Thread: Drill Sharpener

Originally Posted by Michael G

This is my ebay special - Ray suggested that I get it so I could run things like a 110V Biax. I think it was around $70. I must admit I've voided my warranty by opening it and putting a longer supply cord. Originally it was only around 400mm long. Now I have 2m.


Michael

Looks better that I might have guessed for that price, the key thing is the approval.. N16265, must be some of the other ones that look similar are solid state perhaps?

Originally Posted by Chris cba_melbourne

Ray, autotransformers illegal? Can you elaborate on that? There have been Aussie made autotransformers since there is electricity... and they still are made today.

In this instance, yes, under SAA wiring rules it would be illegal.

The relevant SAA clause is this one.. from AS 3000:2007



So to power a 110V rated appliance from a 240v input autotransformer is illegal.

Ray

Originally Posted by RayG

In this instance, yes, under SAA wiring rules it would be illegal.

The relevant SAA clause is this one.. from AS 3000:2007



So to power a 110V rated appliance from a 240v input autotransformer is illegal.

Ray

You are perfectly right Ray. It is illegal using it to power an appliance, that itself is not rated for in our case 240V.

However, the manufacturing, selling and using of an autotransformer is perfectly legal in Australia. The end user is left to find out by himself, if or if not AS3000:2007 4.14.4 is applicable to the particular appliance to be attached.

In this case, the Darex Drill Doctor intended for the American and Japanese market is only fitted with an approval label for 110-120V. It must therefore be hooked up to an isolation transformer. Sorry for having recommended otherwise.



This is an article I found that may help for those considering importing an US appliance to Australia:
http://www.armory.com/~stacey/aus-us-power.html

I have one. I have trouble with drills that have a longer helix pattern.
plus i dont think that the split point works well.
I brought it second hand it was cheap
its the older 750 model

dd3.jpgdd7.jpgdd1.jpgdd2.jpg

Originally Posted by cba_melbourne

I can not recommend the idea with the light bulbs. At least not in the context of hooking up a 110V Drill Doctor. The DD uses a DC motor.

- with an incandescent lamp, when its off the cold filament has a very low resistance. Once the filament get hot and begins to glow, its resistance is much higher. It is said a light bulb has a positive temperature coefficient, it is a PTC resistor. In practice, this means in our case that when the motor is turned on, it will initially be hit with the full 240V, resulting in a very high inrush current. The excessive current is unlikely to damage the motor, as DC motors can support high currents for a short time (think a servo motor, or a car engine starter motor). But the very substantial overvoltage (initially 240V into a device designed for 110V) is possibly not ideal for the rectifier, the power switch and the overall insulation.

- As the motor accellerates the light bulb warms up and limits the voltage as intended. But now, as the motor is on speed and not loaded by grinding, the current drops. The lamp gets again cooler, and causes the voltage to the motor to rise. This is not good. On a DC motor, the motor speed is proportional to the applied voltage. Double the voltage means almost double the rpm. So with the lamp cooling the motor rpm rises in an uncontrolled manner whilst idling.

- Now we load the motor by grinding a drill. The current rises, and the lamp gets hotter. Thereby limiting the current and reducing the voltage to the motor to well below 110V.

This behaviour is exactly the opposite of one would expect a motor controller to do. Instead of limiting top rpm at idle, it increases rpm at idle. And instead of increasing the current when the motor is loaded to keep rpm constant, it decreases the rpm and with it the power output.

Sorry BaronJ, but in the case of a DrillDoctor, it's got to be either a step down transformer, or pay the price for a 240V export model.

Hi Cba, Guys,

I don't disagree at all. The whole idea was to provide a quick and dirty way of providing a power source to run the machine.

All the points made by Cba are perfectly valid ! A step down transformer would be the correct way to go, and provide isolation from the incoming supply.

Originally Posted by Michael G

This is my ebay special - Ray suggested that I get it so I could run things like a 110V Biax. I think it was around $70. I must admit I've voided my warranty by opening it and putting a longer supply cord. Originally it was only around 400mm long. Now I have 2m.
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Michael

This is exactly what I would expect to see in there ! A thumping great transformer. Very heavy, lots of iron and should run quite cool at full load. Not easy to see in the pictures, but I would also expect that the output would be centre tapped and connected to earth.

