Mitsubishi Electric FR-D720S SC EC Instruction Manual page 218

Selection and protection of a motor
NOTES
Fault by electronic thermal relay function is reset by inverter power reset and reset signal
input. Avoid unnecessary reset and power-off.
Install an external thermal relay (OCR) between the inverter and a motor when operating
several motors by one inverter, or when using a multi-pole motor or specialized motor. In this
case, set 0A to the electronic thermal O/L relay setting of the inverter. For the external ther-
mal relay, determine the setting value in consideration of the current indicated on the motor's
rating plate and the line-to-line leakage current. Self-cooling ability of a motor is reduced at
low speed operation. Use a motor with a built-in thermal protector.
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 deterio-
rated. Thermal motor protection must be ensured by external motor protection (e.g. PTC ele-
ment, motor protection switch, etc.).
The operation time of the transistor protection thermal relay shortens when the Pr. 72 "PWM
frequency selection" setting increases.
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Parameter