3 Three Phase Loads

All three phases are typically used in large industrial motors, or efficient air conditioners (e.g. most York units above 2.5 tons are available in 3 phase) as this is the most efficient way to transmit large amounts of electrical power. The greatest power demand is when starting the motor.

Some devices are made which create an imitation three-phase from single phase center-tapped power (240 volts AC in the United States; with phase separation of 180°). This is done by creating a third "subphase" between the two polarities, resulting in a phase separation of 180° - 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.

4 Two phase

Sometimes single phase center-tapped ( split phaseSplit phase is a mains electrical supply system mainly used in America (where it is usually and incorrectly referred to as two phase). It is used occasionally in the UK in distribution situations where only two 11 kilovolt phases are available, but it is) 240 Vac is incorrectly referred to as "two-phase". It should be noted that a two phaseTwo-phase electrical power was used in some early 20th century factories and the distribution systems that served them. It utilized two phases, 90° apart, and was usually supplied using four wires, two for each phase. Less frequently, three wires were use system is a system in which the two voltages are 90° out of phase. For example, if one is and the other is , where t is time, then you have a two phase system, also known as a quadrature system (one being referred to as the real part and the other being referred to as the imaginary part). A two phase system for 120 Vac line to neutral will measure approximately 169.7 Vac line to line. Two phase systems are seldom used for high power because they require the same number of hookup wires as three-phase (i.e. one for sine, one for cosine, and a common wire) delta connection, and the two phase system also does not balance the same amount of electricity in each of the three wires (although the cosine and sine are balanced, the neutral is not the same as the other two). A two-phase system is said to provide complex power and such systems are used at lower voltages (e.g. for communications applications, or to run stepper motors, and the like) but not commonly distributed at high power levels.

5 Complex Power

If we plot phasors of a two phase or three-phase system around the unit circle in the complex plane, we have a form of complex power.

A single phase 240 Vac split phaseSplit phase is a mains electrical supply system mainly used in America (where it is usually and incorrectly referred to as two phase). It is used occasionally in the UK in distribution situations where only two 11 kilovolt phases are available, but it is (center-tapped) power system, when plotted as phasors on the complex plane, can exist entirely along the real axis. It is this lack of complex power capability that impairs its ability to create a rotating magnetic field, and it is the rotating magnetic field that makes motors run very efficiently. For heating, such power is fine, but for example, running an air conditioner, it is far better to use complex power.

6 Testing

Take GREAT CARE when testing anything at mains voltages (or higher) and if you are unsure what you are doing and how to use your equipment safely then DON'T DO IT.

6.1 How to test three-phase electrical supply

A three-phase electrical supply consists of three active conductors and an earth ground.

A three-phase induction motor cannot function correctly if its electrical supply is not within certain parameters.

Typical parameters are 208 or 415 volts between phases, 120 or 240 volts from any phase to earth or ground, voltage within 12% of nominal values, and each phase within 5% of each other.

In a typical three-phase induction motor circuit, an appropriate place to test is at the line side of the direct-on-line motor starter.

Figure 1:

A B C earth/ O O O ground / / / / / / O O O

Tests should be made between A and B, A and C, B and C, A and earth, B and earth, and C and earth.

Note that listed voltages are for countries which use 120 or 240 volts only!

6.2 How to test three-phase devices

Three-phase pumps, compressors, and the like, must be connected in proper phase sequence to avoid damage. Typically such devices will draw less current when connected wrong, and can be easily checked with an amprobe (clip on ammeter) for current draw. For example, testing an air conditioner that has a scroll compressor, one will find that if it's hooked up in the wrong phase sequence, it will draw too little current, and thus any two wires can be switched to change the phase sequence.

There are special small pocket sized motors that are used to check phase sequence by checking the direction of rotation of the motor. These are expensive. A cheaper alternative is to use three neon lights, and whip the head (or eyes) in a quick motion past them to see which way the phase sequence is going.

Topics including testing motor coil resistanceA three-phase induction motor has internal coil windings. These windings must remain the correct value for the motor size and coil configuration, and they must remain the same value as one another. Typical values are six ohms between all phases. In a typi and testing earth fault resistanceA three-phase induction motor has conductors which are insulated to withstand the full rated supply voltage. This insulation may break down so that the insulation resistance falls below certain values. The conductors must have an insulation resistance of are covered separately.

7 3 Phase electrical outlets

The image on the left shows a 3 phase socket. The one on the right a triplex outlet with neons in the top sockets for phase indication. Both are american. if you look carefully you can see that underneath the three phase socket on the left there are breaker positions marked. These are 8, 10 and 12 representing the red, black and blue phases respectively.

Three phase power can be supplied either by the use of a three phase socket, or by triplexing. Most north american receptacles are duplex receptacles. The top and bottom sockets can also be separated, if desired, and, for example, supplied by separate breakers with a common neutral. This is typically done in kitchens where a high load will likely be placed on both sockets. In this case, a common trip 2pole breaker is often used.

The concept of duplexing can be generalized to triplexing, so that three duplex receptacles can be supplied by a common neutral, from a 3phase supply. Typically, a 3 pole common trip 15Amp breaker is used to supply such a socket. This enables three single phase loads to be supplied in a phase-sequenced manner. An example of such a load is a light fixture having three bulbs. For flicker-free operation, three bulbs are each fitted with a separate plug, and driven 120 degrees out of phase with one another, from a triplex receptacle. The top receptacles shown in the figure, are fitted with neon night lights to indicate phase sequence, for triplex loads where proper phase sequence is desired.

8 See also

• single phase electric power
• alternating-current electric power
• polyphase systems.
• For further information on three-phase circuits see: Y-delta transform

Electric power Electrical engineering

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