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VFD shows tripping of Motor High Temperature

If run Variable Frequency Drive average frequency below 20 Hz. The VFD shows tripping of Motor High Temperature. But greater than 25 Hz no such problem occurs.

There are potentially two things that lead to this. I am assuming that the motor has no external cooling fans. The low speed operation at 20Hz leads to almost no self-ventilation in the motor. This in itself will lead to overheating. There is another more subtle force at work too. At low frequencies the impedance (AC resistance) of the motor coils is substantially less than at higher frequencies. This means that as the frequency decreases the current through the coils increases. With the same voltage applied the power dissipation in the coils increases with increased current. That power is converted to heat in the motor. In an extreme example think about applying DC to an AC motor. In that case the only impedance (resistance) is that of the copper and the current consumption will be enormous. The motor will almost immediately heat to the breakdown temperature of the wire insulation.

Follow 3 steps:

Problem due to Motor blower
- check the direction of the motor blower if not correct change the phase connection of motor blower.
- physically touch the motor and conform that is it getting heat.
Problem due to motor bearings..
- de-couple the shaft with the application and rotate it by hand.
Problem due to the thermistor in the motor
- check the Ohmic Value of the thermistor and cross check with the motor manufacturer. I think it will be nearby to 8 ohms(not sure).

Step 2.
check the Parameter settings like..
- current limit
- voltage.
- UP/DOWN Ramp time.
- frequency.
- also check the speed reference path while on the run.
- check for auto tune.. which will tune itself to calculate the stator resistance.

Step 3.
check the input power is stable and conform it that free from harmonics.
If the application is in industry area put a 3 phase input choke.

If nothing work out in these 3 steps...
check with some other motor.. (Since some reworked motor or defected motor doesn't work correctly with drives since drive are sensitive)
if the problem not solved call the VFD manufacturer to check the hardware circuit of the variable frequency drive.

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