The variable frequency drive (VFD) has the capacity of 11kW, it's a rotogravure printing machine which is used in packaging industry (plastic web films). The data is collected from the VFD display when the motor was running:

The set speed or speed command given was 100 m/min via HMI (Touchscreen LCD)

Frequency Reference: 48.83 Hz

Output Frequency: -50.10 Hz to -50.82 Hz (Not stable, fluctuating between this range)

Output Current: 16 Amps to 16.8 Amps (Not stable, fluctuating between this range)

Motor Speed: - 47. 50 Hz to -50.37 Hz (Not stable, fluctuating between this range)

Output Voltage: 405.5 V to 408.2 V (Not stable, fluctuating between this range)

Output Kilowatts: 20 kW to 22 kW (Not stable, fluctuating between this range)

DC Bus Volts: 544 to 548 VDC (Not stable, fluctuating between this range)

Torque Reference: -135 % to - 150 % (Not stable, fluctuating between this range)

Almost between the mid-range frequency (and from the set speed of the machine in meters/minute, from 50m/min to almost 90m/min) the motor takes more amps then its rated FLA (the rated FLA is 30.5 Amps) and it takes 35 Amps sometimes 38 Amps ... and hence the VFD trips the Motor.

The motor should not draw more than 30.5 Amps at any frequency range or at any point, on lower frequency range and on higher frequency range it runs smoothly and perfectly without any tripping. **Let's use a checklist:**

1.a) Did you do a starting study on the system?

1.b) What is your ratio in terms of absorbed vs installed?

2.) If the application or load is of a high inertia or torque application the VFD should be over sized or as a min the Torque setting needs to be at its maximum

3.) Are the drive and the motor more than 150m apart from each other?

4.) Are the load of a vibrating nature, i.e. that a natural system frequency increases the actual load?

5.) Are your motor suitable for VFD control, class H i think?

6.) Do you have harmonic filtering in place?

7.) Do you have a sine filter on the secondary side?

8.) Does this trip only occur during transient states or on a when the feed has settled to a continuous feed?

9.) What is the nominal voltage? What voltage do you measure at the motor terminals and what is the % difference....remember that Torque is proportional to V^2 and if you have a significant V_{drop} the current will increase to compensate for that drop and to maintain the power requirement.

10.) Verify the motor installation...i.e. do a vibration check on it, if you are using V-belts check condition, check gearbox, etc.

11.) Are your switching frequency set right?

12.) What are the individual phase voltages, are they within 5% of each other.

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