Electrical question - vac with power take-off

[mike]

I have a workshop vac with power take-off (ie. plug tool into socket on vac, vac operates when tool trigger pulled). I plugged a 200W sander into it, it fired up for a second and then the vac shut down - although the sander continued to run.

I assumed the vac was knackered (it was cheap) but I plugged an 800W tool in to check and it worked fine. A 250W sander worked fine too.

Then I plugged an extension lead into the vac with no tool connected to the extension - and it fired up for a fraction of a sec and then shut off.

I'm assuming there's a minimum load for the power take-off to work properly and I've never noticed before because I've never connected such a low-wattage tool. Is that right?

But what's the deal with the extension leads?

Thanks for any help.....

[John Rumm]

It would seem to be the case. It might use a current sensor to detect when the tool is in use and the small tool was just not drawing enough continuous current to keep it activated.

It is a so called "Transmission Line" effect. You can think of the length of wire as a long chain of very tiny inductors and capacitors that can store a small amount of energy. When connected to the power you will get a very small current flow into the lead that will in effect cause it to charge and discharge in sync with the AC supply. However since you don't control exactly where you are in the AC cycle when you connect the lead, you may happen to pick a point where the voltage is close to its peek value (340V) - this can result in a very brief but substantial current flow - possibly enough to trigger the current sensor. Since there is no actual load other than the wire this current flow is not sustained and hence it shuts off straight away after.

[Lurch]

That's what I was going to say!

SJW A.C.S. Ltd.

[mike ring]

[e-mail] (mike) wrote in news: [e-mail]:

John's answer appears admirable and I've made a note of it, but just to put my mind at rest....

You said "the VAC shut down"

If that was a slip then I can relax, but why should a sensor close down the vac rather than the take off, specially if it doesn't shut down the take off?

Please put me out of my misery

mike r

[Lurch]

Because the power to the take off is there as long as the vac is switched on, how else would it be able to sense when there is being current drawn if an appliance is plugged into it and switched on? Because the current drawn by the sander is below the minimum amount required to activate the vac then it continues to supply power to the take off waiting for the current to rise above the threshold required to operate the vac.

SJW A.C.S. Ltd.

[N. Thornton]

not at all. your vac has a loose connection.

eh?

Regards, NT

[John Rumm]

IIUC what is being described here then it does actually make sense that way round. The purpose of the power take off on the vac is to trigger automatic starting of the vac when you turn the tool on, not to turn on the tool when you fire up the vac (which could be a bit dangerous!).

[mike]

Thanks, John. Makes sense to me! Trust it also answers Mike Ring's query.

Mike

[mike ring]

John Rumm wrote in news:ZWBSb.209$ [e-mail]:

[posted and mailed]

On 30 Jan 2004, you wrote in uk.d-i-y:

Got it now John, and Lurch.

My elderly Miele has a takeoff for a turbo brush which is live when the cleaner is running, and dead when it isn't; I didn't cotton on to the power tool take off notion

mike r