Enrgy saving by using VFD 5

[abs710524]

Can somebody tell how we can achieve energy saving (in case of induction motors)by using Variable frequency drives, even if speed variation is not required.
request to describe in details and with a example if possible.

regards

 

[electricpete]

Try searching on for DOE motor challenge program.

If load is constant but pump/fan is oversized such that it must be throttled, then reducing the speed using vsd will save the amount of energy wasted in that throttling.

But there may be easier and cheaper ways to do the same thing. For pumps the impeller can often be trimmed to reduce flow wihtout thottling and without significant drop in efficiency. For fan blades, the pitch may be adjustable. Belt driven load speeds can be adjusted by changing sheave sizes.

 

[abs710524]

thanks, is this method useful for power saving in case of the applications like elevators, conveyors etc.

 

[cbarn24050]

Hi, there are inverters around that monitor the power factor of the motor and adjust the output voltage to keep it at the optimum value for maximum energy efficiency.

 

[Marke]

If we look at a typical application, there are two major components: The motor and the driven load. Both of these components can be looked at in terms of energy losses or efficiency.

With many machines operating at less than full load capacity, there can be benefit in slowing them down, thereby reducing losses such as frictional losses, cavitation losses etc. Other machines do not waste as much energy when operating at less than full load capacity. For example, if a fan is operating in a free flow environment and shifting a greater volume of air than is required, then there will be a major benefit in slowing it down such that it only moves the amount of air required, but if the fan is fitted with shutters or dampers, then the energy consumption will be reduced (less work being done) so the energy saved by slowing it will also be reduced, but in may cases still worthwihile. So yes, slowing the speed of some machines will improve their efficiency when operating at less than full load.
The induction motor however, is a different matter. Induction motors operate at a high efficiency and continue to operate at a relatively high efficiency down to less than half load. (except very small motors). If the voltage applied to the motor is reduced when the motor is operating at reduced load, the iron loss of the motor can be reduced, but the copper losses can increase except when the load is very low. Essentially, look at the efficiency curve of the motor in question and you may find that the motor efficiency only becomes an issue at less than 30% load.
The primary losses are usually in the driven load and these are the losses to focus on. If the machine can be slowed down and the efficiency improved by doing so, then that is worthwhile. If the motor is run at constant rated speed, then do not apply an inverter as you are increasing the losses due to an additional 5% or so losses in the inverter, and the high harmonics on the inverter input. If you pay a KVA penalty, expect the inverter to increase that relative to a power factor corrected motor without inverter. - It can cost more to run!!
Best regards Mark Empson

http://www.lmphotonics.com

 

[jOmega]

Marke:

What do you think the speed-torque curve of an ore conveyor would be like ?

Could you expect to source rated motor torque at 20%, 30%, 40% 50% speed ? in such an application.

What about an overhead chain conveyor in a meat processing plant ?

If so, how much energy savings could be achieved by operating the motor with a VFD (AFD) ... ?

What about a plastic or rubber extruder ? ever see the motor delivering full load torque at 5% speed ?
How much energy savings could you realize there with a VFD ?

Or how about reciprocating compressors ? or Screw Compressors ?

I think Mark that when you consider the gammut of CONSTANT TORQUE load appications, you'll realize that there isn't much (if any) energy savings to be obtained from operating the ac induction motor with a VFD.

Energy savings realized from the use of a VFD are primarily the domain of "Variable Torque" or centrifugal type loads where throttling valves or dampers or louvers were the primary means of varying the flow or pressure.

 

[dpc]

I agree with jomega that conveyors and other constant torque loads would not offer much energy-saving potential. But there are good reasons for using an AFD on these loads, such limiting starting torque and speed control. But not saving energy.

If you have a fan or pump that is throttled down a lot of the time, the AFD can save energy. Don't forget to factor in drive losses and increased motor losses due to harmonics in your energy evaluation. Motor efficiencies given by motor manufacturers (NEMA) are based on a clean 60 Hz sine wave. There will be some increases in losses when running on an AFD.

If your only reason for installing an AFD is to save energy, you need to do a careful analysis to make sure it will pay off.

 

[redtrumpet]

jomega - if you have read the thorough posts Mark has made in the past you will realize you didn't tell him anything he didn't already know. I'm still trying to figure out where in Mark's post he mentioned constant torque loads in a way that would cause you to take a run at him in such a condescending manner. The example he gave of a fan application is certainly a variable torque load, and it was my impression he was referring to variable torque loads when looking for energy savings.

