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Selection and protection of a motor
4.8
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 method.
4.8.1
Motor overheat protection (Electronic thermal O/L relay, PTC thermistor protection)
(Pr. 9, Pr. 51, Pr. 561)
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
Name
Number
Electronic thermal O/L
9
relay
Second electronic thermal
51∗1
O/L relay ∗2
PTC thermistor protection
561∗1
level
∗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)
Electronic thermal O/L relay operation characteristic
Pr. 9 = 100% setting of inverter rating*2
Pr. 9 = 50% setting of inverter rating*1, 2
70
30Hz or more
*3
30Hz
or more
20Hz
*3
Operation range
10Hz
20Hz
Range on the right of characteristic curve
60
10Hz
6Hz
Non-operation range
6Hz
Range on the left of characteristic curve
0.5Hz
0.5Hz
50
Characteristic when electronic thermal
relay function for motor protection is turned off
240
(when Pr. 9 setting is 0(A))
180
Range for
120
transistor
protection
60
105%
52.5%
100
50
150
200
Inverter output current(%)
(% to the rated inverter current)
∗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.
100
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Parameter that should be Set
Electronic thermal O/L relay
Pr. 9, Pr. 51, Pr. 561
PTC thermistor protection
Applied motor
Pr. 71, Pr. 80, Pr. 82 to Pr.
Offline auto tuning
84, Pr. 90, Pr. 96
Initial Value
Setting Range
Inverter
0 to 500A
Set the rated motor current.
rated current
Valid when the RT signal is on.
0 to 500A
9999
Set the rated motor current.
9999
Second electronic thermal O/L relay invalid
Set the level (resistance value) for PTC
0.5 to 30kΩ
9999
thermistor protection activates.
9999
PTC thermistor protection is inactive.
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
characteristic in the low-speed range.
2) Set the rated current of the motor in Pr. 9.
Refer to Page
100
Pr. 71
103
105
Description
100%
continuous
torque
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