Selection and protection of a motor
4.9 Selection and protection of a motor
Motor protection from overheat
Use the constant torque motor
Use the constant torque motor
* Applied motor can be used only with FR-B series.
Motor protection from overheat (Electronic thermal relay function) (Pr. 9)
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.
The initial value of the FR-B-750(200V/400V), FR-B3(N)(H)400, 750 is set to 85% of the rated inverter current.
(1) Electronic thermal O/L relay (Pr. 9)
Electronic thermal relay function operation characteristic
Pr. 9 = 50% setting of
Inverter output current (%)
(% to the rated output current)
⋅ Protective function by electronic thermal relay function 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 relay 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 the external thermal relay.
⋅ The operation time of the transistor protection thermal relay shortens when the Pr. 72 PWM frequency selection setting increases.
Electronic thermal O/L relay
Applied motor *
Offline auto tuning
Pr. 9 = 100% setting
of inverter rating*
30Hz or more*
Region on the right of
Region on the left of
electronic thermal relay
function for motor
protection is turned off
(When Pr. 9 setting is 0(A))
Electronic thermal relay
function for transistor
Parameter that must be Set
Pr. 82 to Pr. 84,
Pr. 90 to Pr. 94, Pr. 96
55K or less
0 to 500A
75K or more
0 to 3600A
This function detects the overload (overheat) of the
motor, stops the operation of the inverter's output
transistor, and stops the output. (The operation
characteristic is shown on the left)
⋅ Set the rated current [A] of the motor in Pr. 9. (When
the power supply specification is 200V/220V(400V/
440V) 60Hz, set the 1.1 times the 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
⋅ In FR-B series, when using the Mitsubishi explosion-
proof constant-torque motor
1) Set "1" 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.
When a value 50% of the inverter rated output current (current
value) is set in Pr. 9
The % value denotes the percentage to the inverter rated output current.
It is not the percentage to the motor rated current.
When you set the electronic thermal relay function dedicated to the
Mitsubishi explosion-proof constant-torque motor, this
characteristic curve applies to operation at 6Hz or higher.
Refer to Page
Set the rated motor current.