Purdue University
Purdue e-Pubs
International Compressor Engineering Conference
School of Mechanical Engineering
1996
Three Phase Hermetic Protector Application
Process
J. Petraitis
Texas Instruments
Follow this and additional works at: http://docs.lib.purdue.edu/icec
Petraitis, J., "Three Phase Hermetic Protector Application Process" (1996). International Compressor Engineering Conference. Paper
1192.
http://docs.lib.purdue.edu/icec/1192
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THREE PHASE HERMETIC PROTECTO R APPLICAT ION PROCESS
Jeff Petraitis
Texas Instruments
ABS1RACT
The application of three phase hermetic protectors involve several factors. Information provided
by a compressor manufacturer normally involve the three phase locked rotor current, the rundown trip
current, and the estimated effective protector ambient. Accurate values of these parameters can aid in
finalizing a protector selection, but these parameters are not the only ones that need consideration. A key
item in the selection process that is not always fully understood up front is single phasing (secondary or
primary). Many times the success of meeting the application requirements hinge upon the ability of
passing these tests. The purpose of this paper is to increase the awareness of this issue, and what
directions can be taken to minimize the number of samples with the protector manufacturer, motor
supplier, and compressor customer as well as the time needed to obtain an acceptable application.
INTRODUCTION
Single phasing requirements are usually the most difficult to meet in comparison to the other
application parameters. The reasons for this involve lower locked rotor current in comparison to the three
phase locked rotor and that all of the phases of the motor are not fully energized. This results in a slower
response time in the protector and subsequently higher motor winding temperatures. High motor winding
temperature can become the negating item under single phasing conditions. The difficulty becomes in
what adjustments should be made in the protector selection that will be beneficial in reducing the motor
winding temperature to an acceptable level. Sometimes multiple sample iterations fail to produce
acceptable results and in turn delay the application approval cycle.
PROTECTOR SELECTION PROCESS
The performance of three phase hermetic motor protectors are dictated by the resistive elements
in the assembly. These elements are the bimetal disc and heater. Current passes through both of these
components and by varying the materials, different trip time responses will resUlt from both a low current
condition (rundown) and a high current condition (locked rotor). The protectors are calibrated to a
specific opening temperature (typically from 100 to 170 degrees C). This allows the device to be
temperature sensitive as well as current sensitive. The contribution of each characteristic is dictated upon
the fault condition on the compressor. Locked rotor performance is mostly dependent upon the current
carrying resistive components, while loss of charge is relevant to the opening temperature of the protector.
Rundown or ultimate trip performance is due to a combination of both the opening temperature and the
resistive elements. The selection of a protector for an application is dependent upon the opening
temperature, disc, and heater.
Graphs 1 and 2 are indicative of typical performance curves employed when making an application. The various curves are driven by the heater and disc components and are depicted by the material
resistivities. The ultimate trip curves (Graph 1) are used for rundown performance. Selections are made by
determining the delta temperature from the trip current and disc/beater combination. The delta temp. is
added to the effective protector ambient (EPA) of the compressor to determine the opening temperature of
the device. The three phase trip time curves (Graph 2) are used for approximating the first cycle trip time
performance of the device under three phase locked rotor conditions. These curves are typically compiled
at various opening device temperatures. The curves in Graph 2 are at 135 degrees C opening temperature.
735
APPLICATION EXAMPLE
The ultimate trip and three phase trip time curves are what is usually used in making an initial
sample selection for a motor application. An example of the characteristics given from a compressor
manufacturer for a 230 volt application are as follows:
Rundown: Must trip at 23.0 amps, 45 Degrees C ambient
Three Phase Locked Rotor: 90 amps trip in less than 5 seconds
The ultimate trip curves in Graph 1 indicate the top three curves intersect the 23.0 amp reading
and are considered as possible selections. However, the top two selections from the ultimate trip curves
have long trip times when checked at 90 amps on the three phase trip time curves in Graph 2 (11.5 and
7.5 seconds). The third selection has a trip time of 3.6 seconds at 90 amps and would meet the trip time
requirement specified. Checking the ultimate trip curve of this choice (125 Ohm/CMF disc and 150
Ohm/SMF heater), indicates that a trip of 23.0 amps would occur at a delta temperature of 90 degrees C.
