Electronic – Jerk in Power plants due to breakdown of transmission line

There is more to a SMPS than just a higher frequency. The duty cycle is also changed.

A higher switching frequency does this:

• Smaller transformer/inductor
• Faster transient load response time
• Smaller output capacitors
• Lower output voltage ripple
• LOWER overall efficiency
• HIGHER RF Noise emitted

Generally speaking, the lower the wattage for the SMPS the higher the switching frequency. The disadvantages of a high frequency, lower efficiency and higher EMI, are easier to deal with when the overall wattage is lower. But there are plenty of exceptions to this rule.

But just using the normal 50/60 Hz AC waveform from the wall does not really help you. The reason for this is that you still need to chop the waveform to vary the duty cycle of the signal going to the transformer. In the design of a SMPS, the turns ratio of the transformer gets you close to the ideal output voltage-- but not close enough. Varying the duty cycle in real-time allows the output voltage to be tweaked to be the proper value (usually within a couple of percent). Without this duty cycle adjustment the output voltage might vary by 10% or more.

But that's not all. Varying the duty cycle of what is already an AC waveform is tricky. Sure, it can be done but why? It is easier to convert the AC input to DC and then chop it than to chop the AC input itself. And there really is no benefit to keeping the original AC frequency.

Which brings us to power factor correction. Using the original 50/60 Hz frequency into the transformer really does not help power factor correction. It would still mainly consume power near the peaks of the AC waveform and not when the AC input is at a lower voltage.

Why not three lines all in the same phase?

1. Because then there is no return path.
2. Because single phase has no "rotation". Three phase makes it very simple to make a rotating motor with phase sequence determining the direction of rotation. Swap two phases and the direction is reversed.

Is there less loss when the phases of the three lines are all different?

1. Three phase power distribution requires less copper or aluminium for transferring the same amount of power as compared to single phase power.
2. The size of a three phase motor is smaller than that of a single phase motor of the same rating.
3. Three phase motors are self starting as they can produce a rotating magnetic field. The single phase motor requires a special starting winding as it produces only a pulsating magnetic field.
4. In single phase motors, the power transferred in motors is a function of the instantaneous power which is constantly varying. In three-phase the instantaneous power is constant.
5. Single phase motors are more prone to vibrations. In three phase motors, however, the power transferred is uniform through out the cycle and hence vibrations are greatly reduced.
6. Three phase motors have better power factor regulation.
7. Three phase enables efficient DC rectification with low ripple.

Figure 1. Resultant DC from three-phase rectifier.

8. Generators also benefit by presenting a constant mechanical load through the full revolution, thus maximising power and also minimising vibration.