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International Compressor Engineering Conference
School of Mechanical Engineering
1998
Study of PTCR Behavior for Wide Voltage and
Ambient Temperature Variations and Optimizing
the PTC Start Induction Run Motor & Overload
V. K. Rao
Voltas Limited
Allwyn Unit
Voltas Limited
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Rao, V. K. and Unit, Allwyn, "Study of PTCR Behavior for Wide Voltage and Ambient Temperature Variations and Optimizing the
PTC Start Induction Run Motor & Overload" (1998). International Compressor Engineering Conference. Paper 1271.
http://docs.lib.purdue.edu/icec/1271
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STUDY OF PTCR BEBAVIOR FOR WIDE VOLTAGE AND AMBIENT
TEMPERATURE VARIATIONS AND OPTIMIZING THE PTC START
INDUCTION RUN MOTOR & OVERLOAD.
AUTHOR
V. Koteswara Rao
ALLWYN (A unit of Voltas), Hyderabad- 500 018, India
ABSTRACT
Refrigerator compressor motors using PTC as starting device are in usage
since years. Special care should be taken to optimize the start winding of the
PTCS-IR compressor motor for wide voltage and ambient temperature range
applications and also to select the proper overload protector as the characteristics
ofPTC vary with both voltage and ambient temperature.
This paper presents the study of PTCR behavior under a wide range of voltage
and temperature . Critical values of the PTCR characteristics
considered for optimizing the motor and overload protector.
have to be
NOMENCLATURE
PTC RESISTANCE ( Ro)
RESISTANCE OF PTC RESISTOR AT 25°C
SWITCHING TIME (Ts)
TIME TAKEN FOR PTC TO REACH ITS
SWITCH TEMPERATIJRE (OR)
TIME TAKEN FOR PTC TO CHANGE
FROM CONDUCTING STAGE TO NONCONDUCTING STAGE
RECOVERY TIME (Tr)
TIME TAKEN FOR PTC TO RETURN TO
TWICE THE INITIAL RESISTANCE@
25°C AFTER SWITCHING
391
INTRO DUCTI ON
The PTCR ( positive temperature coefficient resistor ) start Induction run
motors used for refrigerator compressor's application are generally designed to
operate in a particular range of voltage and ambient temperature. Generally the
voltage range considered is ± 10% of rated voltage but for tropical conditions the
voltage variation to be considered is ± 25%. Variation in the ambient
temperature to be considered is from l0°C to 50°C. This wide variation of
voltage and ambients calls for special care to be taken while designing the PTC
start Induction run motors and selecting the overloa d Otherwise the reliability of
compressors gets adversely affected..
When the voltage is varying the switching and recovery times of PTC vary.
Also, the ambient variations cause some variations in the switching and recovery
times of PTC. Specific characteristics of PTC are largely governed by the
combined effect of voltage and ambient temperatures.
A perfect match of motor overload and PTC as a combination works reliably
on the refrigerator at different combinations of voltage and ambient temperatures.
Experim entation
Different sets of experiments are conducted to detennine the effect of voltage
and ambient temperatures individually and in combination on the characteristics
ofPTC.
Simulation of approximate start winding resistance and inductance in series
with the PTC to evaluate the behavior at different voltages at constant ambient
temperature is done. The same set-up is used for evaluating the behavior of PTC
at different ambient temperatures at constant voltage. Then simulating the
different combinations of voltages and ambients the PTC switching and recovery
time's characteristics are recorded. Storage Oscilloscope is used for recording the
characteristics of PTCR in the experiments. The observations of experiments are
listed in table 1. 0.
Considering the critical characteristics of PTCR two motors 118th HP and
116th HP are designed with optimized start winding for refrigerator applications
and also selected the overload protector to suit PTCR behavior. These
compressors fitted in refrigerators are tested for startability at 4 3oc ambient after
392
pulldown to prove the matclring of PTCR and motor design and also compared
with the RSIR motor with electromagnetic relay. Refrigerators are also tested for
power interruption & locked rotor tests at different voltages to see the matching of
overload protector. The observations of experiments are listed in table 2.0.
Description of results
As shown in the fig.1 the switching time of PTCR varies with the ambient
temperature and voltages. The combined effect is that at -1 ooc & 160 Volts the
switching time is maximmn of 2.4 seconds and at 80°C & 260 Volts it is
minimum of0.385 seconds.
·
As shown in fig.2 the recovery time of PTCR varies with the ambient
temperature and voltages. The maximum recovery time that PTCR takes is
another important characteristic of PTCR to be considered for optimizing the start
winding and overload. In this case the maximum ambient temperature ofPTCR
considered is 80°C because the shell temperature of compressor when running in
high ambients like 43°C I 50°C will be about 85°C to 90°C. The maximum
recovery time of 185 seconds recorded is at 80°C & 260 Volts and the minimmn
of 42 seconds at -10°C & 160 Volts.
Analysis
The minimum switching time of0.385 seconds will become only critical if the
motor is not acheiving the stable condition before this time. The motor is
designed to acheive the stable condition before this minimum switching time.
