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Using RCDs
Where residual current devices (RCDs), also known as earth
leakage circuit breakers (ELCBs), are used, comply with the
following:
Use RCDs of type B only which are capable of
detecting AC and DC currents
Use RCDs with an inrush delay to prevent faults
due to transient earth currents
Dimension RCDs according to the system configu-
ration and environmental considerations
Illustration 2.25 Main Contributions to Leakage Current
Illustration 2.26 The Influence of the Cut-off Frequency of the
RCD on What Is Responded to/measured
See also RCD Application Note, MN90G
2.12 Extreme Running Conditions
Short Circuit (Motor Phase – Phase)
The frequency converter is protected against short circuits
by means of current measurement in each of the three
motor phases or in the DC link. A short circuit between
two output phases will cause an overcurrent in the
inverter. The inverter will be turned off individually when
the short circuit current exceeds the permitted value
(Alarm 16 Trip Lock).
To protect the frequency converter against a short circuit
at the load sharing and brake outputs see the design
guidelines.
See certificate in
.
Switching on the Output
Switching on the output between the motor and the
frequency converter is fully permitted. Switching on the
output does not damage the frequency converter in any
way. However, fault messages may appear.
Motor-generated Over-voltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in following
cases:
1.
The load drives the motor (at constant output
frequency from the frequency converter), ie. the
load generates energy.
2.
During deceleration ("ramp-down") if the moment
of inertia is high, the friction is low and the ramp-
down time is too short for the energy to be
dissipated as a loss in the frequency converter,
the motor and the installation.
3.
Incorrect slip compensation setting may cause
higher DC link voltage.
WARNING
The frequency converter must be equipped with a break
chopper.
The control unit may attempt to correct the ramp if
possible (
2-17 Over-voltage Control
).
In the worst case, the inverter turns off to protect the
transistors and the intermediate circuit capacitors when a
certain voltage level is reached.
See
2-10 Brake Function
and
2-17 Over-voltage Control
to
select the method used for controlling the intermediate
circuit voltage level.
Mains Drop-out
During a mains drop-out, the frequency converter keeps
running until the intermediate circuit voltage drops below
the minimum stop level, which is typically 15% below the
frequency converter's lowest rated supply voltage. The
mains voltage before the drop-out and the motor load
determines how long it takes for the inverter to coast.
Static Overload in VVC
plus
mode
When the frequency converter is overloaded (the torque
limit in
4-16 Torque Limit Motor Mode
/
4-17 Torque Limit
Generator Mode
is reached), the controls reduces the
output frequency to reduce the load.
If the overload is excessive, a current may occur that
makes the frequency converter cut out after approx. 5-10
s.
Operation within the torque limit is limited in time (0-60 s)
in
14-25 Trip Delay at Torque Limit
.
Introduction
VLT
®
Refrigeration Drive Design Guide
MG16G102 - VLT
®
is a registered Danfoss trademark
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