Motor electrical question

I'm not sure of the exact mechanism, but on home-style vacuum cleaners when you block the vacuum line the motor speeds up. I assume this means the motor is unloaded, and I assume it's because the turbine is suddenly working in rarefied air.

I'd be interested in hearing truth from one who knows.

A blocked air path will -lower- the current because the fan blades stall and give less lift and drag. Put your hand over the inlet of a vacuum cleaner; the motor pitch rises.

One thing to keep in mind is that the current draw of a centrifugal blower will usually go down as its output is restricted. The current is more closely related to the airflow than to the back pressure. So it's not surprising that you saw an increase in current when you removed all the restrictions.

Tim Wescott fired this volley in news: snipped-for-privacy@giganews.com:

The speed is dependent upon the mass of air being moved. It's pretty large when sucking and blowing freely. The motor does the most work moving the largest volume. When the inlet is blocked, the fan just recirculates the air in the volute, and it's (essentially) unloaded.

The air isn't 'rarified' enough to make a huge difference... most only suck in a few tens of inches of water column... and it takes almost 34' of water to make one atmosphere's worth of pressure.

Lloyd

Oh, yeah ... I remember now, about the current increasing as restriction decreases. Seems paradoxical until you think about it as a matter of mass of air being moved. Thanks for that "model".

Now, back to the original problem: with the "extractor" operational, it was overheating. The mass-of-air model would suggest that the cause of that would be an air _leak_, not a blockage. I'll look into that & be back.

Thanks, Bob

Umm, give it back to Festool for a warranty replacement? I think Festo is one of the companies who take warranties to a new level. Given their outrageous pricing, I'm not surprised.

Gunner Asch fired this volley in news: snipped-for-privacy@4ax.com:

Gunner makes a good point. If the (I'll bet bushings) are even a little snug when cold, you can bet they'll bind up when hot.

Lloyd

Out of warranty. I have heard that they are very good at customer service. As you said, they should be for the prices they charge.

That is one way of looking at it. Normally the air is very dense on the leading edge of the vanes when its plugged it is more rarefied. Its kind of like when you are rowing a canoe and you miss the water. Its a whole lot easier when not pushing against something dense.

You could see if there's a difference between blocking the inlet and the outlet.

Both will lower the load on the motor but not the same amount. The load goes down when the pressure differential on either side of the vane goes down. When the flow is blocked the air just goes round and round and there is not much pressure differential. When the inlet is blocked the air is more rarefied so it does have slightly less resistance.

Also there is probably some air flow through the motor for cooling. If that is on the discharge side blocking the discharge won't stop all flow and blocking the inlet may still cause overheating because the motor cooling flow is also stopped.

Either that or the excess friction in the motor -- it really ought to spin more or less freely (taking bearing seals into account, which I doubt are there).

I recall grabbing the shaft of a 10-horse compressor motor and (slowly) giving it a spin -- I think that would be my baseline.

If Festool is using only bushings in a $650 shop-vac, I'm _sure_ they're more than just over-priced.

...

OK: I put my hand over the intake of the fan, blocking it totally and the current dropped some, from 7.4 to 5.6A. Still way above the spec'ed minimum of 2.9. So the problem is not a leak. Meaning it must(?) be bearings.

I'm going to try to do some cleaning without disassembly. Not Gunner's total immersion, cause I'll use PB Blaster, some such, and I don't have a gallon of that.

Bob

I'm pretty sure that I read that it has bearings. Festool does not consider them serviceable and sells the motor-fan for $95.

Bob

How do they control the speed? i.e. can you check/get in between the speed control and the actual motor?

Just wondering if something went bad in the speed control and it's sinking the extra amperage. Do you have any kind of schematic to work with that might show any of this?

There is a PC board for speed control. No schematic that I could find on the net.

I had this thought too & I put a Fluke clamp-on true RMS meter between the controller and the motor. It read the same as the ammeter on the input.

Bob

Hence Kirchoff's Law

MikeB

I'm guessing that this is the model you're working on:

Per their website they list the amperage as "Power Consumption

350-1200 watts, 2.9-10.0 amps". Maybe the temperature sensing mechanism is at fault and not the motor. Just saying...

Interestingly the manual:

has the amperage as you said:

=== Power consumption (dust extractor only) 2.9 - 8.3 A (350 - 1000 W) see note 1 .

Note 1: 8.3 A (1000 W) is the medium value for different operation conditions, a maximum power consumption of 10 A (1200 W) is possible. ===

Different model comparison chart on this page along with amperage values:

Any loose heat-sink items on the electronics board or something that could mess up the temperature sensing?

One last thought after perusing the manual... Have you experienced the unit overheating? From what I could deduce reading the manual:

=== Max. rating of connected Power Tool = 3.7 A - Note: When the suction power adjuster [1-7] is adjusted to the lowest suction power (see page

10, chapter Settings - Adjusting the suction power) the maximal rating of a connected power tool can be 9.1 Amps. ===

It doesn't say whether this load is being sensed by the over temperature alarm but I think it might be. Did your neighbor have something a bit too greedy plugged into it?