Mitsubishi Electric 800 Series Assembly & Installation Manuallines page 53

A.2.6 Motor overload protection
When using the electronic thermal relay function as motor overload protection, set the rated motor current to Pr. 9 "Electronic
thermal O/L relay".
Electronic thermal relay function operation characteristic
Pr. 9 = 50 % setting of
*1, *2
inverter rating
70
30Hz
*3
or more
20Hz
60
10Hz
6Hz
0.5Hz
50
240
180
120
60
105%
52.5%
50
Inverter output current [%]
(% to the inverter rated current)
CAUTION
The internal accumulated heat value of the 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 motors. And for the setting of the thermal relay, add the line-to line leakage current to the
current value on the motor rating plate (details in the FR-A870 Instruction Manual (Hardware)).
The cooling effect of the motor drops during low-speed operation. Use a thermal protector or a motor with built-in thermistor.
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.
Motor over temperature sensing is not provided by the drive.
Electronic thermal memory retention function is not provided by the drive.
Pr. 9 = 100 % setting of
*1, *2
inverter rating
or more
30Hz
Operation region
20Hz
Region on the right of
10Hz
characteristic curve
Non-operation region
6Hz
Region on the left of
characteristic curve
0.5Hz
Characteristic when electronic
thermal relay function for motor
protection is turned OFF
(Pr. 9 = 0 (A))
Region for
transistor
*3
protection
100
150 230
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.)
*1
When 50% of the inverter rated output current (current value)
is set in Pr. 9.
*2
The % value denotes the percentage to the inverter rated
output current. It is not the percentage to the rated motor
current.
*3
Transistor protection is activated depending on the
temperature of the heatsink. The protection may be activated
even with less than 150% depending on the operating
conditions.
APPENDIX
47