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CHAPTER-6 MEASUREMENT OF SHAFT VOLTAGE AND BEARING CURRENT IN
2, 3 AND 5-LEVEL INVERTER FED INDUCTION MOTOR DRIVE
6.1. INTRODUCTION
Though the research work is concerned with the measurement of CM
voltage, as a spillover the shaft voltage and the bearing current has been
measured.
Due to the CM voltage at the star point of stator winding of an IM, a
voltage is induced in the rotor because of capacitive/inductive coupling.
Since the rotor conductors are short circuited the current will circulate
in the rotor and also tries to flow to the general ground through the
bearing. The fast switching action of the inverter devices can cause high
frequency noise voltage transients which induce capacitive coupling from
rotor to the ground through the bearing and hence called the capacitive
currents. These high frequency currents from the rotor will flow
through the bearing to the ground [23, 36 & 61]. These currents through
the bearing causes electrical discharge machining (EDM) in the inner
surface of the bearing and in turn reduce the life of the bearing.
This chapter presents the experimental measurement of the rotor
shaft voltage and bearing current for a modified 3- phase squirrel cage
IM connected to an inverter. Experiments have been carried out on 2-
level, 3-level and 5-level inverter fed IM drives in SVM scheme. PIC µ-
controller was used to generate SVM pulses along with other associated
electronic interface circuits to operate the 2-level, 3- level and 5-level
121
inverter. Necessary converter circuits were fabricated and tested for
giving the proper DC voltage to the inverter. Standard current probe,
LISN, high frequency 4-channel MSO with differential probes were used
to measure the shaft voltage, bearing current and other parameters. The
graphs were plotted showing Frequency vs shaft voltage in Volts & dBµV
and the bearing current in dBµA using the signal analysis software and
compared the results.
6.2. LITERATURE SURVEY It was observed by the researchers that the occurrence of bearing
failures among IM driven by inverters is much more frequent than those
driven by 50/60 Hz utility supply [26]. A survey conducted by references
[1,3, 4, 5 & 24] indicates that the inverter-fed motors have a greater
probability of bearing breakdown than the 50/60 Hz line-fed motors. The
concept of bearing currents in variable speed drive systems using
Converter-Inverter is due to the existence of CM voltage and also by fast
switching ON and OFF of the inverter devices has been reported for
almost a decade [23, 36 and 43]. Annette Muetze et al. [5] reports that
the induced bearing currents , the ground currents can be from the
influence of CM voltage and the capacitance between stator and rotor
windings with high dv/dt at the input to the IM terminals [1,3 & 67].
D. Busse, et.al [25] has also explained about the characteristics of
induced shaft voltage in the IM due to converter-inverter adjustable
speed drive system.
122
Due to the recent advancement of adjustable-speed drives, with VSI,
mechanism of inducing shaft voltages and bearing currents are due to
the voltage transients exist at the star point of the stator winding of an
IM and the ground. As summarized by Chen et. al. [23], [61], there is
three general types of motor bearing currents (stator to rotor bearing
current, stator winding to ground current, rotor to shaft current) that
can be associated with PWM VSI drive. [3, 5 & 67]
6.3. PROPOSED METHOD OF MEASUREMENT OF SHAFT VOLTAGE & BEARING CURRENT The modified IM is shown in the Fig.6.1. [7, 8] The inner diameter of
the end plates of the existing motor is slightly increased by machining.
Proper insulation is used to isolate the end plates from the main body of
the IM. The fixing bolts of the end plates are also made of non-
conducting material. Hence the whole rotor is isolated from the main
body of the IM [7].
Fig 6.1. Modified IM (Rotor & stator Isolated)
123
With this modification, the rotor is floating and the connections to the
ground through the current probe are done as shown in the Fig.6.1 [9].
The shaft voltage with respect to the ground and the bearing current in
terms of voltage (using current probe) were recorded using the DSO.
