1. DC MOTORS SEE 3433 ELECTRICAL MACHINES

2. DC MOTOR - Shunt motors - Separately excited - Starter

3. DC MOTORS

4. + +- - DC motor DC MOTOR

5. + +- - DC motor Mechanical Load: fans, blowers, Compressors, +Va-+Va- +Vf-+Vf- TmTm T load Load torque opposing the motor torque DC MOTOR

6. + +- - DC motor +V-+V- +V-+V- F = mg TmTm T load

7. DC MOTOR Hoist

8. DC MOTOR - Some applications require the control the speed - Some applications require the control the torque - In order to control the torque or speed we need to know the T-  characteristics of the motor and the mechanical load Intersections between the two characteristics will determine the operating point

9. DC MOTOR Shunt motor V t = I a R a + E a I t = I a + I f E a = k  +Vt+Vt RaRa R cf R cw ItIt IfIf IaIa T e = k  I a  TeTe T load Mechanical load k  = V t - I a R a

10. Three possible methods of speed control: Field flux Armature voltage V t Armature resistance R a

11.  TeTe TLTL Vt ↓Vt ↓ Varying V t Requires variable DC supply

12.  TeTe Ra ↑Ra ↑ TLTL Varying R a Simple control Losses in external resistor

13.  TeTe TLTL Varying   ↓ ↓ Not possible for PM motor Maximum torque capability reduces

14. For wide range of speed control 0 to  base  armature voltage, above  base  field flux reduction Armature voltage control : retain maximum torque capability Field flux control (i.e. flux reduced) : reduce maximum torque capability TeTe  Maximum Torque capability Armature voltage control Field flux control  base Method of speed control in DC motor drives

15. TeTe  Maximum Torque capability  base

16. TeTe  Constant power Constant torque  base 0 to  base  armature voltage, above  base  field flux reduction P = E a I a,max = k  a  I a,max P max P max = E a I a,max = k  a  base I a,max    1/  P

17. 0 to  base  armature voltage, above  base  field flux reduction

18. +Vt+Vt RaRa R cf R cw ItIt IfIf IaIa BUT there are problems !

19. 0 to  base  armature voltage, above  base  field flux reduction +Vt+Vt RaRa R cf R cw ItIt IfIf IaIa Controlling V t will also affect I f Controlling I f via R cf caused losses  I 2 R

20. 0 to  base  armature voltage, above  base  field flux reduction Separately Excited DC motor DC supply for armature DC supply for field What if we have an AC supply ?

21. Separately Excited DC motor AC to DC converter AC to DC converter 3-phase AC source Armature voltage controlField voltage control + V dc - + V dc - 0 to  base  armature voltage, above  base  field flux reduction

22. Starter in DC Motor At stand-still,  = 0  E a = 0 eg, V t = 100, R a = 0.1  I a = 1000 A ! +Vt–+Vt–  I a RaRa

23. Starter in DC Motor We can limit I a at start-up by: 1)Controlling V t using variable supply – e.g. using power electronics converter As speed builds up (so too E a ), R st is gradually reduced +Vt–+Vt–  I a +Ea–+Ea– When Ea = 0 2)Adding external resistor  known as starter RaRa R st

24. Starter in DC Motor As speed builds up (so too E a ), R st is gradually reduced 1234 Starter circuit 1 2 3 4 IaIa t (s) speed I max I min

25. Starter in DC Motor Practical Starter circuit