AMITY SCHOOL OF ENGINEERING AND TECHNOLOGYELECTRICAL MACHINES

ASSIGNMENT -1

1. Draw a neat diagram of a DC machine and label the component parts. Also explain the function of each part.

2. Write short note on: Principle of operation of a DC machine.

3. Explain how commutator works in a DC machine?

4. Briefly describe the function of interploes in a DC machine.

5. Explain the various types of DC generator with their generated voltage equation in terms of load terminal voltage and drop in armature, brushes and various fields.

6. Derive the e.m.f.. equation of a DC generator.

7. Give the comparative summary of Lap and Wave winding and write the uses of these windings.

8. For a 4 poles DC armature with 28 slots and 8 coil sides per slot, find the winding pitches and the commutator pitch for a wave winding. What is the distance between brushes in terms of commutator segments?

9. A shunt generator delivers 450 A at 230 V and the resistance of the shunt field and armature are 50 Ω and 0.03 Ω respectively. Calculate the generated e.m.f.

10. A short shunt compound generator delivers a load current of 30 A at 220 V, and has armature, series field of shunt field resistances of 0.05Ω, 0.30Ω and 200Ω respectively. Calculate the induced e.m.f. and the armature current. Allow 1.0 V per brush for contact drop.

11. A four-pole generator, having wave-wound armature has 51 slots each slot containing 20 conductors. What will be the voltage generated in machine when driven at 1500 rpm assuming the flux per pole to be 7.0m Wb.

12. A separately excited generator, when running at 1000 rpm supplied 200 A at 125 V. what will be the load current when the speed drops to 800 rpm. If If is unchanged? Given that the armature resistance =0.04 Ω and brush drop =2V.

13. A 4 pole long-shunt lap wound generator supplies 25 KW at a terminal voltage of 500 V. the armature resistance is 0.03Ω, series field resistance is 0.04Ω and shunt

field resistance is 200Ω. The brush drop may be taken as 1.0V. Determine the e.m.f. generated. Calculated also the no. of conductor if the speed is 1200rpm and flux per pole is 0.02 Wb.

14. A 4 pole, lap-wound, DC shunt generator has a useful flux per pole of 0.07 Wb. The armature winding consists of 220 turns each of 0.004 Ω resistances. Calculate the terminal voltage when running at 900 rpm, if the armature current

is 50 A.

15. A 440V DC shunt motor has armature resistance of 0.8Ω and field resistance of 200Ω. Determine the back e.m.f. when giving an output of 7.46kW at 85% efficiency.

16. A 25KW, 250V, DC shunt generator has armature and field resistances of 0.06Ω and 100Ω respectively. Determine the total armature power developed when working (i) as a generator delivering 25 kW output and (ii) as a motor taking 25 kW input.

17. A 4-pole, 220 V shunt motor has 540 lap wound conductor. It takes 32 A from the supply mains and develops output power of 5.595kW. The field winding takes 1A, the armature resistance is 0.9Ω and the flux per pole is 30 mWb. Calculate (i) the speed and (ii) the torque developed in Newton meter.

18. A DC series motor takes 40 A at 220V and runs at 800rpm. If the armature and field resistances are 0.2Ω and 0.1Ω respectively and the iron and friction losses are 0.5 kW, find the torque developed in the armature. What will be the output of the motor?

19. A 500 V DC shunt motor when cold has a field resistance of 200Ω and an armature resistance of 0.15Ω. The motor runs at a no-load speed of 1000 rpm under cold conditions. After remaining continuously loaded at a load current of 70 A, field and armature temperatures rise to 40° C above the ambient temperature of 20°C. Calculate the motor speed under hot conditions. The temperature coefficient of copper =1/234.5 per C at 0° C. (Ans: 1068.6 rpm)

20. A 220V DC shunt motor has an armature resistance of 0.3Ω and a field resistance of 200Ω. The motor runs at 800 rpm with an armature current of 40 A. What resistance must be inserted in the field circuit to raise the motor speed to 1050 rpm, the load torque remaining constant? Assume linear magnetization characteristic.

