1. Chapter 5: Speed-Torque Characteristics of Electric Motors

2. Direct Current Machines

3. S N F I V
Brush F
Armature
windings
Rotor w

8. I V

9. Separately Excited Motors

11. DC Machine

14. Speed-Torque CC of DC Motor
Load w o D w } w
Operating
Point
Motor

15. Starting of DC motor
At Starting,  = 0, then Ea = 0

16. Example A dc separately excited motor has the following data:
K  = Vs (Volt second)
Vt = V
Ra = W
Ia = A (armature current at full load)
Calculate the rated torque, starting torque and starting current at full voltage.

17. Solution

18. Starting of DC motor (voltage reduction)

19. Starting of DC motor (starting resistance)

20. Example
Limit the starting current to 6 times the rated current

21. Solution Method 1: Reduce the starting voltage
Method 2: Increase the starting resistance

22. Shunt Motor

23. Series Motors

24. Operating Series Motors in ac Circuits
Assumption:
small inductance
When Vt is reversed, Ia is reversed
When Ia is reversed,  is reversed
 is maintained in the same direction

25. Induction Motor

26. El-Sharkawi@University of Washington
Stator
Rotor ring
Rotor
Stator Windings
Rotor Windings
of Washington

27. Rotation of Induction Motor (IM)
stator ns n
rotor
of Washington

28. Rotating Field

29. i b f c i a f b  c f a  a  b i c

31. Rotating Field
The speed of the airgap flux is one revolution per one ac cycle.
The time of one ac cycle
The speed of the airgap flux

32. Number of Poles

33. Number of Poles

34. What is the Speed of the motor?
The speed of the motor depends on the magnitude of the load torque

35. Slip

36. Equivalent Circuit

38. Equivalent Circuit

39. Torque Characteristics

40. s 1
Torque n s
max n s T st
max

41. T
Torque s 1 st
Small Slip
Maximum Torque s
max T
max
Large Slip

42. Large slip approximation (during starting)

43. Small slip approximation (For steady state, full load)

44. Maximum Torque
Set

45. Summary Maximum Torque region Large slip: Starting region
Small slip: Steady state region
Maximum Torque region

46. Starting of Induction Motor

47. Starting by Reducing Voltage

48. Starting by Adding Rotor Resistance

49. Example
An induction motor has a stator resistance of 3  and the rotor resistance referred to the stator is 2 . The equivalent inductive reactance is 10 . Calculate the change in the starting torque if the voltage is reduced by 10%. Also, compute the resistance that should be added to the rotor circuit to achieve the maximum torque at starting.

50. Solution

52. Synchronous Motor

53. Starting of Synchronous Motor