Adding the EPA of 45 degrees C to the delta temperature of 90 degrees C will result in an opening
temperature of the protector at 135 degrees C. This is the protector selection that was built and sent to the
compressor manufacturer for application evaluation.
The compressor manufacturer conducts various application tests upon receiving the protector
samples. Application test data from the compressor is noted below to demonstrate how well the initial
sample selection meets the requirements. The parameters that have been highlighted in bold type are the
requirements specified by the compressor manufacturer to be met
135 Degrees C Open Temp.
125 Ohm/CMF Disc
150 Ohm/SMF Heater
Initial Sample Selection: Test #1
Rundown: Must trip at 23.0 amps, 45 Degrees C EPA
Must trip voltage of less than 187 volts
22.7 amps
167 volts
Three Phase Locked Rotor: 90 amps trip in less than 5 seconds
1st cycle trip time
1st cycle off time
nominal test voltage
peak motor temp. (350 degrees F max:.)
85 amps
4.2 seconds
0.53 minutes
230volts
244 degrees F (4th cycle)
Secondary Single Phase:
amperage unknown
1st cycle trip time
1st cycle off time
nominal test voltage
peak motor temp. (350 degrees F max.)
72amps
10.5 seconds
0.75 minutes
230volts
292 degrees F (3rd cycle)
Primary Single Phase:
amperage unknown
1st cycle trip time
1st cycle off time
nominal test voltage
peak motor temp. (350 degrees F max.)
86/43 amps
11.0 seconds
1.08 minutes
200volts
395 degrees F (3rd cycle)
Loss of Charge: 350 Degrees F maximum motor temp.
736
293 degrees F
The application test data indicates that all the results are acceptable except for the motor winding
temp. from primary single phase. The single phasing currents were not originally known, but could
have
been estimated. Secondary single phase current is approximately 86.6% of the nominal three phase
current. Primary single phase current is approximately 100% of the three phase current in the unshorted
phase and 50% of the three phase current in each of the shorted phases. The difficulty becomes
in
predicting the protector trip time response under these conditions. Graph 3 shows secondary single phase
trip time responses and is an additional item that can be employed in making applications.
A revised sample selection can be produced based on the original motor test data and the use of
the secondary single phase trip time curves. The rundown current can be reduced by 2.0 amps since
the
protector trip occurred at 167 volts. Reviewing the ultimate trip curves at 21.0 amps (Graph 1), another
option becomes available. The 125 Ohm/C:MF disc and 305 Ohm/S:MF heater curve is nearly 21.0 amps
at
a delta temperature of 90 degrees C. The delta temperature will be increased by 5 degrees C to 95 degrees
C to insure there is sufficient rundown. Checking this new selection on the single phase trip time curve
at
72 amps predicts a trip time of 8.6 seconds. This is an improvement over the original, which the curve
predicted a trip time of 10.5 seconds. The three phase trip time (Graph 2) of the revised selection is
still
acceptable at 3. 7 seconds for 85 amps. The new sample is submitted and the test data is as follows:
Revised Sample Selection: Test #2
Rundown:
140 Degrees COpen Temp.
125 Ohm/C:MF Disc
305 Ohm/S:MF Heater
Must trip at 21.0 amps, 45 Degrees C EPA
Must trip voltage of less than 187 volts
21.2amps
171 to 175 volts
Delta v.s.
Test#!
