Speed control of dc motor

Speed control of dc motor

1. Speed control of DC motor Presented by Mrs. M.P.Sasirekha Unit 3

2. DC Motor Speed Control Theory To derive the speed of a DC motor, we start with the equation for the DC motor’s EMF (Electromagnetic Force). We know that the EMF equation of DC motor is equal to: Hence rearranging the equation: N = 60A E / PZØ

3. DC Motor Speed Control Theory With k = PZ/60A, then: N = E / kØ Hence with E = V – I a R a , we derive the speed of the DC motor (N):

4. Speed of a DC motor Back emf E b of a DC motor is nothing but the induced emf in armature conductors due to rotation of the armature in magnetic field. Thus, the magnitude of E b can be given by EMF equation of a DC generator. E b = PØNZ / 60A (where, P = no. of poles, Ø = flux/pole, N = speed in rpm, Z = no. of armature conductors, A = parallel paths) E b can also be given as, E b = V- I a R a

5. Speed of a DC motor thus, from the above equations N = E b 60A / PØZ but, for a DC motor A, P and Z are constants Therefore, N ∝ K E b / Ø           (where, K=constant) This shows the speed of a dc motor is directly proportional to the back emf and inversely proportional to the flux per pole.

6. Speed control methods of DC motor Speed control of Shunt motor Flux control method Armature control method Voltage Control Method

7. Flux control method it is already explained above that the speed of a dc motor is inversely proportional to the flux per pole. Thus by decreasing the flux, speed can be increased and vice versa. To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram.

8. Flux control method Adding more resistance in series with the field winding will increase the speed as it decreases the flux. In shunt motors, as field current is relatively very small, I sh 2 R loss is small.

9. Flux control method Therefore, this method is quite efficient. Though speed can be increased above the rated value by reducing flux with this method, it puts a limit to maximum speed as weakening of field flux beyond a limit will adversely affect the commutation.

10. Armature control method Speed of a dc motor is directly proportional to the back emf E b and E b = V - I a R a . That means, when supply voltage V and the armature resistance R a are kept constant, then the speed is directly proportional to armature current I a .

11. Armature control method Thus, if we add resistance in series with the armature, I a decreases and, hence, the speed also decreases. Greater the resistance in series with the armature, greater the decrease in speed.

12. Voltage Control Method a) Multiple voltage control : In this method, the shunt field is connected to a fixed exciting voltage and armature is supplied with different voltages. Voltage across armature is changed with the help of suitable switchgear. The speed is approximately proportional to the voltage across the armature.

13. Ward-Leonard System : This system is used where very sensitive speed control of motor is required ( e.g electric excavators, elevators etc.). The arrangement of this system is as shown in the figure at right. M 2 is the motor to which speed control is required.

14. Advantages of Armature Controlled DC Shunt Motor Very fine speed control over whole range in both directions Uniform acceleration is obtained Good speed regulation It has regenerative braking capacity

15. Disadvantages of Armature Controlled DC Shunt Motor Costly arrangement is needed, floor space required is more Low efficiency at light loads Drive produced more noise.

16. Field Rheostat Controlled DC Shunt Motor In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field. An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed. This method of speed control is independent of load on the motor.

17. Field Rheostat Controlled DC Shunt Motor Power wasted in controlling resistance is very less as field current is a small value. This method of speed control is also used in DC compound motor.

18. Disadvantages of Field Rheostat Controlled DC Shunt Motor Creeping speeds cannot be obtained. Top speeds only obtained at reduced torque. The speed is maximum at minimum value of flux, which is governed by the demagnetizing effect of armature reaction on the field.

19. Speed Control Of DC Series Motors The speed control of d.c . series motors can be obtained by two methods Flux control method Armature-Resistance control method .

20. Speed Control Of DC Series Motors

21. Flux control method In this method, the flux produced by the series dc motor is varied. The variation of flux can be achieved in the following ways: Field Diverters Armature Diverter Tapped Field Control

22. Field diverters In this method, a variable resistance (called field diverter) is connected in parallel with series field winding. A part of the line current passes through this diverter and thus weakens the field. Since  N ∝ 1/ϕ  , speed also varies with field flux.

23. Field diverters The lowest speed obtained by this method is the normal speed of motor when the current through diverter is zero, ie , diverter open circuited.

24. Armature diverter In order to obtain speeds below the normal speed, a variable resistance (called armature diverter) is connected in parallel with the armature of dc series motor. The diverter reduces the armature current. As a result flux get increased. So the speed decreases since  N ∝ 1/ϕ .

25. Tapped field control

26. Tapped field control In this method, the flux is reduced (and hence speed is increased) by decreasing the number of turns of the series field winding. The switch S can short circuit any part of the field winding, thus decreasing the flux and raising the speed. With full turns of the field winding, the motor runs at normal speed and as the field turns are   cut out, speeds higher than normal speed are achieved.

27. Armature-resistance Control

28. Armature-resistance Control In this method, a variable resistance is directly connected in series with the supply. This reduces the voltage available across the armature and hence the speed falls. By changing the value of variable resistance, any speed below the normal speed can be obtained. This is the most common method employed to control the speed of d.c . series motors.

29. Thank you