1. When grounding transformer is employed?
a) When neutral is available
b) When neutral is not available
c) When transformer of type Y/D is used
d) Always
Answer
Answer: b [Reason:] In case the neutral of the power transformer is not available for grounding (e.g. when a D-D transformer is used), a special Y-D transformer is employed only for neutral grounding, such a transformer is called a grounding transformer.
2. Grounding transformer is ______________
a) Step-up transformer
b) Step-down transformer
c) Autotransformer
d) Any transformer can be grounding transformer
Answer
Answer: b [Reason:] Grounding transformer is special type of transformer which is employed when neutral is not available in power transformers. It is constructed in Y-D winding format and always a step-down transformer.
3. The secondaries of grounding transformer are generally ___________________
a) Star only
b) Star with neutral taken out
c) Delta
d) Open-delta
Answer
Answer: c [Reason:] The star connected primaries are connected to the system and its neutral is grounded. The secondaries are in delta and generally do not supply any load but provide a closed path for harmonic currents to circulate in them.
4. Which harmonic currents flow through the grounding transformer secondary?
a) Triple
b) Fifth
c) Eleventh
d) Seventh
Answer
Answer: a [Reason:] The secondaries of grounding transformer are in delta and do not supply any load but provide a closed path for triple n harmonic currents to circulate in them. Under balanced conditions the current in a grounding transformer is its own exciting current.
5. Under fault conditions current flowing through the grounding transformer is _____________
a) very large
b) zero
c) some small non-zero value
d) depends on fault
Answer
Answer: a [Reason:] Under balanced conditions the current in a grounding transformer is its own exciting current. Under fault conditions (such as LG fault) large current may flow in it. Hence a grounding transformer should be of sufficient rating to withstand the effects of LG (line to ground) faults.
6. Welding transformer is _______________________
a) step-up transformer
b) step-down transformer
c) auto transformer
d) one-one transformer
Answer
Answer: b [Reason:] Welding transformer is special type of transformer which is basically a step-down transformer. Welding transformer has high reactance both in primary winding and secondary winding.
7. V-I characteristics of welding transformer are _____________________
a) increasing slowly
b) increasing steeply
c) decreasing slowly
d) decreasing steeply
Answer
Answer: d [Reason:] Welding transformer has primary and secondary winding are placed in separate limbs or in the same limbs but spaced distance apart. This high reactance causes steeply drooping V-I characteristics.
8. For welding transformer as current increases induced emf ______________________
a) stays constant
b) increases
c) decreases
d) depends on application
Answer
Answer: c [Reason:] Welding transformers have steeply drooping characteristics. with increase in current, the leakage flux increase and the induced emf will come down. This is why the increase in primary or secondary current increases the reactance voltage drop across the respective windings.
9. Welding transformers work on principle that weld is actually __________________
a) open circuit
b) short circuit
c) circuit with finite resistance
d) circuit with finite reactance
Answer
Answer: b [Reason:] The increase in primary or secondary current of welding transformer increases the reactance voltage drop across the respective windings, which is essential to limit the welding current as the weld is practically a short circuit.
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