Danfoss VLT HVAC Drive FC 100 Series, Руководство по конструкции

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4-19 Max Output Frequency is automatically
limited based on an internal calculation based on
the value of
1-40 Back EMF at 1000 RPM ,
1-25 Motor Nominal Speed and 1-39 Motor Poles .
If it is possible that the motor may overspeed
(e.g., due to excessive windmilling effects) then it
is recommended to equip a brake resistor.
WARNING
The adjustable frequency drive must be equipped with a
break chopper.
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.
See
2-10 Brake Function and 2-17 Over-voltage Control to
select the method used for controlling the intermediate
circuit voltage level.
NOTE!
OVC cannot be activated when running a PM motor
(when
1-10 Motor Construction is set to [1] PM non-salient
SPM).
Line Drop-out
During a line drop-out, the Adjustable frequency drive
keeps running until the intermediate circuit voltage drops
below the minimum stop level, which is typically 15%
below the adjustable frequency drive's lowest rated supply
voltage. The AC line voltage before the drop-out and the
motor load determine how long it takes for the inverter to
coast.
Static Overload in VVC plus Mode
When the Adjustable frequency drive is overloaded (the
torque limit in
4-16 Torque Limit Motor Mode / 4-17 Torque
Limit Generator Mode is reached), the controls reduce the
output frequency to reduce the load.
If the overload is excessive, a current may occur that
makes the Adjustable frequency drive cut out after approx.
5–10 sec.
Operation within the torque limit is limited in time (0–60
sec.) in
14-25 Trip Delay at Torque Limit .
2.13.1 Motor Thermal Protection
This is the way Danfoss is protecting the motor from being
overheated. It is an electronic feature that simulates a
bimetal relay based on internal measurements. The charac-
teristic is shown in
Figure 2.38
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]
175ZA052.11
fOUT = 0.2 x f M,N
fOUT = 2 x f M,N
fOUT = 1 x f M,N
IMN
IM
Figure 2.38 The X-axis show the ratio between I
motor
and I
motor
nominal. The Y-axis is showing the time in seconds before the
ETR cuts off and trips the Adjustable frequency drive. The curves
show the characteristic nominal speed at twice the nominal
speed and at 0.2x the nominal speed.
At lower speeds, the ETR cuts off at lower levels due to
reduced cooling of the motor. In that way, the motors are
protected from being overheated even at low speeds. The
ETR feature calculates the motor temperature based on the
actual current and speed. The calculated temperature is
visible as a read out parameter in
16-18 Motor Thermal in
the Adjustable frequency drive.
The thermistor cut-out value is > 3k
Ω
.
Integrate a thermistor (PTC sensor) in the motor for
winding protection.
Motor protection can be implemented using a range of
techniques: PTC sensor in motor windings; mechanical
thermal switch (Klixon type); or Electronic Thermal Relay
(ETR).
Introduction to VLT® HVAC D...
VLT
®
HVAC Drive Design Guide
MG11BB22 - VLT
®
is a registered Danfoss trademark 2-43
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