VFD related

Not absolutely sure though, but I think, this will inject harmonics into the system.

i dont think harmoincs will inc , bcoz theris 3 core transformer which will mitigate the harmonics..it may be related with torqe and pump aur power requirement

There will be a mechanical limitation on the motor and Pump based on the design.Second the torque starts decreasing when u run above syn . speed with Power remaining constant.

Bear in mind that for a centrifugal pump, the characteristic curves are drawn for specific impeller speeds; doubling the frequency does not produce double the flow, as the system characteristics need to be considered too. The operating point is where the pump characteristic and the system characteristic curves intersect.

Bingo. Good answer vote.

And as also said above, the power (kW or HP) and thereby the torque requirement of the load will increase (although not linearly) with the slightly higher flow, but above full rated speed the motor output power cannot increase and with a flat kW/HP, the motor torque drops off rapidly. Less torque but more load = more slip = higher current draw = eventual overload! You might get a very slightly higher flow out of your pump because although what PWSlack says is true, no pump design matches a given motor speed exactly. You might be able to take advantage of a slight "overshoot" in the design curve, but at the cost of exponentially increase motor loading which effectively shortens your motor life (if it stays running at all)

At frequencies above the base frequency (50 or 60Hz), the motor will have to be derated. I don't have the data here at home though. It's in the office under a pile of other data that I don't need.

You also need to consider that the mechanical equipment that the motor is driving is probably not designed to run too much beyond the nominal speed. It's very possible that the pump could self-destruct if you push it too high (I wouldn't like to be standing next to it when it does).

Then there's the failure rate for bearings and seals.

When asked this question by my workmates, my answer is always "buy a higher capacity pump and the motor to match".