Energy Savings and AC Induction Motors

drive can sense the load. if u r running on full load threre will be no saving but the advantage of soft start and soft stop will be still there. if the load is varing the drive will supply as much as the load demand . For e.g. if load is 40 % it will consume 40 % energy not 100 % as in case of DOL.

The power drawn by the motor varies with load even when not connected via vfd. So if the load is 40% the power consumed shall be less than 100% of the rating, but larger than 40% of course.

And JRaef is right with the answer to the question. Use variable frequency drive for variable loads; also good at starting the motor.

The energy saving of the VFD system is best realized if the ratio of the maximum to minimum loads is high and also if the system operates at less than maximum load for a significant fraction of time.

Maximum percentage saving depends on your application. It can be calculated by different methods. e.g.

W2/W1 = (N2/N1)³

^{N=Speed , W=Power}

Even if you run your motor in full load, it will give u saving, as drive will maintain nearly unity power factor.

"Even if you run your motor in full load, it will give u saving, as drive will maintain nearly unity power factor."

It should be pointed out that this is not ENERGY savings, but may represent a small amount of OPERATING COST savings if your power utility charges penalties for poor power factor. Improving power factor does not save energy.

But in response to the original question, energy savings with VFDs is ONLY related to improving the efficiency of a variable process system. In other words, if you MUST or CAN vary the flow in a pump for example, there are many ways to do that. Other than a VFD, all of those ways involve running the motor at full speed and choking off the output somehow to regulate flow (or pressure). The various methods of choking off the output all represent an efficiency loss in the system. By modulating the motor speed INSTEAD of choking off the output, you eliminate those losses and thus save the energy they represent.

If however you are not regulating flow, you will actually LOSE energy in the conversion process that takes place in a VFD. that is why people mentioned that if you run at full load, there is no savings.

Jraef is right. The other energy saving from motor or cable losses reduction rarely pay for the VFD and the added complexity. GA Jraef .

At constant torque load what will be the energy saving?

Centrifugal pumps and blowers is where VFD's can really save you some energy. Typically if you give a AC motor (Nema B) 50% of its FLA you will usually get something like 80% flow out of it and you can further benefit from the soft start feature (limits inrush current and wear and tear). Edmund

It all depends upon the load.Mostly using VFD results in savings when a load such as a pump has operate on different loads at different times.This happens in pumps, Blowers and fans , conveyors etc whre a motor as a consequence of load operates at diffrent load conditions.An Induction motor has lower eff and PF when operating below 60% Load.The whole operation needs to be studied before using a VFD.In one instance we found that a pump was operating for a short time at a low load thro VFD and By adding a additional pump to meet that load and removal of VFD was found to be a better decision.If your motor is running at full load or near full load u will not gain by VFD.Also remember system eff =VFD EFF* Motor EFF which will be always lower.If you really want to save energy study your motor eff and then try and use energy eff motors which have a higher eff and also a load Vs eff curve which is almost constant till 50%.In short energy saving can be decided only after a detailed study.