whereas, the other conductors placed under the influence of other pole carry the current in opposite direction. A resultant rotor field is produced. Its direction is marked by the arrow head F_r. This rotor field F_r tries to come in line with the main field and torque (T_e) develops. Thus, rotor rotates.

It can be seen that to obtain a continuous torque, the direction of flow of current in each conductor or coil side must be reversed when it crosses the magnetic neutral axis (MNA). This is achieved with the help of a commutator.

Functions of Commutators

The function of commutator in DC motor is to reverse the direction of flow of current in each armature conductor when it crosses the MNA to obtain continuous torque.

Back EMF

It has been seen that when current is supplied to the armature conductors placed in the main magnetic field, torque develops and armature rotates, the armature conductors cut the magnetic field and an emf is induced in these conductors. The direction of the induced emf in the armature conductors is determined by Fleming's Right Hand Rule.

The direction of this induced emf is opposite to the applied voltage. That is why this induced emf in the armature, when the machine works as a motor is called back emf (E_b). The magnitude of this induced emf, is given by relation; E_b = PZ𝟇N/60A

The supply voltage is always greater than the induced or back emf (i.e. V>E_b). Therefore current is always supplied to the motor from the mains and the relation among the various quantities will be; E_b = V - I_a.R_a

SHAFT TORQUE

In d.c. motor whole of the electromagnetic torque (T_e) developed in the armature is not available at the shaft. A part of it is lost to overcome the iron and mechanical (friction and windage) losses. Therefore, shaft torque (T_sh) is some what less than the torque developed in the armature.

Thus, in case of DC motors, the actual torque available at the shaft for doing useful mechanical work is known as shaft torque.

Brake Horse Power (B.H.P.): In case of motors, the mechanical power (H.P.) available at the shaft is known as brake horse power (B.H.P). If T_sh, is the shaft torque in N_m and N is speed in r.p.m. then,

Useful output power = 𝟂 T_sh = 2π N T_sh /60 watts

Output in B.H.P. = 2π N T_sh/60 × 735.5

TYPES OF DC MOTORS

Similar to DC Generators, on the basis of their field excitation, the DC motors can also be classified as:

1. Seperately excited DC motors : These motors whose field windings are excited from some external source of supply. In seperately excited DC motor.

Relations:

V = E_b + I_a.R_a + 2V_b

where, 

I_a = Armature current

      = Line current, I_L

E_b = Back emf developed = V - I_a.R_a

V = Supply voltage

2. Self excited DC motors : Such type of DC motor are excited from their own armature. Their field and armature winding are connected together. These motors can be further divided as:

(i) Shunt Motor : A DC shunt motor is one in which field winding is in parallel with the armature. The field winding consists of many turns of thin wire of copper. The line current is supplied to the motor is divided into two path, one through the shunt field winding and the second through the armature.

Important Relation:

I_sh = V/R_sh 

I_L = I_a + I_sh 

E_b = V - I_a.R_a - 2V_b

Power supplied to the motor, P = VI_L 

Power developed Pm = power input - losses in armature and shunt field.

(ii) Series Motor : A DC series motor is one in which field winding is connected in series with the armature winding. The field winding consists of few turns of thick wire of copper. As the field winding is connected in series with the armature so it will carry the entire current drawn by the motor from supply.

Important Relation:

I_se = I_a = I_L

E_b = V - I_a(R_a + R_se) - 2V_b

Power supplied to the motor = VI_L

Power developed in armature,

P_m = power input - current losses

3. Compound motors : Compound motors are classified into two types:

(i) Cumulative Compound Motor : Compound motor is one in which the field windings are connected in such a way that the direction of flow of current is same in both of the field winding, i.e., 𝟇_t = 𝟇_sh + 𝟇_se

When the motor is loaded, its armature draws more current from the line, thus it strengthens the field.

(ii) Differential Compound Motor : Differential compound motor is one in which the field winding are connected in such a way that the direction of flow of current is opposite to each other i.e., 𝟇_t = 𝟇_sh - 𝟇_se

When the motor is loaded, its armature draws more current from the line, thus it weakens the field.

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