Phase Angle Reference Descriptions
As documentation concerning phase angle relationships begins, I choose to refer to a statement written from an outside engineering firm.
Azima|DLI Engineering: The Concept of a Phase
“Phase is a measure of relative time difference between two sine waves. Even though phase is truly a time difference, it is almost always measured in terms of angle, either degrees or radians.”
The phase difference between two waveforms is often called a phase shift. Phase shift may be considered positive or negative, i.e., one waveform may be delayed relative to another one, or one waveform may be advanced relative to another one. These conditions are called phase lag and phase lead respectively”
“The phase angle can be measured from the reference position either in the direction of rotation or opposite to the direction of rotation, i.e., phase lag or lead, and different equipment manufacturers use different conventions”
What I desire is to focus on is the last statement, “that different equipment manufacturers use different conventions” to describe the phase relationship between two or more sine waves. This philosophical difference has appeared to ENOSERV directly as we work with different test set manufacturers. As a company, we had to deal with this question as the application was written 20+ years ago. Before we go on though, let’s establish what the general concern is from some of our customers, and that is use of Lag terminology (phasor (s) described in reference to the opposite direction of rotation) in RTS vs. Lead terminology (phasor(s) described in reference to the direction of rotation) that may be displayed on test equipment.
The question above just become overwhelming, so let us start with the basics and build up to answer this question.
First, a waveform is used to display the action of one or more sine waves. It displays the action of the sine waves over a period of time.
This view has its value for an overall picture of how the sine waves are reacting over a period of time, but is difficult to see the specifics of what is occurring at a specific moment in time. To do that, we can select any instant in time
and represent that moment on a polar chart for clarity. As this chart is generated, we must choose a reference point 0 on the chart. This is typically at the 12:00 position or the 3:00 position. In the graph below, the reference angle 0 has been selected at the 3:00 position.
Once the reference 0 has been established, all phasors will be drawn from reference 0. To this, we also need to determine a standard in which rotation will be established. Rotation is generally established as a CCW direction. From here phasors will be added per this direction from reference 0.
This is where the problem begins. We have seen 4 specific methods in which our own industry has elected to describe the relationship of a phasor angle value to reference 0.
Engineering standard, with angles entered with values between -180 to 180 Altered standard, with angles values allowed between -360 to 360 degrees All values entered between 0 – 360 as a Leading Angle
Studying these graphs, you will see that all phasors are in the exact same location, only the angle descriptions are listed completely differently. The interesting thing is that they are all correct! How could this be? It is because a decision was made ahead of time as to how the angles would be referenced from 0, using one of the 4 methods described above.
For the engineering standard, angles will always be described as a value between -180 to 180, with a (-) stating the angle is lagging reference 0 (opposite direction of rotation) and (+) stating the angle is leading reference 0 (direction of rotation).
If angles are described as leading values from 0-360, each angle is drawn from reference 0 in the direction of rotation.
If angles are described as lagging values from 0-360, each angle is drawn from reference 0 in the opposite direction of rotation.
Within our own industry, test set manufactures made this determination of how angle values should be interpreted.
Doble 6000 – Display angle values from 0-360, leading reference 0 AVO/Megger – Display angle values from 0-360, lagging reference 0 Omicron – Display angle values from -360 to 360, (-) lagging reference 0 ISA – Display angle values from -180 to 180, (-) lagging reference 0 Manta 1700 – Display angle values from 0-360, lagging reference 0
Manta 5000 – Display angle reference a configuration option. You decide what you desire to see.
SMC – Display angle values from 0-360, lagging reference 0
ENOSERV had to make this decision 20+ years ago as to what would be the standard angle reference for this product. The decision was made to enter all angle values from 0-360 as a lagging quantity. Once this was established, the pattern cannot be changed, as any change would drastically affect existing test procedures.
When writing or examining any RTS test procedure, you must evaluate and enter all phase angle values under this principle, as a positive value from 0-360, lagging reference 0 (opposite direction of rotation). This is true regardless how the test equipment will ultimately display angle values. What I find most helpful is to either draw a vector diagram or table so that you have a reference between the entry in RTS to the value that will appear on the equipment in question. Below is an example of what to expect.
RTS Entry Doble 2000 Omicron Display Doble 6000 Display All AVO/Megger, Manta 1700, and SMC Displays ISA Display 0 0 0 0 0 75 -75 or 285 285 75 -75 90 -90 or 270 270 90 -90 120 -120 or 240 240 120 -120 210 -210 or 150 150 210 150 240 -240 or 120 120 240 120 270 -270 or 90 90 270 90
If applying present angles, expectations can generally be evaluated through internal evaluation. If running a test procedure where the angle(s) will be rotating, it becomes difficult to determine expectations without use of some type of reference information.
RTS Entry Doble 6000 Display
Starting Angle 165 to 195 to
Ending Angle 205 155
Regardless of your own personnel preference or belief as to what is acceptable, understand there is a choice.
ENOSERV made its choice years ago, establishing a pattern that worked well with all equipment marketed in the United States at that time (AVO, Doble, and Manta). Our software aligned with methodology of the AVO and Manta equipment, as well as the display on the Doble 2000. We sent all our values as a negative to satisfy the requirement of the Doble, thus its display would match the values of the ENOSERV product.
Along the way, things have continued as such as more equipment was created. The only unit that has taken a unique display direction is the Doble 6000.
For additional assistance, we have included a polar graph from Keuffel & Esser Co that displays both lead/lag values that could be helpful.
