Two motors on different grid running powering one extractor

[Blutooth]

Hi all,

I'm having a problem understanding an extraction set-up. They have extractors that are used to keep labs constantly on a slightly lower pressure than atmospheric pressure to prevent leaks. To be absolutely sure that the extraction never stops there are two induction motors are fitted on one extractor (both of them connect to the same shaft with their own belts). One of the motors runs on the normal 50 Hz european grid, the other one runs on a local grid powered by a Diesel generator on 50 Hz. The idea is that the diesel powered motor can provide the total required power on it's own should the grid fail.

I'm no electrical engineering expert and I can't get my head around what is happening here. I think the frequency of the local grid can never by as constant as the normal grid's frequency so it will fluctuate slightly over time.

Could this give problems with one of the two motors going in generator mode?

Will both motors constantly power the extraction ventilator or will they just counter-act each other?


Any help on understanding what is going on here would be much appreciated!

(my apologies for any grammar mistakes)

 

[MintJulep]

Is your local generator grid electrically connected to the main grid?

 

[Blutooth]

Thanks for your reply.

To answer your question: No, the two grids are completely de-coupled (apart from the fact that two motors drive the same ventilator shaft)

 

[MintJulep]

Is the diesel generator operating constantly? Or does it only turn on if the main fails?

 

[Blutooth]

Yes the diesel runs constantly, this is done to ensure that the extraction never ever stops (due to safety resons). Also note that the motor on the extractor is not the only consumer of the Diesel grid.

I know there are specialized UPS solutions that will be much less energy consuming than the current setup (which we will advise to implement). I just want to understand physically what is happening with the current system.

 

[MintJulep]

In all likelihood one of the motors is operating at a higher slip angle, so the torque is not being shared equally between them

Depending on the motor ratings and how close they are to the load, that may or may not lead to overheating.

The arrangement you describe has multiple failure modes that will result in the fan not running. It is not even close to providing the operational redundancy that you seem to think it provides.

 

[Blutooth]

OK, thanks for your answer!

 

[cry22]

when we talk redundancy, one should consider complete stand-by systems, i.e. two separate fan systems, not two motors and one fan.
the system must be interlock to some type of make-up air system as well, so that too, needs to track the exhaust systems with the same redundancy.
Then again, we don't know much about your application.

 

[ScottyUK]

You are essentially paralleling your diesel set with the grid, albeit through a loose and inefficient scheme. There are a number of problems with this:

It is highly unlikely that the motors will share load equally.
It is possible that the grid will cause the generator to 'motor' so the grid drives the engine.
It is also possible that the diesel exports power to the grid through the motors.
Your electricity supplier will likely be interested if you have a generator connected to his system.

This is a terrible idea in all respects. Provide dual fans and a source transfer switch between the utility and the diesel set.