Motor overload protection
When using the electronic thermal relay function as motor overload protection, set the rated motor current in Pr.9 Electronic
thermal O/L relay.
Operation characteristics of electronic thermal relay function
Pr. 9 = 50% setting of
Inverter output current (%)
(% to the rated output current)
• 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 multiple motors are driven with a single inverter or when a multi-pole motor or a special motor is driven, install an
external thermal relay (OCR) between the inverter and motors. Note that the current indicated on the motor rating plate is
affected by the line-to-line leakage current (refer to
• 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.
• Set Pr.9 = "0" for vector-control-dedicated motors (SF-V5RU) because they are equipped with thermal protectors.
• Motor over temperature sensing is not provided by the drive.
Pr. 9 = 100% setting
of inverter rating*
30Hz or more*
Range on the right of
Range on the left of
Characteristic when electronic thermal
relay function for motor protection is
turned off (When Pr. 9 setting is 0(A))
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.)
• When using the Mitsubishi constant-torque
(1) Set one of "1", "13" to "16", "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.
68) when selecting the setting for an external thermal relay.
When a value 50% of the inverter rated output
current (current value) is set in Pr.9
The % value denotes the percentage to the
rated inverter current. It is not the percentage to
the rated motor current.
When you set the electronic thermal relay
function dedicated to the Mitsubishi constant-
torque motor, this characteristic curve applies to
operation at 6 Hz or higher.
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.