VFD Efficiency Loss?

[sjohns4]

Does anybody have any ideas on the losses through VFD's? I'm guessing they vary among manufacturers, but is there a rule of thumb, a max that could be expected, etc?

I'm doing an evaluation on blowers for a wastewater treatment plant, which are major power users. The PD type blowers are less expensive, not quite as efficient, and require VFD's to throttle flow. The turbo compressors are significantly more expensive, more efficient, and will utilize soft starts.

I'm trying to do a fair 20-year cost analysis and a loss through a drive should be accounted for.

Any help would be appreciated.

Thanks,

Mike

 

[CJCPE]

At least one major manufacturer advertizes 98% efficiency at rated power. Since there are both fixed and variable losses, efficiency is less at less than base speed and rated torque, but losses are less also. That would be for only the VFD. The VFD causes a small increase in motor losses, probably 0.5% of rated power or less at rated power. If input reactors or output dv/dt filtes are needed to reduce harmonics and/or protect the motor from switching transient voltage ringing, they add a little to the losses.

The losses for an active front end drive or one with more than a 6-pulse rectifier are probably a little higher. With more than 6 pulses, you need input transformers. However, these tend to be larger drives, perhaps medium voltage, and some of the extra front end losses might be offset by reduced losses elsewhere.

 

[LionelHutz]

The efficiency can vary by size too. You could use a number like 4% losses or 96% efficiency to be safe and that should account for any reactors or filters also in use.

Also consider the life of the product. You might have a soft-starter survive the whole 20 years and if not then it likely would just require some repairs. I'd factor in replacing the VFD at least once in that time. Of course, you didn't specify any size - if you're talking 100's of hp or 2.4kV or higher then the VFD starts to be more modular and actually repairable.

 

[macmil]

R and that should be taken ibto considerationemember the space they use up in the electrical room. Some installations require enlargement of the romm, additional ventilation / air conditioning etc,

 

[DickDV]

For a basic 6 pulse VFD, I've found 2% to be a responsible figure. However, it is 2% of LOAD kw, not drive rated kw. This should be fairly equal among any brands that use diode front ends and IGBT outputs.

Reactors are, as mentioned, additional loss items but, again, the losses are relative to load kw.

 

[Skogsgurra]

There used to be also additional losses in the motor. Those losses were because of less than ideal stator current waveforms and also some HF losses. Waveforms are better today, but I have not seen any recent numbers for additional losses in the motor. Anyone got those numbers?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

You need to check what it is you are buying too. What I mean is; some drive manufacturers include EMC filters, DC link chokes and other devices when producing efficiency figures whereas some may consider these 'optional' but essential. Some drive manufacturers require AC line chokes and in some cases output reactors to achieve the same specification. If you are looking at 'drive' efficiency, then you also need to include all the lumps of copper and iron that dissipate heat.

I suggest you look at heat dissipation figures provided by the VFD manufacturers as a reasonable guide to efficiency.
We recently quoted and won a project with 4 x 450kW drives and one deciding factor was lifetime cost based on heat dissipation and the fact that additional AirCon units would be required in the plant room for the nearest alternative compared to our solution. This is a big cost when looking at 10-15yrs min lifetime.

 

[dpc]

Gunnar raises an excellent point. I've heard a lot of numbers thrown around - from 1% to 5% drop in motor efficiency, but I have not seen much supporting data to back this up. But I'm sure there is some loss of motor efficiency when running on a PWM vs a sine wave.

 

[DickDV]

The problem with comparing motor efficiencies is "compared to what?" The only apples to apples comparison can be made at base speed, it seems to me.

But, if you are going to run at base speed,, why is there an inverter? And what are the efficiencies at those other speeds being compared to?

But, Gunnar's point is certainly valid. A motor running at base speed on sine wave power is certainly generating less heat than the same motor under the same loading on an inverter. So that has to be additional losses.

 

[CJCPE]

For anyone with access:

Boglietti, A.; Cavagnino, A.; Knight, A.M.; Zhan, Y., "Factors Affecting Losses in Induction Motors with Non-Sinusoidal Supply," Industry Applications Conference, 2007. 42nd IAS Annual Meeting. Conference Record of the 2007 IEEE , vol., no., pp.1193-1199, 23-27 Sept. 2007