Voltage sag / ride through

At the plant where I work, there is a centrifuge machine, 30Kw, on standard star delta contractors.
When there is a supply "blip" or voltage sag, the contactors "drop out", machine stops and causes havoc.
This machine has a lot of inertia, a heavy rotor i.e flywheel effect and travels at about 3,000 rpm so a couple of seconds freewheeling shouldn't cause it too much grief.

I have a Siemens Logo and a standard small PC UPS at my disposal.

My thinking was:

1. Supply the Siemens Logo from the UPS power output.

2. Use a relay output on the Logo, fed from the UPS output to supply the control voltage to the contactor circuit.

3. Use absence of mains supply as an input to the Logo to operate a 1 second timer, releasing the output. This output is in series with the machine stop circuit.

i.e.

If the mains supply is missing for more than 1 second the contactors drop out, requiring a manual start as per normal operation, but if mains supply is missing for less than 1 second then the contactors stay in.




Anything wrong with the plan ?

Your concept of supplying the plc and machine contactor with a separate UPS is good to eliminate the nuisance dropouts. There "may" be an issue with generated back voltage from the freewheeling motor or maybe not. It depends on residual magnetism in the motor and a few other things like inductive phase shift and so forth. That probably won't be a problem but just want you to be aware if you get some unexplained fuse blowing or circuit breaker tripping on the mains shortly after the power resumes from the "blip". I based my reply on your description of events, I didn't review your logic symbols because I am not that familiar with them.

There are standard capacitor hold-up units that can attach to the coils to maintain them during dips, specifically for this purpose. There are several other technologies that can do this too, without the need and maintenance of UPS batteries, which WILL eventually fail. Look up a Dysc system made by SST (Soft Switch Technologies)

You might also want to find out why you are getting voltage sags. Stopping the sags would seem like the absolute best solution.

Thanks for the replies.

First of all, nothing we can do about the voltage sags/drop out's. This is up to the supply company and their recliner systems i.e not going to change.

Second point is these machines are fed from different panels, not in proximity to each other, so small portable solution is best. We have loads of little ups units, which would fit in a panel and also are cheap, so I guess I will try one and see what happens.

Pablox said:

Thanks for the replies.

First of all, nothing we can do about the voltage sags/drop out's. This is up to the supply company and their recliner systems i.e not going to change.

Click to expand...

Install a meter that will measure EN50160 parameters and present them a report, they may change their mind then....

http://admin.copperalliance.eu/docs...tage-characteristics-in.pdf?sfvrsn=4&sfvrsn=4

Pablox said:

Thanks for the replies.

First of all, nothing we can do about the voltage sags/drop out's. This is up to the supply company and their recliner systems i.e not going to change.

Click to expand...

I'm not familiar with the power supply company term 'recliner systems.'
This was my first thought.

kwade said:

I'm not familiar with the power supply company term 'recliner systems.'

Click to expand...

That's because autocorrect changed "recloser" to "recliner"

Pablox said:

That's because autocorrect changed "recloser" to "recliner"

Click to expand...

iOS devices will also correct "contactor" to "contractor" every time. So it's conceiveable that if you wanted to discuss a "recloser contactor" it might come out as "reclining contractor". That Homer image becomes even more appropriate...