Extreme Running Conditions; Motor Thermal Protection - GE AF-650 GP Design And Installation Manual

Application Setup Examples

6.8 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).
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.
6 6
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 if the moment of inertia is
high, the friction is low and the decel 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.
See B-10 Brake Function and B-17 Over-voltage Control to
select the method used for controlling the intermediate
circuit voltage level.
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 Advanced Vector Control Mode
When the adjustable frequency drive is overloaded (the
torque limit in F-40 Torque Limiter (Driving)/F-41 Torque
Limiter (Braking) is reached), the controls reduces the
output frequency to reduce the load.
6-28
TM
AF-650 GP Design and Installation Guide
If the overload is excessive, a current may occur that
makes the adjustable frequency drive cut out after approx-
imately 5-10 s.
Operation within the torque limit is limited in time (0–60 s)
in SP-25 Trip Delay at Torque Limit.

6.9 Motor Thermal Protection

To protect the application from serious damage, AF-650 GP
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 F-40 Torque Limiter (Driving)
and or F-41 Torque Limiter (Braking) and the time before
the torque limit warning shall trip is controlled in
SP-25 Trip Delay at Torque Limit.
Current Limit: The current limit is controlled in F-43 Current
Limit and the time before the current limit warning shall
trip is controlled in SP-24 Trip Delay at Current Limit.
Min Speed Limit: (F-18 Motor Speed Low Limit [RPM] or
F-16 Motor Speed Low Limit [Hz]) limit the operating speed
range to for instance between 30 and 50/60 Hz. Max
Speed Limit: (F-17 Motor Speed High Limit [RPM] or
F-03 Max Output Frequency 1) limit the max output speed
the adjustable frequency drive can provide
Electronic Thermal Overload: The adjustable frequency
drive Electronic Thermal Overload 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.
Electronic Thermal Overload is an electronic feature that
simulates a bimetal relay based on internal measurements.
The characteristic is shown in Figure 6.28:
DET-767A