Selection And Protection Of A Motor; Motor Protection From Overheat (Electronic Thermal Relay Function) (Pr. 9, Pr. 51) - Mitsubishi Electric FR-A720-0.4K Instruction Manual

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 0.4K and 0.75K 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 using multiple motors with one inverter, or using a multi-pole motor or a specialized motor, provide an external 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.
⋅ 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
55K or lower
Rated inverter
current
75K or higher
*1
55K or lower
9999 75K or higher
*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 187 for the operation
characteristic.)
Refer to Page
183
187
189
199
Description
183
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