Some Problems on Three phase Circuits
1. A balanced star connected load of (8+j6) ohm per phase is connected to a balanced 3-phase,
400V, 50Hz supply. Find the line current, power factor, power and total volt-amperes.
2. A balanced star connected load is supplied from a symmetrical 3-phase 440V system .The
current in each phase is 30A and lags 30 degree behind the phase voltage. Find phase voltage
total power and reactive power.
3. Three identical coils connected in delta across 400V,50HZ 3-phase ac supply, take a line current
of 17.32 A at power factor of 0.7 lagging Calculate phase current , the resistance and inductance
of each coil, power drawn by each coil.
4. 3-coils are connected in star to a balanced 3-phase 440V 50 HZ supply and takes a line current of
10A AT 0.9 pf lagging. Calculate the resistance and reactance of the coil. If the coils are delta
connected to the same supply calculate the line current and real power.
5. The 3-phase voltage and current of a star connected load is 100V and 10 A . The power factor of
the load is 0.8 lagging. Assuming that the power is measured by two wattmeter method. Find
the readings of the wattmeter .
6. Two wattmeter are connected to measure the input to a balanced 3 phase circuit. The readings
of the instruments are 2500W and 500W respectively. Find the power factor of the circuit.
7. The input power to a 3 -phase motor was measured by two wattmeter method whose readings
were 12KW and -4KW and the line voltage was 200V. Calculate the power factor and the line
current.
8. Three coils each with a resistance of 20 ohm and reactance of 20 ohm connected in star across a
3-phase 50 HZ 440 V supply. Calculate line current and readings of the two wattmeter
connected to measure the power.
400V, 50Hz supply. Find the line current, power factor, power and total volt-amperes.
2. A balanced star connected load is supplied from a symmetrical 3-phase 440V system .The
current in each phase is 30A and lags 30 degree behind the phase voltage. Find phase voltage
total power and reactive power.
3. Three identical coils connected in delta across 400V,50HZ 3-phase ac supply, take a line current
of 17.32 A at power factor of 0.7 lagging Calculate phase current , the resistance and inductance
of each coil, power drawn by each coil.
4. 3-coils are connected in star to a balanced 3-phase 440V 50 HZ supply and takes a line current of
10A AT 0.9 pf lagging. Calculate the resistance and reactance of the coil. If the coils are delta
connected to the same supply calculate the line current and real power.
5. The 3-phase voltage and current of a star connected load is 100V and 10 A . The power factor of
the load is 0.8 lagging. Assuming that the power is measured by two wattmeter method. Find
the readings of the wattmeter .
6. Two wattmeter are connected to measure the input to a balanced 3 phase circuit. The readings
of the instruments are 2500W and 500W respectively. Find the power factor of the circuit.
7. The input power to a 3 -phase motor was measured by two wattmeter method whose readings
were 12KW and -4KW and the line voltage was 200V. Calculate the power factor and the line
current.
8. Three coils each with a resistance of 20 ohm and reactance of 20 ohm connected in star across a
3-phase 50 HZ 440 V supply. Calculate line current and readings of the two wattmeter
connected to measure the power.
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