Types

                         

DC MOTOR TYPES

It is important to know the various types of DC Motors that is use in industries for lot of purposes. Let us discuss in detail.

                              

1.Separately Excited DC Motor

In case of a separately excited DC motor the supply is given separately to the field and armature windings. In this dc motor, the armature current does not flow through the field windings, as the field winding is energized from a separate external source of DC current as shown in the figure.

From the torque equation of DC motor,

                                                       Tg = Ka φ Ia 

So the torque in this case can be varied by varying field flux φ, independent of the armature current Ia. 

2.Permanent Magnet DC Motor

The permanent magnet DC motor  consists of an armature winding but does not necessarily contain the field windings. 

The rotor has a conventional DC armature with commutator segments and brushes. The diagrammatic representation of a permanent magnet DC motor is given below.

The torque equation of DC motor suggests

Here φ is always constant for PMDC motor.

Therefore, for a Permanent Magnet DC motor

Where, Ka1 = Ka.φ which is another constant. In this case.

3. Self Excited DC Motor

In case of self excited DC motor, the field winding is connected either in series or in parallel or partly in series, partly in parallel to the armature winding. Based on this, self excited DC Motors can be classified as:

1. Shunt wound DC motor
2. Series wound DC motor
3. Compound wound DC motor

Let’s now go into the details of these types of self excited DC motor.

3.1. Shunt Wound DC Motor

In case of a shunt wound DC motor the field windings are exposed to the entire terminal voltage as they are connected in parallel to the armature winding as shown in the figure below.

 Voltage Equation 

E = E_b + I_aR_a

Torque Speed Characteristics

The shunt wound DC motor is a constant speed motor, as the speed does not vary here with the variation of mechanical load on the output.

3.2. Series Wound DC Motor

In case of a series wound self excited DC motor , the entire armature current flows through the field winding as its connected in series to the armature winding. The series wound self excited DC motor is diagrammatically represented below. 

 Voltage Equation 

                                              E = E_b + I_a (R_a+R_s) 

Torque Speed Characteristics

                                      

In a series wound DC motor, the speed varies with load. And operation wise this is its main difference from a shunt wound DC motor.

3.3 Compound Wound DC Motor

The compound excitation characteristic in a DC motor can be obtained by combining the operational characteristic of both the shunt and series excited DC motor. The compound wound self excited DC motor contains the field winding connected both in series and in parallel to the armature winding as shown in the figure below:

The excitation of compound wound DC motor can be of two types depending on the nature of compounding.

3.3.1 Cumulative Compound DC Motor

When the shunt field flux assists the main field flux, produced by the main field connected in series to the armature winding then its called cumulative compound DC motor.

                                       Ф_Total = Ф_Series+ Ф_Shunt

3.3.2 Differential Compound DC Motor

In case of a differentially compounded self excited DC motor, the arrangement of shunt and series winding is such that the field flux produced by the shunt field winding diminishes the effect of flux by the main series field winding.

Ф_Total = Ф_Series- Ф_Shunt

The net flux produced in this case is lesser than the original flux and hence does not find much of a practical application.

Torque Speed Characteristics

                        

Both the cumulative compound and differential compound DC motor can either be of short shunt or long shunt type depending on the nature of arrangement.

Short Shunt DC Motor

 Circuit Diagram 

If the shunt field winding is only parallel to the armature winding then its known as short shunt DC motor.

Long Shunt DC Motor

Circuit Diagram 

If the shunt field winding is parallel to both the armature winding and the series field winding then it’s known as Long Shunt DC motor.

DC MOTOR

DIRECT CURRENT MOTOR

The electric motor operated by dc is called dc motor. This is a device that converts DC electrical energy into a mechanical energy.

CONSTRUCTION

                                  

 Main Parts of the DC Motor 

1.Yoke

Another name of a yoke is the frame. The main function of the yoke is to offer mechanical support for poles and protects the entire machine from the moisture, dust, etc. 

2.Pole and Pole Core

The pole of the DC machine is an electromagnet and the field winding is winding among pole. Whenever field winding is energized then the pole gives magnetic flux. 

3.Pole Shoe

Pole shoe in DC machine is an extensive part as well as enlarge the region of the pole. Because of this region, flux can be spread out within the air-gap as well as extra flux can be passed through the air space toward armature. 

4.Armature Winding

The armature winding can be formed by interconnecting the armature conductor. Whenever an armature winding is turned with the help of prime mover then the voltage, as well as magnetic flux, gets induced within it. This winding is allied to an exterior circuit. 

5.Commutator

The main function of the commutator is to collect the current from the armature conductor as well as supplies the current to the load using brushes. And also provides uni-directional torque for DC-motor. The Segments in the commutator are protected from thin mica layer.

6.Brushes

Brushes gather the current from commutator and supplies it to exterior load. Brushes wear with time to inspect frequently. 

PRINCIPLE

When a current carrying conductor is placed in a magnetic field, it experiences a torque (mechanical force) and has a tendency to move. The DC motor or direct current motor works on this principal. This is known as motoring action.

                         

The direction of rotation of a this motor is given by Fleming’s left hand rule and its magnitude is given by,

F=B*I*L

Where,
           
                    B = Magnetic flux density

                    I = Current and 

                    L = Length of the conductor within the magnetic field.

WORKING

     The circle in the center represents the direct current motor. On the circle, we draw the brushes. On the brushes, we connect the external terminals, through which we give the supply voltage. On the mechanical terminal, we have a shaft coming out from the center of the armature, and the shaft couples to the mechanical load. On the supply terminals, we represent the armature resistance Ra in series.

When the input voltage E, is applied across the brushes. Electric current which flows through the rotor armature via brushes  produces a torque Tg. 

Due to this torque Tg the dc motor armature rotates. As the armature conductors are carrying currents and the armature rotates inside the stator magnetic field, it also produces an emf Eb . 

The generated Emf Eb is directed opposite to the supplied voltage and is known as the Back Emf. The back emf is represented by,

Where, P = no of poles             

              φ = flux per pole                         

             Z = No. of conductors                         

             A = No. of parallel paths            

             N = speed of the DC Motor

When the application of load reduces the speed of the motor, Eb decreases. Thus the voltage difference between supply voltage and back emf increases that means E − Eb increases. 

Due to this increased voltage difference, the armature current will increase and therefore torque and hence speed increases. Thus a DC Motor is capable of maintaining the same speed under variable load.

Now, armature current Ia is represented by

Now at starting,speed ω = 0 so at starting Eb = 0.

Now since the armature winding electrical resistance Ra is small, this motor has a very high starting current in the absence of back Emf. As a result we need to use a starter for starting a DC Motor.
Now as the motor continues to rotate, the back emf starts being generated and gradually the current decreases as the motor picks up speed.