Transformers Flashcards
7 cards from real EETC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Transformers flashcards as text
An instrument transformer used to measure high voltages safely by stepping them down to a standard 120V level is called a:
Answer: Potential transformer (PT)
A potential transformer (PT), also called a voltage transformer (VT), accurately steps down high voltage to a safe 120V level for metering and protection relays.
The secondary circuit of a current transformer (CT) must never be open-circuited while the primary carries current because:
Answer: It will develop a dangerously high voltage across the open secondary terminals
With the secondary open, the primary MMF is unopposed, driving the core into saturation and inducing a very high dangerous voltage across the open secondary.
In a three-phase transformer bank, what is the primary advantage of using a delta connection on the primary side?
Answer: Third-harmonic currents circulate within the delta, preventing harmonic voltage distortion
The closed delta path provides a circulating path for triplen (3rd, 9th, etc.) harmonic currents, keeping them out of the supply line.
A transformer's short-circuit test (impedance test) is performed to determine:
Answer: Full-load copper losses and series impedance parameters
The short-circuit test is conducted at rated current with the secondary shorted to measure winding resistance, leakage reactance, and copper losses.
What is the typical efficiency of a well-designed modern distribution transformer at full rated load?
Answer: 97% to 99%
Modern distribution transformers achieve efficiencies of 97–99% because both core and copper losses are very small relative to the power transferred.
Which condition represents maximum efficiency in a transformer?
Answer: When core (iron) losses equal copper (I²R) losses
Transformer efficiency is maximum when the variable copper losses equal the constant core losses, which typically occurs between 50–75% of full load.
What is the function of a tap changer in a power transformer?
Answer: To adjust the turns ratio and thereby regulate the output voltage
A tap changer varies the number of active turns on a winding to compensate for system voltage variations and maintain the desired output voltage.