Total Harmonic Distortion (THD)

The high THD on the input side of the VFD will have no direct affect on a motor connected to the output of the VFD. The high input THD may have an affect on the rectifier section of the VFD, but that will not get down to the motor unless there is a catastrophic failure of the drive.

Motors are however sometimes affected by the THD that naturally occurrs in the output of a VFD (which has nothing to to with any line side THD). In generl, you can clean up the output of a VFD with load filters, but my suggestion is to do that only if you see a problem developing. The THD on the output s worst when the frequency is lower, but at the same time, the load on the motor is lower as well, so there is more "head room" in terms of the motor's thermal capacity; its ability to absorb the extra heating effects of the THD. At some point however, the speed often becomes so low that any external cooling system for the motor may itself be compromised, so the heating effects of THD overtake the motor's ability to absorb or dissipate it and the motor is damaged.

Harmonic distortion means that the signal contains not only the base frequency but its harmonics too. The iron core of the motor is designed for operation on the base frequency and higher frequencies will result higher iron loss, those will only heat the motor. 25 % harmonic distortion means that about 1/4th of the invested energy will useless and will only heat the motor.

In airplane industry there are motors and transformers designed for 400 Hz operation but those have special iron core material.

At lower speeds the motor cooling also is redused unless it is a force cooled motor with a separate fan as shat mounted cooling fan will give 1/3 of the air.Load reduction reduces the heating .However the insulation is designed for sine wave and unlss it is a motor designed to operate from vFD u may face insulation failure as VFD out put wave form is not pure sine wave

Are you saying the VFD causes THD of 25% (current distortion)? If so this sounds quite good for a 6 pulse VFD - what power is it?

On the other hand, 25% THD of voltage is catastrophic, if so, you should immediately investigate further as it will cause many problems, so I assume the information relates to the current drawn by the VFD.

Look at it this way, the VFD draws current from the mains supply in 'lumpy spikes' rather than sinusoidally, this can cause some voltage distortion (ripple or notches) on the voltage supply depending on various other factors like how 'strong' the supply is (a weak supply will have its voltage distorted more by such currents).

If the supply voltage is disturbed such that it becomes less sinusoidal (distortion) this will impact on other equipment (e.g. capacitors) designed for a reasonably good supply (e.g. better than 5% THD voltage). If the supply was very poor, this would also impact on the motor as other respondants have stated, increasing motor losses etc.

The fact that the VFD draws current in this way is normal and has no impact on the motor side where the voltage is balanced and the current shape near sinusoidal nowadays. So, really your consideration probably is 'does the VFD affect the supply, if so, how is it affected and what impact might this have?'. As I say, most equipment is designed to operate on tolerably dirty supplies, typically designed to operate OK at 8% with a planned level of 5% max on the plant giving a safety margin.

Does this help or raise more questions?