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the motor (for instance, electromagnetic unbalance caused by asymmetries), which as well provoke current circulation through the bearings. The basic reason for bearing currents to occur within an inverter fed motor is the so called common mode voltage. The motor capacitive impedances become low in face of the high frequencies produced within the inverter stage of the inverter, causing current circulation through the path formed by rotor, shaft and bearings back to earth.
6.9.1 Common mode voltage
The three phase voltage supplied by the PWM inverter, differently from a purely sinusoidal voltage, is not balanced.
That is, owing to the inverter stage topology, the vector sum of the instantaneous voltages of the three phases at the inverter output does not cancel out, but results in a high frequency electric potential relative to a common reference value (usually the earth or the negative bus of the DC link), hence the denomination “common mode”.
The sum of the instantaneous voltage values at the (three phase) inverter output does not equal to zero
This high frequency common mode voltage may result in undesirable common mode currents. Existing stray capacitances between motor and earth thus may allow current flowing to the earth, passing through rotor, shaft and bearings and reaching the end shield (earthed).
Practical experience shows that higher switching frequencies tend to increase common mode voltages and currents.
6.9.2 Equivalent circuit of the motor for the high frequency capacitive currents
The high frequency model of the motor equivalent circuit, in which the bearings are represented by capacitances, shows the paths through which the common mode currents flow.
The rotor is supported by the bearings under a layer of nonconductive grease. At high speed operation there is no contact between the rotor and the (earthed) outer bearing raceway, due to the plain distribution of the grease. The electric potential of the rotor may then rise with respect to the earth until the dielectric strength of the grease film is disrupted, occurring voltage sparking and flow of discharge current through the bearings. This current that circulates whenever the grease film is momentarily broken down is often referred to as the capacitive discharge component.
There is still another current component, which is induced by a ring flux in the stator yoke and circulates permanently through the characteristic conducting loop comprising the shaft, the end shields and the housing/frame, that is often called the conduction component.
I
C er
: Capacitor formed by the stator winding and the rotor lamination (Dielectric = airgap + slot insulation + wire insulation)
C rc
: Capacitor formed by the rotor and the stator cores
(Dielectric = airgap)
C ec
: Capacitor formed by the stator winding and the frame
(Dielectric = slot insulation + wire insulation)
C md
e C mt
: Capacitances of the DE (drive end) and the NDE
(non-drive end) bearings, formed by the inner and
I
CM
I er c the outer bearing raceways, with the metallic rolling elements in the inside. (Dielectric = gaps between the raceways and the rolling elements + bearing grease)
: Total common mode current
: Capacitive discharge current flowing from the stator to the rotor
: Capacitive discharge current flowing through the bearings
These discontinuous electric discharges wear the raceways and erode the rolling elements of the bearings, causing small superimposing punctures. Long term flowing discharge currents result in furrows (fluting), which reduce bearings life and may cause the machine to fail precociously.
Technical guide – Induction motors fed by PWM frequency inverters 21
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Table of contents
- 4 Introduction
- 5 Normative Aspects
- 5 NEMA MG1 - Motors and generators / “United States
- 5 2.2 NEMA - Application Guide for AC Adjustable Speed Drive Systems
- 5 2.3 IEC 60034 - Rotating Electrical Machines / “Internacional
- 5 2.4 Other technical documents of reference
- 5 Induction machines speed variation
- 7 Characteristics of PWM frequency inverters
- 7 General
- 8 4.2 Control Types
- 8 Interaction between inverter and AC power line
- 8 Harmonics
- 9 5.1.1 Normative considerations about the harmonics
- 9 5.2 Line reactor / DC bus choke
- 10 Interaction between inverter and motor
- 10 6.1 Harmonics influencing motor performance
- 10 6.1.1 Normative considerations about the inverter output harmonics
- 11 6.2 Considerations regarding energy efficiency
- 12 6.2.1 The influence of the speed variation on the motor efficiency
- 12 6.2.2 Normative considerations about the efficiency of inverter fed motors
- 13 6.3 Influence of the inverter on the temperature rise of the windings
- 13 6.4 Criteria regarding the temperature rise of WEG motors on VSD applications
- 13 6.4.1 Torque derating
- 14 6.4.2 Breakaway torque
- 15 6.4.3 Breakdown torque
- 15 6.5 Influence of the inverter on the insulation system
- 15 6.5.1 Rise Time
- 16 6.5.2 Cable length
- 17 6.5.3 Minimum time between successive pulses (MTBP)
- 18 6.5.4 Switching frequency (fs)
- 18 6.5.5 Multiple motors
- 18 6.6 Criteria regarding the insulation system of WEG motors on VSD applications
- 18 6.7 Normative considerations about the insulation system of inverter fed motors
- 19 6.8 Recommendations for the cables connecting WEG motors to inverters
- 20 6.8.1 Cable types and installation recommendations
- 20 6.9 Influence of the inverter on the motor shaft voltage and bearing currents
- 21 6.9.1 Common mode voltage
- 21 6.9.2 Equivalent circuit of the motor for the high frequency capacitive currents
- 22 6.9.3 Methods to reduce (or mitigate) the bearings currents in inverter fed motors
- 23 6.10 Criteria regarding protection against bearing currents (shaft voltage) of WEG motors on VSD applications
- 23 6.11 Normative considerations about the current flowing through the bearings of inverter fed motors
- 23 6.12 Influence of the inverter on the motor acoustic noise
- 23 6.13 Criteria regarding the noise emitted by WEG motors on VSD applications
- 24 6.14 Normative considerations about the noise of inverter fed motors
- 24 6.15 Influence of the inverter on the mechanical vibration of the motor
- 24 6.16 Criteria regarding the vibration levels presented by WEG motors on VSD applications
- 24 6.17 Normative considerations about mechanical vibration of inverter fed motors
- 25 Interaction between motor and driven load
- 25 Load types
- 25 7.1.1 Variable torque loads
- 25 7.1.2 Constant torque loads
- 26 7.1.3 Constant horsepower loads
- 26 Speed duties
- 26 7.2.1 Variable speed duty
- 26 7.2.2 Continuous speed duty
- 26 Dimensioning and analysis of actual drive system applications – Practical examples
- 26 8.1 Constant torque application - compressor
- 26 8.1.1 Example
- 26 8.1.2 Solution
- 27 8.2 Squared torque application - centrifugal pump
- 27 8.2.1 Example
- 27 8.2.2 Solution
- 29 8.3 Special application - long cable
- 29 8.3.1 Example
- 29 8.3.2 Solution
- 30 8.4 Variable torque / variable speed application - textile industry
- 30 8.4.1 Example
- 31 8.4.2 Solution
- 32 8.5 Example considering the use of WEG Optimal Flux
- 32 8.5.1 Example
- 32 8.5.2 Solution
- 32 Recommendations for the measurement of PWM waveforms
- 32 Warning
- 32 9.2 Instrumentation
- 33 9.3 Parameter measurements
- 33 Grounding considerations
- 33 9.4.1 Grounding of control
- 33 9.4.2 Grounding of motor
- 33 9.5 Measurement procedures
- 33 9.5.1 Waveform visualization
- 33 9.5.2 Oscilloscope scale setting
- 33 9.5.3 Triggering
- 34 Conclusion
- 35 11 Bibliography