POLARITY TEST OF A SINGLE-PHASE TRANSFORMER

Polarity testing of transformers is vital before connecting them in parallel. Otherwise, with incorrect polarity, it is not possible to connect them in parallel. The rated voltage is applied to the primary and its two terminals are marked as A₁ and A₂, respectively, as shown in Figures 1.44(a) and 1.45(b), respectively. The secondary winding terminals are also marked as a₁ and a₂, shown in Figures 1.44(a) and 1.45(b), respectively. Now a voltmeter is connected across A₂ and a₂. if it measures the difference of E₁ and E₂, A₂ and a₂ are of the same polarity. If it measures the addition of E₁ and E₂, A₂ and a₂ are of opposite polarity.

Figure 1.44 Polarity Test of a single-phase Two Winding Transformer

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Transformer multiple choice questions part 1

Hello Engineer's Q.[1] A transformer transforms (a) frequency (b) voltage (c) current (d) voltage and current Ans : D Q.[2] Which of the following is not a basic element of a transformer ? (a) core (b) primary winding (c) secondary winding (d) mutual flux. Ans : D Q.[3] In an ideal transformer, (a) windings have no resistance (b) core has no losses (c) core has infinite permeability (d) all of the above. Ans : D Q.[4] The main purpose of using core in a transformer is to (a) decrease iron losses (b) prevent eddy current loss (c) eliminate magnetic hysteresis (d) decrease reluctance of the common magnetic circuit. Ans :D Q.[5] Transformer cores are laminated in order to (a) simplify its construction (b) minimize eddy current loss (c) reduce cost (d) reduce hysteresis loss. Ans : B Q.[6] A transformer having 1000 primary turns is connected to a 250-V a.c. supply. For a secondary voltage of 400 V, the number of secondary turns should be (a) 1600 (b) 250 (c) 400 (d) 1250 A

Condition for Maximum Power Developed In Synchronous Motor

The value of δ for which the mechanical power developed is maximum can be obtained as, Note : Thus when R a is negligible, θ = 90 o for maximum power developed. The corresponding torque is called pull out torque. 1.1 The Value of Maximum Power Developed The value of maximum power developed can be obtained by substituting θ = δ in the equation of P m . When R a is negligible, θ = 90 o and cos (θ) = 0 hence, . . . R a = Z s cosθ and X s = Z s sinθ Substituting cosθ = R a /Z s in equation (6b) we get, Solving the above quadratic in E b we get, As E b is completely dependent on excitation, the equation (8) gives the excitation limits for any load for a synchronous motor. If the excitation exceeds this limit, the motor falls out of step. 1.2 Condition for Excitation When Motor Develops ( P m ) R max Let us find excitation condition for maximum power developed. The excitation

Effect of Slip on Rotor Parameters : Part2

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