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SIR C R REDDY COLLEGE OF ENGINEERING
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
Assignment-2 Semester-1 A Y: 2016-17
Subject: PDEM II
Class: III/IV EEE (A)
GROUP 1
1.
Suggest a suitable test to predetermine the efficiency of a 1- phase transformer and
explain it.
2.
Explain why parallel operation of tranformers is necessary under what conditions the
no load circulating current is zero in two single phase transformers operating in parallel.
3. Name the various losses in transformer. How do they vary with load?
4.
Draw the equivqlent circuit of iron core 1-phase transformer. How will the equivalent
circuit be modified if the transformer is an air cored transformer?
5. Draw the phasor diagrams of a transformer on
i)no load and ii)full load with inductive load
6.
A transformer has the same efficiency at 75% full load as it has at 125% full load,
both at unity power factor. What is ratio of copper loss at full load to iron loss?
7. Explain why the open circuit and short circuit test should be conducted on lv and hv
sides respectively.
8. two transformers a and B of different ratings but equal voltage ratios share a load of
500kva at 0.8 p.f lagging at 400v by operating in parallel.transformer a has rating of 500kva,
resistance drop of 1.5% and reactance drop of 5%. Transformer B has rating of
1000kva,
resistance drop of 1% and reactance drop of 4%. Calculate the load sharedby
each transformer and the p.f at which it is working.
GROUP 2
1. Explain the purpose of using consevator and breather in a transformer.
2. Write the emf equation of transformer.
3. Define voltage regulation of a transformer.
4. Give the constructional difference between a core type and shell type transformer.
5. Explain the principle of working of a transformer.
6. Make a comparsion in the wegiht of copper required in an auto transformer and a
two winding transformer of the same rating.
7. Explain with phasor diagram how can two phase supply be obtained from 3-phase mains?
8 Draw the phasor diagram of transformer on i)no load ii)full load with inductive load
GROUP 3
1.
Describe with connection diagrams the test to be done on a single phase T/F to
determine the equivalent circuit parameters.
b)a 20 KVA, 2500/250V, 50HZ,1-PHASE transformer has the following results:
OC TEST: 250V, 1.4A, 105W SC TEST: 104V, 8A, 320W
calculate the efficiency at full load and 0.8 pf lagging.
2
A 5KVA,200/400V,50Hz,1-phase T/F gave the following test results:
OC TEST: 200V,1A,40W
SC TEST:10V,full load current,100W
Calculate the parameters of equivalent circuit of T/F when secondary is refered to primary
and the approximate full load regulation at 0.8pf lagging.
3How do you determine the parameters of the equivalent circuit of a single phase of 230/40V,
50HZ, T/F from the test results.(note:you need not draw the circuit diagrams for the test
to be conducted and explain how the tests are conducted)
4
derive the condition for maximum efficiency to occur in a transformer when it is
supplying a load at constant power factor?
5
define the term "regulation" as applied to T/F derive the expression for approximate
regulation of the T/F when it supplies lagging pf load.
6
Under balanced conditions the T/F rating is 15%greater than that of the teaser T/F
in a scott connected T/F why? Explain?
7Explain with connections and vector diagrams how can a two phase supply be
obtained from 3phase mains?
b)
the maximum efficiency of a 500KVA 3000/500V 50HZ,1-phase T/F is 97% and
occurs at 3/4th full load if the impedence is 10%.calculate the regulation ata full load
and 0.8pf lagging.
8 Explain how 3-phase to 2-phase conversion is done using two 1-phase T/F.
GROUP 4
1.
Draw the internal connections of auto transformer started used with 3-phase cage
induction motor?
2Two single phase T/F of different ratings but equal voltage ratios share a load of
500KVA at 0.8pf lagging at 400V by operating in parallel T/F.A has a rating of
500KVA,
resistance drop of 1.5% and leakage reactance drop of 5%.T/F B has rating of
1000KVA,
resistance drop of 1% and leakage reactance drop of 4%.Calculate the load
shared by each T/F and the pf at which it is working.
3
derive the relation between the rotor power input,
rotor copper losses of a 3-phase induction motor
mechanical power developed and
4a 6-pole 3-phase 50Hz induction motor has a standstill rotor impedance of
(0.02+0.1j) ohm per phase. Determine the speed to give maximum torque. What
external resistance per phase in necessary in the rotor circuit to give half of the
maximum torque at starting?
5 Prove that the tip of the stator current of a 3-phase induction motor lies on a circle.
6a 3-phase 4-pole 50HZ induction motor has a rotor resistance of 0.1ohm per phase
and standstill leakage reactance 0.8ohms per phase if the full load slip is
5%.Determine the full load torque as a percent of maximum torque.
What should be the
value of external resistance per phase to give half the maximum torque at starting.
7derive the expression for the torque developed in a 3phase induction motor and sketches the
torque slip characteristics
8tests on a 15HP (metric), 200V, 3phase, 4-pole, delta connected induction motor gave the
following test figures:
No load test(line values): 200V, 12.5A,0.5PF lagging
Blocked rotor test(line values): 125V,100A,0.25pf lagging
The stator losses at standstill are 55% of total copper losses.
Draw the circuit diagram and determine the full load efficiency.
GROUP 5
1. A 4KVA 1-phase T/F rated at 50Hz 200/400V give the following results:
SC TEST: 9V,6A,21.6W
OC TEST:200V,0.7A,60W for the above T/F.
Draw the equivalent circuit refer to LV side and insert the various parameters on it
compute
the
approximate full load
percentage regulation aat 0.9pf
Neglect no load current.
and
leading.
2.a transformer has same efficiency at 75% load as it has at 125% load both are at
unity power factor what is the ratio of copper loss at full load to iron loss at full load.
3.
Derive the condition for maximum efficiency of a single phase T/F for a given power
factor of the load.
4.
A 3-phase step down T/F is conducted to 6000V mains and takes a current of 20A
from the supply.
Calculate the secondary line voltage,
line current and output for the
following 3-phase connections.
Mesh/star
and
star/mesh The ratio of turns per phase is 12.Neglect losses
5.A three phase step up T/F is connected to 500*1.732 V mains a current of
200A.Calculate the
secondary line voltage,line current and output at unity power
factor,for delta/star connections.Assume the turns ratio to be 12.
6.
1-phase load of 150KW and 100KW at 110V are supplied by the secondaries a scott
connected
T/F from a 3-phase 3.3KW supply.find the currents on the 3-phase side
when the power factors of the load are equal to 0.8 lagging.
7.
A 3-phase T/F has a delta connected primary and star connected secondary.The line
voltage
being 22KV and 400V respectively the secondary has a star connected
balanced load at 0.8pf lagging the line current on the primary side is 5A.Determine
the current in each coil of T/F.
8. Derive the condition for maximum torque.
GROUP 6
1. Prove that the tip of the stator current of a 3-phase induction motor lies on a circle.
2. derive the relation between the rotor power input,
mechanical power developed and
rotor copper losses of a 3-phase induction motor
3. a transformer has same efficiency at 85% load as it has at 125% load both are at
unity power factor what is the ratio of copper loss at full load to iron loss at full load.
4. Draw the internal connections of auto transformer started used with 3-phase cage
induction motor
5. Derive the condition for maximum efficiency to occur in a transformer when it is
supplying a load at constant power factor?
6. Explain the principle of working of a transformer.
7. Describe with connection diagrams the test to be done on a single phase T/F to
determine
the equivalent circuit parameters
8. Suggest a suitable test to predetermine the efficiency of a 1- phase transformer and explain it.