What's the difference between single phase and three phase power?

When you're buying a generator, it helps if you know some of the technicalities - otherwise there's a high chance you'll choose the wrong generator for your needs. So let's start by revealing one of the greatest mysteries of them all: the difference between single phase and three phase power.

three phase power
Single Phase and Three Phase outlets

Here it is:

"A phase is the factional part of the period of a sinusoidal wave, usually expressed in electrical degrees. A single-phase circuit is an alternating-current using only one, sine wave type, current flow. A three phase circuit consists of three different sine wave current flows, different in phase by 120 degrees from each other."

In English? Okay then...

An electrical generator converts mechanical power into a set of alternating electric currents (AC), one from each electromagnetic coil or winding of the power generator. The currents are sinusoidal functions of time, all at the same frequency but with different phases.

In a single-phase motor, the voltages of the supply all vary in unison. Whereas in a three phase system, the phases are spaced equally, giving a phase separation of 120°. In other words, the currents reach their peak in sequence, rather than at the same time. In each cycle of power frequency, first one, then the second and then the third current reaches its maximum value.

Imagine pushing a car uphill. Using a single-phase supply is like one strong man pushing the car. At some point, it will simply become too tough for one guy. But a three-phase supply is like having three equally strong men pushing the car in a relay system - one after another. Because each man is doing one third of the work, together, the three of them push the car further and they will not be as worn out.

It's no surprise, then, that three-phase power has the advantage of being more efficient than single-phase power in certain applications - especially when it comes to motors; a three-phase motor is much more powerful than its single-phase counterpart. A three-phase induction motor has a simple design, high starting torque, and high efficiency. It's also more compact than a single-phase motor - making it perfect for three-phase generators, industrial fans, compressors, and pumps.

So which do you choose? Largely it depends on the type of power your appliances need. Most commonly, three-phase power is used in larger, commercial applications. A three-phase generator allows for greater electrical loads - it distributes the total load in a way that ensures that the current in each phase is lower. Big electric motors, such as those found in large worksite or workshop equipment, will require three-phase power and often large standby three-phase generators are required to back up office buildings and events.

Himoinsa Three Phase Standby Diesel Generator

There are certain three-phase generators that also provide some single-phase power outlets. However, these outlets are added predominantly for convenience to run additional smaller appliances and typically cannot draw as much power as the 3 phase outlets, i.e. it's primarily a three-phase unit with some capacity to run single phase appliances at a lower draw. Many quality 3 phase portable generators will have this flexible outlet configuration and in 3 phase stationary sets outlets can be arranged to fit the required application on request.

There is a whole range of 3 phase generators that are designed and engineered especially for industrial, commercial and rental applications. Check the Three Phase Petrol Generator options, or Three Phase Diesel Generator Range at My Generator.

DISCLAIMER* Please note, this advice is general in nature and we strongly recommend consulting the product manual and where relevant, a professional installer.

5 thoughts on What's the difference between single phase and three phase power?

21 April 2018
I'm planning to build an internet cafe consisting of 10pc(approx500watts each)=approx 5kw 1 airconditon 1hp=appro700-800watts Lighting excluded Can i use 2 single phase alternator which is connected to a flywheel whereas the fly wheel is connected to a single phase induction motor.thanks

13 April 2017
Hi, I have a rental accommodation that has a total power needed by appliances calculated totaled 3000 watts (20 LED lights, 2 water pressure pumps and 8 table fans). I want to buy a single phase 8.5 kva generator with an automatic switch transfer. Is it risky to operate a generator which provides twice the power needed?

My Generator Response
Hi John, for a petrol generator, this should not be an issue. If it was a diesel generator, then issues relating to engine glazing can occur if the power load is constantly less than 30-40% of the unit's power capacity. Hope that helps, thanks.

30 December 2016
hi sean, i have this mini hotel for around 6rooms. and each have tvs, refrigerator heaters and lightings. what should i used a 1phase and 3phase generator?? thank you

My Generator Response
Hi Rolly, you would need to engage an electrician to advise what type of power you need and how much power you need. Once you have this information, you can then get the right generator for your hotel. Many thanks

16 September 2016
An electrician wouldn't know very much about calculating the potential of a generator's alternator, and every product is different. For example..Generac's alternator and AVR on their small commercial 15kW genset would have different characteristics from a comparable Cat, Kohler or Cummins. In addition to the AVR and alternator, excitation circuits play a large role in maintaining the output capabilities under load. In addition, the engine also comes into play. For example you'll be able to block load much more on a diesel genset than a LP or NG genset because of the combustion potential impact on the engine's torque. Short answer...contact a generator manufacturer. At 15kW you're looking at an investment of several thousand bucks..more depending on how good of a genset you want. Most generator manufacturers will size a genset for you and many have sizing software on-line to help you with questions, just like this one. An electrician can tell you about amps and volts...but gensets go into kVAR, kVA, kW, inductive reactance, frequency & voltage stability AND mechanical considerations that most highly skilled "electricians" just don't specialize in. In fact most professional engineering firms turn to generator company application engineers when they design emergency standby power systems. With that, you're better off asking the people that build emergency standby power generation systems for a living, for a hand. They will be glad to lend you one.

16 June 2016
Hi Sean I am in the process of investing in a 15kva genset for my farm house. The genset will be used for lighting, water heaters, refrigerators and maybe a couple of airconditioners. The genset will also power a three phase submersible 5hp motor. I would like to know whether there are any issues in balancing the load between the three phases. Will there be any problems if only the lighting phase is used or is it necessary to keep the other phases on some load as well. With regards and thanks

My Generator Response
Hi Appaiya, bit of a tricky one. You may have to engage an electrician to gauge a reading on your single phase home appliances. A Three Phase generator can provide single phase power, but this is a third of the capacity of the peak and running three phase power rating - whether this will be enough for your single phase appliances may be best decided by an electrician. Thanks

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