Fig.4.2. in Chapter-4 Shows the circuit diagram of a 3-level inverter and
the corresponding switching states of each phase of the inverter is listed
in Chapter-4, Table 4.1. The switching sequence of the 3-level inverter is
similar to that of 2- level inverter as discussed in chapter - 2. The circuit
diagram of a 5-level NPC inverter is shown in chapter-5, Fig.5.1, and the
corresponding switching states of each phase of the inverter are listed in
chapter-5, Table 5.1,
6.4. HARDWARE IMPLEMENTATION
The hardware implementation of the 2-level, 3-level and the 5-level
has been discussed in the chapters 2 to 5. The output of the inverter
bridge is given to the IM (3Phase, 0.37kW, 415VL, 1390 rpm, 50Hz, star
connection) stator terminals. The, line voltage ,CM voltage, shaft voltage,
and the bearing current using current probe (in terms of voltage) has
been monitored and recorded using 4 channel DSO (500MHz) along with
necessary differential probe (200:1). The actual bearing current can be
computed from the current probe output voltage is as follows.
124
6.5 EXPERIMENTAL RESULTS Fig.6.2. shows the 2-level inverter CM voltage, Line voltage, Vector
sum of phase current and the bearing current (in terms of voltage using
the current probe). Fig.6.2. is taken from the published result [7] for the
same modified IM to show the bearing current. Fig.6.3, ch.3 shows the
shaft voltage of 2-level inverter fed IM when the shaft is not grounded.
Fig.6.4 shows the 3-level inverter shaft voltage (ch.3) when there is no
flow of bearing current. Fig.6.5. ch.4 shows the 3-level inverter bearing
current (in terms of voltage using the current probe). Fig.6.6 shows the
shaft voltage (ch.3,1:1)at the instant when shaft is grounded through the
bearing and bearing current (ch.4)in terms of voltage using current probe
for the 5-level NPC inverter. Fig.6.7. shows the shaft voltage (200:1,
ch.2) and the CM voltage (200:1, ch.3) when there is no flow of bearing
current. From the above recorded waveforms it is easy to measure the
magnitude of voltages and it is found to be 142.5Vpeak, 135Vpeak and
130Vpeak for 2,3 and 5-level inverter. Figs 6.8, 6.9 and 6.10 shows the
FFT of IM shaft voltage of 58Vpeak, 43Vpeak and 36Vpeak for 2, 3 and 5-level
inverter respectively. It is observed from the FFT plots that IM shaft
voltage is reduced in 5-level inverter when compared to 3 & 2- level
inverters. Similarly Figs. 6.11, 6.12 and 6.13 shows the shaft voltage FFT
plots for 2, 3 and 5 level inverters in dBµV respectively which can be
used for comparing the results with FCC and CISPR standards in future.
It is also observed that the IM shaft voltage is reduced in 5-level inverter
125
when compared to 3 & 2- level inverters. Hence it is concluded that as
the inverter level increases the shaft voltage reduces.
Fig.6.2 DSO recorded waveform (2-level inverter [7]) Ch 1: CM voltage ( diff. probe 200:1), Ch 2: line voltage (diff. probe 200:1), Ch 3: 10 : 1 vector sum of phase current, Ch 4: 10 : 1 bearing current alone using
current probe.
Fig.6.3. DSO recorded waveforms.(2-level inverter) Ch 3. 200 : 1. Shaft voltage
126
Fig. 6.4. DSO recorded waveforms.(3-level inverter) Ch 1: 200 : 1 Phase voltage.Ch 2: 200 : 1 wave form of Line voltage
Ch 3: 200 : 1 Shaft Voltage with respect to ground.
Fig. 6.5. DSO recorded waveforms.(3-level inverter) Ch 1 200 : 1 Phase voltage. Ch 2 200 : 1CM voltage. Ch 4. 1 : 1 Bearing current using the current probe.
