Wednesday, March 7, 2012

Electrical Interview questions Part 19

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 -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)