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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

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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.

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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)

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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.

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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

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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

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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)

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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)

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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]

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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)

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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

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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.