mains dropping capacitor for 6A

[oldway]

What happens when one fan starts up before another? Not possible, they are dc brushed motors, with the same current. They have the same torque. (dc permanent magnet). Why should one start before the others ?

What happens when they aren't matched? One fan spins faster than another. As resistive torque of a fan is proportional at the third power of rpm and the motor torque is proportional to the current (who is the same in all the motors), the difference stay very small. Nothing occurs.

What happens when one gets stuck or meets higher resistance? Higher resistance ? Never see this occuring in a dc brushed motor. ...If it get stuck (if can't rotate anymore), it's more or less the same as failing short.

What happens when one fails open? They all stop

What happens when one fails short? Others fans have higher voltage (13,7V instead of 12V)

The true problems:
1) No redundancy and lower MTBF
2) Isolation problems: 12Vdc fans are not made to withstand mains voltage between ground and windings.
3) Safety risk for maintenance...

[Simon]

The true problems:
1) No redundancy and lower MTBF
2) Isolation problems: 12Vdc fans are not made to withstand mains voltage between ground and windings.
3) Safety risk for maintenance...

Which is pretty much what i told my boss.

[rolycat]

... for my next work project I could use a 40000W 24V dummy load (seriously).

Well, a cheap'n'cheerful method might be to parallel, say, 10 x 6.5m lengths of 1mm enamelled copper wire. Arrange that in a hundred litres or so of water and feed it 40 kW and my (crude) calculations suggest it will take about 7 minutes to heat from 20 to 60 degrees. If you need more time you could add a cold water supply and a drain.

That doesn't sound like a good idea.  166.7A down a 1mm wire is going to blow like a fuse unless the terminations are also fully immersed.

Oh, you would definitely want to immerse the terminations.

[rolycat]

... for my next work project I could use a 40000W 24V dummy load (seriously).

Well, a cheap'n'cheerful method might be to parallel, say, 10 x 6.5m lengths of 1mm enamelled copper wire. Arrange that in a hundred litres or so of water and feed it 40 kW and my (crude) calculations suggest it will take about 7 minutes to heat from 20 to 60 degrees. If you need more time you could add a cold water supply and a drain.

I think we will stick with a 240V fan thank you

I wasn't attempting to address your problem, Simon. You seem to have enough people doing that.

[Ian.M]

@rolycat:  Your proposed load is operating at approx 0.2W/sqmm.  I believe the commonly accepted maximum surface power density for water heating elements is around half that.

[rolycat]

@rolycat:  Your proposed load is operating at approx 0.2W/sqmm.  I believe the commonly accepted maximum surface power density for water heating elements is around half that.

I can well believe it - I did say it was a cheap and cheerful idea. It was largely a thought experiment, and without knowing NANDBlog's circumstances I have no idea whether it would be appropriate.

For a one-off test, though, I think it might work. Wires immersed in water can cheerfully carry five times their fusing current in free air.

[Simon]

... for my next work project I could use a 40000W 24V dummy load (seriously).

Well, a cheap'n'cheerful method might be to parallel, say, 10 x 6.5m lengths of 1mm enamelled copper wire. Arrange that in a hundred litres or so of water and feed it 40 kW and my (crude) calculations suggest it will take about 7 minutes to heat from 20 to 60 degrees. If you need more time you could add a cold water supply and a drain.

I think we will stick with a 240V fan thank you

I wasn't attempting to address your problem, Simon. You seem to have enough people doing that.

Really ? most of them did not read past the first page if that.

[vk6zgo]

I think we will stick with a 240V fan thank you

I wasn't attempting to address your problem, Simon. You seem to have enough people doing that.

Really ? most of them did not read past the first page if that.
[/quote]

Simon,that's the problem---you didn't give us enough information in your first posting,like what size of fans,how they were being used,etc.
Perhaps you could have made the point  more strongly that you favoured a 240v ac solution,& were looking for information to help you rebut your Employers/customers idea of using 13v dc fans.

[Simon]

Putting fans in series was my idea given the 240V requirement and the 12V fans, that moved to 24V fans and no need for voltage dropping when I found the same version. Later I got to read the emails and they were already talking 240V.

Lots of people seemed to have assumed that I was talking about putting brushless fans in series, which is a silly thing to do.

[mikeselectricstuff]

Either do it properly and use a 240v fan, or us a switchmode PSU - 600W isn't a huge deal, even less so if you can put it in the airflow for cooling.