1. EXPERIMENT NO.: 1. To Study the Variation of Speed and Load Test on Schrage Motor

2. Theory induction motor – Cheap, Rugged, Less Maintenance But low starting Torque In SRIM, T est can be increased by additional r 2 Or AC commutator motors are the other option. AC commutator motors operate near to unity pf. with wide range of speed control

3. ARM Commutator frequency Converter Consider DC Generator operation NS A AA emf E is induced across brushes As armature rotates, Φ + - For same polarity of E Rotate the poles, Anticlockwise Dir n E ● Cond r moves clockwise Dir n

4. Commutator frequency Converter Consider DC Generator operation emf E is induced across brushes As armature rotates, Φ A AA + - S N NS For same polarity of E Rotate the poles, Anticlockwise Dir n E ● Cond r moves clockwise Dir n

5. Commutator frequency Converter Consider DC Generator operation emf E is induced across brushes As armature rotates, Φ + - A AA How much is E ? Zero Again rotate poles S N E ● For same polarity of E Rotate the poles, Anticlockwise Dir n Cond r moves clockwise Dir n ●

6. Commutator frequency Converter Consider DC Generator operation How much is E ? A AA Zero Again rotate poles N S N S E ● ●

7. A AA Commutator frequency Converter Consider DC Generator operation How much is E ? Zero Again rotate poles N S E ● _ + Φ ● ●

8. Commutator frequency Converter Consider DC Generator operation How much is E ? A AA Zero Again rotate poles N S E ● Φ N S _ + ● ●

9. Commutator frequency Converter Consider DC Generator operation How much is E ? A AA Zero Again rotate poles E ● Φ N S _ + ● ● ●

10. ARM Commutator frequency Converter Consider DC Generator operation How much is E ? A AA Zero Again rotate poles E ● Φ N S S N ● ● ●

11. Φ Commutator frequency Converter Consider DC Generator operation How much is E ? A AA Zero Again rotate poles E ● Φ S N + - For N rpm speed of poles Freq of E is ● ● ● ●

12. Commutator frequency Converter Consider DC Generator operation Rotate poles and brushes At same speed and same direction E ● A AA S N + - Relative speed is zero Freq of E will be zero

13. Commutator frequency Converter Consider DC Generator operation Rotate poles and brushes A AA at same speed and same direction S N + - Relative speed is zero Freq of E will be zero Now brush stationary and rotate armature with speed N r rpm and pole speed is N in opposite direction Relative speed betw n air gap field and brushes = N+N r Freq of brush emf E will be E ●

14. Commutator frequency Converter Consider DC Generator operation A AA S N + - Now armature speed is N r rpm and pole speed is N in same direction Relative speed betw n air gap field and brushes = N - N r Freq of brush emf E will be In general emf E is induced across brushes Φ In terms of angle Thus E can be increased by increasing θ and axis of E can be changed by shifting two brushes simultaneously. Φ

15. ERER EYEY EBEB Pole Axis of phase R

16. Consider inverted induction motor 3-phase supply to rotor winding Rotor field rotates at N s wrt rotor As per induction motor action, rotor rotates in opposite direction at N r wrt stator. Therefore, air gap speed is N s - N r wrt stator or wrt stationary brushes So brush emf frequency is Thus whatever may be the speed of rotor, brush emf freq is sf It is suitable to inject into secondary wdg which is having slip freq emf Power related to sf is SLIP POWER.

17. If brush emf is opposite to sE 2, then resultant voltage decreases, Current decreases, torque decrease, Speed decreases Motor runs at sub-synchronous speed If brush emf is added to sE 2, then resultant voltage increases, Current increases, torque increase, Speed increases Motor runs at super-synchronous speed By shifting the axis of brush voltage, power factor can be changed

18. The speed and power factor of slip ring induction motor can be controlled by injecting slip frequency voltage in the rotor circuit. In 1911, K. H. Schrage of Sweden combined elegantly a SRIM (WRIM) and a frequency converter into a single unit. This machine is known as Schrage Motor. Schrage motor is an AC commutator Machine

19. Schrage motor is basically an Schrage motorInduction motor Primary winding on StatorPrimary winding on Rotor Secondary winding on Rotor Secondary winding on Stator If supply is given to stator, rotor rotates in the same direction of rotating magnetic field If supply is given to rotor, rotor has to rotates in the opposite direction of rotating magnetic field, so that rotating magnetic field in the air gap becomes N s -N r = slip speed. inverted polyphase induction motor.

20. Schrage motorInduction motor Ns Nr Ns Nr If supply is given to stator,If supply is given to rotor, If rotor rotates in the SAME direction of rotating magnetic field, then rotating magnetic field in the air gap becomes N s +N r = Addition of speed.

21. Tertiary or adjusting winding, which is housed in the same rotor slots of the primary Schrage motorInduction motor The tertiary winding is connected to the commutator On commutator there are three sets of movable brush pairs, Brush pairs collect the required emf for injection into the secondary circuit for speed and pf control.

22. Schrage motor Slip Rings Supply

23. Schrage motor Slip Rings Supply Primary Winding Tertiary Winding

24. Schrage motor Slip Rings Supply Primary Winding Tertiary Winding Commutator

25. Slip Rings Supply Primary Winding Tertiary Winding Commutator a b Secondary Winding c de f

26. Slip Rings Supply Primary Winding Tertiary Winding Commutator a b Secondary Winding c de f 50Hz sf

27. Schrage motor Arrangement of Three Windings is slots 50Hz sf The primary and tertiary windings, beings in the same slots, are mutually coupled. Therefore, the emfs induced in the tertiary winding are by transformer action and are always at line frequency f, at all the rotor speed.