(Ans : 68Ω)

21. 115 V DC shunt motor an armature current of 25 A when running at 1500 rpm at full- load torque. Motor armature circuit resistance is 0.3Ω. An additional resistance of 0.6 Ω is introduced in the armature circuit when the motor is operating at half full-load torque. Find the % change in the shunt field resistance which would cause the motor to have a speed to 1400 rpm.

(Ans: 3.5% reduction)

22. A 15 kW, 220 V, 1200 rpm, 4-pole, DC shunt motor has 620 conductors connected as wave winding yielding an armature circuit resistance of 0.18Ω. When delivering rated power at rated voltage and speed, it draws a line current of 79.8 A and a field current of 2.6 A. Calculate.

(a) Flux/pole, (Ans: 8.31 mWb) (b) Developed torque, (Ans: 126.6Nm)(c) Rotational losses (Ans: 903W)(d) Total losses and efficiency. (Ans: 2548W, 85.5%)

AMITY SCHOOL OF ENGINEERING AND TECHNOLOGYELECTRICAL MACHINES

ASSIGNMENT -2

1. Find the mechanical & electrical degrees between adjacent poles in a 6-pole electrical machine.

2. Find the mechanical & electrical degrees between adjacent slots in a 4-pole machine stator with 36 slots.

3. A 4-pole AC machine has a 3 – Ø winding wound in 36 slots with coil span of 1400 E. Compute (i) the pitch factor (ii) distribution factor (iii) winding factor.

4. Derive the e.m.f. equation of an alternator and define the factors kp,kd and kw.

5. Explain O.C. Test and S.C. Test on a Synchronous machine.

6. A 3- Ø, 6- pole AC machine with 72 slots. The coil span is 1 to 10 slots. Find (i) kp (ii) kd (iii) kw.

7. A 3- Ø winding for a 4 –pole machine was carried out in 60 slots. The coils are short pitched i.e. if one coil side lies in slot no.1, the other side of the same coil lies in slot no 13. Calculate the winding factor for (i) fundamental, (ii) third harmonic & (iii) fifth harmonic waveforms.

8. A 3- Ø, 16 pole star-connected Alternator has 240 stator slots with 8 conductors per slot and the conductor of each phase are connected in series. The coil span is

1440 E. Determine the phase and lines e.m.f.s if the machine speed is at 375 rpm and the flux per pole is 0.061 Wb.

9. Calculate the emf induced per phase on no load of a 10-pole, 3- Ø, 50 Hz Alternator with 3 slots/ pole/phase and 6 conductors/ slot placed in two layers. The coil span is 1400E. The flux per pole was a fundamental component of 0.06Wb, and a 20% third harmonic component.

10. A 3- Phase, star connected, 50kVA, 415V, 50Hz Alternator has effective resistance of 0.25ohm/phase. The synchronous reactance is 3.2 ohm/phase and leakage reactance is 0.8 ohm/phase. Determine, at rated load and UPF, (i) the internal emf Er (ii) the excitation emf Ef and (ii) the value of reactance which represents armature reaction. (455.44V,588.88V, Xa =2.4 ohm)

11.A 3- phase, star connected, 1.5 MVA, 3.3kV, 50Hz Alternator has effective resistance of 0.08 ohm/ phase. The Synchronous reactance is 1.5 ohm/ phase. Calculate the % voltage regulation for a load of 1.0 MW at rated voltage and at p.f of (i) 0.8 lagging and (ii) 0.8 leading. (11.85 %, -8.47%)

12.Find the Synchronous impedance and reactance of a 3-phase Alternator in which a given field current produces a current of 250 A on short circuits and a generated emf of 1500 V on open circuit. The effective winding resistance is 0.2 ohm/phase. Calculate the terminal (3.464 ohm, 3.458 ohm,7.654kV, 15.97%)

13.The OC and SC test data of a 3-phase, IMVA, 6.6kV, star connected Synchronous generator is given below:

If(A) 60 70 80 90 100 110 Voc(line) (V) 4693 5500 6160 6600 6967 7260 SC (A) 98

Find: (i) unsaturated Synchronous reactance, (ii) adjusted Synchronous reactance, (iii) excitation voltage needed to give rated voltage at full load, 0.8 pf lagging using (ii).(iv) also the voltage regulation for the load specified in part (iii). (27.7 ohm, 26.65 ohm, 9582V,45.2%)

