Electrical machine 1 IOE ALL Subject Notes

 

Electric Machine I (ii/ii) IOE ALL SUBJECT NOTE  notes

Working Principal of D.C. Motor

ð  When a current carrying conductor is placed in a magnetic field then it experiences a mechanical force.

ð  Flux is created by a field winding.

ð  Armature winding plays a role of current carrying conductor.

Every conductor of armature experience a mechanical force that will produce a torque hence armature start to rotate.

So this is simple principle of working of dc motor.

Direction of rotation of motor

Magnitude of the force experienced by armature conductor is: F=BLI (N)

Where,    B=Magnetic flux density produced by field winding.

                  L=Active length of the conductor

                  I= Magnitude of the current carrying by the conductor.

Direction of rotation is determined by Fleming's Left Hand Rule: The rule states that :- "out stretch the three figures of the left hand namely the first finger, middle figure and thumb such that they are mutually perpendicular to each other . Now point the first finger in the direction of magnetic field and the middle figure in the direction of current then the thumb gives the direction of force experienced by the armature conductor."

To reverse the direction of rotation either direction of main field produced by the field winding is reversed (case 1 and 3). Or the direction of current passing throw the armature is reversed (case 2 and 4).

Back E.M.F (E_b): once motor start rotating its conductor will cut the magnetic flux produce by field winding so by faraday law of electromagnetic induction there will be emf induced just like emf induced in case of DC generator so although it is motor but when conductor start rotating because of motoring action it will cut the magnetic flux and produce an emf which is called as back emf.

ð  Back E.M.F. is generated by the generating action (moving conductors  cutting the magnetic flux)

E_b=fPNZ/60A --------- EMF equation of motor

ð  EMF is produced because of Faradays law of electromagnetic induction.  

It direction is given by lenz's law: - Direction will be opposite of supply voltage. 

Voltage Equation (V_s) = I_aR_a+E_b+V_brush

 Role of Back EMF(Eb):

Back  emf(Eb)  plays  very  important  role  in  DC  motor.  Followings  are  the  various roles of  back  emf: 

1. Back emf protect armature from short circuit.
2. if there is no back emf then armature current (Ia) will be very high. it is just like short circuiting applied voltage(V) with smaller value of armature resistance (Ra). as armature conductor are not designed for caring such a high current hence armature winding will be damaged.    
3. Back emf produces required amount of torque according to increase or decrease external load torque .
4. if external load torque is increased then the speed of motor is decreased and therefore back emf will also decreased(as back emf (Eb) is directly proportional to N ). Hence armature current will be increased so motor will be able to  produce required amount of torque. 
5. similarly if external load torque is decreased then the speed of motor will be increased therefor back emf will also increase so armature current will be decreased hence torque produced by motor will also decreased.
6. in any energy converting system opposing agent is necessary in the absence of this opposing agent the system will not work
7. in dc motor back emf (Eb) plays the role of opposing agent.
8. back emf opposes the applied voltage(V) so that applied voltage has to force current through armature to  against back emf
9. the electrical work done in opposition is converted into mechanical work in the form of mechanical rotation of arnature.