127
Fig.6.6. DSO recorded waveforms (5-level inverter) Ch 1. 200: 1 line voltage to IM Ch 2: 200 : 1 wave form of CM voltage at
IM. Ch 3:1 : 1 shaft voltage of IM Ch 4: 1 : 1 Bearing current
Fig.6.7. DSO recorded waveforms (5-level inverter) Ch 1. 200:Phase voltage to IM. Ch 2. 200:1 shaft Voltage.
Ch. 3. 200:1 CM Voltage.
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0 1000 2000 3000 4000 5000
0
20
40
60
Frequency (Hz)
I.M
. sh
aft
& g
nd
. vo
lta
ge
in
vo
lts
Fig. 6.8 FFT of IM shaft Voltage (2-level inverter) (Published results in SPWM scheme)[7]
-1000 0 1000 2000 3000 4000 5000
0
10
20
30
40
50
Frequency (Hz)
I.M
.sh
aft
& g
nd
. vo
lta
ge in v
olts
Fig.6.9. FFT of IM Shaft voltage (3-level inverter)
129
0 400 800 1200 1600 2000 24000
10
20
30
40
Frequency (Hz)
Am
plit
ud
e in
vo
lts
Fig .6.10 FFT of IM shaft voltage in volts (5-level inverter)
0 100 200 300 400 500 600 700 800 900 1000
0
50
100
150
200
250
Frequency (Hz)
Am
plit
ude in d
BµV
Fig. 6.11.FFT of shaft voltage in dBµV (2-level inverter)
130
-100 0 100 200 300 400 500 600 700 800 900 1000
0
50
100
150
200
Frequency (Hz)
Am
plit
ud
e in
dB
µ v
olts
Fig.6.12. FFT of Shaft voltage in dBµV (3-level inverter)
0 400 800 1200 1600 2000 24000
50
100
150
200
Frequency (Hz)
Am
plit
ud
e in
dB
µV
Fig .6.13. FFT of Shaft voltage in dBµV (5-level inverter)
131
Figs. 6.14 to 6.19 show the FFT of Bearing current in mA for 2, 3 and 5-
level inverters which include the expanded views also. Here for the 2-
level inverter the magnitude of bearing current is found to be 18mA for
the fundamental frequency and for other frequencies it is around 10mA
average. For 3-level inverter the magnitude of bearing current is found to
be 17mA for the fundamental frequency and for other frequencies it is
around 0.6mA average. Similarly for 5-level inverter the magnitude of
bearing current is found to be 9mA for the fundamental frequency and
for other frequencies it is around 0.4mA average. Figs. 6.20 to 6.24 show
the FFT of Bearing current in dBµA for 2, 3 and 5-level inverters which
include the expanded views also.
0 1000 2000 3000 4000 5000 6000
0.000
0.005
0.010
0.015
0.020
Frequency (Hz)
beari
ng
curr
ent in A
mps
Fig.6.14. FFT of bearing current in mA (2-level inverter) (Published result)[7]
132
0 1000 2000 3000 4000 5000 6000
0.0000
0.0005
0.0010
0.0015
Frequency (Hz)
be
ari
ng
cu
rre
nt
in A
mp
s
Fig.6.15. FFT of bearing current in mA in expanded view (2-level inverter) (Published result)[7]
0 500 1000 1500 2000 2500
0
5
10
15
20
Frequency (Hz)
be
ari
ng
cu
rre
nt in
mA
Fig.6.16. FFT of bearing current in mA (3-level inverter)
133
0 500 1000 1500 2000 2500
0.0
0.2
0.4
0.6
0.8
1.0
Frequency (Hz)
be
ari
ng c
urr
en
t in
mA
Fig.6.17. FFT of bearing current in mA in expanded view (3-level inverter)
0 500 1000 1500 2000 2500
0
2
4
6
8
10
Frequency (Hz)
be
ari
ng
cu
rre
nt in
mA
Fig.6.18. FFT of bearing current in mA (5-level inverter)
134
0 500 1000 1500 2000 2500
0.0
0.5
1.0
1.5
2.0
Frequency (Hz)
be
ari
ng
cu
rre
nt in
mA
Fig.6.19. FFT of bearing current in mA in expanded view (5-level inverter)
