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Electrical Interview questions Part 13

Q:How to determine alternating current frequency?
A:Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are "n" pulses per second, each with with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.
 
Q:Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv  ?
A:Transformer Induced voltage equation contains 4.44 factor. 
E=4.44*f*T*phi 
E -Induced emf per phase 
T -number of turns 
f -frequency 
phi -maximum flux per pole 

From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.So always transmission voltage is multiple of 11 

Q:Why we use ac system in India why not dc ? 
A:Firstly, the output of power stations comes from a rotary turbine, which by it's nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. thirdly the cost of plant associated with AC transmission 
(circuit breakers, transformers etc) is much lower than the equivilant of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current  occurs. In an AC system this becomes much easier to interupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Q:Which type of motor is used in trains, what is the rating of supply used explain Working principal?
A:Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

Q:Battery banks are in connected in series or parallel and why?
A:Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex : 24 nos. 2V,200Ah batteries connected in series will give 48V,200Ah output (AH = Ampere hours)
Q:What is inrush current?
A:Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.

 Q:In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?
A:Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

Q:Why transformer ratings are in kva? 
A: Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KWatts and include power factor.

 Q: What is difference between fuse and breaker?
A: Fuses are burned at the time of over current flows in the circuit but breakers are just open(not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple number of times.

 Q:What is the difference between delta-delta, delta-star transformer?
A:Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i,e) generally it is used where the Voltage is high & Current is low.Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena.

Q: Capacitor is load free component but why ampere meter shows current when capacitor bank breaker close?
A: As we know that Electrical is having two type of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load,& its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.
 
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