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Motor
10.6 Extreme Running Conditions
Short circuit (motor phase–phase)
The frequency converter is protected against short circuits
by current measurement in each of the 3 motor phases or
in the DC link. A short circuit between 2 output phases
causes an overcurrent in the inverter. The inverter turns off
individually when the short-circuit current exceeds the
allowed value (alarm 16, Trip Lock).
To protect the frequency converter against a short circuit
at the load sharing and brake outputs, see Application Note
for FC 100, FC 200 and FC 300 Fuses and Circuit Breakers.
See certificate in chapter 3 Approvals and Certifications.
Switching on the output
Switching on the output between the motor and the
frequency converter is fully allowed. Switching on the
output does not damage the frequency converter, but fault
messages can appear.
Motor-generated overvoltage
The voltage in the DC link increases in the following cases:
•
When the load generates energy, the load drives
the motor at a constant output frequency from
the frequency converter.
•
During deceleration (ramp-down) when the
inertia moment is high, the friction is low, and
the ramp-down time is too short for the energy
10
10
to be dissipated as a loss in the frequency
converter or motor.
•
Incorrect slip compensation setting can cause
higher DC-link voltage.
•
Back EMF from PM motor operation. If coasted at
high RPM, the PM motor back EMF can
potentially exceed the maximum voltage
tolerance of the frequency converter and cause
damage. To help prevent this scenario, the value
of parameter 4-19 Max Output Frequency is
automatically limited based on an internal
calculation based on the value of
parameter 1-40 Back EMF at 1000 RPM,
parameter 1-25 Motor Nominal Speed, and
parameter 1-39 Motor Poles.
If it is possible that the motor may overspeed,
Danfoss recommends that a brake resistor be
equipped to the frequency converter.
NOTICE
The frequency converter must be equipped with a brake
chopper.
If possible, the control unit may attempt to correct the
ramp (parameter 2-17 Over-voltage Control). The inverter
turns off to protect the transistors and the DC link
capacitors when a certain voltage level is reached. To
select the method used for controlling the DC-link voltage
level, see parameter 2-10 Brake Function and
parameter 2-17 Over-voltage Control.
112
®
VLT
Parallel Drive Modules
NOTICE
OVC cannot be activated when running a PM motor
(when parameter 1-10 Motor Construction is set to [1] PM
non-salient SPM).
Mains dropout
During a mains dropout, the frequency converter keeps
running until the DC-link voltage drops below the
minimum stop level. Minimum stop level typically is 15%
below the lowest rated supply voltage of the frequency
converter. The mains voltage before the dropout and the
motor load determines how long it takes for the inverter
to coast.
Static overload in VVC
An overload occurs when the torque limit in
parameter 4-16 Torque Limit Motor Mode/
parameter 4-17 Torque Limit Generator Mode is reached.
When the frequency converter is overloaded, the controls
reduce the output frequency to reduce the load. If the
overload is excessive, a current can occur that makes the
frequency converter cut out after 5–10 s. Operation within
the torque limit is limited in time (0–60 s) in
parameter 14-25 Trip Delay at Torque Limit.
10.6.1 Motor Thermal Protection
To protect the application from serious damages, the
frequency converter offers several dedicated features.
Torque limit
The motor is protected from being overloaded
independent of the speed. Torque limit is controlled in
parameter 4-16 Torque Limit Motor Mode and
parameter 4-17 Torque Limit Generator Mode. The time
before the torque limit warning trips is controlled in
parameter 14-25 Trip Delay at Torque Limit.
Current limit
The current limit is controlled in parameter 4-18 Current
Limit, and the time before the current limit warning trips is
controlled in parameter 14-24 Trip Delay at Current Limit
Minimum speed limit
Parameter 4-11 Motor Speed Low Limit [RPM] or
parameter 4-12 Motor Speed Low Limit [Hz] limit the
operating speed range to between 30 and 50/60 Hz.
Parameter 4-13 Motor Speed High Limit [RPM] or
parameter 4-19 Max Output Frequency limit the maximum
output speed the frequency converter can provide.
ETR (electronic thermal relay)
The frequency converter ETR function measures actual
current, speed, and time to calculate motor temperature
and protect the motor from being overheated (warning or
trip). An external thermistor input is also available. ETR is
an electronic feature that simulates a bimetal relay based
on internal measurements. Illustration 10.11 provides the
following example, where the X-axis shows the ratio
Danfoss A/S © 6/2016 All rights reserved.
+
mode
MG37N102
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