Safely discharging a capacitor

[Guest]

Why do they have to be able to reverse the drum? I've never seen a dryer
that did that, obviously they must exist but I'm not sure of the benefits.

They tangle then untangle the washing. Our first one did that, Our current
one only goes one way, It's much less efficient, and is more work separating
at the end.

 

[david lang]

How many tumble driers use brushed motors?

I don't know, never had one apart, but I assumed they all did. Every
washing machine I've ever repaired had a brush motor, why would they use
induction on a tumble fryer as a matter of interest?

Dave

 

[paulfoel]

How many tumble driers use brushed motors?

How many brushed motors also have a capacitor?

Now this assumes that it _is_ a capacitor of course. If it does have a
brushed motor, I wouldn't be surprised if it had a line filter instead
and these can look very similar to capacitors.

Nope. Definitely an 8 micro-Farad capacitor.

 

[paulfoel]

There is a TB regarding capacitors on White Knight TD's, the fault is caused

by the capacitor being affixed too close to the motor, the spare part now
includes a bracket to move it further away. Sometimes even when the part is
replaced the motor subsequently burns out if the windings have been damaged
by a stalled motor.

TB ? (Technical Brief?)

Where can I see this info?

 

[powerstation]

TB ? (Technical Brief?)

Where can I see this info?

Sent to your email if it genuine
Peter

 

[Andy Dingley]

I don't know, never had one apart, but I assumed they all did. Every
washing machine I've ever repaired had a brush motor, why would they use
induction on a tumble fryer as a matter of interest?

You use induction motors whenever you can - they're quieter (this is a
domestic appliance after all) and they're cheaper. The advantage of the
brushed motor is that they have much higher torque at low speeds or when
stalled (why they appear on power tools) and they're also more easily
controllable for variable speeds etc. There's a lot of things a washing
machine does that a tumble drier just doesn't need.

 

[Tony Williams]

I don't know, never had one apart, but I assumed they all did.
Every washing machine I've ever repaired had a brush motor, why
would they use induction on a tumble fryer as a matter of
interest?

Relatively constant speed operation under variable load,
without the expense of speed-sensor and controller?

 

[david lang]

You use induction motors whenever you can - they're quieter (this is a
domestic appliance after all) and they're cheaper.

Odd. The only experience I have is with high pressure cleaners. The el
cheapo DIY jobbys have brush motors which make a terrible screaming noise -
but they use them to reduce costs.

The advantage of
the brushed motor is that they have much higher torque at low speeds
or when stalled (why they appear on power tools) and they're also
more easily controllable for variable speeds etc. There's a lot of
things a washing machine does that a tumble drier just doesn't need.

I can see the logic in that, but I've always assumed (from my HPC
experiences) that induction motors were much more expensive.

Dave

 

[Andrew Gabriel]

Odd. The only experience I have is with high pressure cleaners. The el
cheapo DIY jobbys have brush motors which make a terrible screaming noise -
but they use them to reduce costs.

My 4-5 year old el-cheapo "Deltajet" (cheapest I could find
at the time, for hosing mud off the bicycles) is a 1200W
induction motor.

In the case of lawnmowers, I suspect the use of universal
motors is to make mowers into consumables, which people
will thus tend to change every few years. Just about every
characteristic of a universal motor is inappropriate for
a lawn mower. You can find mowers with induction motors,
but they tend to be in the better quality less well known
makes, which you have to buy from specialists rather than
DIY sheds.

You can also get a more powerful universal motor in a smaller
physical size, but generally they are significantly less efficient.
E.g. you need around 30-50% more powerful universal motor to get
same cutting power out of a lawn mower as you do from an induction
motor, and the universal motor will run too hot to touch even
with forced air cooling, whilst the induction motor only runs
luke warm. I don't know if this is an inherent design issue, or
if it just reflects that the induction motors are found in better
specified/designed products. Also, universal motors generally run
much faster, indeed much too fast for many applications where they
need to be geared down (e.g. mowers, drills, etc).

I can see the logic in that, but I've always assumed (from my HPC
experiences) that induction motors were much more expensive.

I don't see why they should be technically. It may be a question
of supply and demand, what the market will bear, etc.

 

[soup]

Why do they have to be able to reverse the drum? I've never seen a
dryer that did that, obviously they must exist but I'm not sure of
the benefits.

To stop the clothes getting tangled, also the stopping then going the
other way will help stop areas of stagnation (and hence slow drying)
occuring. Mine a "Tricity Bendix TM 220 W" certainly has this drum
reversing indeed it is sold as a feature of this model and is called
"reverse action".

 

[Andrew Gabriel]

STOP IT STOP IT STOP IT

You're giving me ideas

I've got this terrible urge to dig out some perspex
sheet I've got left over in the garage...

I built a Van de graff generator when I was at school,
but that was rather disappointing. (On later reflection,
I'm not sure the belt material I used was a good enough
insulator.)

Then I built a high voltage generator using a car ignition
coil, EHT valve rectifier, and a capacitor made from
kitchen foil and a large roll of cellulose acetate
overhead projector film. I got really 'cracking' 4"-5"
sparks from it for a day, until the cellulose acetate
broke down.

