Current Reduction Thru' VFD

Yes, though undesirable, as all it does is turn the motor into a convection heater until such time as something trips/fails/burns out/blows up.

Before a lot of us jump to conclusions about what you asked, maybe you could enlighten us on what you are trying to get at. Is this related to energy consumption, limited current capacity from your line, what?

Reduction in Energy Consumption.

Ok then.

Fist off, recognize that the reduction of current alone does not automatically reduce ENERGY consumption. This is a very common trick used to sell unnecessary equipment. Be VERY WARY of anyone who attempts to convince you otherwise. There is just no truth to it. Current is a PART of the energy equation, but not the whole thing.

Energy is simply defined as POWER required to accomplish a task through a defined period of TIME, nothing else. So in order to SAVE energy within the same time period, you have to reduce the power. POWER to do a task can be defined as the WORK performed, plus any LOSSES in the conversion of one source of energy to another. So in the case of an electric motor, you are converting electrical energy to kinetic energy, or movement (usually rotational but the recent advent of linear motors messed up that statement).

So to save energy, one must either reduce the losses, or reduce the amount of work being performed. In many cases unfortunately, sales people use VFDs to show a user the reduction of current and claim this is a reduction of energy, but in reality it is a reduction of the work being performed. So yes, you are saving energy in a relative sense, but you are doing less work as well. The ultimate expression of this concept is what my colleagues were teasing about: turning it off completely is the ultimate "energy savings" technique.

That is not to say, however, that VFDs cannot save real energy, they can. But they do so by eliminating LOSSES in a system. So before you can ASSume that a VFD can save energy, you must FIRST identify where the losses are in an overall system that is running at design speed without a VFD. Therein lies the devil of the details. Without knowing the ENTIRE story of your application, it is impossible to say whether a VFD will be able to save real energy or not.

One REMOTE and minor possibility of energy savings potential in ALL AC motor applications is that motors have losses inherent in them from just the basics of the iron and copper becoming a working motor. A small fraction of those losses are related to the applied voltage. IF THE MOTOR IS UNLOADED but for some unknown reason must be spinning anyway, then there is a small fraction of that small fraction of the losses that can be reduced by reducing the voltage at the motor terminals. But as the losses are reduced, so is the work capacity of the motor, hence the requirement that the motor be unloaded. So to take advantage, a system must be capable of detecting that the motor needs tp work harder again and get out of that energy saving mode. All VFDs are capable of doing this. However whether or not your application can take advantage of that is subject to evaluation, and must always be compared to just turning it off when not it use.

It can however be said that it will NEVER be worth installing a VFD just for this purpose. There must be some other advantage to using variable speed. If there is, the application must ALSO be required to be running when there is virtually no load on the motor. Those circumstances are much more rare than the salesmen want you to believe.

Not so fast . . . ! An idling motor does not do any work, but it does have all kinds of losses that can be reduced - without actually stopping the motor. These loses are the copper losses in the winding and the hysteresis core loses. These can be substantially reduced by running the motor at a reduced voltage - using a VFD or other means.

Think of it this way, . . . a loaded motor runs at 99% of synchronous speed, while a unloaded motor runs at 99.99% of synchronous speed. You can safely reduce the voltage to a value that will reduce the motor speed down to 99% synchronous without suffering any ill effects. Depending on the design of the motor, this is likely to be less then 1/2 the nameplate voltage. With half the voltage running the motor - you will have about 1/2 the current, . . . low and behold, the copper losses have just been reduced by 75% - down to 25% of losses. The same savings apply to the hysteresis losses in the core.

Similar savings can be obtained when running a partially loaded motor at reduced voltage

Try it, . . . you will like it!!

No, no ,no, noooo. If a motor is not doing any work then it has zero efficiency. The copper and core losses may be reduced in the motor design process to the point that self heating damage of a stalled motor will not be a problem but a VFD does not change the motor design. Now a VFD may improve a particular induction motor by keeping the slip in the sweet spot of the torque curve for the rotor design. If you have instead a brushed universal motor then you've wasted your money. Changing the supply frequency on these motors is pointless.

I Agree that YOU got very close to the Truth. Can I take it specific that my question replies to a "NO" from you?

