Exam Question
With Reference to Power transformers onboard:
(a) Explain how the efficiency and regulation of a transformer can be assessed by open circuit and short circuit tests?
(b) What is meant by equivalent resistance?
Class 2 (2nd Engineer)
MET
📅 Nov 2018
Reference Answer
(a) Assessing transformer efficiency:
Open Circuit Test: One winding is connected to a normal voltage supply, while the other is left open-circuited. The input power (P₀) is measured using a wattmeter, and the no-load current (I₀) is measured with an ammeter. Voltmeters measure the primary and secondary voltages. The wattmeter reading directly indicates the core losses (iron losses), which are primarily due to hysteresis and eddy currents in the transformer core. These losses are relatively constant regardless of the load.
Short Circuit Test: One winding is short-circuited through an ammeter, and a reduced voltage is applied to the other winding. The applied voltage is adjusted to circulate the full-load current through the short-circuited winding. Because the core flux is proportional to the applied voltage, and the voltage is kept low, the core losses are negligible. The wattmeter reading primarily represents the copper losses (I²R losses) in the windings. These losses are dependent on the load current.
Open Circuit Test: One winding is connected to a normal voltage supply, while the other is left open-circuited. The input power (P₀) is measured using a wattmeter, and the no-load current (I₀) is measured with an ammeter. Voltmeters measure the primary and secondary voltages. The wattmeter reading directly indicates the core losses (iron losses), which are primarily due to hysteresis and eddy currents in the transformer core. These losses are relatively constant regardless of the load.
Short Circuit Test: One winding is short-circuited through an ammeter, and a reduced voltage is applied to the other winding. The applied voltage is adjusted to circulate the full-load current through the short-circuited winding. Because the core flux is proportional to the applied voltage, and the voltage is kept low, the core losses are negligible. The wattmeter reading primarily represents the copper losses (I²R losses) in the windings. These losses are dependent on the load current.
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