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4.13 Selection and protection of a motor
Purpose
Motor protection from overheat
Use the constant torque motor
The motor performance can be
maximized for operation in magnetic
flux vector control system
High accuracy operation unaffected
by the motor temperature and stable
operation with high torque down to
ultra low speed are performed
4.13.1 Motor protection from overheat (Electronic thermal relay function) (Pr. 9, Pr. 51)
Set the current of the electronic thermal O/L relay to protect the motor from overheat. This feature provides the
optimum protective characteristics, including reduced motor cooling capability, at low speed.
Parameter
Name
Number
Electronic thermal
9
O/L relay
Second electronic
51
thermal O/L relay
*1
The initial value of the FR-A720-00030 and 00050 (FR-A740-00015 and 00025) is set to 85% of the rated inverter current.
*2
When parameter is read using the FR-PU04, a parameter name different from an actual parameter is displayed.
(1) Electronic thermal relay function operation characteristic (THM)
[Electronic thermal relay function operation characteristic (E.THM)]
Pr. 9 = 50% setting of
inverter rating*
1.2
70
30Hz
or more*
3
20Hz
60
10Hz
6Hz
0.5Hz
50
240
180
120
60
105%
52.5%
50
Inverter output current(%)
(% to the rated inverter current)
CAUTION
Fault by electronic thermal relay function is reset by inverter power reset and reset signal input. Avoid unnecessary reset and
power-off.
When running multiple motors with one inverter or operating a multi-pole motor, provide a thermal relay (OCR) between the
inverter and motor. And for the setting of the thermal relay, add the line-to line leakage current (refer to page 52) to the current
value on the motor rating plate. For low-speed operation where the cooling capability of the motor reduces, it is recommended
to use a thermal protector or thermistor-incorporated motor.
When the difference between the inverter and motor capacities is large and the setting is small, the protective characteristics of
the electronic thermal relay function will be deteriorated. In this case, use an external thermal relay.
A special motor cannot be protected by the electronic thermal relay function. Use the external thermal relay.
Since a thermal protector is built in a motor dedicated for vector control (SF-V5RU), set "0" in Pr. 9 to use the motor
Electronic thermal relay may not function when 5% or less of inverter rated current is set to electronic thermal relay setting.
Parameter that must be Set
Electronic thermal O/L relay
Applied motor
Offline auto tuning
Online auto tuning
Initial Value
200V class (400V class)
02150 (01100) or less
Rated inverter
current
02880 (01440) or more
*1
02150 (01100) or less
9999
02880 (01440) or more
*2
Pr. 9 = 100% setting
of inverter rating*
2
30Hz or more*
3
20Hz
Operation region
Region on the right of
10Hz
characteristic curve
6Hz
Non-operation region
Region on the left of
0.5Hz
characteristic curve
Characteristic when
electronic thermal relay
function for motor
protection is turned off
(When Pr. 9 setting is 0(A))
Electronic thermal relay
function for transistor
protection
100
150
Selection and protection of a motor
Pr. 9, Pr. 51
Pr. 71
Pr. 82 to Pr. 84,
Pr. 90 to Pr. 94, Pr. 96
Pr. 95, Pr. 574
Setting Range
0 to 500A
Set the rated motor current.
0 to 3600A
0 to 500A
Valid when the RT signal is ON.
Set the rated motor current.
0 to 3600A
9999
Second electronic thermal O/L relay invalid
This function detects the overload (overheat) of the
motor, stops the operation of the inverter's output
transistor, and trips. (The operation characteristic is
shown on the left)
Set the rated current [A] of the motor in Pr. 9. (If the
motor has both 50Hz and 60Hz rating and the Pr. 3
Base frequency is set to 60Hz, set the 1.1 times of the
60Hz rated motor current.)
Set "0" in Pr. 9 when you do not want to activate the
electronic thermal relay function, e.g. when using an
external thermal relay with the motor. (Note that the
output transistor protection of the inverter functions
(E.THT).)
When using the Mitsubishi constant-torque motor
1) Set "1" or any of "13" to "18", "50", "53", "54" in Pr. 71.
(This provides a 100% continuous torque characteristic
in the low-speed range.)
2) Set the rated current of the motor in Pr. 9.
*1
When 50% of the rated inverter current (current value) is set in Pr. 9
*2
The % value denotes the percentage to the rated inverter current. It
is not the percentage to the motor rated current.
*3
When you set the electronic thermal relay function dedicated to the
Mitsubishi constant-torque motor, this characteristic curve applies
to operation at 6Hz or higher.
Refer to Page
185
189
192
202
Description
185
4
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