Dispersion Motor PWM Operation

The answer to the question on vortices has more to do with the equipment that the motor is connected to rather than the drive system applied to it. If the load is dropping, then it would indicate a dropping of the viscosity and a corresponding increase in the Reynolds Number in the process as the viscosity is in the denominator; whether or not the vortex remains after 10 seconds depends on the size of the equipment, the Reynolds Number achieved, and how well or poorly the flow is baffled, none of which can be seen from here.

The answer to the second question is that the process will take whatever power it needs from the supply given that the motors are programmed to run at constant speed. Any attempt to save energy may well be met by the product going out of specification due to the process changes inherent in running the motors at a lower speed. The equipment is there to produce in-spec product, though, so savings in energy are trivial in comparison with the loss of sales income, and the cost of disposal or re-work of off-spec product. As the motors are driven by VFDs, which manage the use of power quite effectively, it is safer to disregard any potential energy savings in this instance until trials have been completed on pilot-scale equipment with process materials that do not form part of the income stream. So, hand the issue over to the Product Development Laboratory, and walk away.

PWSlack has provided you with the right answer, consistant with the answer in the previous thread. He is pointing out that you are incorrectly ASSuming that just because a motor is slightly over sized that it is therefore wasting energy. It is not, the energy consumed in accomplishing the task is determined by the task, not the size of the tool. Does a large hammer "waste energy" when driving a small nail? Not really, it just depends on whether or not the hammer hits it with only the force that it takes. In an AC motor, that is already inherent in the system, it only consumes what it needs to consume.

The only caveat is waste energy, the enegy it takes to make a motor into a motor instread of an iron boat anchor. There is a SLIGHT amount of added magnetizing energy consumed in making that 45kW motor work compared to a 37kW motor. But the first moment that you over load that 37kW motor, ALL savings achieved by under sizing it may go right out the window. And using a 1.25SF motor? Same thing. The motor may be CAPABLE of 1.25SF, but if you read the fine print, the efficiency values go right out the window at anything above 1.0 SF anyway, so the idea of trying this to save energy is absolutely pointless compared with just using a larger more energy efficient motor.

As to the VFD. I mildly disagree with PWSlack on this issue, only in that if the load REQUIRES a fixed speed, a VFD adds it's own level of inefficiencies and that might be unnecessary. If the process can be accomplished at a speed naturally attained by the motor, then the VFD adds NOTHING to the equation, it only lowers the throughput efficiency by the losses in the VFD itself, usually 3% or so. If however you have to adjust speed with a belt drive or gear box, there are efficiency losses in those as well. So bottom line, you cannot make any universal assumption about a VFD running a FIXED SPEED load more efficiently or not, it all depends. Where PWSlack's statement makes absolute sense if it the process MUST vary the speed, because a VFD is ALWAYS more efficient than any other methood of variance.

Now as to YOUR application of a VFD supposedly providing constant torque. Your question in the other thread; "What is the VFD doing here?" (sic), was actually very valid. A VFD, DTC or not, runs in EITHER velocity mode or torque mode. If you command the VFD to maintain torque, it will on occasion sacrifice velocity to do so. If you command it to maintain a specific speed, it will provide whatever torque possible to do so. The concepts are essentially mutually exclusive. The only advantage you have in using the VFD in DTC or (Sensorless Vector Control if it were anything but an ABB drive), is that the VFD vector algorithm can BOOST torque temporarily in the motor while maintaining speed accurately. But if you need a CONSISTANTLY higher level of torque than the motor is designed for, the VFD will likely protect it from overload by tripping if you continuously attempt to get more from it than it is rated for. So this brings you full circle back to where you started. 45kW or 37kW? I would have stuck with the 45kW.