Three-phase testing

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This article covers basic testing of three-phase power and equipment. For general information on the mathematics behind three-phase please see three-phase. For information on where how and why three-phase is used see three-phase electric power. Testing, especially of energized equipment, requires suitable safety precautions.

[edit] 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 may not start or may overheat if the supply voltage is not balanced on all three phases.

Voltage of the supply varies by country. Typical nominal line-to-line voltages may be 208V, 230V, 400V, 480 V, or 600 V. Between each phase and neutral will be 120 V, 230,277 V, or 347 V. Each voltage should be within 5% of the nominal supply, and each phase to neutral voltage should be within 1% of the average phase to neutral. Where the supply is fed froma delta connected transformer, the line to earth ground voltage may not be balanced but phase-to-phase voltages must be balanced for proper motor operation.

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. If the supply has a neutral, A and earth, B and earth, and C and earth.

[edit] 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 resistance and testing earth fault resistance are covered separately.