To increase motor speed on VSD when working at nameplate current 1

[SA07]

Hi

For process reason, there is a request to increase the speed of a boiler ID fan motor which is on VSD. Plz see some data below. If we increase the frequency, the current will increase and the motor will trip. Is there a way to do this in the VSD without tripping? Thks

RPM Current/A kW
Nameplate 992 644 360
70% Scada 680 547
On VSD 760 645 281
95% Scada 757 644
On VSD 760 644 281

 

[waross]

As I kept reading and re-reading this thread I kept thinking:
If you are up against the VFD current limit, you are also at an HP limit.
You need more HP.
If the maximum current is fixed, then for more HP you will need more voltage.
Then I found this:

Does the VSD supply input voltage have an effect on the output current? The actual voltage is 388 V instead of 400 Vac.

The current limit is also a torque limit.
Higher voltage will result in greater HP.
But that greater HP will be in the form of higher speed at the same torque.
In order to take advantage of the greater HP you will have to change the fan curve by either trimming the impeller or by restrickting the discharge.
Some thoughts:
We haven't mentioned density or temperature.
Testing with the boiler stopped the flow will be cooler and more dense putting more load on the motor at a given speed.
Testing with the boiler stopped the there will be no fly-ash and the flow will be less dense.
Conclusion; testing with the boiler off-line may not be a valid test.

Solution:
Determine the maximum allowable supply voltage of the VFD.
Consult with the fan designers.
Real world data is not agreeing with the curves.
This may be caused by a greater flow density than the design assumptions.
It is not the fault of the designers if you operate their fan outside of the design parameters.
The best attitude is:
"FIX THE PROBLEM, NOT THE BLAME."
Share your operating data (Values when the boiler is in operation, not off-line) and request assistance.
They should be able to suggest a combination of motor speed and fan modifications that will allow you to take advantage of the full HP of the motor.
You and the fan designers should be working to a torque limit, not a HP limit.
At nameplate current, a motor supplied with over voltage and over current may safely develop more than nameplate HP.

I suggest doing calculations based on 440 Volts if the VFD will accept that.
This will give you a 10% increase over nameplate speed and nameplate HP.
Ask the fan designers for suggested modifications to the fan so as to be able to use that HP at that speed.




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

 

[LionelHutz]

The motor is only at 76% of it's rated speed and power. Why talk about running the motor over rated speed when it can't even get to rated speed?

 

[waross]

That is a big part of the problem, 24% of the HP is presently unusable.
That is why I suggested sharing the data with the fan designers to see what can be done to extract the extra HP.Maybe the fan may be trimmed or replaced to allow the motor to efficiently run faster.
I suggest that makes it worth while to consider the option of running overspeed.
110%/76% = almost 145% of present usable HP

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

 

[waross]

What is the purpose of the boiler.
An instance such as recently occurred in Texas where the spot price for energy went to extremes, measures may be considered for short time use that would be unthinkable for continuous use.
For a short time in Texas, recently, a generating plant may reap excess profits in the range of a few million dollars per hour.
Under those economic conditions the managers may be pushing to over-fire the boilers and overload the motors and generators badly.
Are you generating electricity in Texas?

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

 

[SA07]

No it is a small power plant in a small island in the Indian Ocean. Turbo-alternator is 950 kW. The purpose of the plant is to burn coal fly ash & bottom ash. The residue is used as additive in cement. The purpose is to re-use these waste instead of dumping as landfill.
Ok we will discuss with the fan manufacturer and also check VSD parameters.
I am not sure we can raise our voltage to 440 V. All motors, alternator are designed for 400 Vac.

 

[waross]

If your motors are inverter rated they may easily take 440 Volts. That is only 10% over rated voltage.
But consult with the motor manufacturer. Be sure to mention that you will be going over-frequency when you go over-Volts.
They may be wound with inverter grade wire that will withstand quite high voltages.
That is another issue that may be addressed, possibly with filters if the run from the VFD to the motor is very far.
Auto-transformers will economically boost the voltage, if not to 440, at least to 400 Volts.
If you do go over rated voltage, you MUST go over frequency by the same percentage.
Your Volts per Hertz ratio is 400V/50Hz, or V/Hz = 8 Volts per Hertz.
To run at 440 Volts you should be at 55 Hz.

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

 

[SA07]

Hi
We stopped the boiler, did an inspection inside, repaired holes inside ductings. After restarting the plant, we have the following readings.

ID fan damper opening 73 %
Speed 710 rpm
Current 562 A
Power 226 kW
Boiler pressure -8.33 mmH2O

Before repair, readings were:
The boiler pressure 7.02 mmH2O
ID fan damper is opened 100 %
On the VSD: Torque =96.7% Speed = 757.4 rpm Current = 643.61 A Output power= 280.68 kW

 

[LittleInch]

Thank you for coming back and advising us of the latest situation.

They must have been pretty big holes to make that much difference.

The only thing I can see that would severely impact your power is temperature and density of the air if colder air was allowed to mix before entering the fan. The curves were based on 210 C and seem to assume a lower atmospheric start pressure, i.e. no sea level.

Still interesting.

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

 

[FacEngrPE]

Holes in the duct work are a likely cause of the high boiler firebox pressure. Closing the holes off in the boiler exhaust will allow a higher firing rate with the same gas flow at the fan. Reason - all of the hot exhaust gas the fan is moving now comes from the boiler.

That is a dramatic improvement. When you have a scheduled boiler tuneup, you will likely be able to run the tuning all the way to boiler nameplate load now.

Fred