Shaft Current; Braking Resistors - Siemens LMV5 Series Technical Instructions
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NOTE: The DC bus runs at voltages substantially higher than the incoming voltage to the drive (about
35% higher) and typically employs large capacitors. These capacitors remain charged for a period of
time after the incoming power to the drive is de‐energized, and are a shock hazard until they
discharge. See the VFD manufacturer's recommendations for minimum waiting time to work on the
drive after the drive is de‐energized.
If wire length cannot be kept to less than 150 feet on the drive output, correction options are available.
These are listed in Figure 5‐2:
Wire Length ‐ up to (ft)
Consult Motor OEM
Figure 5‐2: Correction Options for Long Wire Length between VFD and Motor
Shaft Current
As was mentioned earlier, the fast switching or "firing" of the IGBTs enable the VFD to produce modified
sine waves of different frequencies and different voltages in order to speed up or slow down a motor.
The fast switching of the IGBTs does have electrical side effects, some of which are detailed on the
previous pages.
This fast switching of the IGBTs can also cause "shaft current" on the motor. When this happens, a
voltage charge builds up on the motor's shaft. When this voltage gets high enough, it will arc to ground
through the path of least resistance. The path of least resistance is typically the ball bearings that
support the rotor of the motor. When this arcing occurs in the bearings, damage occurs to the bearings.
Over time, the bearings will be destroyed, and the motor will fail.
Shaft current can be mitigated by using a grounding ring, which is typically bolted to the motor housing
and has some type of conductive filament that contacts the shaft, thus grounding the shaft. Some
motor OEMs have grounding rings built into the motor, so an external ring is not necessary.
Braking Resistors
Three‐phase AC induction motors can also function as three‐phase AC generators if they become driven
by what they typically drive. In the case of a blower, the motor drives the blower wheel when the speed
of the wheel is increased (accelerated). Conversely, the blower wheel can drive the motor when the
speed of the blower wheel is decreased (decelerated) with a closed air damper. When the motor is
driven by the blower wheel, it will act as a generator and "push" electrical energy back to the VFD. This
energy will be seen as a voltage increase on the VFD's DC bus.
SCC Inc.
Correction Option
150
300
Load Reactor at VFD Output
650
Load Reactor at Motor Input
2000
dV/dT Filter on VFD Output
Inverter Duty Motor
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