 

[jbartos]

Suggestion to abs710524 (Electrical) Dec 12, 2002 marked ///\\Can somebody tell how we can achieve energy saving (in case of induction motors) by using Variable frequency drives, even if speed variation is not required.
request to describe in details and with a example if possible.
///Since you request details, it will be best to consider your specific application. Set it up with a VFD and without the VFD, and measure the power consumption of each and record it. Then, decide according to power consumption records. Alternately, an "accurate" simulation may be performed by using suitable computer software.\\

 

[electricpete]

Well said, RT.

jOmega - from looking at all your posts it looks like you have a lot of good info to contribute on motors and drives. But sometimes those good ideas are presented in an overly argumentative manner. I'm probably also guilty of the same thing on all threads. We all benefit if comments and disagreements are expressed calmly and respectfully.

 

[abs710524]

thanks everybody.
jbarots:
Can you tell me the name of some simulation software. Are these software are freely available on any website.

 

[Marke]

jOmega

I totally agree. There are many applications where a VSD will not save energy, infact a VSD will consume more energy due to the losses in the VSD and the additional losses due to harmonics. There are however, some applications where a VSD will actually pay for itself in terms of energy saved, although I am very sceptical of some of the claims made in this regard.
As per my previous post, the energy that can be saved is predominantly a reduction in mechanical losses where the machine can be slowed down and still achieve the same useful work output. Slowing a conveyor carrying the same amount of material will result in less work done due to a smaller distance travelled in a unit of time. The actual work output is reduced. Fans and Pumps are the most likely candidates for energy saving with a VFD due to the cavitation losses when not achieving full flow rate.

Best regards, Mark Empson

http://www.lmphotonics.com

 

[jOmega]

Mark,
Most VFDs today post efficiency numbers between 95 - 98% (if you can believe them)... But that's at rated output.
Some years ago, Maury Peltola of ABB, produced some curves for the ACS 500 that I thought were rather interesting. The curves had a very shallow drop-off down to about 30% load at which point the efficiency dropped off quite steeply. Not too surprising if you consider that as the output load decreases, a point is reached at which the fixed losses of the VFD become a greater percentage of the input current draw.

I think you recognize, Mark, that at some point it becomes a numbers game that skews away from reality. Not to digress, but consider a VFD that introduces 47% THiD into a distribution transformer....but at a total amp draw of 5 amps on a 1500 kVA transformer.... how significant is it really.

My previous post, Mark was not an attack upon you as some have erroneously stated. The interrogatories were intend to examine other ramifications over a broader spectrum of applications.

Bottom line: Energy Savings with VFDs .... high probability with variable torque loads. Low probability with constant torque loads. However, in constant torque applications, operating efficiencies may be realized to justy the VFD in the process.

Enjoyed the discussion with you sir.

Kind regards,
jOmega

For instance, consider conveyors with belts... at lower speeds there can be increased frictional losses as more of the belt is in contact with the rollers. At higher speeds, the laws of aerodynamics come in to play and the belt actually lifts off the rollers.

The extruder application.....draws close to 100% torque at 5%-40% speed, at which point the frictional heating of the screw adds to the melt temperature, reducing the viscosity of the material in the barrel; at which point the load drops off with increasing speed.

The energy savings with centrifugal pumps is realized by the elimination of the flow/pressure control valves that are throttled to achieve process operating points. The drop across such valves can be significant.

Positive displacement pumps have little realization of energy savings because they operate in constant torque mode.

Mark, I want to close this with a point that you made with regard to motor losses. In particular the iron and copper losses. That was a concern maybe 10 years ago, but today, with the advent of vector technology...i.e. space vector, voltage vector, etc..... and improved modulation techniques,plus improvements in motor designs, these losses are minimal because the applied current waveform from the VFD is virtually sinusoidal.
I recently made some measurements of several brands of VFDs currently in the market and the data shows that the harmonic content in the output fell within the range of 1.5 - 2.5%. The harmonics occurred a much higher frequencies with very low amplitude than the units of years past. Also, improvements continued to be made in the domain of harmonics introduced into the plant power distribution system. The new Siemens SED2 drive eliminates the dc link inductor, and reduces the bus capacitance to an extremely low value as compared to conventional dc bus topology. As a consequence, the THiD and THvD are extremely low. If you are used to seeing the line current of a typical PWM being two gulps...the SED2 has no distinct gulps. The current is almost continuous because of the small time constant of the dc bus. That is, it begins to draw current from the ac line almost as soon as the line sinewave voltage gets off zero. This has really given cause to VFD designers to re-think the bus topology and yesterday's rationale.

 

[bigamp]

Thanks Mark Epson and JOmega for the helpful and well presented information. The SED2 sounds interesting. Will it be a good alternative to AFE for minimising harmonics on the supply system do you think?

Regards