14 replies to this topic

[GGOSS]

Hello All,

Are there any issues that must be considered when operating a variable frequency drive (VFD) from a generator power supply?

For example:

How do I size the generator for a VFD application?

What are the effects of harmonics created by the VFD on the generator?

I would greatly appreciate advice from anyone who may have experience in this area.

Regards,
GGOSS

[Harry Dampers]

Mr Goss ......be carefull , basically if more than 30% of the VA rating of the Generator goes into 6 pulse drives then there MAY be a possibility of over heating the damper windings(due to harmonic content).

There are some reasonably priced solution for that .

Just remember as well that the the generator will see the true PF of the drive ..........not the fundamental PF of .97 that the rep usually quotes the customer.

[GGOSS]

Hi Harry,

It appears you know a little bit about this subject...that's great!

Your response implies that the kVA rating of the gen-set must be at least 3 to 3.5 times greater than the kW rating of the motor controlled by the VFD. Although I have no grounds to dispute this, I have been advised by others that 5 times is more appropriate.

I guess what I am looking for is a method to calculate gen-set sizing for VFD (and for that matter soft starter) applications. As you know, the prime mover consumes fuel and therefore it is important that gen-set size is minimised.

Are you aware of any such calculations?

Thanks in advance.
Regards,
GGOSS

[theDOG]

I beleive that this number is so high because of the large source impedence of a generator (typically 20%) in comparison to a mains supply transformer (typically 3-5%).

Voltage drop in a circuit being proportional to its impedence, the generator output voltage drops most during the portion of the cycle when the current is at its highest. For non-linear loads, this occurs at the peak of the sine wave causing a severe distortion of the genertors output voltage waveform.

A VFD will incorporate a soft charge circuit to limit the inrush current drawn by the DC bus capacitors. This circuit is then bypassed when the DC bus volatge reaches a predetermined level. If the input voltage level drops, the pre-charge circuit will again be switched on. With the situation descibed above, this can lead to a "chasing" situation (on/off/on/off etc) of the pre-charge circuit and eventual failure.

There are also voltage harmonic issues with an electronic AVR, causing the AVR to increase (or decrease) the output voltage falsly becasue of the multiple zero crossings of the voltage waveform.

The only way to be gauranteed operation of a VSD on ANY generator (assuming current delivering ability), is to use an AFE drive. This is an expensive option, but may be cheaper than oversizing the generator 3-5 times.

[Harry Dampers]

Dog ........it's alarming these days that AFE,s are being touted as "cure alls"for any Harmonic problems ......there at least two cheaper and acceptable alternatives to something that is :

Expensive , tricky dicky and physically large.

The only place you should automatically consider using an AFE is where you have a large regenerative load .

For other applications look around and weigh up the alternatives .....thats what engineers are for .

[theDOG]

Harry;
The reason AFE's are being touted as "cure all's" is because they are! In ten to fifeteen years time, I would be very suprised if AFE's aren't the standard for all VSD front ends. Think about it:
1. There are no harmonic problems.
2. Increased Power Factor.
3. Automatic regenerative braking.
4. No need for oversizing of alternators.
5. Power savings.

Wouldn't your arguement be similar to that of people with the introduction of VSD's? I would have thought that Engineers (of all people), would embrace the technology.

I'm not disputing that the AFE has a higher purchase price, but add the true cost of the alternative (Engineering time/cost, harmonic filters, brake resistors, power usage etc.) and the difference will be very small.

[Harry Dampers]

The current thinking in Europe(Amongst Drive ENGINEERS as opposed to Reps) .....and has been for some time, has swung around from the "cure all" position to a more rational one of using them for the designed purpose . ...........AFE's may well be used for ALL applications some years down the line ...........when they are

1. Cheaper

2. Less complex and more reliable.

3. Have a smaller footprint.

In essence what I am saying is that at present ......weigh up ALL the alternatives using Engineering principles as opposed to sales jargon...........IF after following a proper procedure the AFE comes out on top ........then use it .........if it doesn't .......then use something else.

Currently I cannot think of many applications where I would automatically use an AFE .....UNLESS it was a load at reasonably high power which required heavy and frequent Braking .

[tom]

What is an AFE????

[BigMax]

Tom,

An AFE is an 'Active Front End', i.e. instead of an uncontrolled bridge rectumfrier ooops, I mean rectifier, the VSD input looks electrically similar to the output. This allows energy flow from the DC buss back into the mains power supply, and allows more sinusoidal current draw, amongst other benefits.

I'm with TheDog here, I look forward to AFEs becoming 'standard' on all (most?) VSDs as this feature alone would negate the requirement for most so called 'Engineering'. No need to consider the effects of harmonic distortion on 'soft' power supplies, no need to 'calculate' brake resistor sizes etc etc.

Utopia for motor control perhaps? If AFEs become more common, the average punter / electrician / contracter would be able to approach a VSD application without fear of mismatch / performance issues / power supply THD problems.

Like everything else, it's a question of technology development and cost. Both issues I expect will resolved in the not too distant future.

BigMax;f;

[theDOG]

Hi BigMax,

I am also looking forward to the "DC less" VFD:o. Sounds like a fairy take right? I know of at least one company that has developed a DC less drive, and it IS operational. Think about the benefits...cheaper, smaller, reduced harmonics, inherent braking.
What's next?

[BigMax]

Hey Mr TheDog,

I'm curious about what you mean by "DC less" VFD?

I was thinking V V V F VSD technology with AFE, ie constant Voltage / DC buss link / IGBT-Diode In/Out.

What type of "DC less / AFE VSD is currently available? Sounds exciting!

Cheers!

BigMax

[theDOG]

BigMax,

Sounds pretty interesting doesn't it? I know very little about it (if I knew more, I'm sure I wouldn't be able to talk about it!).

There are currently two approaches to the AC-AC convertor. One has no capacitance but is very heavy on signal processing. The other incorporates capacitance, but is reduced by 80% over typical AC-DC-AC convertors.

As I stated previously, there is one company that I know of that has this AC-AC VSD working...I'm sure we will find out more about it very soon!!

[BigMax]

Mr TheDog,

In your cryptic way, I'd guess you're alluding to the 'matrix converter'. I've done a little surfing and can find nothing to lead me to believe that this technology is about to come to fruition soon......

Oh well, we'll just have to wait and see who's first with the Holly Grail ;b;

BigMax

[SITIN]

Hello All
Where I can found any good issue about ACTIVE FRONT END RECTIFIER with full explanations and time diagrams?
Regards


[Edited on 15/10/02 by SITIN]

[Edited on 15/10/02 by SITIN]

[gwosun]

I have two questions

1) It was mentioned that there were 2 less expensive alternates to using an AFE with a VFD which is powered by a generator. Can you point me to a site where I can read about these ?

2) I have a 400HZ generator. Can I use this with a VFD if the VFD takes its power for the controller from the DC bus? If I cannot feed 400HZ into the input of a VFD, can I rectify the 400HZ and feed this into the DC bus?

-Thanks in advance
-gwosun