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To increase motor speed on VSD when working at nameplate current

To increase motor speed on VSD when working at nameplate current

To increase motor speed on VSD when working at nameplate current

(OP)
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

RE: To increase motor speed on VSD when working at nameplate current

No.
A faster fan speed requires more torque.
Current is closely related to torque.
Possible options:
A larger motor.
If the ID fan is belt driven you may try changing the belt ratio and boosting the voltage.
An explanation is available if your fan is belt driven.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

Increase in speed means increase in power consumed which means more current.

Your figures are a bit confusing as some lines have two number and others three, but are you saying that at 760 rpm, the motor is already at nameplate Amps, but nameplate for the motor is 992 rpm and 644 Amps?

What frequency is the motor at for the 760 RPM? what frequency and voltage is the motor nameplate?

We're only dealing with a small part of the data here.

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

RE: To increase motor speed on VSD when working at nameplate current

You can get more HP out of a VFD driven motor if you increase both the voltage and the frequency in the appropriate V/Hz ratio,
You must adjust the drive mechanical ratio to take advantage of the added HP.
eg: A 100 HP, 1760 RPM 230 Volt motor will develop 200 HP if supplied with 120 Hz at 460 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

(OP)
When speed is 760 rpm current is 644 A, motor torque is 96.8%, motor frequency 38.1 Hz

Motor nameplate 400 V delta frequency 50 Hz

RE: To increase motor speed on VSD when working at nameplate current

No, there is no way to change parameters in the VFD and get what you want.

You are running the motor at 76% of it's possible rpm and HP, but you are already at 100% of it's rated torque. So, you do have 24% more available speed and power, but no more available torque to get there.

This means if it's direct drive, no there is nothing you can do.

But, if it's belt drive, changing the belt ratio to increase the motor frequency for the same fan speed can get you more fan rpm. Quick math says to change the ratio so so that when the motor is running at 50Hz the fan is spinning 10% faster than it currently does at 38Hz.

RE: To increase motor speed on VSD when working at nameplate current

Is it just me or for ~300kW that seems an awful lot of amps?? The cables must be HUGE!

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

RE: To increase motor speed on VSD when working at nameplate current

What Lionel said. Change the drive ratio.

360 kW, at a combined efficiency and PF of 80% is about 650 Amps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

(OP)
Thanks all for your reply

RE: To increase motor speed on VSD when working at nameplate current

If you are operating at (38.1/50) Hz, and at 100% torque, fan laws https://www.axair-fans.co.uk/news/applications/und... us that increasing speed on a centrifugal blower will increase power requirements by the cube of the speed increase.
If there is a set of drive drive parameters that make the motor do what you want it might last at most a few weeks.

380 kW = 509 HP - this is a big motor.

You might consult with the fan oem. A different fan wheel diameter (smaller OD, same housing) might get you the result you need.

RE: To increase motor speed on VSD when working at nameplate current

FacEngrPE That is another valid option and that will work with direct drive.
At 38.1 Hz out of 50 Hz, you have approximately about 24% of your HP not used due to current limit.
A fan wheel designed to draw the motors full HP at the motors 50 Hz rated speed will allow you to utilize that idle HP.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

It doesn't sound like the fan is properly matched to the motor.

What's the fan duty curve look like?

What are you trying to increase - volume or velocity?

Can you reduce the air resistance form air to the fan?

I.e. start looking at the process the motor is driving, not the motor

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

RE: To increase motor speed on VSD when working at nameplate current

Wear off of the fan tips does the same thing as trimming the OD of the wheel.
If speed is held constant the fan curve moves inward.
If shaft speed is increased so that fan wheel tip speed is the same as new then performance will be similar to new.
In this case wear off of the fan wheel tips makes it possible to run the fan a bit faster before fully loading the motor (Amps = Nameplate FLA). While you might be able to run into the service factor (Amps = Nameplate FLA*S.F.) at nameplate speed, you are operating at a lower speed so the cooling air moving through the motor is less than design, this risks shortening the motor life.

The fan OEM should be able to provide a set of fan curves showing fan OD vs head and flow at motor nameplate speed (I am assuming the blower is direct connected to the motor). The best selection would be one that fully loads the motor at nameplate speed (Amps = Nameplate FLA). If the motor is fully loaded (Amps = Nameplate FLA) at less than name plate speed you are running in the service factor. and power (Watts) is significantly lower than nameplate.

Posting the fan curve for you current configuration will result in better answers here.

This might not really be a problem with either the fan, motor or VFD, Large industrial boiler systems are a rather complex system problem, where troubleshooting requires steeping back from the immediate problem, and looking at how the observed problem relates to other parts of the plant.

