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DEPARTEMENT : ELECTRICAL

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Presentation on theme: "DEPARTEMENT : ELECTRICAL"— Presentation transcript:

1 DEPARTEMENT : ELECTRICAL
Dr. S.&S.S. GHANDHY GOVERNMENT ENGINEERING COLLEGE,SURAT. TOPIC NAME : DC MOTOR. DEPARTEMENT : ELECTRICAL SEM : 3

2 GROUP MEMBER : 140230109031 : PATEL ANKIT 140230109032 : PATEL HARSHAD
: PATEL JAY : PATEL KARTIK : PATEL KRISHNA FACULTY : PROF. M R VASAVADA

3 Working principal of DC motor
An electrical motor is a machine which converts electrical energy into mechanical energy . It action is based on current carrying conductor is placed in a magnetic field , it experience a mechanical force whose direction is given by Fleming's right hand rule and whose magnitude is given by F=BIL Newton .

4 SIGNIFICANCE OF BACK E.M.F.
The motor armature rotates , the conductor also rotates and hence cut the flux . In accordance with the law of electromagnetic induction , e.m.f. is induced in them whose direction ,as found by Fleming's right hand rule , is in applied voltage because of opposing direction , it is referred to as counter e.m.f. or back e.m.f. Eb . The equivalent circuit of a motor is as shown in fig.

5 The rotating armature generating the back e. m. f
The rotating armature generating the back e.m.f. Eb is like a battery of e.m.f. Eb put across a supply mains of V volts . Obviously , V has to drive Ia against the opposition of Eb . The power required to overcome this opposition is EbIa . In the case of cell , this power over an interval of time is converted into chemical energy , but in the present case , it is convert into mechanical energy ‘ It will be seen that Ia= Where , Ra is the resistance of armature circuit

6 As pointed out above , Eb = ФZN*(P/A) Volt is in r.p.s. Back e.m.f. depends , among other factor , upon the armature speed . If speed is high , Eb is large , seen armature current Ia , seen from the above equation , is small . If the speed is less , then Eb is less hence more current flows which develops motor torque .so, we fine Eb acts like a governor i.e., it makes a motor self regulating so that it draws as much current as is just necessary .

7 VOLTAGE EQUATION OF A MOTOR
The voltage V applied across the motor armature has to (1) Overcome the back e.m.f. Eb and (2) Supply the armature ohmic drop IaRa V=Eb+IaRa This is known as a voltage equation of motor . Now , multiplying both the side by Ia ,we get VIa =EbIa +I2aRa As shown in fig.

8 VIa = Electrical input to the armature
EbIa = Electrical equivalent of mechanical power developed in the armature I2aRa = Cu loss in armature Hence , out of armature input ,some is wasted in I2R loss and rest is converted into mechanical power within the armature . It may also be noted that motor efficiency is given by the ratio of power developed by armature to its input i.e., EbIa/VIa=Eb/V . Obviously ,higher the value of Eb as compared to V , higher the motor efficiency .

9 STARTER Starter is external device which is required to be connected along with a dc motor , in order to start the motor in a safe manner . That means starting a dc motor without a starter is not safe for the motor .so we should understand the need of starter

10 NEED OF STARTER : We know that , V=Eb+IaRa…for a dc shunt motor
and V=Eb+Ia(Ra+Rs) …for a dc series motor Hence the expression for Ia as follow : …dc shunt motor and …dc series motor

11 At the time of starting the motor ,speed N=0 and hence the back emf Eb=0 .hence the armature current at time of starting is given by , …dc shunt motor …dc series motor

12 METHOD OF SPEED CONTROL
Speed control method Flux control Rheostatic control Voltage control Change the flux by changing the filed current Change the armature path resistance which in turn changes the armature voltage Change the supply voltage V

13 SOLID STATE SPEED CONTROL
Solid state armature voltage control The block diagram of closed loop armature voltage control method for speed control of dc shunt motor is as shown in fig. I this method the filed winding is exited by a constant dc voltage and the armature voltage is varied in order to vary the motor speed . This method is applicable only for speed variation below the rated speed of motor .

14 The reference voltage is passed through a unit called “ramp” unit to provide the “soft start” i.e. gradual acceleration of the motor . The output of ramp unit now acts as speed reference voltage is compared with the speed feedback signal obtained from the dc tachogenerator . The difference between reference speed and feedback speed is known as the “speed error” . The speed controller will act on this speed error to produce a control voltage Vc which changes the firing angle α in a such way that the armature voltage is automatically increased or decreased . This will automatically reduce the speed error .

15 Solid state field current control :
This method is used for varying the speed of dc shunt motors above the rated speed the armature voltage is kept constant and field current is varied . The field current control scheme is basically same as the armature control scheme except for one change . The output thyristorized controlled rectifier drive the field winding of the motor

16 Thank you


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