Internal Drive Problems; Overtemperature Faults; Current Sensor Faults; Electromagnetic Compatibility - Danfoss VLT FC Series Service Manual

Drive and Motor Application...
the 3 output phases. Refer to chapter 4.6 Drive Inputs and
Outputs.
Usually, a current limit warning indicates an incorrect
mechanical load. If possible, disconnect the motor from the
load to determine if the load is incorrect.
Often, the indications of motor problems are similar to the
problems of a defective drive. To determine whether the
problem is with the drive, disconnect the motor from the
motor terminals. Perform the test in chapter 8.5.9 Output
Imbalance of Motor Voltage and Current. If the 3 voltage
measurements are balanced, the drive functions correctly.
If the voltage measurements are not balanced, the drive
malfunctions. Typically, 1 or more output IGBTs do not
function correctly. This problem can be a result of a
defective IGBT or gate signal.

7.2 Internal Drive Problems

To identify most problems related to failed power
components, perform a visual inspection and the static
tests as described in chapter 8.4 Static Test Procedures.
However, the following problems must be diagnosed in a
different manner.

7.2.1 Overtemperature Faults

When an overtemperature indication occurs, determine
whether this condition exists within the drive or whether
the thermal sensor is defective. If an overtemperature
condition is present in the drive, the outside of the unit is
warm. If the exterior is not warm, check the temperature
sensor with an ohmmeter.

7.2.2 Current Sensor Faults

An overcurrent alarm that cannot be reset, even with the
motor cables disconnected, sometimes indicates current
sensor failure. The drive experiences frequent false ground
fault trips due to the DC offset failure mode of the sensors.
An explanation of the internal composition of a Hall-effect
type current sensor helps to explain these faults. Included
inside the device is an op-amp to amplify the signal to
usable levels in the receiving circuitry. The output at 0
input level (0 A flow being measured) is 0 V, exactly
halfway between the plus and minus supply voltages. A
tolerance of ±15 mV is acceptable. In a 3-phase system
that operates correctly, the sum of the 3 output currents is
always 0.
When the sensor becomes defective, the output voltage
level varies by more than the 15 mV. The defective current
sensor in that phase indicates current flow when there is
MG94A502
Service Guide
none. This condition results in the sum of the 3 output
currents being a value other than 0. If the deviation from 0
(current amplitude) approaches a specific level, the drive
assumes a ground fault and issues an alarm.
To determine whether a current sensor is defective,
disconnect the motor from the drive and observe the
current in the drive display. With the motor disconnected,
the current must be 0. A drive with a defective current
sensor indicates some current flow. Because the current
sensors for the drives with higher power ratings have less
resolution, an indication of a fraction of 1 A is tolerable.
However, that value must be considerably lower than 1 A.
If the display shows more than 1 A of current, the current
sensor is defective.
To determine which current sensor is defective, measure
the voltage offset at zero current for each current sensor.
Refer to chapter 8.5.12 Current Sensors Test.

7.3 Electromagnetic Compatibility

The following is an overview of general signal and power
wiring considerations when addressing the electromagnetic
compatibility (EMC) concerns for typical commercial and
industrial equipment. High-frequency RF emissions and
immunity are discussed. Compliance to national and
European CE EMC directives are required.

7.3.1 Effects of EMI

While electromagnetic interference (EMI) disturbances to
the operation of the unit are uncommon, the following
detrimental EMI effects can be seen:
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A disturbance resulting from other nearby equipment is
more common. Generally, other industrial control
equipment has a high level of EMI immunity. However,
non-industrial, commercial, and consumer equipment is
often susceptible to lower levels of EMI.
Detrimental effects to these systems can include the
following:
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Motor speed fluctuations
Serial communication transmission errors
Drive CPU exception faults
Unexplained drive trips
Pressure/flow/temperature signal transmitter
signal distortion or aberrant behavior
Radio and TV interference
Telephone interference
Computer network data loss
Digital control system faults
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