Due to their operating principle, AC drives produce a series of unwanted secondary effects:
l Motor winding isolation stress
l Bearing stress
l Acoustic switching noise in the motor
l Electromagnetic interference
In most applications, these effects are at an acceptable level, but sometimes these effects must be mitigated. For the mitigation of these
effects, filters are installed at the output of the drives. The most commonly known filters are dU/dt filters, sine-wave filters, and common-
mode filters.
The steep switching rate of the AC drive output voltage combined with the inherent common-mode voltage produced by the AC drive
causes shaft voltage. Motor asymmetries, or the use of asymmetric motor cables especially in high-power applications where the motor
current exceeds 100–200 A can also cause shaft voltage.
Table 53: Mitigating Bearing Current Effects with Filters
Type of filter
dU/dt filters dU/dt filters reduce the slew rate of the voltage pulses at the drive output to rates which are typically below
500V/µs. This reduces the stress of the motor winding isolation. The voltage shape remains pulse-width modu-
lated. The optional dU/dt filters also protect the motor insulation system and reduce bearing currents.
Sine-wave filters A sine-wave filter reduces bearing currents and voltage reflections, and it also reduces motor noise. If an out-
put transformer is used, the sine-wave filter eliminates high-frequency components that could stress the trans-
former. The sine-wave filter also allows the use of considerably longer motor cables.
Common-mode fil-
ters
Common-mode filters reduce high-frequency common-modecurrents between AC drive and motors. High-
frequency common-mode filters are a good solution for reducing electric bearing current stress, but the use of
such filters does not eliminate the need of an EMC-compliant installation.
7.7.5 Motor Thermal Protection
During operation, the motor connected to the drive can be monitored to avoid overheating.
Depending on the criticality of overheating, different monitoring methods can be used:
l Built-in Electronic Thermal Motor monitoring
l Externally connected sensors (PTC according to DIN 44081)
Electronic thermal relay function
The electronic thermal relay (ETR) function protects the motor from thermal overload without connecting an external device by
estimating the motor temperature based on present load and time.
The ETR function meets the relevant requirements of UL 61800-5-1, including the Thermal Memory Retention requirement, and ensures
a class 20 protection level.
ETR is an electronic feature that simulates a bimetal relay based on internal measurements. The characteristic is shown in Figure 45.
86 | Danfoss A/S © 2024.08 AJ402315027937en-000401 / 130R1239
Design Guide | iC2-Micro Frequency Converters Electrical Installation Considerations
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