-1.5 amps
+4to 8 volts
Three Phase Locked Rotor: 85 amps trip under 5 seconds
1st cycle trip time
1st cycle off time
nominal test voltage
max. motor temp. (350 deg. F)
85 amps
3.8 seconds
0.80 minutes
230 volts
228 deg. F (4th cycle)
Secondary Single Phase:
72 amps
1st cycle trip time
1st cycle off time
nominal test voltage
max. motor temp. (350 deg. F)
72amps
8.5 seconds
1.01 minutes
230volts
263 deg. F (3rd cycle)
86/43 amps
1st cycle trip time
1st cycle off time
nominal test voltage
max. motor temp. (350 deg. F)
86/43 amps
10.0 seconds
1.46 minutes
200 volts
334 deg. F (3rd cycle)
-61 deg. F
305 degrees F
+12deg.F
Primary Single Phase:
Loss of Charge: 350 degrees F maximum motor temp.
-0.4 seconds
+0.27 minutes
-16 deg. F
-2.0 seconds
+0.26 minutes
-29 deg. F
-1.0 second
+0.38 minutes
The revised sample reduced the primary single phase motor temperature to an acceptable level
while still meeting the other application requirements. This was achieved by increasing the device
opening temp. and heater resistance to maintain an acceptable trip for rundown performance. The higher
resistance heater allowed the protector to become more responsive and at the same time generate more
heat in the assembly which increased the off time. The initial sample selection that did not meet single
phasing could have possibly been avoided if the secondary single phase trip time curves were used
to
compare potential selections. These curves are a tool that can be used to predict the protector on times.
737
RECOMMENDATIONS
There are several factors in the protector selection process that can help reduce the single phase
motor winding temps. before an application is even evaluated. Incorporating these factors in the selection
will increase the probability of success with an initial sample. The considerations are as follows:
- Employ a low closing device temp. (61 or 57 degrees C nominal). This will increase the off
times when the protector is cycling and will help reduce the peak motor winding temperature.
- Use the option of a higher resistance heater with a higher opening temperature instead of a
lower resistance heater with a lower opening temperature. This is demonstrated in the
application example where the revised sample reduced the on time and increased the off time
which both have an effect of decreasing the peak motor winding temperature. Maximum
opening temperature will be limited by the loss of charge condition.
- Refer to the secondary single phase trip time curves if available to compare potential selections
at 86.6% of the three phase current. If secondary single phase curves do not exist, then single
phase trip times can be approximated by using the three phase trip time curves at 70.7% of the
three phase current The goal is to have secondary single phase trip times less than 10 seconds.
These guidelines should be applied where it is deemed as necessary. Compressor applications
vary and some may not need to be screened to this extent. Past experiences within a compressor model
line can add insight as to what extent these guidelines need to be followed.
CONCLUSIONS
There are areas where all of the parties involved can help meet the requirements for single
phasing. This includes the protector supplier, motor manufacturer, and compressor manufacturer.
- The protector supplier publishes secondary single phase trip time curves (See Graph 3) in
addition to the standard three phase trip time curves on the newer three phase protectors. These
curves are intended to be used in the application process. They provide frrst cycle on times
for a given amperage and can be used in comparing potential protector selections.
- The motor manufacturer should provide motor temperature rate of rise data when submitting a
motor to a compressor manufacturer for application work. This would help provide information
on how quickly the protector would need to trip under single phasing conditions.
- Past application test data from the compressor manufacturer should be reviewed of similar
models to provide insight on how difficult it may be meeting the single phasing requirements.
The type of protector chosen on previous models would also give guidance as to if something
similar may be needed for a new application.
Three phase hermetic protector applications involve passing various tests prior to being released
for production. The ability to complete this cycle on a timely basis can hinge upon minimizing the amount
protector sample iterations needed to complete the application. The level of success can be enhanced by
following the actions outlined in this discussion that will result in the single phasing motOr peak
temperatures being at an acceptable level
738
ULTIMATE TRIP
ULTIMATE TRIP CURRENT VS. DELTA TEMPERATURE BETWEEN
PROTECTOR OPENING TEMPERATURE AND PROTECTOR AMBIENT TEMPERATURE APPROXIMATE,
TO BE USED ONLY FOR SELECTING SAMPLE FOR MOTOR VERIFICATIO N TEST
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