The relative influence of the frequency converter's harmonics on the motor and its bearings

Update:02 Sep 2020

Frequency converter adjusts the voltage and frequency of the output power supply by the internal IGBT switch, and provides the required power voltage according to the actual needs of the motor, so as to save energy and speed.
Influence of Frequency Converter Harmonic on Electric Machine
The effect of energy saving transformation of frequency converter is obvious, but the harmonic pollution of frequency converter has become a serious problem at present. It is necessary to install filters such as reactors to solve the harmonic problem.
The influence of output harmonics on the motor is mainly caused by the additional heating of the motor, which leads to the additional temperature rise of the motor. The motor often needs to be used down. Due to the distortion of output waveform, the repetitive peak voltage of the motor is increased, which affects the insulation of the motor. Harmonics also cause the motor torque pulsation and noise increase.

Case Study on the Impact of Frequency Converter on Electric Machine
Motor problem:
1. The motor stator has two opposite positions
2. The stator and rotator wipe the center and the rotator outer circle is damaged
3. Bad measurement of bearing chamber, damaged bearing chamber
After investigation by the motor production enterprise, the stator is burnt out facing each other.From the photo provided by the customer, the stator and rotor have obvious friction marks; the root of the stator outlet line is seriously black and some copper wires are bare.The preliminary conclusion is that the burning of the stator two-phase windings caused by the rubbing of the stator and rotor centers.The high current and high heat produced by the instantaneous burning result in serious damage to the stator outlet.

Bearing Damage and Rotor Cleaning Chamber Analysis
Due to the damage of the front-end bearing, a low point is bound to be formed at the front-end bearing, causing the form of low front and high back of the rotor. At the same time, the low point of the front-end bearing becomes a fulcrum because the back-end bearing is not damaged (which can also explain the black ablation marks of the bearing inner sleeve). The unbalanced pivot movement of the back-end bearing causes friction between the bearing outer sleeve and the bearing chamber, resulting in friction.After the bearing chamber is destroyed, an enlarged conical track is finally formed during the rotation of the rotator, and the final contact between the rotator and the stator causes burnout.
Customer provided bearing photos clearly show that the clearance of bearing holder has changed, indicating that the bearing rollers have been seriously damaged, the bearing inner ring is obviously black, indicating that the bearing is not damaged due to the clearance of the jacket.
The analysis is as follows:

Bearing due to frequency conversion harmonic current is too large, which causes the shaft current to be too large, which causes the bearing to be corroded, causes the bearing to slide, drags the bearing jacket for friction motion, and causes the bearing chamber to be deformed.Therefore, the ablation of the inner jacket is more severe than that of the outer jacket.
The motor bears more radial loads than deep groove ball bearings can accept, causing the bearing jacket to be dragged and displaced, thus causing bearing damage.
The final appraisal conclusion is that the burning of bearings at non-bearing ends is the direct cause, which causes serious damage to the bearing holder and the burning of the stator after the rotor loses its supporting balance.The causes of bearing burnout are: due to the non-axle extension, which is a non-loaded part, it should be maintained for customers, frequency conversion usage caused by excessive motor shaft current.Therefore, the motor is not a manufacturing quality problem.

From another point of view, if the quality of motor products is a problem, the following situations will occur:
1. If the bearing chamber is too poor due to manufacturing quality, the bearing can not keep running for a period of 14 months.
2. Copper leakage at the outlet should be caused by the instantaneous huge current impact when the motor is burned out, resulting in the breakage of the insulation layer.
3. The clearance between the stator and the rotator is about 1.5mm. If the clearance of bearing chamber is 40 micrometers too large, it is not enough to cause the stator and the rotator to wipe the center.
Harmonic effect of frequency converter may cause damage to electrical equipment and even more serious impact in the actual process. Considering the harmonic problem, it is necessary to install devices including filters, reactors, etc. to ensure the normal operation of the equipment.

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