3.5 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 please see the
design guidelines.
Switching on the Output
Switching on the output between the motor and the
frequency converter is fully permitted. The frequency
converter will not be damaged in any way by switching on
the output. 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 (
1-62 Slip
Compensation
) may cause higher DC link voltage.
The control unit may attempt to correct the ramp if
possible (
2-17 Over-voltage Control
.)
The inverter turns off to protect the transistors and the
intermediate circuit capacitors when a certain voltage level
is reached.
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.
3.5.1 Motor Thermal Protection
To protect the application from serious damages offers
several dedicated features
Torque Limit: The Torque limit feature the motor is
protected for being overloaded independent of the speed.
Torque limit is controlled in
4-16 Torque Limit Motor Mode
and or
4-17 Torque Limit Generator Mode
and the time
before the torque limit warning shall trip is controlled in
14-25 Trip Delay at Torque Limit
.
Current Limit: The current limit is controlled in
4-18 Current
Limit
and the time before the current limit warning shall
trip is controlled in
14-24 Trip Delay at Current Limit
.
Min Speed Limit: (
4-11 Motor Speed Low Limit [RPM]
or
4-12 Motor Speed Low Limit [Hz]
) limit the operating speed
range to for instance between 30 and 50/60Hz. Max Speed
Limit: (
4-13 Motor Speed High Limit [RPM]
or
4-19 Max
Output Frequency
) limit the max 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.
The characteristic is shown in
1.2
1.0
1.4
30
10
20
100
60
40
50
1.8
1.6
2.0
2000
500
200
400
300
1000
600
t [s]
175ZA052.12
fOUT = 2 x f M,N
fOUT = 0.2 x f M,N
fOUT = 1 x f M,N(par. 1-23)
I
MN
(par. 1-24)
I
M
Illustration 3.4 ETR: The X-axis shows the ratio between I
motor
and I
motor
nominal. The Y- axis shows the time in seconds before
the ETR cut of and trips the drive. The curves show the charac-
teristic nominal speed, at twice the nominal speed and at 0,2 x
the nominal speed.
At lower speed the ETR cuts of at lower heat due to less cooling
of the motor. In that way the motor are protected from being
over heated even at low speed. The ETR feature is calculating
the motor temperature based on actual current and speed. The
calculated temperature is visible as a read out parameter in
16-18 Motor Thermal
in the
FC 51 Micro Drive Programming Guide,
MG02CXYY
.
Introduction to VLT Micro D...
VLT Micro Drive Design Guide
MG02K102 - VLT
®
is a registered Danfoss trademark
25
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