SIEMENS
Introduction
General technical data
1/6
Siemens D 86.2 · 2007
1
Bearings and bearing currents
When operating multiphase AC machines by a converter, an
electrical bearing stress results from a capacitive induced volt-
age via the bearing lubricating film, depending on the principle
being used. The physical cause of this is the common-mode
voltage at the converter output: The sum of the three phase-to-
neutral voltages is not zero at all times, unlike with direct on-line
operation. The high-frequency, pulse-shaped common-mode
voltage brings about a residual current, which closes back to the
converter's DC link via the machine's internal capacitances, the
machine housing and the earthing circuit. The machine's inter-
nal capacitances include the main insulation winding capaci-
tance, the geometric capacitance between the rotor and stator,
the lubricating film capacitance and the capacitance of any
bearing insulation that may be present. The current level via the
internal capacitances is proportional to the common-mode volt-
age regulation (
i(t)
=
C
·
du/dt
).
In order to apply currents to the motor which are sinusoidal as far
as possible (smooth running, oscillation torques, stray losses), a
high pulse frequency is required for the converter's output volt-
age. The related (very steep) switching edges of the converter
output voltage (and also, therefore, of the common-mode volt-
age) cause correspondingly high capacitive currents and volt-
ages on the machine's internal capacitances.
In the worst-case scenario, the capacitive voltage induced via
the bearing can lead to random punctures of the bearing lubri-
cating film, thus damaging the bearing/causing premature wear.
The current pulses caused by the puncture in the lubricating film
are referred to as EDM (Electrostatic Discharge Machining) cur-
rents, although this is not primarily a question of an electrostatic
effect, but more of (partial) punctures of insulating material, i.e.,
of partial discharges.
This physical effect, which occurs in isolated cases, has mostly
been observed in connection with larger motors.
EMC-compliant installation of the drive system is a basic prereq-
uisite for preventing premature bearing damage via bearing cur-
rents.
The most important measures for reducing bearing currents:
•
Use of cables with a symmetrical cable cross-section
(see Figure below)
•
Use of motor reactors (converter option L08)
•
Preference given to a supply with insulated neutral point
(IT system)
•
Use of grounding cables with low impedance in a large fre-
quency range (DC up to approximately 70 MHz):
for example, plaited copper ribbon cables, HF litz wires
•
Separate HF equipotential-bonding cable between motor
housing and driven machine
•
Separate HF equipotential-bonding cable between motor
housing and converter PE busbar
•
360° HF contacting of the cable shield on the motor housing
and the converter PE busbar. This can be achieved using
EMC screwed glands on the motor end and EMC shield clips
on the converter end, for example.
•
Common-mode filters at the converter output
•
The HT-direct motors are equipped with an electrically insu-
lated bearing housing at NDE.
Thermal limit characteristic and field-weakening range
Due to the speed-independent cooling of the HT-direct motors,
no torque reduction or only a relatively minor torque reduction
(depending on their speed range) is required for operation at
constant load torque and with wide speed ranges.
Guide values for the maximum load torques at various speeds
can be obtained from the diagrams below:
L1
L2
L3
L1
L2
L3
L1
L2
L3
PE
PE
PE
PE
Concentric
copper or aluminium shield
Steel armor
G_D011_EN_00005
n
n
/
N
M
/
M
N
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
G_D086_EN_00007a
Field weakening
0.2
1.2
1.0
0.4
0.6
0.8
Speed
Torque