The maximum recovery time obtained from the experiment (table.1.0 & :fig.2) is
185 seconds at 80°C ambient & 260 Volts. The resistance of the PTC after 185
seconds is the recovery resistance. Therefore recovery resistance is considered
for calculating the starting torque and optimized the start winding to give
required starting torque at 260 Volts. The minimum restarting time allowing the
system to balance is generally taken as 3 minutes, which is more than the
minimum recovery time of 42 seconds recorded at -1 ooc & 160 Volts. However
to enable the compressor to start at 160 Volts after system balancing time and
80°C PTC ambient, motor is designed to produce required minimum starting
torque at 160 Volts taking the PTC recovery resistance into considerations (not
PTC resistance). Therefore the compressor starts between any voltage from 160
Volts to 260 Volts after 3 minutes at any ambient between -10°C to 50°C.
393
The overload characteristics are nmed to reset only after 185 seconds (maximum
recovery time obtained from experiments) so that the compressor (PTC) will be
ready for starting between any voltage ranging from 160 Volts to 260 Volts, after
tripping in case of power interruptions in pulldown test or after ultimate trip.
CONCLUSION
The compressors are made with new design motors of 1/8 hp & 1/6 hp. Tests
on appliance are conducted with PTC and Overload . Appliance tests for low
voltage running, startability and high voltage running,startability are conducted.
Reliability tests under extreme conditions are also conducted. The performance of
these compressors is found satisfactory in all above tests for the variation of
voltages from 160 to 260 Volts and ambient temperatures from l0°C to 50°C.
Depending on the PTCR behavior for different voltage and ambient variation.
The maximum recovery time obtained from similar experiments the motor start
winding and overload can be optimized.
ACKNOWLEDGEMENTS
1) M/s Annapuma Electronics , Hyderabad, India
2) M/s Siemens, New Dellii, India
3) M/s Texas Instruments, Singapore
REFERENCES
1. Robert F Blaha " The PTC Resistor ". 21st Electronic Components
Conference, 1971.
2. Veinott, C. G. "Theory and Design of Small Induction Motors"_ Mc.GrawHill
publications, 1979.
3. William R Hoffmeyer. ~~A Motor designer looks at Positive Temperature
Co-efficient Resistors". General Electric Company.
394
PTCR SWITCHING TIME RECOVERY TIME CHARACTERISTICS AT DIFFERENT
VOLTAGES AND AMBIENTS
VOLTAGE SWITCIDNG TIME=11ILLI-SECONDS RECOVERY TIME (SECONDS)
-10°C 10°C 25°C 5ooc 1 sooc -10°C l0°C 25°C 50°C 80°C
160
42
50
I 2400 2110 1937 1500/ 935
81
104
144
230
1200 1092
890
600\ 480
48
60
89
117
170
260
960
813
685
500/ 385
52
65
99
134
185
TABLE 1.0
STARTABILITY TESTS ON REFRIGERATOR
RELAY
RESTART TIME (SECS)
VOLTS\TIME 60 120 150 180
160
NS OK
OK OK
170
OK OK
OK OK
ELECTRO
180
OK OK
OK OK
MAGNET
190
OK OK
OK OK
-IC
200
OK OK
OK OK
RELAY
220
OK OK
OK OK
230
OK OK
OK OK
250
OK OK
OK OK
260
OK OK
OK OK
PTCR
RELAY
NS
NS
NS
NS
NS
NS
NS
NS
NS
160
170
180
190
200
220
230
250
260
NS
NS
NS
NS
NS
OK
OK
OK
OK
TABLE2.0
OK:COMPRESSORSTARTED
NS : COMPRESSOR NOT-STARTED
395
NS
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
OK
PTCR AMBIENT TEMP VS SWITCHING TIME
CURVE AT DIFFERENT VOLTAGES
00. 2500
Q
z
0
\,.)
2000
'"•"
00
~
~
::2 1500
-+- 160 VOLTS
~
--230VOLTS
......,....260VOLTS
-
::2
'"" 1000
f-4
-=~
500
~
··;,c'··
..
.. .
~
0
-I0°C
-~
·:.. ·.
I0°C
50°C
25°C
800C
AMBIENT TEMPERATURE
FIG. 1
PTCR AMBIENT TEMP VS RECOVERY TIME CURVE AT
DIFFERENT VOLTAGES
zoo~~--~--~~~~~~~----~~~
180+;;~~~~:~;~~~;;~.;,.;;~+-~-~~;.~.~.;,~¥,~~~~.;~;,0·:~+~,;<:;;·<:~·~'~''·~·+····'··~~
00.
1604~•o4~~;0;UL;,;,::;f~0~~~~~~,.;;,~~;~;.;~:~~·~+~~~;~,;~~~~;~;~,~~·~r~<l
~140~)2.~;;~;~d~:~;.~..c•••;,,~;~~.;;~~;~~~)l.~,""0;;~,.;;~;,;,;~;""~~~'~'~"·~'~'74
0120~·>.:..:;";.::;c,c:c.,;o;.;;;,;.c;,,;,.~c:~:c ...•..cc.c,~:.,~ .. ,.•..,,.•. +,~~••0 ::... ~
~
00.}00~,~~~;~~.,o;.; __ ,.~~~~;~··~"';,;;~;;~.;;~;-~•+0~;~~~
~ 80 -~~;~;_,~~;.,;",;''·"''··~""·'··'·'·''•'·'··'·'""""'"""~:;;~,.;;;;.,:._;;~;:..~;;e.;,;;,;__,,;;,;,;";';,"''''•'"~"·.:.:l
::2
'"" 60
~
40
20
0
-10°C
10°C
50°C
25°C
AMBIENT TE:MPERATURE
FIG.2
396
80°C
-+-160 VOLTS
-D-230VOLTS
._...,_260VOLTS