s.v.vijayaraj

 Profile Answers by s.v.vijayaraj Questions by s.v.vijayaraj

• Jul 14th, 2008

 

Hi
    Several factors influence the choice of frequency in an AC system. Lighting, motors, transformers, generators and transmission lines all have characteristics which depend on the power frequency.
The first applications of commercial electric power were incandescent lighting and commutator-type electric motors. Both devices operate well on DC, but DC cannot be easily transmitted long distances at utilization voltage and also cannot be easily changed in voltage.

With AC, transformers can be used to step down high transmission voltages to lower utilization voltage. Since, for a given power level, the dimensions of a transformer are roughly inversely proportional to frequency, a system with many transformers would be more economical at a higher frequency.

If an incandescent lamp is operated off a low-frequency current, the filament cools on each half-cycle of the alternating current, leading to perceptible change in brightness and flicker of the lamps; the effect is more pronounced with arc lamps, and the later mercury-vapor and fluorescent lamps.

Commutator-type motors do not operate well on high-frequency AC since the rapid changes of current are opposed by the inductance of the motor field; even today, although commutator-type universal motors are common in household appliances, they are universally of low ratings less than 1 kW. Once the induction motor had been developed, it was found to work well on frequencies around 50 to 60 Hz but with the materials available in the late 1800s would not work well at a frequency of, say, 133 Hz. There is a fixed relationship between the number of magnetic poles in the induction motor field, the frequency of the alternating current, and the rotation speed; so, a given standard speed limits the choice of frequency (and the reverse).

Electric power transmission over long lines favors lower frequencies. The effects of the distributed capacitance and inductance of the line are less at low frequency.

Generators operated by slow-speed engines will produce lower frequencies, for a given number of poles, than those operated by, for example, a high-speed steam turbine. For very slow prime mover speeds, it would be costly to build a generator with enough poles to provide a high AC frequency. As well, synchronizing two generators to the same speed was found to be easier at lower speeds.

Generators can only be interconnected to operate in parallel if they are of the same frequency and wave-shape. By standardizing the frequency used, generators in a geographic area can be interconnected, providing reliability and cost savings.

Direct-current power was not entirely displaced by alternating current and was useful in railway and electrochemical processes. Prior to the development of mercury arc valve rectifiers, rotary converters were used to produce DC power from AC. Like other commutator-type machines, these worked better with lower frequencies.

All of these factors interact and make selection of a power frequency a matter of considerable importance. The best frequency is a compromise between contradictory requirements.

Manohar Mathur

 Profile Answers by Manohar Mathur 

• Nov 13th, 2008

 

Our eyes have special property of retentivity (capacity of retaining object for few moments even when removed). Advantage of this has been used in cinema. If frequency of power supply goes below certain level, we get flickering effect in lighting. Also in case of motors, rhythmic rise and fall of speed will be both visible and audible. If we choose frequency very high, line resistance will increase due to skin effect (self induction of line conductor). As a consequence, unwanted power loss and voltage drop will take place. Therefore, a bargain has been struck in considering 50 Hertz in India. In USA, it is 60 Hertz.

bittu123

 Profile Answers by bittu123 Questions by bittu123

• Nov 9th, 2011

 

You are right, its all bout standerisation. Once standerisation is fixed for indian (50Hz) by british, so its continuing and In america its 60 Hz according to their standerisation.

But real question is "why standrisation is 50-60Hz only, why not 150Hz or 10Hz?"
Its only because, too low value will create fluctuation in apparatus like Bulb starts blinking(persistence of eye is 15 Hz) and too high value will lead more skin effect,proximity effect,
inductive reactance, which will create transmission problem
So, one need to choose something considering both the factors. Someone choosed 50Hz and someone choosed 60 Hz.