28. The secondary phase winding on stator are not connected to each other but are connected to brushes a, b, c, d, e, f. Alternate brushes a, c, e, 120 electrical degrees apart, are mounted on one brush rocker and brushes b, d, f on the second brush rocker. The angle between brushes ab, cd, and ef can be controlled by means of rack and pinion arrangement and one hand wheel provided outside the motor frame. Operation

29. At standstill, due three phase supply Rotor rotates at speed N r, air gap field speed ( N s – N r ) i.e. slip speed. slip frequency emfs E 2 in the stator winding. slip frequency emf E j is induced across brushes to primary. field rotates at synchronous speed N s with respect to primary and secondary conductors. opposite direction since primary winding is on the rotor.

30. Equal slip frequency voltages, At the time of start, the brush pairs are shorted, The shorted secondary winding (brush angle zero) is also condition for starting the Schrage motor. a b injection into the secondary winding i.e. stator winding is possible

31. a b E2E2 Φ at N s wrt rotor N r < N s Φ at (N s - N r ) wrt stator E j =0 a θ b sE 2

32. a b Φ at N s wrt rotor N r < N s Φ at (N s - N r ) wrt stator E j =0 a b EjEj Φ at (N s - N r ) wrt stator sE 2 E 2 and E j are in opposite direction E 2 - E j, resultant voltage decreases. speed decreases. (sub-synchronous speed). Slip is POSITIVE. N r < N s sE 2 θ E2E2 E2E2

33. b a Φ at N s wrt rotor N r > N s Φ at (N r - N s ) wrt stator a b EjEj Φ at (N s - N r ) wrt stator sE 2 E 2 and E j are in opposite direction E 2 - E j, resultant voltage decreases. speed decreases. (sub-synchronous speed). Slip is POSITIVE. N r < N s sE 2 (-s) θ EjEj Change the position of a & b. Axes of voltages are coincidence But E 2 and E j are in same direction. E 2 + E j, resultant voltage increases. speed increases. (super-synchronous speed), Slip is NEGATIVE. E2E2 E2E2

34. Speed θ θ=180 0 θ=0θ=-180 0

35. a b EjEj Φ at (N s - N r ) wrt stator sE 2 N r < N s θ E2E2 Now move a, b brushs bodily towards left Φ at N s wrt rotor There is angle ρ between axes of voltages. E2E2 sE 2 EjEj I2z2I2z2 ρ I2I2 I1I1 Φ Power factor angle Thus power factor is controlled. ρ a b

36. Circuit Diagram Experimental set-up for studying the Variation of Speed and Load Test on Schrage Motor A V ML C V R Y B L1 N L3 L2 10A 300V A AA G 1000Ω 2A F FF LOADLOAD A 10A + - V 300V + - Schrage MotorDC Generator

37. Apparatus Required 1. Schrage Machine: Rating:………………… 2. DC Shunt Generator: Rating…………………… 3. AC Ammeter: one, 0-10 amps 4. AC Voltmeter: one, 0-300 volts. 5. Wattmeter: one, 300V, 10A 6. DC Ammeter: one, 0-10 amps. 7. DC Voltmeter: one, 0-300 volts. 8. Rheostat: one, 1000 ohms, 2 amps. 9. Tachometer or speedometer: one 10. Rheostatic Load: 7.5kW

38. Procedure Speed Test: 1.Note down ratings of the machines and make the connection as shown in Fig. 2. Put the DC generator field rheostat at maximum resistance point. 3. Keep the brush angle pointer at zero (condition for starting the Schrage motor). 4. Switch on AC supply to Schrage motor by I. L. T. P. switch. Press green button of ILTP to start motor. 6. Now increase brush angle from zero to 360 0 gradually at regular intervals and note down the speed for each interval. 5. Note down speed for zero brush angle.

39. Procedure Load Test: 7. Run the Schrage motor at rated speed of DC shunt generator. 8. Excite the DC shunt generator by decreasing the field rheostat resistance and build up to its rated voltage. Maintain this voltage CONSTANT through out the experiment. 9. Increase the load gradually and note down the speed and meters readings for each load.

40. Observations and Calculations Speed Test:

41. Observations and Calculations Load Test: CONSTANT

42. Results and Conclusions Speed Test: Speed θ θ=360 0 θ=0 0 1. The speed of motor increases as the brush angle is increased. Speed is directly proportional to brush angle.

43. Load Test: Speed Motor Output 0 Power factor N Pf Im η Im Efficiency Results and Conclusions 2. The speed of motor decreases as the load on motor is increased.

44. Load Test: Speed Motor Output 0 Power factor N Pf Im η Im Efficiency Results and Conclusions 3. The power factor of motor increases or improves as the load on motor is increased.

45. Load Test: Speed Motor Output 0 Power factor N Pf Im η Im Efficiency Results and Conclusions 4. The current of motor increases as the load on motor is increased.

46. Load Test: Speed Motor Output 0 Power factor N Pf Im η Im Efficiency Results and Conclusions 5. The efficiency is zero at no load. It increases as the load increases and is maximum when variable losses are equal to constant losses.

47. Load Test: Speed Motor Output 0 Power factor N Pf Im η Im Efficiency Results and Conclusions 6. The efficiency at rated output is less than maximum value and the rated operating point is after maximum efficiency point.

48. Write down all the precautions which are listed in Laboratory and attach after Index page