# What is the difference between three phase current and direct current?

I am currently reading about three phase current and its advantages. One of these advantages is that the voltage of a balanced three phase system will be constant, $$\ V_{AN}(j\omega) + V_{BN}(j\omega) + V_{CN}(j\omega) = \text{constant} \$$. If this is the case then how is three phase any different to just sending a direct current voltage down a line?

• Because it's still AC so it will still work with transformers etc. Aug 27 '21 at 11:09
• Unfortunately, $V_{AN}+V_{BN}+V_{CN}=\text{zero}$ for a balanced 3-phase supply so, it's meaningless even if it is constant. Aug 27 '21 at 11:13
• Induction motors won't work with DC. Along with transformers was the reason AC distribution won. 100 years ago the way to increase/decrease DC voltage was with dynamotors. Very lossy. Transformers on the other hand..... Aug 27 '21 at 11:32
• Voltage is measured between two points so your equation doesn't mean very much in the sense you are thinking about it from. Aug 27 '21 at 13:52

## 2 Answers

The voltage is not constant. The voltages of each individual phase varies sinusoidally. What is constant is the power. Although the power transferred by each phase varies sinusoidally, the sum is constant. As a result, the mechanical power driving the generator is constant rather than pulsating and the mechanical power delivered by a three-phase motor is constant rather than pulsating.

All AC motors require a rotating magnetic field. Three-phase power provides a smoothly rotating magnetic field.

DC power can be more efficiently transmitted over long distances. DC power is also more easily controlled and can reasonably easily be converted to AC for motor operation. Single-phase AC also has advantages. Systems with two phases and more than three phases are used for some purposes. Each power configuration can be converted to others and used for purposes for which they are suited.

https://en.wikipedia.org/wiki/High-voltage_direct_current

Transformers are more reliable and still cheaper than DC-DC converters. But for "long haul", HVDC is more cost-effective for line loss.

China and Swiss (ABB) have 1MV technology but China has installed 1.2 MV UHVDC links. Partial Discharge is a critical technology for insulation here and electrification of very high E-fields. (Makes the hair stand up on your head like an old TV glass tube could do)

• GV? As in 1000 MV? Where? What is the required air gap for that? Aug 27 '21 at 12:43
• I had GV ionization crosslinked on my brain Aug 27 '21 at 14:17
• That's fine, Tony. I thought I might have missed a three-order of magnitude technology jump. Aug 27 '21 at 15:06
• It only exists in special gas tubes and outer space. (electron beam welders) Aug 27 '21 at 16:01
• ABB is Swedish, not Swiss. HVDC was originally developed for undersea cables to reduce armor losses. Aug 27 '21 at 16:21