PHEP23 Power Transformers Flashcards
A power transformer transfers electric power from one circuit at a certain frequency and voltage, an to a second circuit at:
A different voltage and same frequency.
Transformer polarity markings show:
The ends of the windings at which the induced voltages have the same instantaneous polarity.
The efficiency of a well designed transformer is:
An average of 98 percent.
A single phase transformer may have a/an:
Air core
Compared to the secondary of a loaded step-down transformer, the primary has:
Higher voltage and lower current.
Potential transformers are instrument transformers used to:
Step voltage down.
The secondary of a potential transformer should never be:
Short-circuited to remove the device it operates from service.
Current transformers are instrument transformers used to:
Step current down.
Current transformers are used with:
Ammeters
Watt meters
Protective relays
When the number of turns or voltage on the secondary of a transformer is greater than that of the primary, it is known as a:
Step-up transformer.
Transformers are classed as static AC machines.
True.
Transformers having only one winding are called:
Autotransformer.
In an energized transformer iron losses are present even if ther is no load on the secondary.
True.
An auto transformer has:
One winding which is composed of both the primary and secondary winding.
The core of a transformer is designed:
To carry the magnetic field for the induction of voltages between the primary and secondary windings.
The secondary of a current transformer should never be:
Opened to remove the device it operates from service.
Eddy currents are circulating currents that cause:
A loss of energy.
In a core-type transformer:
A section of the primary and secondary windings are wound around each leg of the core.
Hysteresis and eddy currents are known as:
Iron losses.
Hysteresis can be explained as:
Friction of the molecules due to alternating current.
Increasing the rate of cooling on a transformer will increase its capacity.
True.
The rating of a transformer is usually given in:
VA (volt-amperes)
KVA (kilovolt-amperes)
MVA (megavolt-amperes)
Transformer oil will lose its dielectric strength rapidly when:
Exposed to air or moisture.
Bushings are used on transformers to:
Serve as entrances into the transformer for the main leads.
Taps on a transformer allow the:
Output voltage to be varied.
The condition(s) that must be met when transformers are operated in parallel are:
Ratio of transformation approximately the same
Voltage rating approximately the same
Polarity must be the same
The conservator system is used on transformers to:
Preserve the dielectric strength of the transformer oil.
A series street lighting system requires what type of transformer?
A constant current transformer.
In a transformer the winding which receives the energy inductively is called the:
Secondary winding.
In a transformer, excluding the small amount of losses, the induce voltage per turn in the secondary winding is:
Equal to the voltage across each turn of the primary winding.
In a transformer a ______________________ must always exist between the primary and the secondary fields to maintain the required primary current to supply the transformer losses and secondary load.
Balanced condition
Transformers permit the transfer of electricity between two circuits which are:
Completely insulated from each other.
A transformer with more turns on the secondary than on the primary is called a/an:
Step-up transformer.
When a transformer’s windings are connected delta:
The line current will be greater than the phase current.
When a transformer’s windings are connected wye:
The line current will be the same as the phase current.
In a transformer, both the primary to secondary voltage and turn ratio will vary inversely as the primary:
To secondary current ratio.
