Squirrel cage amperage related question

" if you block off inlet air completely, the amperage decreases. I thought it was the complete opposite"

When you block the inlet of any blower, it is no longer doing any work. So the amperage will decrease. The more air it moves, the harder it's working. Less air, less work, less current.

no longer doing any work

I don't get it - If the intake is blocked, isn't the impeller still turning and effectively trying to create a vacuum ? ie, it's load increases and it would burn out ? I'd like some further explanation/re-assurance before I test this on something. From your description, it sounds like I can duct-tape the inlet to Mrs K's hair-dryer, set it to cold, and all will be OK ?

It's true! However, no longer doing any work is an overstatement though. You are still doing some work.

I've actually proved this. I had a manager make the statement that a dirty filter will cause a motor to draw excessive current. I said no. So, I had to prove it, with a 60hp blower. I cut a piece of plywood the same size as the filter. I them put on a clamp meter. We took a reading with no filter, then with the plywood in place of the filter. The current went way down with the plywood in place.

It's somewhat counter-intuitive, but when you think about it, you need a lot more power to move a large volume of air, than not.

But don't try it with the wife's hairdryer. The airflow in that unit also provides cooling for the motor. That's not the case with an industrial blower. The motor has it's own impeller to do that.

Ah, it becomes clearer. Thank you Bricktop. Damn shame I got impatient to test the hairdryer.

Bricktop G.A.

Is the same principle true for a water pump ?

Regards

Mr. W.A Snow

For a centrifugal pump ,yes, normally. It will damage the pump though, through cavitation (Kris' vacuum doing some destructive work). Flow reduction is best done on the discharge side via a valve. Is it an efficient method, no. That is why VSD devices are built.

Having said all that and fully agreeing with Bricktop, some pumps and fans behave in an odd manner. Just a small percentage, but they are beasts.

To the OP to satisfy yourself and anyone else with an interest, use Bricktop's test method (your digital multimeter will record the current if you don't have a tong that is accurate at low current) and let us all know how you went.

Yup!

You will also hear the speed of the motor INCREASE as the load is reduced (no more air passing though!!)

Just a supplement to Bricktop's explanation: if you stop the airflow, air doesn't move into and out of the region swept be the fan blades - so the fan isn't accelerating anything much. That means the fan blades are freeer to rotate. If DC powered (or AC powered and slipping), the motor can accelerate and generate a larger back-EMF; if it's AC driven and not slipping, the phase of the back EMF will advance and the effect will be similar.
Sorry if I've used the terms loosely.

N.B. I thought I might find you in this custom-built cage - but I'd hoped you might obey the law and refrain from smoking indoors.

Good day Compressit,

some motors have thermo cut-off fuses which are ampere rated & thermally rated.

I think that a clogged filter would increase the internal drive temperature & thus would cause the fuse to do its job. Drives contain senitive electronic components that are sensitive to things such as power surges, cooling & mechanical shock. Most drives have certain ventilation sepcifications that have to be adhered to when installing.

I hope this clears matters a bit.

kind regards

I like the accurate answers I see. Furthermore, overheating due to blocked filters can contribute to a blown fuse by preventing flow through (if applicable) and certainly body cooling of the motor(s). Check the amperage while running under full load as well as the voltage and compare it to the nameplate. Also check start amps & duration. There's always a reason a fuse opens. Is this direct drive?

Very good boys and girls, we have all discovered that as a conductor heats up resistance increases causing less current to flow. However, we have failed to answer the original question by Compressit.

Why did the fuse blow (overcurrent protection) or has Compressit mistaken a thermal protection device as a fuse?

I dunno uncle goat.

I are not the sharpest tool in the shed so I wait to see what compressit says.

Right on GROG. You're up to your usual 100 years of experience! (compliment intended)

How many "technicians" just replace the fuse?

Once more into the fray of chaotic diatribe with some cited information. From the Littlefuse designer's guide of 1988. "The current carrying capacity tests of fuses are performed at 25 °C and will be affected by changes in ambient temperature. The higher the ambient temperature, the hotter the fuse will operate, and the shorter its life will be."

Enough said.

The conductors heating is a secondary effect, which will take some time to occur. Cutting off the airflow has an immediate effect - far faster than the heating. This is best illustrated by covering the inlet to a vacuum cleaner hose.

As Andy said, when you block off the airflow, the motor immediately runs faster (indicating it is doing LESS work pumping air). With many vacuum cleaners, the same airflow that does the cleaning also cools the motor, so if the airflow is blocked very long, the motor will overheat and either shut itself off or burn out.

Unless the fuse is located in the air stream the heating should not blow the fuse, because less current is flowing, not more.

Now if the VFD has a sensor that tries to regulate air flow, it may have tried to increase current to keep the same flow under restricted conditions. That could definitely blow a fuse.

AHHHHHHHHHHH! No no no no. How many of you remember about phase angle involving reactive loads? How many of you even have the faintest idea what I'm talking about? The real power drawn by the motor is P=VIcos(Θ). The magnitude of the current drawn by the motor does drop in a non-linear fashion as the slip speed approaches zero. The slip speed is the difference between the synchronous rotational speed and the rotor speed. But until one externally drives the motor shaft to convert the motor to a generator, the internal losses are not compensated to cause a significant drop in current. So the motor is still producing mechanical work.

Now that I've gotten that tantrum over with, the lack of air movement did cause the motor and fuse to overheat. All fuses have a temperature degradation of their holding current rating. This overheating may have increased the drag slightly by dimensional changes of the bearings and thus once again slightly increased the drawn current closer to full current draw. Possibly the motor overheating started some mild breakdown of the winding insulation and caused a sharp increase of current drawn but when this happens it is usually is a permanent failure.

My conclusions come from studying the textbook, "Electric Machinery Fundamentals" by Stephen J. Chapman published by McGraw-Hill. While their maybe a web site that does publish this kind of information, I do not know of a web site that does. Since a little bit of knowledge is dangerous, some things are best learned in a classroom.

Good answer, both from and Tomad but you were first and backed it up with documentation. If I were teacher I would give you an A.

I know this is simple minded but you do deserve some praise from someone.

Compressit,

The current drawn by a squirrel cage motors (induction motors) practically does not change if the total output power remains the same, irrespective if it is ventilated via a fan or not. But the balance between the heat developed in a motor (due to losses in windings, mechanical losses, etc) and the heat dissipated via its fan is distorted in case fan air input is blocked, so the created heat leads to an increase in the temperature of the motor and ultimately may lead to damaging the electrical insulation, etc.

If the motor is driving a fan, the output power of the motor has to match the required power input of the fan. This power is dependent on the fan characteristics (power (HP) versus air flux (cfm). When gradually blocking the air input of the fan, the power required increases up to a point and, for a total input block, it decreases.

So, with air input totally clogged, the power drawn by the driving motor is less than the rated output power (less current drawn). So basically your customer was right.

Thanks to Tomad and redfred and all others for their input. To clear up any confusion, yes it was an overcurrent protection fuse (8 amp to be exact) that blew not a thermal protection device (unless there is something called a thermal protection device that looks like a fuse and says time delay and 8 amps...). I do feel slightly dumb for thinking lack of airflow would increase motor amp draw even though in my head this seems like a logical explanation. Anyway, the motors show fine now when tested with a multimeter. Amp draw, voltage balance and windings all check out fine. I'm just going to have to assume that the heat of the frequency drive was what took it out. I really dont see any other way to look at it. Oh, yes one more thing. The contacts of the motor starters also chout out fine. Once again thanks so much for all the replys even though this question seemed kind of sophomoric.

Travis