Introduction

• Schematic Diagram of a transformer with core and windings.
• Primary side takes the input voltage while put voltage is obtained at the secondary side.
• Ratio of primary turns (N₁) to secondary turns (N₂)   i.e. N₁/N₂ is called turns ratio.
• For step-up transformer N₁> N₂, while for   step-down transformer N₁< N₂.
• Need to first ascertain number of coils, voltage ratio and polarity of coil windings.

]]> Run 1: Continuity Test

• Determine number of coils in primary and secondary.
• Use multimeter to check continuity across coil terminals.
• When the coils are continuous, the deflection is zero.
• When the coils are discontinuous, the deflection is infinity.

]]> Run 1: Voltage Ratio Test

• Nameplate shows 3KVA, 110:220V, 50Hz.
• 110V is the input voltage, while 220V is obtained at the output.
• Apply 110V input across one primary coil and note voltage across the other coil terminals using the voltmeter.
• Range of voltmeter used is 0-300V.
• 110V induced across the other primary coil and 220V across the secondary.

]]> Run 1: Polarity Test

• Need to determine polarity of each coil before connecting coils in series or parallel.
• Short any one terminal of one of the two primary coils with one terminal of the second coil, and measure the voltage between the other two terminals.
• If the terminals with opposite instantaneous polarity are shorted, there will be a series addition of voltage.
• If the terminals with same instantaneous polarity are shorted, there will be a series subtraction of voltage.

]]> Run 2: Parallel Operation

• For parallel operation with proper polarity, connect two primary coils such that the dotted ends are shorted together, and the two undotted ends shorted together.
• Parallel operation with incorrect polarity leads to a buildup of e.m.f.s across the primary coils, which leads to a circulating current within the closed path formed by the two coils.
• 5A Fuse used here blows off due to the circulating current.
• Never connect coils with opposite polarity for parallel operation.

]]> Run 2: D.C. Supply

• With D.C. supply, inductive reactance is absent in the primary ciruit(as frequency f = 0).
• Coils generally have low resistance.
• When connected to 110V D.C., low impedance leads to a high current that blows off the fuse.

]]> Run 3: Loading of Transformer

• Rated Current for Transformer = KVA Rating / Source Voltage = 3 KVA/110V = 30 A approx.
• Use a fuse with rating for minimum 30A ( i.e. have two or three 15A fuse wires in parallel).
• Connect as a two winding step-up transformer, with two primary coils in parallel and connect the secondary side to a load box.
• Load the secondary in steps of 2A, up to rated currentand measure the current in the primary and secondary using ammeters.

]]> Run 4: Step Up Autotransformer

• A single coil used as both primary and secondary.
• Connect all the coils in series.
• Input is 110V across one of the primary coils.
• Output voltage can be measured across different terminals.

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