Danfoss VLT HVAC Drive FC 100 Series, Руководство по конструкции

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NOTE!
If a short circuit in the brake transistor occurs, power
dissipation in the brake resistor is only prevented by using
a line power switch or contactor to disconnect line power
for the Adjustable frequency drive. (The contactor can be
controlled by the Adjustable frequency drive).
WARNING
Do not touch the brake resistor, as it can get very hot
during/after braking.
2.12.3 Control with Brake Function
The brake is protected against short-circuiting of the brake
resistor, and the brake transistor is monitored to ensure
that short-circuiting of the transistor is detected. A relay/
digital output can be used for protecting the brake resistor
against overloading in connection with a fault in the
Adjustable frequency drive.
In addition, the brake makes it possible to read out the
momentary power and the mean power for the latest 120
seconds. The brake can also monitor the power energizing
and make sure it does not exceed a limit selected in
2-12 Brake Power Limit (kW) . In 2-13 Brake Power Monitoring ,
select the function to carry out when the power
transmitted to the brake resistor exceeds the limit set in
2-12 Brake Power Limit (kW) .
NOTE!
Monitoring the braking energy is not a safety function; a
thermal switch is required for that purpose. The brake
resistor circuit is not protected against ground leakage.
Overvoltage control (OVC) (exclusive brake resistor) can be
selected as an alternative brake function in 2-17 Over-
voltage Control . This function is active for all units. The
function ensures that a trip can be avoided if the DC link
voltage increases. This is done by increasing the output
frequency to limit the voltage from the DC link. It is a very
useful function, e.g., if the ramp-down time is too short
since tripping of the Adjustable frequency drive is avoided.
In this situation, the ramp-down time is extended.
OVC cannot be activated when running a PM motor (when
1-10 Motor Construction is set to [1] PM non-salient SPM).
2.12.4 Brake Resistor Cabling
EMC (twisted cables/shielding)
To reduce the electrical noise from the wires between the
brake resistor and the Adjustable frequency drive, the
wires must be twisted.
For enhanced EMC performance, a metal shield can be
used.
2.13 Extreme Running Conditions
Short Circuit (Motor Phase – Phase)
The Adjustable frequency drive 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 Adjustable frequency drive against a short
circuit at the load sharing and brake outputs, please see
the design guidelines.
See certificate in
2.6.1 Electrical terminals .
Switching on the Output
Switching on the output between the motor and the
Adjustable frequency drive is fully permitted. Switching on
the output does not damage the Adjustable frequency
drive in any way. However, fault messages may appear.
Motor-generated Overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in the following
cases:
1. The load drives the motor (at constant output
frequency from the Adjustable frequency drive),
i.e., 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 Adjustable frequency
drive, the motor and the installation.
3. Incorrect slip compensation setting may cause
higher DC link voltage.
4. Back-EMF from PM motor operation. If coasted at
high rpm, the PM motor back-EMF may
potentially exceed the maximum voltage
tolerance of the adjustable frequency drive and
cause damage. To help prevent this, the value of
Introduction to VLT® HVAC D...
VLT
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HVAC Drive Design Guide
2-42 MG11BB22 - VLT
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