IES Paper # 48
A LAMP LIFE PREDICTOR FOR FREQUENTLY SWITCHED INSTANT-START
FLUORESCENT SYSTEMS
N. Narendran, T. Yin, C. O’Rourke, A. Bierman and N. Maliyagoda
Lighting Research Center
Rensselaer Polytechnic Institute, Troy, NY 12180.
Tel: (518) 276 4803; Fax (518) 276 4835
ABSTRACT
Although T8 fluorescent lamp life is typically rated at 20,000 hours, it can significantly vary depending on
the ballast product. The current industry practice for testing fluorescent lamp life requires these lamps to be
subjected to 3-hours on/20-minutes off cycle, which is also known as the standard cycle for life testing.
This method requires over 2 years of testing, which is expensive and also delays the development of new
products. Therefore, researchers have been trying to expedite the procedure for lamp life testing by
increasing the switching frequency. However, none of these attempts have shown much promise for
correlating the lamp life on faster cycles to the lamp life on the standard cycle. Alternatively, researchers
have been trying to correlate certain starting and operating electrical parameters of the lamp-ballast system
to lamp life. As an example, it has been shown that the ratio of the hot cathode resistance to cold cathode
resistance correlates well with lamp life for rapid-start ballast fluorescent systems. Instant-start ballasts
comprise a larger market share than rapid-start ballast. A literature survey shows that at the present time
there is no such parameter available for predicting lamp life of instant-start systems. It is also shown in
literature that within a range, lamp life shortens with increasing switching frequency. At very fast switching
frequencies such as the 5-minutes on/5-minutes off cycle, the major portion of the electrode damage takes
place due to starting, which ultimately leads to lamp failure. At standard operating cycles, the electrode
damage takes place during starting and operating periods. To predict lamp life on any switching cycle, one
has to know the amount of electrode damage that takes place due to starting and the amount of electrode
damage that take place due to normal operation. This study deals only with starting effects. The goal of this
initial study is to analyze the starting electrical properties and identify a parameter that correlates well to
high-frequency switching life, which provides insight to the amount of electrode damage that takes place
during starting. It is shown here that the time integrated value of the lamp voltage over the starting
start
period,
³ Vdt , correlates well with high-frequency switching life for the lamps tested. The details of the
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experimental study and the results are explained in this manuscript.
INTRODUCTION
Of all the different ballast types available for linear fluorescent lamps, instant-start ballasts are the most
commonly used, over 70%, especially with F32T8 lamps [1]. The widespread use of instant-start ballasts is
mainly attributed to lower cost, lower energy consumption, and its ability to start lamps without delay.
Over the past several years, the number of manufacturers supplying instant-start ballast products to the
marketplace has been growing steadily. Although having many choices for ballast products is a positive
trend for the fluorescent lamp industry, it is confusing for the end users who have to select a suitable lamp
and ballast system for their target applications. Past research has shown that fluorescent systems with
incompatible lamps and ballasts result in shortened lamp life [2-8]. Therefore, it is very important to
appropriately combine a lamp with a ballast to obtain longer lamp life. Traditionally, lamp life is
determined by subjecting fluorescent lamps to a 3-hours on/20-minutes off cycle, which is known as the
standard cycle for life testing. Testing fluorescent lamps that have median life of 20,000 hours, on the
standard cycle can take up to 2.5 years. This is expensive and also delays the development and introduction
of new products. Therefore, researchers have been trying to expedite the procedure for lamp life testing by
increasing the switching frequency [9]. However, none of these attempts have shown much promise for
correlating the lamp life on faster cycles to the lamp life on the standard cycle. Alternatively, researchers
have been trying other methods such as measurement of certain electrical parameters, use of laser induced
fluorescent (LIF) or use of x-ray fluorescence measurement techniques to study coating loss from the
1
electrodes, which is then used to predict lamp life [3-4, 6-8, 10-17]. Some of these approaches have shown
limited success while others have shown greater success in predicting lamp life. As an example, it has been
shown that the ratio of the hot cathode resistance to cold cathode resistance, Rh/Rc, correlates well with
lamp life for rapid-start ballast fluorescent systems [4,8,18]. In 1998, Ji et al systematically studied the
Rh/Rc for different lamp/ballast combinations in the market place and showed that Rh/Rc correlated well
with lamp life [8]. D. Hitchcock in 1983, conducted a study with instant-start fluorescent systems to relate
operating electrical parameters to lamp life [7]. However, at present there isn’t any published information
that relates starting electrical parameters to lamp life for instant-start systems. Therefore, an experimental
study was conducted to investigate the possibility of identifying a lamp life predictor for fluorescent lamps
operated on instant-start ballasts by analyzing the starting electrical parameters.
In practice, fluorescent lamps undergo different on-off cycles. As an example, lamps used in open-plan
office spaces, on an average, can be on for 18 hours and off for 6 hours in a day [19], whereas in
conference rooms, on an average they are on for 5 hours and off for 19 hours [20]. End of life for a
fluorescent lamp is defined when the lamp fails to start. Usually, a fluorescent lamp reaches end of life
when the emissive coatings of its electrodes evaporate or sputter away completely. There are times,
especially with instant-start systems, when a lamp would operate even after the emissive coatings have
disappeared. The authors of this paper assume that the time of operation after complete loss of emissive
coatings is small. The depletion of emissive coatings takes place during starting and operating periods. The
rate at which the emissive materials are lost during the starting process is different to the rate at which they
are lost during operation. Assuming that the lamp life is determined by the coating losses taking place
during starting and operating periods only, the total coating loss, L in percentage, can be written as,
L = Ns . D + Nhrs . E
(1)
where, Ns and Nhrs represent the number of starts and number of operating hours respectively, D and E
represent the coating loss per start and coating loss per hour of operation respectively. When L is equal to
100% the lamp reaches end of life. The authors suspect that there may be other factors that contribute to
lamp life, but those factors may be small compared to the ones shown in equation (1). The life of a
fluorescent lamp that operates continuously for a long period depends mainly on the damage that occurs
during lamp operation and the life of frequently switched fluorescent lamps depends mainly on the damage
that occurs during lamp starting. Assuming that D and E are the two dominant factors and knowing their
values for a particular lamp-ballast system one can estimate the life of the system for any on-off cycle.
Subjecting similar lamp and ballast systems to two different on-off life testing cycles, as an example 5minutes on/ 5-minutes off cycle and 3-hours on/ 20-minutes off cycle, one can determine D and
E $ sample estimate of D and E values for six different instant start ballast and lamp combinations is
illustrated in Table 2. These values were calculated from the results of a life study experiment that took 3
years to complete. This process takes a very long time. Ideally, if one can determine the percent damage
due to starting, D and operating, E by measuring certain electrical parameters during these two periods, it
would significantly reduce the time and cost involved in life testing fluorescent lamps. The end use
community can significantly benefit by using this method to select a lamp-ballast system optimum for their
target application. Likewise the manufacturing community can significantly benefit by using this method to
rapidly improve product performance.
EXPERIMENT - Lamp Life Test
As part of our ongoing lamp-ballast system compatibility research, several F32T8 fluorescent lamp and
ballast systems were put on a life test rack in 1996. In that study, several lamp and ballast combinations
were subjected to two different life-testing cycles, namely 3 hours on/20 minutes off and 5 minutes on/5
minutes off. That previous study was aimed at collecting life data for different starting methods with
concentration on rapid-start systems. However, two groups of instant-start systems were included in that
study for comparison with rapid start systems. The two instant-start electronic ballasts, named B3 and B9,
had significantly different life on the two switching cycles and therefore they are the ones of interest for our
study presented in this manuscript. Table 1 summarizes the median life data for the different lamp and
ballast combinations subjected to the 5 minutes on/5 minutes off and 3 hours on/20 minutes off cycle tests.
2
In addition, Table 1 summarizes the number of lamp-ballast systems used in each case. Table 2 illustrates
the calculated D and E values of the six different systems.
Prior to starting the life test in 1996, LRC researchers measured certain ballast parameters for the instantstart lamp and ballast systems. Some of the measured parameters were lamp power, open circuit voltage,
current-crest factor, and ballast factor [8]. Most of these parameters show very little difference and do not
explain the difference in life data for the lamps operating on the two different ballast products. In one case,
the lamp life on B9 ballast exceeded the lamp life on B3 ballast by 290% when they were subjected to the
5-minutes on/5-minutes off cycle. Since the above measured electrical parameters were not capable of
explaining the lamp life differences of the instant-start systems, further research was needed to identify
electrical parameters that correlate well to lamp life. Understanding the reason for the difference in lifetime
and identifying a parameter that correlates well to lamp life for high-frequency switching cycles became the
goal for our study presented in this manuscript. This study is limited to high-frequency switching cycles
because complete life data was available only for the fast cycle when this study was started.
Ballast
Type
Lamp
Median
Lamp
Life (hrs) for 5min on/5-min off
cycle.
Median
Lamp
Life (hrs) for 3hrs
on/20-min
off cycle.
B3
B9
A
B
C
A
B
C
1,487
711
981
1,597
1,191
2,064
n=12
n=12
n=12
n=12
n=12
n=12
24,737
13,842
14,162
21,120
19,536
>25,000
n=4 *
n=6
n=6
n=6
n=6
n=6
Table 1: The median lamp life data for instant start systems, n refers to the number lamps used in each
case. * Due to early ballast failure for two pairs, the sample size is reduced to 4 from 6.
Ballast
Type
Lamp
B3
B9
A
B
C
A
B
C
5.42 x10-5
11.44 x10-5
8.13 x10-5
4.96 x10-5
6.76 x10-5
3.81x10-5
2.24 x10-5
3.41 x10-5
Table 2: Calculated D and E values.
4.35 x10-5
3.08 x10-5
2.87 x10-5
2.70 x10-5
D (%/ start)
E (%/hr)
EXPERIMENT – Starting Electrical Parameter Measurement
Figure 1 illustrates the schematic of the experimental setup utilized for measuring the starting electrical
parameters. All ballasts used in this study are two lamp ballasts. The test setup and procedures conformed
to American National Standard Institute (ANSI) requirements [21,22]. A digital thermometer was placed at
a point not more than 3 feet away from the lamps and at the same height as the lamps to monitor the
ambient temperature. The ambient temperature throughout the measurement period was in the range of
25qCr1qC (77qFr1.8qF). The starting aid distance, 0.5 inch, was kept constant for all the measurements.
3
An ac power source supplied the necessary input voltage, 120 volts r 0.12 volts, at 60 Hz frequency to the
experimental setup. A four-channel digital storage oscilloscope was utilized for capturing starting voltage
and current waveforms from the blue and red wire lamps. Two differential voltage probes (attenuation ratio
– 1/200) were used for voltage measurements and two current monitors (0.1 Volts/Amp) were used for
current measurements.
When power was applied to the ballast the fluorescent lamps were started and the digital storage
oscilloscope captured all the starting waveforms. The digital storage oscilloscope was set to trigger at
160V dc and the sampling rate was set at10 million samples per second. The acquired waveforms were
directly transferred to a computer via IEEE-488 interface and a commercial software package, LabVIEW,
was utilized to further analyze the data.
A total of 6 lamp-ballast systems were studied in this experiment. The ballasts are 2-lamp ballasts and each
system had 3 sets of lamp samples and 3 ballast samples. All ballasts and lamps used in this experiment
were from the same manufacturing batch as the ballasts and lamps that were life tested. The lamps were
seasoned for 100 hours prior to making measurements. The same sets of lamps were used with the various
ballasts. Two lamps were connected to the same ballast for which 4 repeated measurements were made on
each of the 6 ballast samples. Altogether 144 measurements were made for all the lamp-ballast
combinations.
Lamp 1
Ballast
/
Line
1
Regulated AC Power
Supply
Lamp 2
Differential
Probes
&+
&+
&+ &+
Oscilloscope
Figure 1: Experimental setup utilized for measuring the starting electrical
parameters
DATA ANALYSIS
Figure 2 illustrates the ANSI described starting current waveform for the instant-start fluorescent system
[22]. As shown in the figure 2, t0 is the time when power is applied, t1 is the time when glow current first
appeared and t3 is the time when peak of the first half cycle is at least 90% of the waveform and is sustained
at that value. From ANSI definitions the starting time, t, can be written as
t t 3 t1 .
(2)
4
Figure 2: ANSI recommend starting current waveform for the
instant-start fluorescent system [22]
However, in our experiment it was noticed that the starting waveform looks very much different than what
ANSI illustrates. Figure 3 illustrates a sample current waveform obtained from our experiment. The ANSI
illustration of the waveform may fit typical 60Hz magnetic ballast operation, but it does not reflect the
more complex operation on a high frequency ballast.
Figure 3: A sample current waveform
obtained from our experiment.
Initially, this discrepancy posed a challenge in deciding how to measure the starting time for the lamp and
ballast systems. Figure 4 illustrates a sample starting voltage waveform and the corresponding current
waveform for lamp-ballast system using B9 ballast. Likewise, Figure 5 illustrates a sample starting voltage
waveform and the corresponding current waveform for lamp-ballast system using B3 ballast. From figures
4 and 5 it can be seen that lamps on ballast B3 takes longer to stabilize compared to the lamps on ballast
B9. It was noted in many cases that the starting current and the starting voltage didn’t stabilize at the same
time. In our analysis the starting time was taken to be the time needed for both, lamp current and lamp
voltage, to stabilize. Typically, the current waveform took longer than the voltage waveform to stabilize.
Starting time was calculated according to equation 2, where t1 is the time when the first glow current
5
appeared and t3 is the time when the lamp current stabilized. The current was considered stabilized when
the rms value after time t3 did not vary more than 10 percent from the rms value of the operating current. It
also became apparent that the starting time varied every time a system was started. Therefore, for every
lamp-ballast system, 8 repeated measurements were made, with 1 hour off period in between
measurements. The average value of these 8 measurements was taken as the starting time along with its
standard deviation value for a given lamp ballast combination.
Figure 4: Sample starting voltage waveform and the corresponding current
waveform for a lamp-ballast system using B9 ballast
Figure 5: Sample starting voltage waveform and the corresponding current
waveform for a lamp-ballast system using B3 ballast
6
The other starting parameters such as the root mean square (rms) values of the lamp voltage and current
applied over the starting time period were calculated once the starting time was decided. The starting
power, P, is the time-averaged value of the instantaneous power, calculated from the instantaneous values
of V and I, over the starting time period.
Correlation between the starting parameters and system life was explored. Figures 6a and 6b illustrate the
correlation between lamp life when frequently switched and starting time and starting voltage. In both these
plots the y-axis represents the lamp life and the x-axis represents the corresponding starting parameters. In
addition best fit regression lines are drawn through the data points and the corresponding variance values,
R2, are utilized to estimate how well the parameters correlate to lamp life. It can be seen that the starting
time and starting voltage correlate well with lamp life with respective correlation values 0.89 and 0.91 [2].
However the starting current and starting power showed much lower correlation. From this analysis it
appears that starting time and starting voltage are two parameters that can be used for predicting lamp life.
Of the two parameters, starting time is not a physical cause for electrode damage, but starting voltage could
influence physical damage. Furthermore, voltage is not constant throughout the starting time, and the
electrode damage depends on the magnitude of the starting voltage. Therefore, it appears that integrating
start
the starting voltage,
³ Vdt , over the starting time would better represent the cumulative damage caused
0
by the starting voltage during the starting period.
start
The
³ Vdt value which is the area under the curve of the voltage waveform, was calculated for each
0
system. The data acquisition software seperated the cathode and anode half cycle voltage waveforms and
start
took the absolute value of the voltage when calculating
³ Vdt .
Since lamp failure would take place when
0
start
either of the two electrodes lost all of its emissive material, the larger
³ Vdt
value of the two electrodes
0
start
was used for analysis. For every ballast the
³ Vdt values from each lamp were averaged.
0
start
Figure 7 illustrates the plot of lamp life versus
³ Vdt .
It should be noted that V inside the integral is not
0
the rms starting voltage, instead it is the instaneous value of the starting voltage at each time value. A beststart
fit regression line shows a very high correlation between
³ Vdt
and lamp life, with a R2 value of 0.94
0
[2].
7
(a)
2
R = 0.89
Life (hrs)
2 5 00
2 0 00
1 5 00
1 0 00
500
0
0
10
20
30
40
50
60
Starting tim e (ms)
Starting Time (ms)
(b)
R
0 .9 1
R22 == 0.91
Life (hrs)
2 5 00
2 0 00
1 5 00
1 0 00
500
0
0
50
100
150
200
250
300
350
400
Starting Vo ltage (V)
Figure 6: Plot of lamp life versus a) starting time; b) starting voltage [2]
Life (hrs)
2
R = 0.94
2500
2000
1500
1000
500
0
0
2
4
6
8
10
start
Vdt (Volts.sec)
³ Vdt (V.S)
0
Figure 7: Plot of lamp life versus the time integrated value of the lamp
start
voltage over the starting period,
³ Vdt .[2].
0
8
DISCUSSIONS
The electrode structure of a fluorescent lamp has an emissive coating. As mentioned earlier, once the
emissive coating is totally lost the lamp is close to end of life. The lamp electrode performs the function of
transferring charges between the external circuit and the positive column. In an a-c discharge, the electrode
structure performs three functions, supplying electrons to the discharge and collecting ions on the cathode
half-cycle, and receiving electrons on the anode half-cycle. A cathode coating that reaches thermionic
emission rapidly will supply electrons to the discharge promptly and minimize cathode erosion during
starting. This is important since the high ion bombardment of the cathode during starting, when it is not at
emission temperature, will greatly erode the cathode coating [23]. The ANSI recommends a starting time of
less than 100ms for instant-start systems. If all other parameters are equal, shorter starting time will result
in less sputtering damage to the electrode emissive material. Therefore, the lamp life will be longer. This
explains the good correlation between lamp life and starting time, t, observed in our experiment. The results
show that longer starting time results in shorter lamp life.
