EL25 Auto-transformers and Instrument Transformers

Questions and Answers

What is a significant advantage of auto-transformers over ordinary transformers?

They require more copper than an ordinary transformer.

They require a smaller magnetising current than a 2-winding transformer of the same rating. (correct)

They have poorer voltage regulation than a 2-winding transformer.

They operate at a lower efficiency than a 2-winding transformer.

Which of the following statements is true regarding the construction of auto-transformers?

They cannot work on both AC and DC.

They have a larger physical size compared to regular transformers.

They require less copper than an ordinary transformer of the same rating. (correct)

They have a more complex design than two-winding transformers.

For what primary purpose are instrument transformers typically used?

To generate power in high-voltage circuits.

To step down the voltage for household appliances.

To serve as a backup power supply.

To provide precise measurement and isolation of electrical quantities. (correct)

Where should the primary winding of a potential transformer be connected?

In parallel with the supply. Signup and view all the answers

What action is crucial for the safe removal of a current transformer?

Short the secondary winding. Signup and view all the answers

Which of the following statements is correct regarding the efficiency of auto-transformers?

They operate at a higher efficiency than a 2-winding transformer of the same rating. Signup and view all the answers

What is a common characteristic of auto-transformers concerning their physical size?

They are generally smaller than an ordinary transformer of the same rating. Signup and view all the answers

Which of the following describes a disadvantage of using auto-transformers?

They may pose safety risks due to common windings. Signup and view all the answers

What is the suitable current transformer ratio to provide a metering current of 0-5A for a 1000kVA load?

75:5 Signup and view all the answers

Which potential transformer ratio can provide a metering voltage of 110V from an 11,000V line?

100:1 Signup and view all the answers

If a current transformer of ratio 100:5 is used, what metering current range can be expected?

0-5A Signup and view all the answers

Which of the following combinations is NOT suitable for metering a 1000kVA load from an 11,000V line?

80:1 CT and 50:1 PT Signup and view all the answers

Why is it important to select the correct ratios for instrument transformers?

To ensure accurate voltage and current measurements Signup and view all the answers

What would be the voltage and current readings on the meters when the line voltage is 10,950V and the line current is 140A?

109.5V, 3.5A Signup and view all the answers

What is the primary function of a current transformer?

Metering purposes Signup and view all the answers

In which environments are instrument transformers commonly utilized?

Test laboratories Signup and view all the answers

Which of the following is NOT a typical use of current transformers?

Starting up large motors Signup and view all the answers

If the line voltage is 10,950V, which value represents a potential reading on an instrument transformer?

109.5V Signup and view all the answers

What type of measurement is primarily supported by instrument transformers in electrical installations?

Voltage and current measurement Signup and view all the answers

Which statement about instrument transformers is accurate?

They are essential for accurate metering in high voltage systems. Signup and view all the answers

When used in high voltage installations, what is the expected outcome of using instrument transformers?

Safe and accurate measurement of high voltages Signup and view all the answers

Which transformer ratio provides a secondary current of 5A from a 200A primary current?

200:5 Signup and view all the answers

What potential transformer ratio would be suitable for a metering voltage of 110V from a higher voltage primary?

100:1 Signup and view all the answers

If a load requires a measurement of 3000 kVA, which primary-to-secondary CT ratio would be appropriate?

200:5 Signup and view all the answers

Which configuration is NOT correct for producing a secondary voltage of 5V from a primary voltage of 300V?

400 Primary Volts: 5 Secondary Volts Signup and view all the answers

Which combination of current transformer and potential transformer would meet the requirement of a 0-5A metering current and 110V metering voltage?

Current transformer with 50:5 and potential transformer with 100:1 Signup and view all the answers

Which of these is an incorrect statement regarding the effects of primary voltage on secondary amperage in transformers?

50 primary amps can yield 500 secondary amps. Signup and view all the answers

When operating at full load, which of the following ratios converts 300 amps in the primary to 5 amps in the secondary?

300:5 Signup and view all the answers

What happens to the secondary current when the primary current of a transformer is increased?

It increases proportionally. Signup and view all the answers

What is a key disadvantage of an auto-transformer when compared to a double-wound transformer?

Auto-transformers pose a higher safety risk when reducing voltage. Signup and view all the answers

Which of the following is NOT considered an advantage of using auto-transformers?

Higher voltage regulation capabilities. Signup and view all the answers

Which statement accurately reflects the efficiency of auto-transformers?

Auto-transformers have the lowest efficiency of all transformer types. Signup and view all the answers

What factor primarily contributes to the higher copper loss in auto-transformers?

The single winding design which results in greater current flow. Signup and view all the answers

Which application is least appropriate for the use of an auto-transformer?

Isolating circuits for safety. Signup and view all the answers

In which scenario would adopting an auto-transformer be particularly beneficial?

In applications demanding lightweight and compact designs. Signup and view all the answers

How do auto-transformers compare to traditional transformers in terms of voltage adjustment?

Auto-transformers allow for smooth voltage transitions. Signup and view all the answers

What ongoing maintenance consideration should be taken into account with auto-transformers?

Their insulation must be regularly tested. Signup and view all the answers

Study Notes

Auto-transformers

Auto-transformers are more efficient than traditional transformers, requiring less copper for similar ratings.
They are beneficial for high voltage transformations and possess better voltage regulation.
Smaller size compared to traditional transformers makes them more space-efficient.
A significant disadvantage includes high risk when stepping down voltages due to lack of isolation.

Instrument Transformers

Instrument transformers, including current and potential transformers, are essential for accurate metering and protection in electrical circuits.
Current transformers (CTs) mainly serve for metering and circuit protection by scaling down high currents.
Potential transformers (PTs) reduce high voltages to manageable levels for measurement and processing.

Transformer Connections

The primary of a potential transformer is connected in parallel with the supply for accurate voltage readings.
To safely remove a current transformer from service, short the secondary winding to prevent damage and ensure safety.

Key Transformer Ratios

Proper transformer selection involves matching ratios for accurate metering, e.g., CTs and PTs need to be selected for designated metering currents and voltages.
Example ratios include:
- For a 3000 kVA load: Current transformer with a 200:5 ratio and potential transformer with a 100:1 ratio.
- For a 1000 kVA load: Current transformer with a 75:5 ratio and potential transformer with a 100:1 ratio.

Measurement Interpretation

It is critical to understand that the current and voltages experienced in the secondary must correspond accurately with those on the primary side, e.g., primary 300 A may produce secondary 5 A in a properly designed CT setup.
Instrument transformers are commonly employed in test laboratories to ensure precise measurements are maintained.

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Description

Test your knowledge on Auto-transformers and Instrument Transformers in this quiz designed for the EL25 course. The assessment includes various questions that evaluate your understanding of transformer connections and functionalities. Perfect for reinforcing your learning in electrical engineering.