0 1000 2000 3000 4000 5000
0
50
100
150
200
250
Frequency (Hz)
Be
ari
ng
cu
rre
nt in
dB
µ A
Fig.6.20. FFT of bearing current in dBµA (2-level inverter)
135
Fig .6.21 FFT of bearing current in dBµA (3-level inverter)
0 500 1000 1500 2000 2500
0
50
100
150
Frequency (Hz)
Be
ari
ng
cu
rre
nt in
dB
µ A
Fig. 6.22. FFT of Bearing current in dBµA (5-level NPC inverter)
136
0 500 1000 1500 2000 2500
0
2
4
6
Frequency (Hz)
Be
ari
ng
curr
en
t in
dB
µ A
Fig. 6.23 FFT of Bearing current in dBµA
(5 level NPC inverter Expanded view Y -Axis)
-100 0 100 200 300 400 500 600 700 800 900 1000
0
2
4
6
Frequency (Hz)
Be
ari
ng
cu
rre
nt in
dB
µ A
Fig.6.24.FFT of Bearing current in dBµA (5 level NPC inverter Expanded view X -Axis)
137
6.6 CONCLUSION In this chapter the experimental measurement of the rotor shaft
voltage and bearing current for the modified 3- phase squirrel cage IM
connected to an inverter is discussed. Experiments have been carried out
on 2-level, 3-level and 5-level inverter fed IM drives in SVM scheme. It is
noted that from the recorded waveform of shaft voltage (Figs.6.3, 6.4 and
6.7) for 2, 3 and 5-level inverter is 142.5V peak, 135Vpeak and 130V peak
respectively. From this it is noted that 5-level inverter shaft voltage is
less by 5Vpeak with respect to 3-level inverter and 12.5Vpeak with respect
to 2-level inverter. It is also observed from the FFT plots that the IM
shaft voltage is 130 dBµV (Fig.6.13), 175dBµV (Fig.6.12) & 220 dBµV
(Fig.6.11) for 5, 3 & 2-level inverters fundamental frequency. Similarly
Figs 6.14 to 6.19 shows that the bearing current in mA (in the form of
pulses) 18mA, 17mA and 9mA for 2, 3, and 5 level inverter respectively.
Figs. 6.20, 6.21 and 6.22 shows the FFT plots of bearing current in dBµA
for 2, 3 and 5 level inverters. The values are 220 dBµA, 140 dBµA and
100 dBµA respectively. It is observed that IM bearing current is less in 5-
level inverter when compared to 3 & 2 level inverters. Hence it is
concluded that as the inverter level increases the shaft voltage and
bearing current reduces.
Note:-Justification for Figs.6.4 and 6.5
Fig.6.4. gives the actual readings of the inverter output line voltage, CM
voltage, shaft voltage and the bearing current measured in terms of
138
voltage. Observing the DSO recorded waveform, at the time of grounding
the shaft voltage, there will be the flow of bearing current. Due to the
flow of bearing current the CM voltage magnitude is diminished (ch.2).
The ch.3 of Fig.6.4 is the shaft voltage which is shorted to ground is
diminished, however the sharp pulses exists which is in agreeable with
CM voltage peaks. Fig 6.4 clearly shows that once there is a flow of
bearing current, which is decreasing the CM voltage due to capacitive
flow of current and hence the CM voltage is also reduces in magnitude
which can be seen in Fig. 6.4 ch. 2.
Observing the DSO recorded wave form of Fig.6.5 ch.2 and Fig.6.4 ch.3
the CM voltage is agreeable in phase and magnitude with that of the
shaft voltage before grounding the shaft.
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