calculating power handling capability of a toroid core / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

calculating power handling capability of a toroid core

previous 1 2

Move Thread

LAN_403

Dr. Slack

Wed Oct 20 2010, 08:09AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The ratio of dimensions looks plausible from the eBay photograph.

However, he also gives you some vital approximation data, the core weighs 6lbs. Assume a similar amount of copper, and the completed transformer could weigh 12lbs.

Now weigh some of your MOTs, and also weigh a commercial "proper" big wound toroid. They should give you an upper and lower limit for power handling for that weight of transformer.

Quickly doing "stretching" exercises in my head, I'm pretty sure that power handling is linearly related to weight, other things being equal. So if you increase the height of the core, the weight increases linearly, as does the volts per turn, and so the power. As you stretch the diameter, the weight goes up as the square, but so does the copper area and hence the current you can deliver.

Again imagining some of the transformers I have, and trying to weigh them in my head, I reckon a 1kw MOT goes about 12lbs or less (somebody with access to a MOT and scales correct me on this one), and I have a "proper" 600VA transformer that weighs somewhat more.

Actually, for all that I slag off MOTs, they are the optimum solution to a very accurately stated problem - how to maximise the power of a transformer that has free forced air cooling, no efficiency requirements, never runs off-load, and must be as cheap and small as possible. So your power will vary by a factor of possibly as much as 10 between a conservative continuous rating, and edgy low duty cycle running.

Steve Conner

Wed Oct 20 2010, 08:53AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sounds about right. I've got an old 50 watt valve amp whose power transformer is about the size of a MOT, if not a little bigger.

That would be a design that has to supply only about 100-150W, but do it 24/7 with no forced air cooling, no audible hum and low flux leakage. And the old full-wave tube rectifier that only uses half of the winding at a time. The amp was made by Sansui, who were originally a transformer company.

klugesmith

Wed Oct 20 2010, 08:05PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Dr. Slack wrote ...
...Now weigh some of your MOTs, and also weigh a commercial "proper" big wound toroid. They should give you an upper and lower limit for power handling for that weight of transformer.

Quickly doing "stretching" exercises in my head, I'm pretty sure that power handling is linearly related to weight, other things being equal.
...

Slightly better than that, according to basic principles -- try a power law with exponent = 1.33.
If you double the linear dimensions you get 8 times the weight and 16 times the volt-amps, assuming constant flux density and current density.
That's why utility scale transformers easily reach efficiencies that are impossible in hand-held transformers.

Of course power handling ability is also roughly proportional to frequency.

Dr. Slack

Fri Oct 22 2010, 06:19AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Slightly better than that, according to basic principles -- try a power law with exponent = 1.33.
If you double the linear dimensions you get 8 times the weight and 16 times the volt-amps, assuming constant flux density and current density.

Yes, a 2x stretch each side is 4x the core area for volts, and 4x the winding area for current. So the "current per area" rated power has gone as weight^1.33. However the winding length and core volume at flux has increased so dissipation rises, and the surface area has only gone up 4x, so I'm not sure what that does for a dissipation limited rating.

Which explains why utility scale transformers use convection cooling arrangements.

Which means if you want to estimate the power of a core by weighing it, you'd better calibrate it by using a known rated transformer that's close to your proposed finished weight.

Steve Conner

Fri Oct 22 2010, 09:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, let's see. A 100VA transformer weighs about 1kg.

A 1kVA weighs about 10kg.

Our new 100kV, 10kVA, oil-filled test transformer weighs 200kg and had to be dangled into place by a forklift. (A lower voltage 10kVA transformer would be lighter: 100kV means a lot of insulation, which requires a bigger winding window, and that means a much bigger core.)

According to GE's brochure DEA-269, a 1MVA padmount transformer weighs 9400lbs, which is 4270kg.

According to this

a 10MVA transformer weighs 17000kg.

And finally this interesting brochure from Siemens

says that a 40MVA transformer weighs 70-80 metric tons (=1000kg) and a 100MVA unit comes in at "several hundred tonnes". The Siemens ones are available in a range of attractive colours too.

Areva make transformers up to 1000MVA, but I have no idea how much one of those weighs or how on earth they deliver it to the customer.

I'll leave it to someone else to try to fit a 1.33 power law to these data points...

klugesmith

Sun Oct 24 2010, 12:30AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Steve McConner wrote ...
... but I have no idea how much one of those weighs or how on earth they deliver it to the customer.

The bottom of this page shows a delivery failure, probably of equipment under 100 MVA based on Steve's data.

followed by a perceptive comment under "UPDATE FROM OUR READER".
Cargolaw is my favorite site for pictures of -big- wrecks.

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob