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Basic functions (Pr. 0 to Pr. 9)
3.3.6 Motor overheat protection (Pr. 9, Pr. 452, Pr. 876 )
<Setting>
• When not using an external thermal relay, set the rated current value [A] of the motor in Pr. 9 (Pr. 452) to make
the electronic thermal relay function valid.
(Normally set the rated current value at 50Hz. When the rated current value of 50Hz is not indicated on the name
plate, set the value obtained from multiplying the rated current value of 60Hz by 1.1.)
• Setting "0" in Pr. 9 (Pr. 452) deactivates the electronic thermal relay function (motor protective function). (The
inverter's output transistor protective function is activated.)
When using the dedicated motor, set "0" since the thermal relay protector is onboard (outside).
• When using a Mitsubishi constant-torque motor, first set "10" in Pr.71 "applied motor". (This provides a 100%
continuous torque characteristic in the low-speed region.) Then, set the rated current of the motor in Pr. 9
"electronic thermal O/L relay".
• The electronic thermal relay function of the second motor (Pr. 452 "second electronic thermal O/L relay" is made
valid by:
Turning on the RT signal; and
Setting other than 9999 in Pr. 450.
(The value set in Pr. 9 is valid when Pr. 452 = 9999.)
z Selection for whether to use an external thermal relay or not (Pr. 876 "thermal relay
protector input")
When an external thermal relay is not used, protect the motor from overheat by integration processing of the
inverter output current. This feature provides the optimum protective characteristics, including reduced motor
cooling capability, at low speed.
Parameter Name Factory Setting Setting Range Remarks
9 Electronic thermal O/L relay 0 0 to 500A Extended mode
452
Second electronic thermal
O/L relay
9999 0 to 500A, 9999
Extended mode
9999: Without second
electronic thermal relay
function
876 Thermal relay protector input 1 0, 1 Extended mode
Pr. 876 Setting Motor with encoder (e.g. SF-JR)
0 When thermal relay etc. is not used (thermal relay protector input invalid)
1
(factory setting)
When thermal relay etc. is used (thermal relay protector input valid)
CAUTION
• When two or more motors are connected to the inverter under V/F control, they cannot be protected by
the electronic thermal relay function. Install an external thermal relay to each motor.
• When a 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.
speed torque position
Set value 1Set value 0
IM
U
V
W
OH
SD
Not used
IM
OH
SD
OHT error occurs when terminals OH-SD are open.