The described situation can be caused by incorrect setting of fan inlet air spin (http://www.chicagoblower.com/Resources/FanDoctor.a...) ie incorrect setting of inlet guide vanes or dampers, or perhaps a baffle near the fan inlet is loose or out of position. This situation is highly dependent on duct layout and damper position - upstream vs downstream of the fan.
The described situation can also be the result of excessive restriction in the dust collection system (damper set more closed than it should be, stopped up cyclones or blinded bags, incorrect coordination in the control system between dampers and fan speed control).

Do you have a record of how this fan operated when the plant was commissioned? If it ran at design speed and a reasonable motor load, look for what might be different now vs when the plant was commissioned.

Fred

RE: To increase motor speed on VSD when working at nameplate current

Do you have any dampers that you can easily close?
It sounds counter-intuitive but restricting the flow should reduce the current.
With the current reduced you will be able to run the motor faster.
I wonder if someone has not opened a damper trying to get more flow?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

(OP)
They operate the fan with damper opened at 100%

RE: To increase motor speed on VSD when working at nameplate current

Tuning industrial power boilers is painful and time consuming. I agree with waross (Electrical) that your Induced draft fan is likely seeing too little restriction. This could be a bit more complicated than just closing the ID fan damper, as in a boiler everything reacts to everything else.
The following applied to a pulverized coal fired boiler I am familiar with.
The part of the process train you need to follow here is
FD fan is controlled from the boiler master and fixes the gas flow through the furnace. The ID fan controls are not directly linked to the FD fan controls.
The purpose of the ID fan is usually to keep the boiler gas path slightly below atmospheric pressure (prevents boiler exhaust leakage into the boiler room) using a differential pressure sensor between the boiler gas path and the boiler room. I suspect this control loop needs to be re-tuned in the same manner that it was at plant commissioning.

I suggest that you start by looking at the program and see how the program is reacting to changes in boiler gas flow (measuring against boiler master % load usually works). If the program is changing both damper position and fan speed across the entire operating range, there is a high risk of getting the tuning incorrect.

Getting the fan speed correct may be as simple as putting the damper in hand control, closing it (small adjustments only) and let the operating control increase the fan speed, until the desired operating speed is reached. Motor amps should not change much, but should be monitored. You will need to do this for several firing rates, one being the desired maximum firing rate. If you can get the full operating range with a fixed damper position, the damper can be locked in place.

The only future reason readjustment of the damper will be needed is if the gas path friction changes between the boiler and the chimney.

Fred

RE: To increase motor speed on VSD when working at nameplate current

Quote:

They operate the fan with damper opened at 100%
That is probably causing the problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

Randhir,

You're quoting a motor speed of 760 RPM, but the curves you posted have a min and max of 2815 to 2980 RPM??

This doesn't make sense. Please explain.

Also what the static pressure and what do you think is the current volumetrioc flow rate?

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

RE: To increase motor speed on VSD when working at nameplate current

(OP)
On the curve it is labelled @815 rpm, @980 rpm
It is not 2815 rpm, 2980 rpm

RE: To increase motor speed on VSD when working at nameplate current

Ok. The circle made it look like a 2...

Anyway at 760 rpm,I.e below the 815 curve, max shaft power seems to be about 150kW (30 x 5.085). Yet your motor is pulling 280. You need to find out why.

What is the differential pressure across the fan?

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

RE: To increase motor speed on VSD when working at nameplate current

I suspect that everything was working well until an operator noticed that the damper was partly closed.
He thought that there was more capacity available and so he opened the damper.
He may have done so under direct instructions from a manager.
By opening the damper, he actually reduced the capacity.
The damper should be returned to the original position and the capacity should be controlled by changing the VFD speed setting.
I have seen motors burned out by operators who would not neither accept nor understand this.
The easy part is trimming the damper so that the motor is able to develop full HP.
The hard part is getting the operators to leave it alone.
It may be easier to trim the fan diameter than to educate the operators and managers.




Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

I bet you are totally correct Bill. Wide opened dampers by a neanderthal.

Keith Cress
kcress - http://www.flaminsystems.com

RE: To increase motor speed on VSD when working at nameplate current

Too many years in the field and it's hard to get good help. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: To increase motor speed on VSD when working at nameplate current

(OP)
Hi
We have just replaced the fan with a new identical one and put into service. However the operation does not seem to have improved.
The boiler depression is now 7.02 mmH2O. It should be - 3mmH2O
The 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

When we close the damper a little bit, the motor speed can be increased a bit. However this does not improve the boiler depression.
We are limited by the VSD current.
Plz advise what we can do?
Does the VSD supply input voltage have an effect on the output current? The actual voltage is 388 V instead of 400 Vac.

