VFD 18 Pole?

You probably mean 18 PULSE.

A basic VFD uses 6 diodes in a bridge configuration to convert the incoming AC to DC. This is referred to as "6 pulse" because each of the 6 diodes "pulses" power into the bridge as the AC sine wave passes through it. Because this power consumption is non-linear compared to the normal sine wave, this creates harmonic distortion on the incoming supply. If you split the incoming power into 2 separate sources, one through a Delta-Delta transformer, the other through a Delta-Wye and run it into 2 separate 6 pulse bridges ("12 pulse"), the power consumption from the separate bridges creates harmonics that are shifted by 90 degrees respective to each other, so some of the harmonics cancel each other out, reducing the overall Total Harmonic Distortion (THD). If you split it into 3 service points with phase-shifted Delta-Wye transformers going into 3 bridges with 6 diodes each ("18 pulse"), more of the harmonics cancel each other out and reduce the THD even further, which lowers the cost of mitigating them.

If its the VFD you mean 18 pulse as J Raef describes accurately, if really poles, you are talking about the motor in which case the speed is determined by the number of poles and the frequency. This is normally done to achieve a gearless solution and is common in some applications e.g. elevators

More the number of pulses in a Power Electronic Switching Device, the lesser will be Harmonic Distortion. The first predominat harmonics order in any power electronic system is "mn +/- 1", where 'm is any integer and 'n' is the number of pulses. So, in as 18 pulse VFD, the first predominant harmonics will start from 17 and so on ( 17, 19, 35, 37, 53, 55, etc.). Also, as the order of the harmonics increases its magnitude reduces (magnitude of any harmonics is: magnitude of fundamental/order of the harmonics). Lesser order harmonics will be hard to manage as the magnitude will be significant. So, to remove this lower order harmonics the number of pulses in the power electronic switching is increased. Now, you only have to manage higher order harmonics which is relatively easier as the magnitude will be lesser now.

BUT A WORD OF CAUTION: Any higher pulse device will not automatically reduce the harmonics. It is possible ONLY and ONLY when fed by a transformer with star and delta secondaries. For example, if you have a 12 Pulse VFD, it is usually 2 x 6-Pulse Rectifiers. And, one of these 6-Pulse Rectifiers MUST be fed from a STAR Connected Secondary and the second 6-Pulse Rectifier MUST be fed from a DELTA Connected Secondary of the feeding transformer. So, this feed-in transformer should have two secondaries, one connected in STAR and the other connected in DELTA. Then only you can avoid the lower order harmonics. Otherwise even if you have a 100 Pulse Drive there is no use. This must be understood clearly.