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
All values entered between 0 – 360 as a Lagging 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 2000 – Display angles with a value from -360 to 360, (-) lagging reference 0
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
0
75
90
120
210
240
270
0
-75 or 285
-90 or 270
-120 or 240
-210 or 150
-240 or 120
-270 or 90
0
285
270
240
150
120
90
All AVO/Megger,
Manta 1700, and
SMC Displays
0
75
90
120
210
240
270
ISA Display
0
-75
-90
-120
150
120
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.
Starting Angle
Ending Angle
RTS Entry
165 to
205
Doble 6000 Display
195 to
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.