VFD

You could look at constant voltage transformers, or interruptible power supplies.

I don't know what type of VFD you are using, but connecting an external supply directly to the DC bus doesn't sound very practical to me. Also, depending how the VFD senses the low voltage, it may still react to a voltage dip on the AC input.

Getting a Page Not Found.

Strange, I downloaded the pdf, it's from PG&F. Can I post it?

Try it now.

http://www.pge.com/includes/docs/pdfs/mybusiness/.../vfd_sagimpact.pdf

Thanks.

Excellent article. If the OP is fortunate, he may be able to reprogram some of the VFD parameters. (Check the documentation carefully and/or consult with a representative of the VFD manufacturer.)

Dear Unredundant, the link you have sent is not working pl. check and re-send

See #7

Most of the VFDs I've used had terminals designed to be connected to external DC bus capacitors. With an appropriately sized external capacitor bank, you should be able to "ride" over the shorter line droop events.

Initial charging of large DC bus capacitors must be done correctly or the high surge currents can destroy the bridge diodes. Also, a large capacitor bank will store a LARGE amount of energy and extreme caution MUST be used at all times when working on or near the drive/capacitor assembly.

I suggest you contact the drive manufacturer first to see if they have any recommendations for (or against) using external bus capacitors.

Yes you can use a DC bus to supply most of the VFDs in the market. If this is a Spinning Frame, the load will be ~15 kW minimum(?). Your DC supply will have to be substantial: 3 phase rectifier bridge with diodes for ~ 40A at least and 1200V rating.

The Capacitor will have to be rated at the same voltage or at least 1000 V, with capacity ~40,000 microfarad to cover your 200 ms dip without tripping.... I am guessing since I did not read the article mentioned. The DC bus will be at ~585V peak after rectifying.

I think that all this might not be needed: A spinning frame should not have end breaks when you loose power: the speed will be droping relatively slowly. It is when you pick-up speed after re-setting automatically that you might have the problem: Too fast start-up(?). ADJUST the Starting RAMP for the Final (High speed) speed (or running speed) so that it starts slowly.

If you are willing, you can pick-up the problem dip and re-start the frame sequentially, going through the ramping of the increase speed properly.

Maybe a flywheel or a tensioned accumulator (idler) for the yarn would cure this?

Expense?

I think the OP is talking about a Yarn spinning machine in the textile industry. These frame have somewhere between 300 and 1000 spindles. There are 2 main types: Ring spinning and Open end spinning. The ring spinning type could be his type.

this consists of a steel ring having a cursor that is loose to travel round the internal diametre. A tube is in the middle of the ring and spinns at between 5000 and 10000 rpm depending on the ring diam etc. The yarn comes froma drafting system and is wound on the tube after going through the cursor (which is draged along to insert a twist on the fibres...).

The breaking of the yarn ends is, amongst other causes, due to a sudden speed increase from very low to high. In his case, if the main motor and the frame inertia is big enough, slowing down in 100ms is nothing to worry about except that the VFD might be shutting down completely then restarting after too long a stop.

As suggested by (RedFred?) a ride through should be parametred and not a shut down. the inverter can take a short dip without stopping to feed the motor. His setting looks unusual to me.

Thanks for the clarification, my suggestions for a mechanical "cure" were based on my experience with high speed rolling stock automation and the use of drive train flywheels and accumulators to "ride through" fluctuations in speed. But these features were designed in to the original machine.

In this case VFD adjustment is more practical.

There should be a parameter called "momentary power loss ride through" in your VFD. If you enable it, it would take care of such momentary power loss conditions.

there is not any parameter (user accessible) like, "Under Voltage trip" in VFD

In spinning mill the thread breakage is a problem.This happens even if u use a old system of speed change by pulleys or 2 speed motors.One way is to go in for a no break set(Rotary Ups).There are many variations.one type is a motor driven generator supplying power to the PCC supplying the ring frame motors.It has a fly wheel on the other end with a centrifugal clutch connecting to a diesel engine.In normal operation motor gets its power from the mains but when there is interruption the flywheel maintains the speed for about 22 second when the diesel engine comes to speed and connects to generator which continues to run.The load does not see the break in supply.This is better as a electronic ups will need a huge batter bank depending upon the number of motors u have in the mill.