 

[Andy Dingley]

Odd. The only experience I have is with high pressure cleaners. The el
cheapo DIY jobbys have brush motors which make a terrible screaming noise -
but they use them to reduce costs.

Induction is cheaper for a specific power, or for almost all large
motors. Brushed is cheaper for small low powered motors that still need
high torque. If you can go to a direct-drive design, then using a
brushed motor and no gearbox is usually cheaper, but you need a motor
that can provide the required torque on its own shaft.

 

[James Sweet]

In the case of lawnmowers, I suspect the use of universal
motors is to make mowers into consumables, which people
will thus tend to change every few years. Just about every
characteristic of a universal motor is inappropriate for
a lawn mower. You can find mowers with induction motors,
but they tend to be in the better quality less well known
makes, which you have to buy from specialists rather than
DIY sheds.

I suspect the main reason there is power to weight/cost ratio. You can
get a tremendous amount of power out of a compact universal motor but an
induction motor suitable for that would be a lot heavier and more
expensive. A universal will burn up at the drop of a hat though, a
friend of mine retrofitted his electric mower with a surplus pool pump
motor when that happened, he said it worked really well but those motors
are expensive new.

 

[James Sweet]

To stop the clothes getting tangled, also the stopping then going the
other way will help stop areas of stagnation (and hence slow drying)
occuring. Mine a "Tricity Bendix TM 220 W" certainly has this drum
reversing indeed it is sold as a feature of this model and is called
"reverse action".

Well damn, I cleaned up and tested out the Creda dryer which is the mate
for the washer I've been refurbishing and it does in fact reverse the
drum periodically. Nifty little machine, not like anything I've used
before. If the washer was a bit larger capacity and I didn't have a big
Neptune front loader already I'd definitely keep this pair but a friend
of mine has more of a need for them than I do.

 

[david lang]

In the case of lawnmowers, I suspect the use of universal
motors is to make mowers into consumables, which people
will thus tend to change every few years.

Ditto cheap high pressure cleaners. The £30 B&Q jobbies have a motor life
of 10-12 hours. Even a cheap Chinese induction motor will run for 50-60
hours. Qua;ity industrial machine will have an induction motor rated at
800-1000 hours.

Just about every
characteristic of a universal motor is inappropriate for
a lawn mower. You can find mowers with induction motors,
but they tend to be in the better quality less well known
makes, which you have to buy from specialists rather than
DIY sheds.

Ditto cheap high pressure cleaners.

Also, universal motors generally run
much faster, indeed much too fast for many applications where they
need to be geared down (e.g. mowers, drills, etc).

Indeed. High pressure pump wants to run at 1400 rpm, or 2800 at a pinch.
The cheapo HPCs use a ring gear to slow the motor down.

I don't see why they should be technically. It may be a question
of supply and demand, what the market will bear, etc.

Maybe mass production. Many el cheapo HPC's use domestic vacuum cleaner
motors.

Dave

 

[James Sweet]

Ditto cheap high pressure cleaners. The £30 B&Q jobbies have a motor life
of 10-12 hours. Even a cheap Chinese induction motor will run for 50-60
hours. Qua;ity industrial machine will have an induction motor rated at
800-1000 hours.

1000 hours sounds awfully low for a good induction motor. The 3HP main
pump in my spa runs 4 hours a day for the filtration cycles and it's 10
years old. Isn't used year round but it must have at least 5000 hours on
it and the bearings felt like new when I replaced the shaft seal. Theres
well used equipment at a friend's machine shop with motors dating back
to the 1940s that are still running strong.

 

[david lang]

1000 hours sounds awfully low for a good induction motor.

Depends on what you pay for them. On a cold water HPC the motor will be
about 65% of the total build cost, so they don't use the top grade -
besides, the pump etc won't last much more than 1,000 hours anyway.

Dave

 

[Ascro]

A capacitor with a digital readout? Now they've gone over the top!

I think that you need to understand the market they are trying to sell
into. When you see the claim "A CAPACITOR stores current." you see what
level they are aiming at!

 

[William R. Walsh]

Hi!

Would be nice, but I'm afraid back in '84, '87 and '88 respectively,
cars didn't come with CD players of any sort, hence my need to install
aftermarket units in mine.

I have seen some nicely designed car CD players (simple displays and simple
controls) for sale in various stores. One place I know to carry a few is
crutchfield.com (in the US, not sure if they sell overseas). They still
carry some fairly simple car audio devices.

This is a fairly new radio:
http://greyghost.dyndns.org/2003s10/radio-far.jpg

and to me it has a very simple, logical layout without all the flashing
widgets, whirligigs and doodads...I can run it with just one finger... Yes,
I replaced the factory CD player...my CDs are kept safely stored in a disc
changer and I like the concept of having a "direct" line in through the tape
deck OR being able to produce 120 minute mix tapes from any source I can
play into my stereo...turntable, CD, computer, whatever...

The tone quality of the factory radio was poor...bass, bass, bass all the
way. Had it read CD-RW discs I might have left it alone.

Not to mention I'm still not aware of any OEM units that will play MP3
discs, an essential feature to me that made all earlier CD players
virtually obsolete. 10 hours of music on one disc, no more fumbling with
CD's in traffic.

There do exist a few. I know that Chevrolet has one in their Aveo compact
car (not sure where all this is sold) and it will play MP3s.

William