How does DC motor work?and working principle and type of dc motor

Explanation of Construction of Dc motor, types of dc motors series, shunt and compound dc motors

fig: dc motor working principle

If a current carrying conductor is placed into the field of a permanent magnet, as shown in Fig, a force will be exerted on the conductor to push it out of the magnetic field. 

PRACTICAL D.C. MOTORS
 

Fig: dc motor construction

Direct Current Machines:In a d. c.machine, the field winding is on the stator and the armature winding is on rotor.

The constructional features of a typical two-pole d. c. machine are depicted in above figure

STATOR: The stator consists of (i) yoke (or frame) made of unlaminated ferromagnetic material, such as cast iron or fabricated steel (ii) the salient field poles bolted to the inner periphery of the yoke and (iii) bearings, brush-rigging carrying brush-holders, end-covers etc. The yoke is made, to rest on a supporting base.

The field poles are made of a stack of steel plats (1 to 1. 5 mm thick), riveted together. The pole core, where the  exciting or field winding is wound, is usually of smaller cross-section than the pole shoe (or pole face), 

due to the following reasons:

(a) The reduced cross-section of the pole core requires less copper for the field winding.

(b) The large pole shoe area increases the flux per pole entering the armature, due to the reduction in air-gap reluctance.

(c) plot shoes provide mechanical strength and support to the field winding 

ROTOR: The armature core consists of a stack of circular steel laminations about 0. 4 to 0. 6 mm thick. The periphery of these laminations is slotted to receive the distributed armature winding, These laminations are insulated from one another so as to decrease the eddy-current losses. In case of small machines, the laminations are assembled tightly on the shaft, but on the cast-iron spider in case of large machines.
In addition to the field and armature windings, a d. c. generator must have a commutator, to serve as a mechanical rectifier for the alternating e. m. f. generated in the armature winding to direct e.m.f. At the brush terminals. For a d. c. motor, the commutator serves as a mehanical inverter to invert the direct applied voltage to alternating voltage in the armature winding. These requirements of mechanical-rectifier and mechanical inverter operations demand that the armature and commutator be placed on the rotor and field winding in stator 

Contact with the external circuit is made through carbon brushes rubbing on the commutator segments.

Type of dc motor:
1) dc series motor
2) dc shunt motor
3) dc compound motor

Direct current motors are classified by the way in which the field and armature windings are connected, which may be in series or in parallel. 

1) dc Series Motor: Series motor The field and armature windings are connected in series  share the same current. The series motor has the characteristics of a high starting torque but a speed which varies with load. Theoretically the motor would speed up to self-destruction, limited only by the windage of the rotating armature and friction, if the load were completely removed. 

speed  load characteristics

Figure  shows series motor connections and characteristics.For this reason the motor is only suitable for direct coupling to a load, except in very small motors,such as vacuum cleaners and hand drills, and is ideally suited for applications where the machine must start on load, such as electric trains, cranes and hoists.

Reversal of rotation may be achieved by reversing the connections of either the field or armature windings but not both.

This characteristic means that the machine will run on both a.c. or d.c. and is, therefore, sometimes referred to as a ‘universal’ motor.

2) Dc shunt motor: Shunt motor The field and armature windings are connected in parallel . Since the field winding is across the supply, the flux and motor speed are considered constant under normal conditions.

speed  load characteristics

 In practice, however, as the load increases the field flux distorts and there is a small drop in speed of about 5% at full load, as shown in Fig. The machine has a low starting torque and it is advisable to start with the load disconnected. The shunt motor is a very desirable d.c. motor because of its constant speed characteristics.It is used for driving power tools, such as lathes and drills. Reversal of rotation may be achieved by reversing the connections to either the field or armature winding but not both.

3) dc compound motor: Compound motor The compound motor has two field windings – one in series with the armature and the other in parallel.

 If the field windings are connected so that the field flux acts in opposition, the machine is known as a short shunt and has the characteristics of a series motor. If the fields are connected so that the field flux is strengthened, the machine is known as a long shunt and has constant speed characteristics similar to a shunt motor. The arrangement of compound motor connections is given in Fig. The compound motor may be designed to possess the best characteristics of both series and shunt motors, that is, good starting torque together with almost constant speed. Typical applications are for electric motors in steel rolling mills, where a constant speed is required under varying load conditions.