Electrical Measurements

1.The bridge method commonly used for finding mutual inductance is

(A) Heaviside Campbell bridge (B) Schering bridge

(C) De Sauty bridge (D) Wien bridge

Ans:Heaviside mutual inductance bridge measures mutual inductance is terms 

of a known self-inductance.

2.A dual trace oscilloscope is set to operate in the ALTernate mode. The

control input of the multiplexer used in the y -circuit is fed with a signal

having a frequency equal to

(A) the highest frequency that the multiplexer can operate properly

(B) twice the frequency of the time base (sweep) oscillator

(C) the frequency of the time base (sweep) oscillator

(D) haif the frequency of the time base (sweep) oscillator

Ans: option(C)

3.An ammeter has a current range of 0-5 A, and its internal resistance is 0.2

Ω. In order to change the range to 0-25 A, we need to add a resistance of

(A) 0.8 Ω in series with the meter

(B) 1.0 Ω in series with the meter

(C) 0.04 Ω in parallel with the meter

(D) 0.05 Ω in parallel with the meter

Ans:opion(D)

4.The pressure coil of a dynamometer type wattmeter is

(A) Highly inductive (B) Highly resistive

(C) Purely resistive (D) Purely inductive

Ans:Option (B) is correct.

Since Potential coil is connected across the load terminal, so it should be

highly resistive, so that all the voltage appears across load.

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How do you extend the range of ammeter and voltmeter?

The ranges of electrical measuring instruments (whether ammeter, voltmeter or any other type of meters) are limited by the currents, which be carried by the coils of the instruments safely. For example, the moving coils of the instruments can carry maximum current of about 50 mA safely and the potential drop across the moving coil is about 50 mV. Hence, it becomes necessary that the current and voltage being measured be reduced and brought within the range of instrument.

There are four common devices used for extending the range of the instruments, namely

1. Shunts
2. Multipliers
3. Current transformers
4. Potential transformers

The let us discuss the first two topics i.e. shunts and multipliers

1. Shunts: The range of an ammeters can be extended by connecting a low resistance, called shunts, connected in parallel with ammeter. The shunt by passes the extra current and allows only safe curren
to flow through the ammeter

Properties of Shunts:

1. Temperature coefficient should be low.

2. Its resistance should not be vary with time.

3. Should carry the current without excessive temperature rise.

4. Should have low thermal e.m.f with copper.

2. Multipliers: The range of voltmeter can be extended by connecting a high resistance, called multiplier in series with the voltmeter coil. The multiplier limits the current through the meter so that it does not exceed the value of full scale deflection and thus prevents the movement from being damaged.

Note: Materials used for multipliers are manganin and constantan.

Properties of Multipliers:

1. Their resistance should not change with time.

2. The change in their resistance with temperature should be small i.e. TVR should be small.

3. They should be non-inductively wound for A.C meters

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flemings left hand rule

for motor






gate eee

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gate eee basics

flemming right hand rule for generator






gate eee

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Difference between Neutral and Earth or ground

Neutral:

It is the return path for an AC circuit which is supposed to carry current in normal condition. This current may be because of many reasons, primarily because of phase current imbalance and some time because of 3rd and 5th harmonics also. This neutral wire is given to ground (by grounding) to make the second terminal of neutral wire at zero potential.


Earth or Ground:

It is for safety concerns against leakage or residual currents on the system via least resistance path. While phase and neutral is connected to main power wiring, earth may be connected to the body of equipment or to any system which in normal condition doesn’t carry current but in the case of some insulation failure, is supposed to carry some minor current.

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