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

 

[LittleInch]

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.

 

[SA07]

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

 

[LittleInch]

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.

 

[waross]

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

 

[itsmoked]

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

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

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

 

[SA07]

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.

 

[itsmoked]

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

 

[LionelHutz]

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.

 

[SA07]

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.

 

[itsmoked]

So by this:

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

 

[SA07]

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.

 

[LittleInch]

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.

 

[SA07]

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

 

[SA07]

 

[SA07]

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.

 

[LittleInch]

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.

 

[LionelHutz]

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.

 

[LionelHutz]

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.

 

[SA07]

kW is the power displayed on the VSD screen.