What is DC Motor? Working and it's types

 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.