"Can I take it specific that my question replies to a "NO" from you?"

If I interpret your question correctly, that you want to reduce ENERGY (from your subsequent response) simply by reducing the voltage and frequency, then the answer would be a QUALIFIED no. If there is some reduction in losses that corresponds to the reduction in speed, then the answer would be yes.

For example, let's say it is a centrifugal pump. The Affinity Law is often quoted as a magical energy savings benefit of putting a VFD on a pump, because it holds that the energy consumed when flow is reduced by changing the shaft speed of the pump drops at the cube of the flow reduction. So by reducing the flow with a VFD, it saves energy. But reducing the flow WITHOUT a VFD, for example with a valve, ALSO saves energy, or rather REQUIRES less energy inherently to begin with. They just fail to bring that up.

But that does not mean a VFD does not save energy in that situation, because it does. The use of the Affinity Law by changing the motor speed drops the energy consumption FASTER within the middle of the speed range than just reducing the flow by restriction. Plus with the valve, the drop in flow has a corresponding drop in pressure across the valve as well as losses in turbulence in the pipe that is expressed in heat (although difficult to notice because it is spread out in the flow). So reducing the flow by reducing the speed with a VFD, eliminates the LOSSES in the valve restriction as well. So using a VFD does indeed save energy COMPARED to reducing the flow with a valve. But then again, if you do not need to VARY the flow and just want a permanent reduction, guess what? The VFD then will WASTE energy compared to trimming the impeller! Trimming the impeller has the exact same Affinity Law effect on power consumption. The only problem is, it is a permanent solution; you can't "un-trim" it when you want more flow again.

So my real answer to your question of can I save energy on a motor by reducing the frequency and voltage is, it just is not that simplistic.

Kindly Clarify with respect to " beyond its No-Load Current / Magnetising Current " part of my question.

I like it when you explain these things in so much detail.

Thank you.

What do you need a VFD for, you can reduce the current to zero by opening the contactor.

No current leads to no motion thus there's no problem.

I assume that you can't afford stopping and starting your load at certain times, either for current surges or mechanical inconveniences; you can drop the freq. output of your drive, but do program an auxiliary output to turn on when running at idle speed and use it to start a an external fan to cool your motor.

Compare the savings against the consumption of your fan before you proceed.

Regards

Sure, but the motor won't do any work, or it will burn up.

So, if your goal is to wring the last penny out of your fixed costs, I'd look somewhere else to cut corners.

Otherwise, use a smaller motor.

If its running purely unloaded then yes you can do a moderate supply voltage and related no load power consumption drop a without causing any issues with the motor.

most 415 VAC 50 Hz (480 VAC American 60HZ) can stay running at near full speed unloaded at as little as 1/3/ to 1/4 of their normal drive voltage with no issues at all.

Its rather easy to do as well. All thats needed for the basic circuit is a contactor set that switches the power from the normal 415 VAC circuit to a lower voltage circuit of the same phasing configuration once the motor is up to speed but not carrying any load.

Good induction motors are designed to operate most efficiently in the 80-100% load range. They become much less efficient when operated at reduced loads - typically very inefficient when operated near zero load.

Such induction motors, operating at reduced load, will become more efficient when operating at reduced voltage. You do not want to change the frequency - as this will change the speed of the motor.

Thus for example, a motor operating 50% load may be most efficient when operating near 90% voltage. Similarly this motor may be most efficient at 20% load when operating near 75% voltage.

The actual improvement in efficiency when operating at reduced load and reduced voltage will depend on many factors like the copper losses, iron losses and air gap.

In summary, then, just turn the thing off.

Dear Mr. Kashinath Saha

The answer for your Question is NO, and should NOT ATTEMPT to reduce Voltage.

Secondly you have not mentioned, whether the Motor is suitable for Variable Frequency/Speed operation. You should NOT put ordinary motor for VFD operation, especially below 60% of Rated speed - otherwise, the cooling of Motor will be affected.

DHAYANANDHAN.S

Respected Sir,

The Motor in my question is suitable for Variable Frequency/Speed operation. Shall be thankful if you clarify a bit more in favour of your answer "No".