EXPERIMENT NO 1 To Study the Variation of

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

Theory induction motor – Cheap, Rugged, Less Maintenance But low starting Torque In SRIM, Test 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

Commutator frequency Converter Consider DC Generator operation A N + ARM AA - S E● As armature rotates, emf E is induced For same polarity of E across brushes Rotate the poles, Φ Anticlockwise Dirn Condr moves clockwise Dirn

Commutator frequency Converter Consider DC Generator operation S A + N S AA - E● N As armature rotates, emf E is induced For same polarity of E across brushes Rotate the poles, Φ Anticlockwise Dirn Condr moves clockwise Dirn

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

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

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

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

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

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

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

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

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

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

ER Axis of phase R Pole EB EY

Consider inverted induction motor 3 -phase supply to rotor winding Rotor field rotates at Ns wrt rotor As per induction motor action, rotor rotates in opposite direction at Nr wrt stator. Therefore, air gap speed is Ns - Nr 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.

If brush emf is opposite to s. E 2, then resultant voltage decreases, Current decreases, torque decrease, Speed decreases Motor runs at sub-synchronous speed If brush emf is added to s. E 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

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

Schrage motor is basically an inverted polyphase induction motor. Induction motor Primary winding on Stator Secondary winding on Rotor If supply is given to stator, rotor rotates in the same direction of rotating magnetic field Schrage motor Primary winding on Rotor Secondary winding on Stator 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 Ns-Nr= slip speed.

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

Induction motor Schrage motor Tertiary or adjusting winding, which is housed in the same rotor slots of the primary 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.

Schrage motor Supply Slip Rings

Schrage motor Supply Tertiary Winding Primary Winding Slip Rings

Schrage motor Supply Tertiary Winding Primary Winding Slip Rings Commutator

Secondary Winding a b Supply Tertiary Winding Primary Winding c f e d Slip Rings Commutator

Secondary Winding sf a b Supply Tertiary 50 Hz Winding Primary Winding 50 Hz c f e d Slip Rings Commutator

Schrage motor Arrangement of Three Windings is slots sf 50 Hz 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.

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

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

Equal slip frequency voltages, injection into the secondary winding i. e. stator winding is possible 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

E 2 s. E 2 b a Ej=0 Φ at Ns wrt rotor Nr < Ns a Φ at (Ns - Nr) wrt stator θ b

E 2 s. E 2 b a Ej=0 Φ at Ns wrt rotor Nr < Ns a s. E 2 θ Ej Φ at (Ns - Nr) wrt stator Nr < Ns b Φ at (Ns - Nr) wrt stator E 2 and Ej are in opposite direction E 2 - Ej, resultant voltage decreases. speed decreases. (sub-synchronous speed). Slip is POSITIVE.

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

Speed θ=-1800 θ=1800 θ

E 2 a aθ Ej s. E 2 Now move a, b brushs bodily towards left bb There is angle ρ between axes of voltages. Φ at Ns wrt rotor Nr < Ns I 1 Φ at (Ns - Nr) wrt stator Φ Power factor angle ρ ρ Ej s. E 2 Thus power factor is controlled. E 2 I 2 z 2

Circuit Diagram L 1 M C 10 A L V + A- A V 300 V N 10 A R A F G L 3 L 2 B Y AA FF + V - 300 V 1000Ω 2 A Schrage Motor DC Generator Experimental set-up for studying the Variation of Speed and Load Test on Schrage Motor L O A D

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, 300 V, 10 A 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. 5 k. W

Speed Test: Procedure 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. 5. Note down speed for zero brush angle. 6. Now increase brush angle from zero to 3600 gradually at regular intervals and note down the speed for each interval.

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.

Observations and Calculations Speed Test:

CONSTANT Load Test: Observations and Calculations

Results and Conclusions Speed Test: Speed θ=00 θ=3600 θ 1. The speed of motor increases as the brush angle is increased. Speed is directly proportional to brush angle.

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

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

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

Results and Conclusions Power factor Load Test: N Pf Im η Speed Im Efficiency 0 Motor Output 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.

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

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