Unit 28: Single Phase Transformers Flashcards
What is a transformer?
A transformer is a magnetically operated machine that can change values of voltage, current, and impedance without a change of frequency.
What are common efficiencies for transformers?
Transformers are the most efficient machines known. Their efficiencies commonly range from 90% to 99% at full load.
What is an isolation transformer?
In an isolation transformer the secondary winding is physically and electrically isolated from the primary winding. There is no electric connection between the primary and secondary winding.
All values of a transformer are proportional to its?
Turns ratio.
What is an autotransformer?
Autotransformers are one-winding transformers. They use the same winding for both the primary and secondary.
What is a disadvantage of an autotransformer?
Because the load is connected to one side of the powerline, there is no line isolation between the incoming power and the load. This can cause problems with certain types of equipment and must be a consideration when designing a power system.
Explain the difference between a step-up and a step-down transformer.
A transformer that has a higher secondary voltage than primary voltage is a step-up transformer, whereas a transformer that has a lower secondary voltage than primary voltage is a step-down transformer.
A transformer has a primary voltage of 240 V and a secondary voltage of 48 V. What is the turns ratio of this transformer?
5:1
A transformer has an output of 750 VA. The primary voltage is 120 V. What is the primary current?
A good rule for any type of transformer is that power in must equal power out. 6.25 A
A transformer has a turns ratio of 1:6. The primary current is 18 A. What is the secondary current?
If the primary current is 18 A and it is a step up transformer with the ratio of 1:6 the voltage increases by 6 and that will cause the current to decrease by 6. If the transformer were a step down transformer the current would increase, with the voltage drop in the secondary. 18A/6 = 3A
What do the dots shown beside the terminal leads of a transformer represent on a schematic?
When a transformer is shown on a schematic diagram, it is common practice to indicate the polarity of the transformer windings by placing a dot beside one end of each winding.
A transformer has a primary voltage rating of 240 V and a secondary voltage rating of 80 V. If the windings were connected subtractive, what voltage would appear across the entire connection?
160 V
A transformer has a primary voltage rating of 240 V and a secondary voltage rating of 80 V. If the windings were to be connected additive, what voltage would appear across the entire winding?
320 V
The primary leads of a transformer are labeled 1 and 2. The secondary leads are labeled 3 and 4. If polarity dots are placed beside leads 1 and 4, which secondary lead would be connected to terminal 2 to make the connection additive?
3
