Danfoss VLT FC51, Руководство по конструкции, Страница: 27 / 94

Motor-generated over-voltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in following
cases:
•
The load drives the motor (at constant output
frequency from the frequency converter), that is
the load generates energy.
•
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.
•
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 frequency converter
to coast.
3.5.1 Motor Thermal Protection
Motor thermal protection can be provided in 2 ways.
One method uses a motor thermistor, via one of the
following
•
Thermistor input on a standard AI
•
Sensor input MCB 114
•
PTC Thermistor input MCB 112
The frequency converter monitors motor temperature as
the speed and load vary to detect overheating conditions.
The other method calculates motor temperature by
measuring current, frequency, and operating time. The
converter displays the thermal load on the motor in
percentage and can issue a warning at a programmable
overload setpoint. Programmable options at the overload
allow the frequency converter to stop the motor, reduce
output, or ignore the condition. Even at low speeds, the
frequency converter meets I2t Class 20 electronic motor
overload standards.
This method is called Electronic Thermal Relay (ETR).
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 3.8 ETR Characteristics
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 frequency converter. The curves
show the characteristic nominal speed, at twice the
nominal speed and at 0,2 x the nominal speed.
At lower speed, the ETR cuts off at lower heat due to less
cooling of the motor. In that way, the motor is 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 product specific Programming Guide .
A special version of the ETR is also available for EX-e
motors in ATEX areas. This function makes it possible to
enter a specific curve to protect the Ex-e motor. The
Programming Guide takes the user through the set-up.
Product Overview
VLT
®
Micro Drive FC 51 Design Guide
MG02K302 - Rev. 2013-12-03 25
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