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Selection and protection of a motor
4.8 Selection and protection of a motor
4.8.1 Motor overheat protection (Electronic thermal O/L relay, PTC thermistor protection)
(Pr. 9, Pr. 51, Pr. 561)
∗1 The above parameters can be set when Pr. 160 Extended function display selection = "0". (Refer to page 162)
∗2 When parameter is read using the FR-PU04, a parameter name different from an actual parameter is displayed.
(1) Electronic thermal O/L relay (Pr. 9)
Purpose Parameter that should be Set Refer to Page
Motor protection from overheat
Electronic thermal O/L relay
PTC thermistor protection
Pr. 9, Pr. 51, Pr. 561
100
Use the constant torque motor Applied motor Pr. 71
103
The motor performance can be
maximized for operation in magnetic
flux vector control method.
Offline auto tuning
Pr. 71, Pr. 80, Pr. 82 to Pr.
84, Pr. 90, Pr. 96
105
Set the current of the electronic thermal relay function to protect the motor from overheat. This feature provides the
optimum protective characteristics, including reduced motor cooling capability, at low speed.
Parameter
Number
Name Initial Value Setting Range Description
9
Electronic thermal O/L
relay
Inverter
rated current
0 to 500A Set the rated motor current.
51∗1
Second electronic thermal
O/L relay ∗2
9999
0 to 500A
Valid when the RT signal is on.
Set the rated motor current.
9999 Second electronic thermal O/L relay invalid
561∗1
PTC thermistor protection
level
9999
0.5 to 30kΩ
Set the level (resistance value) for PTC
thermistor protection activates.
9999 PTC thermistor protection is inactive.
Electronic thermal O/L relay operation characteristic
This function detects the overload (overheat) of the
motor 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 operate the
electronic thermal O/L relay, 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 a Mitsubishi constant-torque motor
1) Set "1" or "13", "50", "53" in any of 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 inverter rated output current (current value) is set to Pr. 9
∗2 The % value denotes the percentage to the inverter rated output current. It is not the percentage to the motor rated current.
∗3 When you set the electronic thermal O/L relay dedicated to the Mitsubishi constant-torque motor, this characteristic curve applies to operation at 6Hz or higher.
NOTE
The protective function performed by the electronic thermal O/L relay is reset by inverter power reset and reset signal
input. Avoid unnecessary reset and power-off.
When multiple motors are operated by a single inverter, protection cannot be provided by the electronic thermal
function. Install an external thermal relay to each 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 an external thermal relay.
The operation time of the transistor protection thermal shortens when the Pr. 72 PWM frequency selection setting value
increases.
Electronic thermal relay does not function when 5% or less of inverter rated current is set to electronic thermal relay setting.
Operation range
Range on the right of characteristic curve
Non-operation range
Range on the left of characteristic curve
Range for
transistor
protection
Inverter output current(%)
(% to the rated inverter current)
52.5%
105%
50
100
150
60
120
180
240
50
60
70
(min) unit display in this range
Operation time (min)
Pr. 9 = 50% setting of inverter rating*1, 2
Pr. 9 = 100% setting of inverter rating*2
(s) unit display in this range
Operation time (s)
Characteristic when electronic thermal
relay function for motor protection is turned off
(when Pr. 9 setting is 0(A))
6Hz
20Hz
10Hz
6Hz
0.5Hz
30Hz or more
*3
30Hz
or more
*3
20Hz
10Hz
0.5Hz
200