Transformers Flashcards
What is a transformer?
It is a magnetically operated machine that can change values of voltage, current, and impedance without changing frequency.
What are the three classifications of transformers?
Isolation transformer, autotransformer, and current transformer
All values of a transformer are proportional to its ________
Turn ratio
How do you calculate the turns ratio of a transformer?
Divide the number of turns of the primary wire by the number of turns of the secondary wire
What is primary winding?
The winding that is connected to the incoming power supply
What is secondary wiring?
It is the winding that is connected to the driven load.
What are isolation transformers?
Transformers that have secondary wiring that is physically and electrically separated from primary wiring.
Why is isolation desirable in a transformer?
It reduces voltage spikes that originate on the supply side.
What is the excitation current?
The amount of current required to magnetize the core of a transformer. It remains constant from no load to full load.
How is mutual induction formed?
The magnetic field produced by the primary winding cuts the windings of the secondary which then produces a voltage.
What is mutual induction?
The ability of one coil to induce a voltage in another coil.
The amount of voltage induced in the secondary is determined by _________
The ratio of the number of turns of wire in the secondary to those in the primary.
Multiple tapped windings
Isolation transformers that have more than one set of lead wires connected to the primary and secondary.
What is a distribution transformer?
Type of transformer that changes the high voltage of a power company to the standard 240/120 volts that runs most homes and businesses.
If you measured the voltage across the entire secondary of a 240/120-volt line, what would the voltage be? What would it be if you measured each line to the center tap?
240 volts. 120 volts.
What is a control transformer?
A transformer that is used to reduce the line voltage to the value needed to operate control circuits.
The most common type of control circuit contains ________ and __________.
Two primary windings and one secondary winding
Why are laminated cores preferred in transformers?
A thin layer of oxide forms on the surface of each lamination which insulates the core and reduces the formation of eddy currents.
The magnetic coupling between the windings and the efficiency of the transformer is determined by the ________ and _______ of the core
Type and shape
How are the windings arranged in a core-type transformer?
Windings are placed around each end of the core material. Low voltage winding is closest to the core and high voltage winding is over the low voltage winding.
How are the windings arranged in a shell-type transformer?
Primary and secondary windings are wound around a centerpiece of the core (there is a window in the core).
How are the windings arranged in a H-type transformer?
Similar to shell-type core in which windings are wound around a centerpiece; however, the core surrounds the windings on four sides instead of two.
How are the windings arranged in a tape-wound core transformer?
Winding is one long continuous steel tape that is wound into a spiral. Flux leakage is kept to a minimum with this design.
What is inrush current?
The amount of current that flows when power is initially applied to the circuit.
What is the difference between reactors and transformers when it comes to inrush current?
Reactors are used to prevent inrush currents while transformers produce high inrush currents.
What are autotransformers?
Transformers with one winding
What are the pros and cons for autotransformers?
Pros: High efficiency and cheaper than an isolation transformer
Cons: Does not provide line isolation
How do you determine volts per turn of a transformer?
Divide the applied voltage by the number of turns of the primary winding.
How can you determine the secondary voltage if the volts per turn is known?
Multiply the number of secondary turns by volts per turn
What is a Variac or Powerstat transformer?
A transformer with a sliding tap that allows the turns ratio to be adjusted.
How is polarity of a transformer indicated on a schematic?
Dots on the end of a winding
How can the polarity of transformer windings be determined?
By connecting one lead of the secondary to one lead of the primary and measuring the voltage across both windings.
If the voltmeter indicates a sum of the two voltages, the windings are connected in additive (boost). If the voltmeter indicates a difference between the two voltages, the windings are connected subtractive (buck).
What are the eight things that are required on a transformer nameplate?
- Name of a manufacturer
- Rated Kilovolt ampere
- Frequency
- Primary and secondary voltage
- Impedance of transformers rated 25 kVA and larger
- Required clearances of transformers with ventilating openings
- Amount and kind of insulating liquid when used
- The temperature class of the insulating system of dry-tape transformers
What factors determine transformer impedance?
- Amount/type of core material
- Wire size used to construct windings
- Number of turns
- Degree of magnetic coupling between windings
How is impedance expressed and measured?
It is expressed as a percentage. It is measured by connecting a short circuit across the low-voltage winding and connecting a variable voltage source to the high-voltage winding. The impedance is determined by calculating the variable voltage as compared to the rated voltage of the high-voltage winding.
What is a constant current transformer?
They are a special type of isolation transformer that delivers a constant output current under varying load conditions.
