
The Difference Between Single vs. Three-Phase AC Voltage

Single Phase Power is Limited
Three Phase AC Voltage is Typically Used for Higher Power Loads
How to Determine Required Line to Line Voltages
The 120° phase shift between each waveform can be done in one of two phase rotations – A -> B -> C or A -> C -> B. Phase rotation does not affect most loads except for three phase AC motors which will turn in opposite direction if the phase rotation is changed. Changing phase rotation can be accomplished by swapping any two of three phase connections. When using a programmable AC power source like the AFX Series, phase angles for phase B and C can be programmed to either 120° and 240° or 240° and 120° respectively to change phase rotation. The AFX also allows phase imbalances to be programmed to study the effect of phase variations on a unit under test.
Delta and Wye Relationship
While the ‘normal’ three phase delta and wye voltage relationship is easily captured in a simple formula, this applies only with equal line to neutral voltages, perfect phase balance and sinusoidal voltages. For this ideal case, the relationship between Line to Neutral RMS voltage and Line to Line RMS voltage can be expressed by the following formula:
Figure 4 below shows two typical examples of three phase utility grid voltage configurations used in the United States. Europe and Asia typically use 220/380V or 230/400V configurations instead. The 120VLN per phase is equivalent with the 208VLL vector sum:
VLL = 120VLN * 1.732 = 207.84VLL
Note that the 480V delta grid configuration has no neutral connection and is referred to as a 3Wire + Ground Delta connection. To simulate this type of grid with an AC power source, the 3 phase load is connected as a delta between the three output phases only with no connection to the neutral output terminal.
This ratio of √3 is important when using a programmable three phase AC power source as all T&M style AC power sources are only programmable in Line to Neutral voltage.
Thus, if any of the stated conditions are not true, you cannot just rely on this formula to determine Line to Line voltage:
- Identical VLN voltages on all three phases
- Balanced Phase angles on phase B and C
- Low distortion, pure sinewave
Why Does it Matter?
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When testing three phase loads, pay close attention to voltage and phase parameters when making assumptions about the Line to Line voltages applied to the unit under test. If you need help, contact our experts in test by chat, email, or phone.