Do Frequency Drives Save Power?

please find out the duty cycle of the motor approx.

that is how much time it is under loaded to the total cycle time if you could write the application then it will be better.

the drive normally reduces the voltage applied to the motor to suit the load , if the drive parameters are set to the power saving mode.

you should take care for speed reduction as the efficiency of the fan decreases in square proportion to the speed thus cooling of the motor will get affected by that, also the motor normally heats up more due to the high frequency component of the drive output take care and best luck

Not enough information. Generally if you slow a motor down, you also reduce its power (torque) to some extent. You could compensate with increased voltage or current or both at a lower frequency, but the motor will likely overheat under these conditions. And, not all motors are the same. If you really want a variable speed motor, you should buy one designed for that mode of operation. They will have more capacity to shed the heat.

Most motors are air cooled. So when you run them slower, they don't get as much air. And, the coils are sized around a given power source (i.e. 50 or 60 Hz). For lower frequencies, you may need larger wire, or coils. That is not-to-say that you will have immediate problems. You may be able to get away with this idea for several days or months. It all depends on the loading, the motor type, and the drive type. Like I said, not enough information. Any energy savings will be lost when you have to replace the motor due to premature failure.

Hello JRaef,

Thanks for the clarification. It seems you have had more positive experiences with matching up well designed VFD drives with cheap motors than I have. I was answering the question with the understanding that "not all VFD's are the same" and "not all AC motors are the same."

Perhaps some people in life never have to follow up after some other "engineers" half baked solution to a problem. I have not been so lucky as to get into every application from the begining.

I can appreciate your frustration. As you will see in later postings even in this simple thread, there is a LOT of misunderstanding out there on how VFDs and AC motors work together and unfortunately, a lot of that exists in the Engineering community. That often leaves Technicians and Electricians to try to figure out what went wrong on their own, especially if an arrogant Engineer was convinced that they didn't make a mistake in an application. So your experiences are no doubt real, but I just wanted to clarify the understanding of what SHOULD be happening. There are a LOT of nuances in all of this, so for sure there are a number of places where things can go horribly wrong. Sounds as though you found some.

One problem I see is that the education system with regards to Electrical Engineering is all over the map on what defines an "Electrical Engineer" and a lot of people get degrees relating to electronics (computer circuits and such) with no exposure to power concepts such as this. To make matters worse, proper understanding of VFD applications needs a modicum of knowledge in Mechancial Engineering as well, and cross disciplinary education is now a thing of the past.

I appreciate your not being angry with my earlier response (or at least your not showing it), I meant no disrespect. It's just that I occasionally get on a crusade to try to make sure that misinformation stops somewhere, and a lot of people read these forums without ever posting or questioning what they see here.

Spot on, JRaef. Too many engineers consider only "their" side of the coupling, without looking at the entire process.

If the application allows for a constant operation at a lower speed you could eliminate the inrush currents from turning it on multiple times saving energy.

ChazL

Inrush current is not energy. Energy is defined as power over time. Power is defined as kW, kW is defined as the product of voltage, current AND POWER FACTOR. Starting current is high on a DOL start because the power factor is extremely low, but the net POWER in the equation is relatively constant.

Sometimes utilities charge extra for instantaneous demand current or kVA increases, but most of the time they use a "demand window" of 15 or 30 minutes (at least here in the US they do), so reducing inrush does not really even reduce money either. That is a common misconception fostered by unscrupulous Soft Starter vendors. Don't get me wrong, I love soft starters and they have lots of benefits, but energy savings is not one of them.

The motor seems to be am asynchronous motor(1440 rpm). If you reduce the frequency/the rpm of the motor you will surely reduce the torque of the motor. Try to study the characteristic of the motor, torque as function of rpm, and you will see that the torque tends to decrease as the rpm decreases, down to the value called starting torque, which is attained at zero rpm.To find out the power the formula is power=torque/rpm. The frequency drive does save power at startup by reducing starting current. The so called soft starter. In the rest of the time it only helps you to choose whatever rpm you need and just that because the conversion of the energy from 50/60Hz to lower or higher frequencyes uses power as the static switches heat up.

Almost nothing that you have said here is correct.For future readers who are trying to learn about using VFDs and AC motors:

1. Reducing the frequency and voltage together (as a VFD does) does NOT reduce TORQUE, it KEEPS THE TORQUE CONSTANT! But because the speed is reduced and mechanical power is a function of SPEED and torque, it reduces POWER.
2. Starting torque is HIGHER than full load torque of a motor and has very little to do with applications involving VFD driven AC motors. In fact, some types of VFDs can make the motor provide 100% torque at zero speed.
3. A VFD does NOT save any appreciable POWER by reducing starting current. The amount of POWER, defined as kW, used in starting a motor is always exactly the same no matter what method you use. It is a function of mass, inertia and change in speed. A VFD will use less CURRENT than a Soft Starter or DOL stating, but current is not power. A VFD can also stretch out the acceleration time as long as necessary to keep the current low, whereas a SS or any other starting method is heating up the motor rapidly and consuming thermal capacity at a rate that may prevent it from starting successfully without tripping the Over Load relay. And by the way, a Soft Starter does not save POWER in any way, it just reduces current and torque. But because the power factor is very low when starting a motor at a fixed frequency, the POWER, as always, stays constant.
4. The only thing you got right was the fact that as speed is reduced, the switching losses in the VFD do go up (what you refered to as "the static switches heat up"). But that is AS A PERCENTAGE OF TOTAL POWER and since the POWER is reduced at lower speeds, so then are the total switching losses in watts. So while this is relevant in terms of throughput efficiency, it has almost no bearing in a consideration of evaluating a VFD's performance against a fixed speed application, the total amount of switching losses is less than a few percent of a lower total power amount.

Please please please refrain from posting on things you are guessing about.

In response to the first part of your question. Yes VFDs can mean a huge savings in energy use. Depending on how you set it up, it can also help in reducing maintenance and repairs on the motor. Motors have a bottom end, meaning they can only be reduced in speed to a certain percentage. Refer to your motor specs.

In response to the second, third, and fourth questions. If you don't know the basic principles to how a VFD works, how are you going to calculate the savings? If you do want to know how the VFD works and need to calculate the savings. Contact the your preferred VFD manufacturer and he will spend all the time you need to understand how his equipment works.

^{"... Contact the your preferred VFD manufacturer and he will spend all the time you need to understand how his equipment works."}

Ahhh, if only that were true... Far too often lately I am finding that under the pressure to maintain sales volume, even the VFD manufacturers are allowing their people to sell based on false ideas or allow ignorance to prevail...

JRaef has provided excellent information.

Most flux vector drives can provide rated torque through all operating speeds, including zero and negative rotations.

For smaller motors TENV motors will provide cost effective operation over all speed ranges without ventilation problems.

I recommend inverter duty AC motors to withstand the stresses the VFD puts on the insulation system. (Many manufacturers only make inverter duty motors these days, to reduce product line inventory.)

I often hear people trying to justify buying a VFD to save power, but as Raef points out it all depends on the application. Many applications are actually more efficient using a high efficiency motor straight across the line.

As soon as variable speed (flow) applications are required the VFD makes sense, and may save power against other alternatives.

In some applications I still prefer DC VSD's, especially where torque regulation instead of speed regulation is required across a wide speed range, or regenerative capabilities for extended periods are required (four quadrant operation). (But it is always nice to get rid of the commutator!)

Regenerative VFD's are expensive and overall losses are higher than non-regen with more devices employed. However, if regeneration is a requirement, there is definite energy savings over using friction braking. The cost benefit still needs to be calculated for a ROI.