Danfoss VLT AutomationDrive FC 300 Design Manual page 38

FC 300 Design Guide
Step-by-step Description:
Step 1: Select only Proportional Control, meaning that the Integral time is selected to the maximum
value, while the differentiation time is selected to zero.
Step 2: Increase the value of the proportional gain until the point of instability is reached (sustained
oscillations) and the critical value of gain, K
Step 3: Measure the period of oscillation to obtain the critical time constant, P
Step 4: Use the table above to calculate the necessary PID control parameters.
General Aspects of EMC Emissions
Electrical interference is usually conducted at frequences in the range 150 kHz to 30 MHz. Airborne in-
terference from the drive system in the range 30 MHz to 1 GHz is generated from the inverter, motor
cable, and the motor.
As shown in the illustration below, capacitive currents in the motor cable coupled with a high dV/dt from
the motor voltage generate leakage currents.
The use of a screened motor cable increases the leakage current (see illustration below) because
screened cables have higher capacitance to earth than unscreened cables. If the leakage current is not
filtered, it will cause greater interference on the mains in the radio frequency range below approx. 5 MHz.
Since the leakage current (I
only be a small electro-magnetic field (I
The screen reduces the radiated interference but increases the low-frequency interference on the mains.
The motor cable screen must be connected to the frequency converter enclosure as well as on the motor
enclosure. This is best done by using integrated screen clamps so as to avoid twisted screen ends (pig-
tails). These increase the screen impedance at higher frequencies, which reduces the screen effect and
increases the leakage current (I
If a screened cable is used for Fieldbus, relay, control cable, signal interface and brake, the screen must
be mounted on the enclosure at both ends. In some situations, however, it will be necessary to break the
screen to avoid current loops.
If the screen is to be placed on a mounting plate for the frequency converter, the mounting plate must
be made of metal, because the screen currents have to be conveyed back to the unit. Moreover, ensure
good electrical contact from the mounting plate through the mounting screws to the frequency converter
chassis.
NB!
When unscreened cables are used, some emission requirements are not complied with, al-
though the immunity requirements are observed.
In order to reduce the interference level from the entire system (unit + installation), make motor and
brake cables as short as possible. Avoid placing cables with a sensitive signal level alongside motor and
brake cables. Radio interference higher than 50 MHz (airborne) is especially generated by the control
electronics.
38
Introduction to FC 300
, is reached.
u
) is carried back to the unit through the screen (I
1
) from the screened motor cable according to the below figure.
4
).
4
MG.33.B8.02 - VLT ® is a registered Danfoss trademark
.
u
), there will in principle
3