In recent decades, the demand for AC motors in railway and especially industrial applications has grown rapidly. The three-phase induction motor is the most
commonly used type of traction motor in the railway industry. Classical DC traction motors are still in demand by some railway operators, but requests are continually decreasing.
Frequency converters
Power-switching semiconductor devices used in frequency converters have changed from thyristors to gate turn-off transistors (GTOs) and further to the insulated gate bi-polar transistors (IGBTs). These IGBTs are used to create the pulse width modulated (PWM) output voltage waveform and thereby improve efficiency and dynamic performance of the drive. However, there is no advantage without compromise. So apart from the classic voltages and currents generated by the motor itself, new effects have been observed when the motor is supplied from a PWM converter
(frequencies of 3 to 12 kHz, depending on the power range).
It now has been discovered that bearing damage is caused by a high frequency (5 kHz to 10 MHz) current flow that is induced by these fast-switching (100 ns) IGBT semiconductor devices. These IGBTs also cause a very rapid voltage rise (du/dt) up to 2,5 to 8 kV/µs or even up to 10 kV/µs at the converter output.
Traction motor bearing insulation requirements
Frequency converter AC traction motors DC traction motors
Usage Most commonly used Requirements from some operators
Resistance requirements Impedance1) Ohmic resistance
Solution Hybrid bearings
INSOCOAT bearings depending on the specific application requirements
INSOCOAT bearings
1) Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with DC, there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.
U
t U
t
500 V/µs 2 500 V/µs
Voltage pulse of a GTO thyristor compared to an IGBT transistor
GTO thyristor IGBT transistor
Spikes
4
The basic causes and sources for bearing currents are:
magnetic flux asymmetries in the motor
•
asymmetrical, non-shielded cabling
•
fast-switching frequency converters and
•
their common mode voltage
The first two sources are potential risks for all electrical motors, whether they are main-fed or converter-main-fed motors.
The last source only exists for converter-fed motors. Potential problems arise because of different parasitic currents:
high frequency shaft grounding currents
•
high frequency circulating currents
•
capacitive discharge currents
• [23]
When a rolling bearing is operating correctly, an oil film separates the rolling elements from the raceways. From the electrical point of view, this film acts as a dielectric, which is charged by the rotor voltage. For high
The basic three traction motor propulsion concepts and potential causes for electrical erosion
Direct current
propulsion system
Alternating current single phase propulsion system
AC frequency converter propulsion system
Magnetic flux asymmetries in the motor
yes yes yes
Asymmetrical,
non-shielded cabling no no yes
Common mode voltage no no yes
Magnetic flux
asymmetries in the motor:
The three phases and their sum is not zero but lead to the common mode voltage
V common
time[s]
frequencies, it forms a capacitor in which the capacitance depends on various parameters such as the type of lubricant, temperature and viscosity, plus film thickness. If the voltage reaches a certain limit, called the breakdown or threshold voltage of the lubricant, the capacitor will be discharged and a high frequency capacitive discharge current occurs. In this case, the current is limited by the internal stray capacitances of the motor, but it will occur every time the converter switches.
Obviously, an induction motor fed by a frequency converter is a very complex drive system, which is influenced by many parameters. The whole drive, including supply, DC link, switching elements, cables, motor and load, has to be regarded as a total system consisting of inductances and distributed capacitances.
Uu [V DC]
Uv [V DC]
Uw [V DC]
[V DC]
Bearing raceway in very large magnification
Bearing raceway, no electrical
current passage Bearing raceway, damaged by electrical current passage
DC propulsion system AC frequency converter propulsion system
Traction motor principle with inherent stray capacitances [24]
2B5
Shaft Rotor Shaft
Gearbox
Protective earth line 1 Protective earth line 2 Protective
earth
Protective earth Non-drive-end
bearing Drive-end
bearing
Influence of the electrical parameters There is a distinction between DC and AC electrical regimes and the behaviour of INSOCOAT bearings in these applications.
In DC applications, an INSOCOAT bearing acts as a normal (pure ohmic) resistor. The aluminium oxide layer is an insulator and, therefore, only the ohmic resistance R of the layer is the important quantity. The break-down voltage of the standard layer is stated as 1 000 V DC and the resistance is bigger than 50 MΩ, which provides efficient insulation of the bearing.
In AC applications, especially at high frequencies produced by PWM-converters, this is no longer valid. An equivalent electrical circuit diagram of the whole bearing that considers all elements of an INSOCOAT bearing, such as inner and outer ring, rolling elements, the cage, the lubricant and the contact surface area between rolling elements and raceways and the ceramic coating has to be developed.
One possible approximation of the bearing equivalent electrical structure is shown in the illustration above.
It is difficult to create a precise equivalent circuit of the bearing as an electrical system.
There are two main reasons for this:
Lubricant Schematic electrical
circuit model of a mounted INSOCOAT bearing
The massive metal elements in high
•
frequency electrical fields have a very complicated three-dimensional structure.
The possible presence of eddy currents within this structure has to be considered.
The contacts between outer ring and
•
rolling elements and between rolling elements and inner ring create capacitances. The values of these capacitances change stochastically according to the dynamics in bearings, for example, due to vibrations.
Modelling of electric insulation behaviour An electrically insulating layer such as the aluminium oxide Al2O3 coating has to be modelled as a parallel connection of a resistor and a capacitor. Therefore, the impedance Z must be considered, which is described as
where:
Z = impedance j = imaginary unit
R = DC (ohmic) resistance of the system [Ohm]
C = capacitance [F]
ƒ = frequency [Hz]
The value of the impedance can be obtained from:
This equation illustrates that with increasing frequency the term incorporating the capacitance becomes stronger and causes a decrease of the impedance. To increase the impedance of the bearing, the capacitance of the coating should be kept as small as possible. The capacitance of an INSOCOAT bearing depends on the size (coated surface area) of the bearing, on the thickness of the insulating coating and on the coating material, as indicated in the following equation [23]
where:
0 = dielectric constant in vacuum
r = permittivity1) constant of the insulating coating
A = coated contact surface area s = thickness of the ceramic coating
1) In electromagnetism, permittivity is the measure of how much resistance is
encountered when forming an electric field in a medium.
4
Selection of electrical insulation SKF offers three basic design solutions for electrical insulation of traction motor bearings, depending on the application requirements:
Hybrid bearing designs
•
INSOCOAT bearing designs with coated
•
inner ring, which is mainly used for traction motor bearing units and industrial electrical machines
INSOCOAT bearing designs with coated
•
outer ring, which is used for traction motors and generators in railways
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Impedance [kΩ]
Capacitance [nF]
Frequency [Hz]
Frequency [Hz]
Measured impedance and capacitance of an INSOCOAT 6316/VL0241 deep groove ball bearing (outer ring coated)
Hybrid bearing design
Ohmic resistance Impedance Function of frequency
and capacitance
INSOCOAT inner ring coated INSOCOAT outer ring coated
Bearing designs for different ohmic resistance, frequency and capacitance.
The electrical impedance is a vector function based on the ohmic resistance, frequency and capacitance.
The capacitance is a measure of the amount of electric charge stored for a given electrical potential.