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#144388 11/19/05 11:04 AM
Joined: Dec 2004
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"As they say, if it ain't broke, don't fix it."


Words to live by

[This message has been edited by IanR (edited 11-19-2005).]

#144389 11/19/05 12:31 PM
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The point about universal series motors is a good one. In fact I was looking at Porter Cable routers a few weeks ago and the specs. on the website made a point of emphasizing that the tools are suitable for D.C. operation.

I'd go with the flow. I don't see any problem with your current setup.



[This message has been edited by pauluk (edited 11-19-2005).]

#144390 11/19/05 12:48 PM
Joined: Mar 2005
Posts: 1,803
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Quote
If it ain't broke, don't fix it

I went on a Ford Motor Co. Seminar on just this subject 15 years back. Ford Engineers had proved that 'planned maintenance' did more damage to machines by upsetting bearing-bedding, intoducing mis-alignment on reassembly and by the introduction of fresh crap to moving parts, than letting them run on. Not 'till they exploded from lack of care, of course! Their new method was called 'Condition Maintenance'; routine examination of lube quality, (ie metal content, additive breakdown, presence of contaminants etc.), checking the temperature of drives, bearings and slides with thermocouples, and analysis and comparison of vibration patterns from machines by microproceesors, all these would indicate when maintenance was actually required within close limits. Result: Better quality, less downtime, lower costs.
Lots of 'old-wives-tales' have a good core of common sense.
Needless to say, the Dinosaurs on our plant carried on taking everything to bits once a year regardless!

Alan


Wood work but can't!
#144391 11/20/05 07:13 PM
Joined: Jul 2005
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Re DC operation of appliances with universal motors, while the motors will be ok with it, the switches in modern appliances generally won't be. The gap between the contacts when the swtich is opened is likely to be insufficent to quench the arc.
Anyway it's more hypothetical than anything as DC mains no longer exist. There was however a commercially made gadget a few years ago which provided 240V DC from your car alternator. It was recommended for various heating appliances, power tools and incandescent lighting.
As the output from a car alternator is unfiltered rectified DC, any switch arc will be extinguished as per AC during the zero voltage part of the cycle.
I have never seen 50 vs 60 cycle operation of induction motors being a problem except for speed critical applications.

#144392 11/21/05 01:24 AM
Joined: Feb 2003
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While it is unlikely that an occasionally-used motor will burn out if connected to the wrong frequency, one which runs continuously will not last long. I think this applies to induction motors only. Universal motors are safe.

Here is a better explanation than I could come up with.

I can recall connecting 50Hz induction motors in industrial situations here in the US. They always had transformers due to the difference in design voltage, but I wonder if the voltage was tweaked to compensate for the difference in current as well? That was so long ago I don't remember.

Has anybody ever monitored the output of a VFD with a voltmeter? Clearly an induction motor can be run at a variety of frequencies without harm, since VFDs are widely used.

#144393 11/23/05 05:16 AM
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Quote
Re DC operation of appliances with universal motors, while the motors will be ok with it, the switches in modern appliances generally won't be.

That's the thing to watch. I assume that those Porter Cable tools specified for DC operation are fitted with the appropriate switches.

Quote
Anyway it's more hypothetical than anything as DC mains no longer exist.

I think India is still supposed to have DC in some areas.

Quote
While it is unlikely that an occasionally-used motor will burn out if connected to the wrong frequency, one which runs continuously will not last long. I think this applies to induction motors only. Universal motors are safe.

That's pretty much the way I would take it. Lightly used power tools etc. run on 50Hz aren't going to be a problem. I'd be wary of trying to run something like a refrigerator or air-con compressor on the wrong frequency.

#144394 11/23/05 07:08 PM
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I think India is still supposed to have DC in some areas.

I had seen reference to this in various tables of world electricity supplies up until the early 80's, but nowadays I'd be surprised if it does still exist, especially for domestic supply. I haven't seen any mention of DC rated domestic appliances for many years. Though given India's population it probably would be economical to still manufacture for a small market there.
In Sydney the last DC mains were killed off in 1985. It was a 480V centre tapped system feeding parts of the CBD mainly around the harbour. The +/- 240V supplies were used for lighting and appliances up until the 1950's but the 480V was still used for things like lift motors into the 1980's. By then the motor generator AC-DC converters had given way to mercury arc rectifiers.
Many country towns, particularly in Victoria and Western Australia also had DC supplies into the 50's. Interesting stories I've heard include when a light bulb filament went o/c whilst switched on....apparently the arc travelled up out of the light bulb, and up the flex to the ceiling rose. Then there was the electric jug with exposed element plugged into DC...the resulting boiled water had a strange taste with the electrolytic action that had occurred. And of course there's the usual burnt up power transformers when people took their mantel radios to these places and plugged them in.

#144395 11/24/05 09:24 AM
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DC supplies were still in use in some parts of Britain in the early 1960s, mostly the older parts of urban areas which had been the first to get electricity, so transformerless AC/DC radios and TVs were very common. Some models were made in an AC-only xfmr version as well though. I have an old Bush set which belonged to my grandparents and the service sheet refers to a field conversion kit which was sold.

I'm not sure of the exact date of the very last DC public supply here.

#144396 11/24/05 01:14 PM
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Vienna's last DC supply was shut down in 1955. A radio technician told me that DC used the same round pin plugs as AC, so correct polarity was not ensured. It was a 220/440V system.

#144397 11/25/05 06:45 AM
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A radio technician told me that DC used the same round pin plugs as AC, so correct polarity was not ensured

That was the same here. If an AC/DC radio was plugged in the wrong way round, the dial would light up, as would the tube filaments, but there would be no B+ supply so it would remain silent. Some instruction booklets added a note to reverse the plug if this happened.

As the live would be positive with respect to ground/neutral in some houses, negative in others, when fitting a 3-pin plug to a radio it was sometimes necessary to deliberately reverse the connections from the usual. And of course, when taken to a house wired on the opposite pole of the supply, it wouldn't work again until the connections were swapped over!

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