Question 1: Draw the equal circuit and phasor diagram of the potential transformer. Deduce the expressions for its ratio and phase angle errors.
Question 2: Explain the effect of the given on the characteristics of the potential transformer:
a) Burden (VA) of secondary winding circuit.
b) Power factor of secondary winding circuit.
c) Frequency.
d) Supply voltage.
Question 3: A current transformer consists of a single turn primary and a 200 turn’s secondary winding. The secondary winding supplies a current of 5A to a non-inductive burden of 1Ω resistance.
The requisite flux is set up in core by an mmf of 80A. The frequency is 50 Hz and net cross section of the core is 1000 mm2. Compute the ratio and phase angle of the transformer. As well determine the flux density in the core. Avoid the effects of magnetic leakage, iron losses and I2R losses.
Question 4: The exciting current of a ring core current transformer of ratio 1000/5 A when operating at full primary current and with a secondary burden of non-inductive resistance of 1Ω is 1A at a power factor of 0.4. Compute:
a) The phase displacement between primary secondary winding currents.
b) The ratio error at full load, supposing that there has been no compensation.
Question 5: Current transformer with a bar primary has 300 turns in its secondary winding. The resistance and reactance of secondary circuit are 1.5Ω and 1.0Ω correspondingly comprising the transformer winding. With 5A flowing in the secondary winding, the magnetizing mmf is 100A and the iron loss is 1.2W. Find out the ratio and phase angle errors.
Question 6: A current transformer consists of a bar r primary and 200 secondary winding turns. The secondary winding burden is an ammeter of resistance 1.2Ω and reactance 0.5Ω, the secondary winding consists of a resistance 0.2Ω and reactance 0.3Ω. The core needs the equivalent of an mmf of 100A for magnetization and 50 A for core losses.
a) Determine the primary winding current and ratio error when the ammeter in the secondary winding circuit points out 5A.
b) How many turns could be decreased in the secondary winding in order that the ratio error be zero for the condition.
Question 7: The primary winding exciting current of a current transformer with bar primary, nominal ratio 100/1, operating on external burden of 1.6Ω non-inductive, the secondary winding resistance being 0.2Ω, is 1.9A, lagging 40.6° to the secondary voltage reversed there being 100 secondary turns. With 1A flowing in the secondary winding, compute:
a) Actual ratio of primary winding current to secondary winding current.
b) Phase angle between them in minutes.
Question 8: A 100/5A, 50 Hz current transformer consists of a bar primary and a rated secondary burden of 12.5 VA. The secondary winding consists of 196 turns and a leakage inductance of 0.96mH. With a purely resistive burden at rated full load, the magnetization mmf is 16A and the loss excitation needs 12A. Determine the ratio and phase angle error.