When the electrode is negative with respect to the surrounding discharge, emitted electrons are accelerated
through the cathode sheath, into the negative glow of the lamp discharge due to the potential difference
across the lamp. These electrons collide with atoms of mercury and argon in the negative glow region,
giving up their energy to produce mercury ions. The ions that reach the cathode strike it with energy equal
to the kinetic energy gained through the cathode fall region. Therefore, the ions closer to the cathode will
typically have less kinetic energy and the ions further away will have greater kinetic energy. During the
starting period of the instant-start system the major portion of the starting voltage is cathode-fall voltage
and typically this is higher than the cathode-fall voltage during lamp operating period. When the cathode
fall voltage is high, the incoming ions are able to physically eject material from the cathode surface,
resulting in severe sputtering, because they possess higher kinetic energy that results in higher momentum
[3,24]. Sputtering is the main reason for the loss of emissive material during starting. This explains the
good correlation between the lamp life and lamp starting voltage, V, observed in our experiment. The
results show that higher starting voltage results in shorter lamp life.
Although starting time and starting voltage showed good correlation, 0.89 and 0.91, with lamp life,
start
respectively, the time integrated starting voltage over the starting period,
³ Vdt , showed the best
0
correlation, 0.94, with lamp life [2]. In addition, as seen in figures 6a & b some data points overlap because
start
of the variations in starting time and starting voltage. However, in the case of lamp life versus
³ Vdt ,
0
start
the overlap between
³ Vdt
values is minimal. As mentioned earlier, during the starting period significant
0
portion of the starting voltage is cathode fall voltage. Integrating the instantaneous voltage values over the
starting period provides a better estimate of the accumulated electrode damage since it accounts for the
voltage fluctuations that are directly related to the energy gained by the ions that cause the sputtering
start
damage. Therefore,
³ Vdt
is a good predictor of lamp life for frequently switched instant start
0
fluorescent systems.
The correlation between lamp life and time integrated value of the starting current over the starting period,
start
³
Idt , is low. At first glance it looks surprising that starting current doesn’t correlate well with lamp life,
0
since high current means more electrons in the discharge region of the lamp, which means more ions and
9
finally more damage to the electrode. However, there are many factors that influence the number of ions
that bombard the electrode. These factors include the probability that an emitted electron produces an ion in
the negative glow region, the probability that the ion will reach the cathode and the energy distribution of
the ions [25]. Therefore, a high starting current does not necessarily mean high ion bombardment. Also,
during starting the current is carried by the ions and ions near the cothode will count as current but may not
cause much damage to the cathode for the reasons explained earlier. This explains the poor correlation
between the starting current and lamp life. Since starting current has poor correlation, when multiplied with
starting voltage to produce power P, it too gives a poor correlation with lamp life.
SUMMARY
As part of an ongoing lamp-ballast system compatibility research, in 1996 several 4-foot F32T8 fluorescent
lamp and ballast systems were subjected to two different life-testing cycles, namely 3 hours on/20 minutes
off and 5 minutes on/5 minutes off cycles. The goal of the 1996 study was to gain better understanding of
lamp life for different starting methods. Although, the study mainly dealt with rapid-start systems, two
groups of instant-start systems were included in that study for comparison. The two instant-start electronic
ballasts showed significantly different lifetimes than each other on both switching cycles. Understanding
the reason for the differences in lifetime and identifying a parameter that correlates well to lamp life for fast
switching cycles became the goal of our study presented in this manuscript. This study was limited to highfrequency switching cycles, where most of the electrode damage takes place due to starting. The main
reason for selecting only the rapid cycle test data is, when this study was started complete life data was
available only for the fast cycle. It is shown in this study that the time integrated value of the lamp voltage
start
over the starting period,
³ Vdt , correlates well with fast switching life for the lamps tested.
0
Having only two ballast types is a limitation of this study. Further, investigation is needed with many more
ballast types to confirm the results observed in this study. A second phase of this research is currently
start
underway and one of the goals is to use many different instant-start ballasts and verify that
³ Vdt
0
correlates well with high-frequency switching life. As mentioned earlier, high-frequrncy switching lamp
life cannot be correlated to standard cycle lamp life. To be able to predict lamp life on any cycle one has to
know the amount of electrode damage due to starting and operating seperately. Therefore, another goal for
the second phase study is to analyze the operating parameters and identify a parameter that can predict
emissive coating loss due to operating. The results of the second phase study will be published at a later
time when it is completed.
ACKNOWLEDGEMNTS
The authors gratefully acknowledge the support of ESEERCO and NYSERDA. The authors are grateful to
Dr. Victor Roberts for his valuable discussions.
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