Electrical Machines by S. K. SAHDEV | Holooly

Electrical Machines

Electrical Machines [EXP-707]

397 SOLVED PROBLEMS

Question: 6.26

The following test results were obtained on a 345 kVA , three-phase, 6600 volt, star connected non salient pole type alternator. Open circuit characteristics. Short circuit characteristics : Stator current 35 A with an exciting current of 50 A . Leakage reactance on full load is 8% Neglect armature resistance. Calculate the exciting current (for f u l load ) at p.f. 0-8 lagging and at unity.

Verified Answer:

Reducing all line voltages to phase voltage, for a...

Question: 9.28

400 V, 3-phase, 50 Hz, 6 pole induction motor is supplying a load of 20 kW, when the frequency of rotor currents is 2Hz. Estimate (i) Slip and speed at this load (ii) Rotor copper loss (iii) Speed of motor when supplying 30 kW load assuming torque-slip curve to be a straight line.

Verified Answer:

Here,

f = 50 Hz; P = 6; f_{r} = 2Hz; Outpu...

A 1 kVA, 220/110 V, 400 Hz transformer is desired to be used at a frequency of 60 Hz. What will be the kVA rating of the transformer at reduced frequency?

Verified Answer:

We know that

E_{1}=V_{1}=4.44\: \phi _{m}\:...

Question: 6.16

A 1-phase 60 kVA , 220 V , 50 Hz, alternator has an effective armature leakage reactance of 0.07 ohm and negligible armature resistance. Calculate the voltage induced in the armature when the alternator is delivering rated current at a load power factor of 0.7 lagging.

Verified Answer:

Here, Rated power

=60 kVA =60 \times 10^{3...

Question: 5.12

A 10 b.h.p. 7.46 kW, 200- V shunt motor has full-load efficiency of 85 % . The armature has a resistance of 0.238 Ω . Calculate the value of the starting resistance necessary to limit the starting current to 1.5 times the full-load current at the moment of first switching on. The shunt current

Verified Answer:

Full load motor current,

I_{f l}=\frac{7.4...

Question: 10.3

A 10 H.P. 3-phase induction motor with full load efficiency and p.f. of 0.83 and 0.8 respectively has a short circuit current of 3.5 times full load current. Estimate the line current at the instant of starting the motor from a 500 V supply by means of star delta starter.

Verified Answer:

Output of the motor = 10 H.P. = 7355 W...

Question: 9.12

A 10 H.P., 4 pole, 25 Hz, 3-phase, wound rotor induction motor is taking 9100 watt from the line. Core loss is 290 watt, stator copper loss is 568 watt, rotor copper loss in 445 watt, friction and windage losses are 100 watt. Determine; (a) power transferred across air gap; (b) mechanical power

Verified Answer:

Power input to motor or stator = 9100...

Question: 5.42

A 10 k W, 250 V D C self excited shunt generator driven at 1000 rpmhas armature resistance 0.15 ohm and field current 1.64 A. Find rated load, armature induced emf, torque developed and efficiency when the rotational losses are 540 watt. Assume constant speed operation.

Verified Answer:

\text { Here, } \quad \text { output power ...

A 10 kVA, 2500/250 V, single phase transformer gave the following results: Open circuit Test: 250 V, 0.8 A, 50 W Short circuit Test: 60 V, 3A, 45 W Determine (a) Equivalent circuit parameters referred to LV side (b) The efficiency at full load and 0.8 power factor lagging (c) Load (kVA) at which the

Verified Answer:

Transformer rating

=10\: kVA;\: E_{1}=2500\...

A 10 kVA, 500/100 V transformer has the following circuit parameters referred to primary: Equivalent resistance, R_{eq}=0.3\: ohm; Equivalent reactance, X_{eq}=5.2\: ohm. When supplying power to a lagging load, the current, power and voltage measured on primary side were 20 A, 8 kW and 500 V, respectively. Calculate the voltage on the secondary terminals under these conditions. Draw the relevant phasor diagram.

Verified Answer:

Rating of transformer

=10\: KVA=10*10^{3}\:...

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