variable speed efficiency

[Takhinen]

Hi,
I am wondering if there is any study showing the AC drive chain efficiency?
Lets assume that we have the following AC Drives parts:
EMC filter , Frequency converter PWM type ,Output Filter ( for example - sine wave former) , long cables,AC motor , load
So , what is the losses ( approx. in percentage % of the total input kW)after each part of the above mentioned AC drive chain !What about the KVAR percentage of the total kVA on the output of PWM type frequency converter ?
Is it acceptable to assume that the power losses in form of heat in the AC motor fed from F.C PWM type by harmonics( 5 , 7 , 11 , etc) is about (5-7)% of the output power of PWM type frequecy converter ,in case of there is no output filter( sine wave former)?
And even if there is a filter , could the losses in it reach 5% of the F.C PWM type output power?

Regards

 

[alehman]

I don't know of any one source for all this information unless you can find it in a published study of some sort. This seems like a very general question. Can you ask such that a more specific answer can be provided.

 

[ozmosis]

takhinen
are you actually asking a question about total PWM drive+component efficiency or are you simply trying to highlight the efficiency of the NFO type drive?
This is a site for engineering tips, not a sales leverage platform for products.

 

[Takhinen]

Thanks Alehman,
It is not a general question ,Lets think that we need for example 10 kW power to be used for (X) load on the output of an AC motor , with efficiency 0.83. in this case your calculation will lead you to have 12 kW , (or 13,3 KVA if the motors cos f =0.9 ) power on the input of such AC motor. Now , if you have long cables to feed the AC motor from PWM type F.C , say 800m long, it is possible to add 1 kW for the losses in the cables ,so we have 13 kW power needs to be fed from PWM FC . but we know that the wave form on the output of PWM converter is not sinusoidal , and it contains harmonics( 5,7 ,11,13,17,19,etc )those harmonics are heating the AC motor and shaking the load movement .
My question above was: Is it possible to assume that the total power spent on harmonics is equal to (5-7)%of the nominal power of the PWM FC , say we add to the above 13 kW more 1 KW needed .Then we need to caculate the losses in the PWM FC itself , say about 5%,and then the reactive power used by the converter diode bridge , for example 20%
In general , for such VSD system ,the total power consumption from the network will be about 18-20 KVA .
Is that right?
So dear Ozmosis !the above mentioned is important issue to be taken in account and not to highlight any product as there is no end user customers to market any thing to them!!
Some times ,especially, when using old motors in retrofitting projects or even new standard types motors fed from FC , you need to include dV/dT filter , or sine wave filter between the FC and the AC motor . Have you asked your self , Why? I am asking about the losses in such filters and in the VSD chain elements !!
I was wondering , when looking FC manufacturers catalogue in which for example given the following data:Input 460VAC,3ph , 32A --- Output 20 HP ,460VAC,3ph ,15kW ,24,1KVA, 29A
regards

 

[dpc]

All power conversion systems have losses. People trying to sell these systems often tend to gloss over or ignore some of these losses. This is all well known to most of the engineers in this forum.

Beyond that, I have no idea what point you are trying to make or what your question is, if you have one.

 

[CJCPE]

I don’t think that there is such a study. Individual drive manufacturers have efficiency/loss data for their products. They may also have estimates for the motor loss increase due to VFD operation. The losses vary from one motor to another. I believe that the increase is usually less than 0.5% of the motor power. For a motor that has 5% losses on sine wave, an increase to 5.5% would be a 10% increase in losses. The motor KVAR is pretty much the same as for sine wave operation and is not reflected to the line side of the VFD.

The KVAR on the line side of a VFD is determined by the drive’s input circuit and the total AC source impedance. It is mostly distortion KVAR which adds to the total KVAR but is not added arithmetically to the displacement KVAR. The total KVAR is the square root of the sum of the squares of the distortion and displacement KVARs.

The filter manufacturers can provide loss data for their products. Some may have the information on their web sites or in downloadable instruction material. Filter losses will vary a little with different drives and different power system characteristics. I doubt that filter losses reach 5% for input and output filters combined. Something like 1% or 2% seems more likely.

 

[itsmoked]

Ditto!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[alehman]

My question above was: Is it possible to assume that the total power spent on harmonics is equal to (5-7)%of the nominal power of the PWM FC

No, I don't think it correct to make such an assumption. To have a meaningful calculation you will need to obtain the specific data from the drive mfg. The number they provide will likely be based on operation at a given load (like 100%). In reality the number varies as some function of load (and input voltage and drive speed).