SESSION:7

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1)       Which Transformer will give more Output, Transformer operated at 50 Hz or 60 Hz? 

It will give more output when we operate a transformer (of same rating) on 50 Hz instead of 60 Hz.

Suppose,        

When Transformer operates on 50 Hz Frequency

Transformer = 100kVA, R=700Ω, L=1.2 H, f= 50 Hz.

XL = 2πfL = 2 x 3.1415 x 50 x 1.2 = 377 Ω

Impedance Z = √ (R2+XL2) = √ (7002+ 3772) = 795 Ω

Power factor Cos θ = R/Z = 700/795 =0.88

Transformer Output (Real Power) = kVA x Cos θ

When Transformer operates on 60 Hz Frequency

Transformer =100kVA, R=700Ω, L=1.2 H, f= 60 Hz.

XL = 2πfL = 2 x 3.1415 x 60 x1.2 = 452.4 Ω

Impedance Z = √ (R2+XL2) = √ (7002+ 452.4 2) = 833.5 Ω

Power factor = Cos θ = R/Z = 700/833.5 =0.839

Transformer Output (Real Power) = kVA x Cos θ

2)       Can we operate a 60HZ Transformer on 50Hz Supply Source and Vice Versa?

Yes, we can operate but for that we need to use external current limiter in series with the primary as when Transformer designed to operate at 60 Hz is operated at 50 Hz, it will draw more current for which it may not be designed.

3)       What would happen if a power transformer designed for operation on 50 Hz (frequency) were connected to a 5Hz (frequency) source of the same voltage?

Power Transformer is made to operate at one particular frequency, usually 50 Hz. If it is operated at low frequency then primary will have very less inductance and therefore for creating working mmf, primary will take excessive current because of which transformer may burn.

4)       Explain the statement that ” In Inductive circuit, when Inductance (L) or inductive reactance (XL) increases, the circuit Current (I) decrease”

As we increase the Inductance, the Impedance of the circuit increases and I= V/Z and hence current in the circuit will decrease.

5)       What would happen if a power transformer designed for operation on 50 Hz (frequency) were connected to a 500 Hz (frequency) source of the same voltage?

Power Transformer is made to operate at one particular frequency, usually 50 Hz. If it is operated at higher frequency then because of high inductance, primary won’t be able to draw current from the source and there will be excessive Hysteresis and Eddy Current Loss.

Primary Current is inversely proportional to supply frequency.

6)       Under what condition is DC supply applied safely to the primary of a transformer?

As we know that Transformer works only on A.C, in case of D.C Supply, the primary of transformer may start to smoke and burn. But if we want to connect DC to the Transformer primary then we need a high value of Resistor to be connected in series with Primary to limit the high current through the primary.

7)       What is the Role of Capacitor in AC and DC Circuit?

Role of Capacitor in AC Circuits: In an AC circuit, capacitor reverses its charges as the current alternates and produces a lagging voltage (in other words, capacitor provides leading current in AC circuits and networks)
Role and Performance of Capacitor in DC Circuit
In a DC Circuit, the capacitor once charged with the applied voltage acts as an open switch.

8)       Why AC Needs More Insulation Than DC for the Same Working Voltage Level?

For the same working voltage, the potential stress on the insulation is less in case of DC system than that of AC system. Therefore, a DC line requires less insulation.

When we talk about DC, let say 220 Volt DC, it means the maximum value (peak value) of the voltage is 220V, but when we talk about 220 Volt AC, then it is basically 220 RMS (Root Mean Square = √2) value, i.e. it is the RMS value of 220V AC.

The peak value of AC voltage is equal to √2xVRMS = 1.414 x VRMS (where √2 = 1.414)

In other words. The peak value of AC voltage = VRMS / 0.707

 Now, in our case, the peak value of 220V AC = 220V/0.707 or 220V x 1.414 = 311V AC peak.

It’s mean 220 VDC = 311V AC peak. That’s why AC Voltage needs more insulation than DC voltages for the same rating of Voltage and Power.

9)       What is the difference between AC and DC Resistance?

Resistance in AC circuits is called Impedance. Or

The Overall resistance (Resistance, Inductive reactance and capacitive reactance) in AC circuits is called Impedance (Z).

We know that there is no concept of Inductive and Coactive reactance in DC Circuits i.e. capacitive and inductive reactance in DC circuits zero because there is no frequency in DC circuits, i.e. magnitude of DC current is constant. Therefore, only the original resistance of wire comes into picture.

10)    Bulb connected in strange way? Will it glow? What is the Current through Ammeter?

 The Circuit does not make sense as the circuit is not complete, so Bulb will not glow and the Current in Ammeter is Zero (0).

11)    What will happen if Ammeter is connected in parallel?

Ammeter is connected in series with load. If it is connected in parallel then because of very low resistance of Ammeter, whole current will flow through the Ammeter and it will burn as Ammeter coil is not designed to carry large current.