RE: To increase motor speed on VSD when working at nameplate current

Yes it does as the output is power less volts means more current. That said 388 is only 3% off of 400V which is not enough to matter usually.

Can you please explain what YOU mean with regard to "boiler depression"?

Keith Cress
kcress - http://www.flaminsystems.com

RE: To increase motor speed on VSD when working at nameplate current

In theory, the motor power at 757rpm is 757rpm/992rpm*360kW = 275kW. So, your motor operating point is correct and what is expected when the motor is fully loaded at 757rpm. So, there is still nothing you can do in the VFD parameters to fix it.

Power is required to move air. At 757rpm you are limiting the motor to 275kW of power but at 992rpm you have the full 360kW of power. So, if you get the motor rpm up to 992rpm you can move 128% more air.

Try closing the damper again, this time more than just a little bit.

RE: To increase motor speed on VSD when working at nameplate current

(OP)
Boiler depression is pressure inside the boiler which is less than atmospheric.
I will see with operation dept if they can further close the damper.

RE: To increase motor speed on VSD when working at nameplate current

So by this:

Quote:

The boiler depression is now 7.02 mmH2O. It should be - 3mmH2O

You're saying the flu is extracting faster than the blower can provide air so there is a greater sub-ambient 'depression' than desired?

Keith Cress
kcress - http://www.flaminsystems.com

RE: To increase motor speed on VSD when working at nameplate current

(OP)
The ID fan is not extracting enough flue gas from the boiler. It should extract more to bring the pressure inside the boiler to -3mmH2O.

RE: To increase motor speed on VSD when working at nameplate current

What's the temperature of the air being extracted?

What is the differential pressure accross this fan.

As said before, the fan appears to be taking more power than the curves provided say it should be. So you need to figure out why that is.

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

RE: To increase motor speed on VSD when working at nameplate current

(OP)
Temp of air extracted is 190 deg C
We do not have a differential pressure transmitter across the ID fan. There is a pressure indicator at the inlet of the ID fan reading - 600 mmH2O

RE: To increase motor speed on VSD when working at nameplate current

(OP)

RE: To increase motor speed on VSD when working at nameplate current

(OP)
Plz see test we have done with various opening of the ID fan damper.
Is it normal that the current remains fixed at 644 A? Should the current not vary proportional as the kW varies.

RE: To increase motor speed on VSD when working at nameplate current

SA07,

Finally some decent data!

I return to my point some months ago - Based on the curves you supplied, the shaft power required by the fan at the lower speeds should be about 150kw.

41 tonnes per hour at 190C is 55,000 m3/hr.

Using your graph this equates on the X axis to 7.5 (x 7310)

At less than 815 rpm this should therefore be 1bout 150kW, yet your system claims over 250. So there is a very large gap between what the fan supposedly needs for the flow at that speed and what the motor is consuming.

So find the missing power.

Where / how are you measuring power? Is this electrical power into the VFD? or into the motor? VFD losses can be high, but not usually that high

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

RE: To increase motor speed on VSD when working at nameplate current

Your question just showed that you still don't understand the basic motor relationships with a VFD.

The VFD makes the motor more or less a constant torque device.

Motor torque is roughly proportional to the current change from no-load current to full-load current. No matter what rpm, no-load on the motor should cause approximately no-load current to flow and rated torque loading on the motor will cause full-load current to flow.

Motor power is proportional to torque and rpm. At rated torque, the shaft power will vary proportionally with rpm.

Your VFD is being told to run at 100% speed, but it must also have a current limit programmed. The current limit works by reducing/limiting the rpm so the current doesn't go over the setting. In other words, the VFD is trying to accelerate to 100% speed but it stops at 668rpm because that's the rpm where it hit the current limit.

As for the fan data, hopefully someone else has some insight because its not making a lot of sense to me.

RE: To increase motor speed on VSD when working at nameplate current

I see LittleInch has already commented. Double check the basic motor settings in the VFD. Also check the base or knee frequency, this is the frequency the VFD reaches rated output voltage and sets the V/Hz ratio.

RE: To increase motor speed on VSD when working at nameplate current

(OP)
kW is the power displayed on the VSD screen.

RE: To increase motor speed on VSD when working at nameplate current

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:

Quote:

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!

RE: To increase motor speed on VSD when working at nameplate current

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?

RE: To increase motor speed on VSD when working at nameplate current

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!

RE: To increase motor speed on VSD when working at nameplate current

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!

RE: To increase motor speed on VSD when working at nameplate current

(OP)
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.

RE: To increase motor speed on VSD when working at nameplate current

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!

RE: To increase motor speed on VSD when working at nameplate current

(OP)
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

RE: To increase motor speed on VSD when working at nameplate current

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.

RE: To increase motor speed on VSD when working at nameplate current

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

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