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]

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

 

[LittleInch]

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.

 

[waross]

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

 

[SA07]

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

 

[LionelHutz]

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.

 

[LittleInch]

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.

 

[waross]

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

 

[SA07]

Thanks all for your reply

 

[FacEngrPE]

If you are operating at (38.1/50) Hz, and at 100% torque, fan laws

https://www.axair-fans.co.uk/news/applications/understanding-basic-fan-laws/tell

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.

 

[waross]

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

 

[LittleInch]

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.

 

[SA07]

As soon as I receive the fan duty curve I will send.
We would like to increase the air flow.
The issue is that the fan impeller is worn out a bit due to flow of coal ash through a bag filter.[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1604211197/tips/ID_fan_wear_on_impeller_pictures_yajzgw.docx[/url]
Plz see attached pictures of fan impeller.

 

[FacEngrPE]

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.aspx)

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

 

[SA07]

Plz see attached the fan curves from the OEM

 

[SA07]

Plz see attached curves

 

[waross]

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

 

[SA07]

They operate the fan with damper opened at 100%

 

[FacEngrPE]

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

 

[waross]

They operate the fan with damper opened at 100%

That is probably causing the problem.

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