14. Why a Synchronous motor called as a Synchronous condenser?

15. What are the applications of the Synchronous motors? Explain the dual purpose of a Synchronous motor.

16. Why synchronous motor is not self-starting?

17. A Synchronous motor absorbing 20kW is connected in parallel with a factory load of 20kW having a lagging a p.f. of 0.8. If the combined power in the supply lines has a p.f. of 0.92, what is the value of leading reactive power taken by the motor and at what p.f. is it working. (56.26 kVAR,0.335 leading)

AMITY SCHOOL OF ENGINEERING AND TECHNOLOGYELECTRICAL MACHINES

ASSIGNMENT -3

1. A 3- phase I/motor runs at almost 1000rpm at no load & 950 rpm at full load when supplied with power from a 50 HZ, 3-phase line. Calculate

(i) How many poles has the motor? (6)(ii) Slip at full load ? (5%)(iii) Frequency of rotor voltage? (2.5 HZ)(iv) Speed of rotor field w.r.t motor? (50rpm)(v) Speed of rotor w.r.t stator? (950 rpm)(vi) Speed of rotor field w.r.t stator field? (zero)(vii) Rotor frequency at the slip of 10% ? (5 Hz)

2. A 4 – pole, 50 Hz, 3-I / motor when running on full load develops a useful torque of 100 N-m while the motor emf is observed to make 120 cycles/min. It

is known that the torque lost on account of friction & core loss is 7 N-m. Calculate (if the total core loss is given to be 700W)

(i) Shaft power output,(ii) Mechanical power developed,(iii)Rotor Cu loss,(iv)Motor input, & (v) Motor efficiency.

(Ans: 15.07 kW 16.12kW 0.67kW, 17.49 kW, 86.16%)

3. A 6.6 kV, 20 pole, 50 HZ, 3-phase star –connected I/ motor has a rotor resistance of 0.12 Ω. The motor has a speed od 292.5 rpm at full load. Calculate:

(a) Slip at maximum torque, and (b) The ratio of maximum to full load torque. Neglect stator impedance.

(Ans: 0.17,2.26)

4. A3- phase, 40 V,25kW,50 Hz, 8 pole I/ motor yielded the following data on testing:

No load : 400V,8.7A,1210WBlocked motor: 200V,47.8A, 6050W

The stator phase resistance is 0.42Ω (star basis)Estimate the parameters of the circuit model ?

(Ans: r1=132.2Ω, Xm=27Ω, r1+r2 = 0.833Ω, X1+X2=2.25Ω)

5. The efficiency of a 400 V,3-phase, 6- pole I/motor drawing a line current of 80 A at 0.75 p.f at 4% slip is 85%. Calculate the shaft output & shaft torque.

(Ans: 35.33kW, 351.5 N-m)

6. A 415V, 4- pole, 3- phase I/motor develops 15.25 kW inclusive of mechanical losses when running at 1462 rpm. The supply frequency is 500.5 Hz and the power factor equals 0.89. Calculate:

(i) Slip,(ii) Rotor Cu loss,(iii) Total input if the stator losses are 1.4 kW, and(iv) Line current. (Ans: 0.035, 0.533kW, 17.2k W, 26.89A)

7. A22 kW, 3-phase I/motor operates with a slip of 3.5 % on full load. The stator losses are 1200 W and the mechanical losses are 500 W. If the current per phase in the rotor winding is 28A. Find:

(i) Resistance per phase of the rotor (ii) Efficiency of the motor? (Ans: 0.347Ω,89.74%)

Write the short notes on:

8. Explain why an Induction machine is called a generalized transformer?

9. In a poly-phase Induction motor, show that the rotor must rotate in the direction of rotating magnetic field.

10. Explain why the rotor of poly phase Induction motor can never attain synchronous speed?

11. Explain the similarities and dissimilarities b/w the slip ring (wound rotor) & squirrel cage poly-phase Induction motor.

12. What are the advantages of inserting external resistance in the rotor circuit of a wound rotor Induction motor at the time of starting?

13. What are the types of starting of a poly-phase Induction motor?