Can we start an ID fan motor 700 kW 690 Vac direct on line? 5

[SA07]

Hi
Our VSD is faulty. Our spare soft starter is being used on a second plant. We do not have another spare VSD or soft starter

The motor is supplied from the following double winding transformer transformer

3.63 MVA 11kV/400V 630 kVA 690V/3MVA Dynyn11 50 Hz Primary 1 11500 2. 11275 3. 11000 4. 10275 5 10450 Sec BT1 715V BT2 415V Current 190.5 A 2422A

 

[LittleInch]

You can try, but will probably trip on under volts or low frequency.

Is it connected via a motor starter?

You're going to be pulling 6+ FLA on a DOL start but your TX is only about 4 so don't think this will work.

How hard is it to get a spare VSD?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[crshears]

Is the VSD part of the draft/airflow control of the ID fan? If yes, will any other control devices associated with the unit even be be able to regulate the flow within an acceptable range?

It may not be worth the trouble to run it sans VSD; it may even be unsafe . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

First, post full motor nameplate details.

Your motor FLC is likely around 700 A and a DOL start would take around 6xFLC = 4200 Amps.

Now, do the math.

Your system is too weak is the reason there was a VFD in the first place.

Muthu

http://www.edison.co.in

 

[iop95]

Needed start torque being very low may use RL series circuits to limit current and short them after.
A smaller VFD with synchronous transfer feature may be an option.

 

[waross]

The soft supply may be to your advantage as far as starting the motor.
The transformer impedance will act as a series impedance type of reduced current start method.
Considering the fairly reactive nature of the starting current I would use the %impedance of the transformer rather than the regulation for estimates.
Estimate the impedance of the starting motor.
Series that with transformer impedance.
Use the result to estimate the starting terminal voltage and the starting current.
This is an estimate, not a rigorous solution as the impedances re at different phase angles, but it may give a suggestion of the voltage dip that other equipment on the transformer may experience during a DOL start.
I don't know if inertia may make a DOL start inadvisable even with the possible reduced current.
With low enough inertia for a successful start, my second concern will be the effect of the voltage dip on other connected equipment.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

Is there anything else on that transformer? If not, it MIGHT work in that it will saturate and drop the voltage to the motor, acting like a primary reactor starter. Then IF the reduced torque is still enough to accelerate it, it will slowly come up to speed and stop saturating. You also likely will need to bypass any overload relay during starting. The other tricky part is in holding in the contactor coils during that process.

If there is anything else on that transformer, then no.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[SA07]

On the transformer, there are SA fan 350 kW, FD fan 450 kW, FA fan 110 kW. But it is the ID fan which starts first.

 

[waross]

This is the first time that I can remember disagreeing with you Jeff.
Transformers saturate from over-voltage not overload.
As the impedance of the load drops, the current increases and the terminal voltage drops until the load impedance reaches zero Ohms and the current reaches the available Short Circuit Current as determined by the the transformer impedance.
Apart from that I am in agreement with you.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

A further explanation;
A primary reactor starter may saturate and may be designed to saturate.
The more current that is passed through the primary reactor, the greater the voltage drop across the reactor.
With an external supply, the voltage drop across the reactor may reach saturation level.
When a supply transformer is overloaded it may act as a primary reactor but the voltage produced by the transformer starts below saturation level and then drops further so there is no possibility of saturation.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

On the transformer, there are SA fan 350 kW, FD fan 450 kW, FA fan 110 kW. But it is the ID fan which starts first.

I ask again: is the VSD part of the draft/airflow control of the ID fan? If yes, will any other control devices associated with the unit even be be able to regulate the flow within an acceptable range?

What I'm getting is that if you are successful in getting the ID fan running DOL/ATL, it will be rotating at some slightly sub-synchronous speed and attempting to extract flue gases from the boiler at some point near its full capacity, absent any dampers or other devices to regulate its through flow [fan affinity laws?].

Depending on fuel used, obtaining ignition may require extensive turndown of SA [secondary air] fan and FA [forced air? I'm used to either forced draft or primary air] fan so as to attain sufficiently stable and quiescent air flow for combustion to be established; if the ID fan draw-through cannot be sufficiently reduced, excessive negative pressure will be placed on the boiler and all intermediate ducting/trunking. This can cause not only failure to ignite but actual equipment damage.

Just sayin'.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

Aren't ID fans high inertia fans with DOL starting time running up to 20 to 30 seconds?

Muthu

http://www.edison.co.in

 

[SA07]

One boiler is actually operating with the ID fan supplied through a soft starter. After start up, the motor is supplied through the bypass; so DOL. There is a damper to control the flue gas flow. The boiler is operating well now although with a VSD the kW consumed would be much less. Our normal way of operating is both ID fans supplied through VSD.
One VSD failed in January. We have a soft starter as a back up for the VSD. Now a second VSD has failed. We have no back up. We must start the plant in 1 week after annual maintenance. A technician is supposed to come to repair the first VSD. Some spare parts are already on site. But we are getting lots of problem with the VSD manufacturer. There is no firm date of arrival of the technician on site.
That is why we are looking for another solution to start the plant.

At another plant, the ID fan is supplied DOL from 11 kV.

 

[LittleInch]

"After start up, the motor is supplied through the bypass; so DOL"

Errr, no. That is not DOL.

DOL means direct on line, no VFD, no soft starter,just apply 690V 50 Htz and watch the lights dim.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SA07]

Sorry for the mistake. I meant supply directly to 690 Vac 50 Hz and operating at nominal fixed speed and the damper is regulating the flue gas flow.

 

[wayne440]

...we are getting lots of problem with the VSD manufacturer. There is no firm date of arrival of the technician on site.
That is why we are looking for another solution to start the plant.

Good luck. To me, "lots of problem" is reason to consider looking for a different VSD manufacturer after this crisis has been resolved.

 

[TugboatEng]

I think this was a very valid question. The pandemic shockwave combined with some of our new wars are really putting a pinch on availability of so many components. You can choose to invest in spares or develop alternatives. Maintaining spares is a very expensive option.

I have to explain this to our auditors annually. It doesn't present a safety issue, only a revenue issue and the lost revenue costs less than maintaining spares.

 

[crshears]

Hey SA07, thanks for the additional info. So I'm hearing you don't actually need a VSD, only a soft starter; correct?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[SA07]

In 1 week we must start the plant after annual maintenance. Else we will get penalty from utility company. We export on the grid. If we get a soft starter we can start the plant to avoid penalty.
In the long run, we will order a vsd to save energy. Even a second hand soft starter will be helpful.

 

[waross]

A possible solution:
Rent a diesel generator large enough to start the motor.
If you connect the motor before you run up the generator, you can probably get away with generator capacity of 150% to 200% of motor KVA.
Once up to speed, sync with the grid and disconnect the generator.
Once started, don't stop the motor until the VFD is repaired.
The consideration of this method will depend on the cost of the penalties you are facing.
If your plant has a large enough backup generator that may be your salvation.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!