Справочник Пользователя для Mitsubishi Electronics FR-D740-012

4.8 Selection and protection of a motor

∗1

The above parameters can be set when Pr. 160 Extended function display selection = "0". (Refer to page 162)

∗2

When parameter is read using the FR-PU04, a parameter name different from an actual parameter is displayed.

Set the current of the electronic thermal relay function to protect the motor from overheat. This feature provides the
optimum protective characteristics, including reduced motor cooling capability, at low speed.

Inverter

rated current

0 to 500A

Set the rated motor current.

9999

0 to 500A

Valid when the RT signal is on.
Set the rated motor current.

9999

Second electronic thermal O/L relay invalid

9999

0.5 to 30k

Ω

Set the level (resistance value) for PTC 
thermistor protection activates.

9999

PTC thermistor protection is inactive.

This function detects the overload (overheat) of the
motor 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 operate the
electronic thermal O/L relay, 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 a Mitsubishi constant-torque motor

1) Set "1" or "13", "50", "53" in any of 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 inverter rated output current (current value) is set to Pr. 9 

∗2

The % value denotes the percentage to the inverter rated output current. It is not the percentage to the motor rated current.

∗3

When you set the electronic thermal O/L relay dedicated to the Mitsubishi constant-torque motor, this characteristic curve applies to operation at 6Hz or higher.

NOTE

The protective function performed by the electronic thermal O/L relay 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
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 an external thermal relay.
The operation time of the transistor protection thermal shortens when the Pr. 72 PWM frequency selection setting value
increases.
Electronic thermal relay does not function when 5% or less of inverter rated current is set to electronic thermal relay setting.

Operation range
Range on the right of characteristic curve
Non-operation range
Range on the left of characteristic curve

Range for  
transistor  
protection

Inverter output current(%)

(% to the rated inverter current)

52.5%

105%

50

100

150

60

120

180

240

50

60

70

(min) unit display in this range

Operation time (min)

Pr. 9 = 50% setting of inverter rating*1, 2

Pr. 9 = 100% setting of inverter rating*2

(s) unit display in this range

Operation time (s)

Characteristic when electronic thermal  
relay function for motor protection is turned off 
(when 

Pr. 9 setting is 0(A))

6Hz

20Hz
10Hz

6Hz

0.5Hz

30Hz or more 

*3

30Hz  
or more 

*3

20Hz
10Hz

0.5Hz

200