Dear serdman,

Due to the non linear fan curves, it is not a surprise that the “fan laws” did not work and especially when you increase the rpm by 120% (example: 1000 rpm + 1200 rpm = 2200 rpm)

Moreover, the increased flow induces a higher backpressure and the expected flow is therefore reduced until the increased pressure is reached in the new operating point on the curve.

Have you taken into account the flow resistance curve of the system?

Did you use the manufacturers data sheets and curves?

Looking at the FLA of 5.8 A and the, obvious, low airflow, I would say that the flow resistance curve is very steep, causing the observed result.

Have a nice day

Teus ■

Dear serdman,

The consumed power of an electro motor is:

kW = 3 * Delta voltage/SQR(3) * Amps * cos(phi)

Therefore, the relation between power and Amps is also depending in the cos(phi) (power factor or phase angle between current and voltage)

In your case, the power factor increased significantly and therefore the consumed Amps increased less.

The fan laws are explained in the link:

http://www.engineeringtoolbox.com/fa...aws-d196.html

The fan laws explain the change in the operating curves and not the response of a complete system as the flow resistance curve plays also a role.

Why was the original rpm chosen so low? (original rpm = approx 0.6/1.4 * rpmmax = 0.43 * rpmmax)

Success

Teus ■

The reason is, You might be simply watching at current AMPs only while comparision.

As you told, you have experienced increased air flow due to increased blower speed.

Now as per the mass transfer theory, when air flow rate increases, load on the motor will also increase. This is for sure.

Let me explain why the motor current did not increased as per the fan law.

Please note,

increase in speed might be only calculated one, based on motor speed and drive ratios.

I want to say that there are somany cases in which blower may be running at predicted speed or new speed. Like....

1. Speed transmission efficiency of the drive may not be same as earlier and so transmitted speed or actual speed of the blower may be lower. belt slipage.

2. Motor speed may not be obtaining its rated speed i.e. full speed due to over load or voltage fluctuations. In somany cases it is seen that the amp reading of overloaded motors are showing unrealistic current reading i.e. below the noload current reading. This is because, the actual work done is lesser than the predicted one due to reduced speed of the motor in overloaded condition.

I hope there should have any or all of above reasons for your doubt towards fan laws.

Please also note that you have only felt that the air flow is increased but did not have measured the difference.

So i would like to advise you to measure the actual air delivery or actual blower speed.

But you should be careful, if your motor is really overloaded then there must be significant temprature rise of the motor. so please ensure for protection of your motor first and do not run it more.

Vikesh Patel

Exe. Engineer

www.amarfabricators.co.in ■

Belt slip is not something to be missed. The noise and heat generated will soon draw attention to itself.

At 7.5 HP I would expect the motor to be a 3 phase squirrel cage induction motor so if it was overloaded the standard characteristics would result in an increase in current. The minimum current draw would be when the motor is off load running at near synchronous speed. Of course it could be "something completely different" as the originator has not specified the motor type.

It's quite possible to feel an insrease in airflow!

If the motor is overloaded then I would have expected the thermal overloads to trip.

Actually what we need is a copy of the fan characteristic graphs for the fan in question, and full details of the motor driving it. ■

Dear Mr. Designer,

You are right, that if slippage takes place, it gives abnormal sound and heat.

That is also true that if motor overloads, it generates heat in itself. And also in the situation of high overloading, motor will be blown in minutes.

Regarding thermal protection, yes you are right it must break the supply to motor due to higher current draw.

But the originator has not cleared his side on any of above points and he is asking CAN FAN LAWS BE FAILED?

According to me, there may be a chance for overloading b'coz 20% increase in speed will 72% increase in power. So the new load is near to double. So I think this may create slippage as well as overloading of motor .

But still this is a guess only, we don’t know what actually happened there and nobody can conclude that fan laws had or had not failed there by knowing information given by the originator.

Regards,

Vikesh Patel ■

Dear Serdman,

The best thing for exact prediction is to measure the air delivery and pressure.

I hope this will solve your doubt.

Regards,

Vikesh Patel ■

Have you checked that the fan is running the right way?

Michael Reid. ■

Dear Vikesh,

Attached an example of a fan curve on which the fan laws were applied.

flow1 = n2/n1 * flow1

pressure2 = (n1/n2)^2 * pressure1

power2 = (n1/n2)^3 * power1

Try to reconstruct these curves for your case and see what tha consequences are.

Success

Teus

Attachments

fanlawscurves model (1) (PDF)

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Assuming its 3 phase, did you measure ach of three legs for current draw? They may not all draw same current. ■

What is needed is what we don't have, the performance characteristic graphs for the fan. ■