Inverter Fan Control

[frank]

We have a situation at the moment where a number of Air Handling Units have been supplied but they are not achieving the correct air volume.

Obviously, this has an effect on the air change rate in the conditioned space and also the amount of cooling we can input to the air stream.

The manufacturers are telling us that the fans should be connected to frequency inverters to speed the fans up.

The supply fans are fitted with 2.5kW direct drive motors with a plate rating of 2880RPM and the extract fans have a 1.5kW direct drive motor rated at 1420RPM.

To achieve the design duties, the speed of the motors/fans must be increased by up to 38% at the extreme. Upto 3940RPM on the supply fans.

Has anyone experience of increasing the plate rating by this amount?

It doesn't sound right to me. What about increased heat generated, noise, vibration etc?

Any thoughts appreciated.

[superswill]

a little out my comfort zone here frank,but iam willing to have a bash

but can you decrease (its increase or decrease) the motor pulley size to increase the air flow?

[grahamgfm]

Your 2 pole motors (2880rpm) would have to run at 3974rpm and your 4 pole motors (1420rpm) would have to run at 1959rpm. You would require 240v 70hz to acheive this. How many 4 pole and 2 pole motors do you have to run? This sounds an expensive job!

[nike123]

a little out my comfort zone here frank,but iam willing to have a bash

but can you decrease (its increase or decrease) the motor pulley size to increase the air flow?

The supply fans are fitted with 2.5kW direct drive motors with a plate rating of 2880RPM and the extract fans have a 1.5kW direct drive motor rated at 1420RPM.

................

[Brian_UK]

Never mind the electrics Frank, that sounds like a hell of a speed to be running the fans at.

Don't know about inverter power outputs but a simple power calc at that speed increase takes your 2.5kW up to 6.4kW.

What about duct velocities at the new volume ?

Sounds like the has been a serious miscalculation at the design stage.

[TRASH101]

I agree with Brian,

Are you sure that there isn't a resistance problem and that the fans/motors aren't rated for the design volume. If its a design flaw then are the coils suitable for the increase of face velocity if you uprate the fans/motors ? (i.e. eliminators carry a resistance penalty too).



It doesn't sound right to me. What about increased heat generated, noise, vibration etc?


It isn't right unless the motor and fan manufacturer agree that their product is suitable for that amount of "thrashing" and I think you know what they would say, as the dynamic balancing of both motor shaft and fan would be a huge factor in noise, vibration, bearing wear. The heat and additional power required to run and the motor inefficiencies at that speed should really put the option out of the solution.

This is assuming of course that the fans going the right way and dampers are open (especially fire dampers);)

[casstrig]

Using an inverter to increase the motor speed,this dose not sound right,the maximum speed will be 2880 and can be reduced by an inverter.To increase the fan speed you must change the pulley ratio after consulting the fan curve as you can increase static pressure by too much.

[chemi-cool]

Hi Frank.

Increasing the speed will shorten the life of the ball bearings in the motors, will create more noise and will cost hell of a lot [inverters, cables, controls]

I would check the design of the AHU and amount of air required.

The AHU manufacturer's answer smells of a***s covering.

Smells bad to me.:mad:

[grahamgfm]

Hello Casstrig, You are correct in saying that you can run a motor from 0hz up to 50hz. Provideing it is a syncronous motor. Meaning that the speed of the motor is controlled by the frequency of the electrical supply. In our case 50hz. 50hz x 60 seconds divided by the number of pairs of poles.EG., 1x pair/2x pairs or 4x pairs. = RPM. You can also speed up a syncronous motor by increasing the frequency by using a power inverter. This however would be expensive in this case and would depend on the motor bearings ect., being able to run at the increased speed. In this case I think there was a misscalculation on the sizeing of this plant? Someone, as has been sugested, is covering their arse....

[grahamgfm]

The correct name for increasing the frequency is a power frequency converter. 2 types, solid state and rotary. Just getting the terms right :-)

[frank]

Thanks guys.

Next week, after the holiday, I'll post the manufacturers recommended data for the fans.

A message has been sent to the manufacturer asking him to provide fans and motors of the correct size to do the duty without having to resort to installing power frequency converters

Thanks for the heads up Graham :D

[grahamgfm]

Good decision Frank, put the ball firmly in their court.

[Brian_UK]

Using an inverter to increase the motor speed,this dose not sound right,the maximum speed will be 2880 and can be reduced by an inverter.To increase the fan speed you must change the pulley ratio after consulting the fan curve as you can increase static pressure by too much.:off topic:

Please read posts 1 and 4.