Assignment 12 - Single Phase Transformers

All values of a transformer are proportional to its . (Two words).

turns ratio

Why do we call them "isolation transformers"?

They are called isolation transformers because the secondary winding is electrically and physically "isolated" from the primary winding. Meaning that they are magnetically coupled and greatly reduces voltage spikes due to having the same input and output voltage with a turn ratio of 1:1 "The transformers shown in Figure 27-1 and Figure 27-2 are isolation transformers. This means that the secondary winding is physically and electrically isolated from the primary winding. " Pg. 625, 7th edition

A tape-wound core is also called a(n) _____ core. a. toroid b. solid c. H d. E

a. toroid

What is a range of common efficiencies for iron-core transformers?

90% to 99% at full load is the range of common efficiencies for iron-core transformers . "Transformers are the most efficient machines known. Their efficiencies commonly range from 90% to 99% at full load." Pg. 624 7th edition

Give a definition of an electric transformer.

An electric transformer can change values of current, voltage and impedance, but frequency remains the same. Transformers are highly efficient machines that are magnetically operated and their values are proportional to its turns ratio. "A transformer is a magnetically operated machine that can change values of voltage, current, and impedance without a change of frequency." Pg. 624 7th edition

How is an autotransformer constructed differently from an isolation transformer?

Autotransformer are constructed differently from an isolation transformer because they use the same winding for primary and secondary winding, meaning it only has one-winding, whereas an isolation transformer has two windings, one connected to load and the other to AC supply. "Autotransformers are one-winding transformers. They use the same winding for both the primary and secondary." pg 647, 7th edition "one winding of an isolation transformer has been connected to an AC supply, and the other winding has been connected to a load. "pg 627, 7th edition

The power input winding of a transformer is called the _____ winding. a. tapped b. primary c. secondary d. tertiary

b. primary

A transformer has a 240 V primary and a 120 V secondary. With a 30 ohm load connected, what is the primary volt-amps? a. 4 b. 8 c. 480 d. 960

c. 480

The output winding of a transformer is called the _____ winding. a. tertiary b. tapped c. primary d. secondary

d. secondary

Why are cores of transformers laminated?

Cores of transformers are laminated because it helps prevents power loss caused by induction of eddy currents. "The core is generally made of laminations stacked together. Laminating the core helps reduce power losses caused by eddy current induction." Pg 627, 7th edition

A transformer has a primary voltage rating of 240V and a secondary voltage rating of 80 V. If the windings were connected subtractive (or series opposing), what voltage would appear across the entire connection?

If the windings were connected subtractive (or series opposing), 160V would appear across the entire connection. V = P-S V = 240V - 80 V = 160V

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?

Lead 4 would be connected to terminal 2 to make the connection additive.

When a transformer is constructed to step up voltage from primary to secondary, how does the current in a loaded secondary compare to the current in the primary?

The current is step down compared to the current in the primary.

A transformer has a turns ratio of 1:6. The primary current is 18A. What is the secondary current?

The secondary current is 3A. Is/Ip=Np/Ns Is= (Np/Ns)Ip Is= (1/6)18A = 3A

If the windings of the transformer in Question 12 were to be connected additive, what voltage would appear across the entire winding?

If the windings were connected additive, 320V would appear. V = P+S V = 240V + 80 V = 320V

What do the dots shown beside the terminal leads of a transformer represent on a schematic?

The dots shown besides the term terminal leads of a transformer indicate polarity on a schematic. "Polarity dots are often added to schematic diagrams to indicate transformer polarity." Pg 666, 7th edition

What is the greatest disadvantage of an autotransformer?

The greatest disadvantage of an autotransformer is that it can cause problems with certain types of equipment because the load is connected to one side of the power line, thus there is no isolation between the load and incoming power. "The autotransformer does have one disadvantage. 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." Pg 650, 7th edition

A transformer has an output of 750 VA. The primary voltage is 120V. What is the primary current? Assume 100% efficiency.

The primary current is 6.25A. I= VA/V I= 750 VA/120V = 6.25A

A transformer has a primary voltage of 240V and a secondary voltage of 48V. What is the turns ratio of this transformer in whole numbers?

The turns ratio of this transformer is 5:1. P/S = 240V/48V = 5/1