Originally Posted by BaronJ

This is exactly what I would expect to see in there ! A thumping great transformer. Very heavy, lots of iron and should run quite cool at full load. Not easy to see in the pictures, but I would also expect that the output would be centre tapped and connected to earth.

Yes, it is heavy!
The centre tapping can be seen as the white wire in the last picture soldered on to a tag secured to the case using the fuse holder (also visible in the 3rd picture). The factory can't be able to afford coloured wire, this thing is so cheap. It does the job and works but I can't say that the quality of workmanship fills me with great confidence.

Michael

Originally Posted by BaronJ

This is exactly what I would expect to see in there ! A thumping great transformer. Very heavy, lots of iron and should run quite cool at full load. Not easy to see in the pictures, but I would also expect that the output would be centre tapped and connected to earth.

Why would you expect an earth center tap on an isolation transformer?

Originally Posted by Stustoys

Why would you expect an earth center tap on an isolation transformer?

If you ground the center tap, the the appliance (say a handheld drill) still sees 110V. But if you touch either live wire (say because the power cord breaks), you only get zapped with 55V - which will not kill you, even if you were working barefoot on a wet floor. If you take both live wires each in one hand, then you would of course still feel 110V - but that is rather unlikely to happen inadvertently.

If you think at a construction site, where extension cords get on a daily basis dragged about unfinished concrete floors and around sharp unfinished edges, you appreciate the high risk. On Victoria construction sites the law does not require to use insulation transformers, but all electric tools and extension leads need to be safety tested and tagged every 3 months.

But(as I understand it), if you have no center tap and no earth connection on the secondary you are isolated of the supply(i.e no potential to ground. To something like 6.5kV if I remember correctly). So to use your analogy you can hold either wire while standing barefoot on a wet floor and you wont get zapped at all. Though you'll still get 110V if you manage to get across both lines. In either case case(of being across both lines) an RCD isnt going to save you. Though I assume the chances of getting across both lines at near as damn it exactly the same time would have to be pretty slim......which maybe why the secondary is earthed.........I guess with an isolated secondary, the bigger the system gets, the more chance of two opposite faults* going undetected and meeting(through someone) increases.

Stuart

*along the lines of two pistol drills one with A shorted to earth, one with N shorted to earth. Either fine to use by itself, just dont pick up one in each hand.

Originally Posted by Stustoys

..... In either case case(of being across both lines) an RCD isnt going to save you. .............

Yes, but with the center tap of the isolation transformer secondary grounded, there is still a return path for the secondary earth leakage current back to the RCD. The RCD will still work, as it is triggered by current not by voltage.

Originally Posted by Stustoys

Why would you expect an earth center tap on an isolation transformer?

The only reason is that the UK regulations require a center tapped ground. I'm open to either argument, we used to argue this back and forth for years without ever acheiving a consensus... my argument is that if all the mains is floating with respect to ground, you can never get electrocuted by a fault to ground, unless you have two faults, one to ground and one to you. If all mains is referenced to ground ( like in Australia ) then only one fault is required to get zapped... so to mitigate that risk we mandate the use of RCD's .... One reason (perhaps) to reference mains to ground is to minimize lightning strike damage... the discussion then usually moves on to solar storms and coronial mass ejection.. then I make the sign of the cross and slowly back away... Oh.. and most portable power tools aren't earthed anyway. So go figure.

Ray

Originally Posted by cba_melbourne

Yes, but with the center tap of the isolation transformer secondary grounded, there is still a return path for the secondary earth leakage current back to the RCD. The RCD will still work, as it is triggered by current not by voltage.

Sure the path maybe there but if you're across both lines and happen not to be earthed there wont be any current to earth. No current to earth no imbalance to trip the RCD

Originally Posted by RayG

The only reason is that the UK regulations require a center tapped ground. I'm open to either argument, we used to argue this back and forth for years without ever acheiving a consensus..

Might even make sense for 110V. Not sure I see the point with 240V though.(which is why they are using 110V tools I guess )


I can see what you say about going around in circles lol

Originally Posted by RayG

the discussion then usually moves on to solar storms and coronial mass ejection.. then I make the sign of the cross and slowly back away.

Will be a good time to be in the transformer business..... or will it?

Stuart

http://www.diynot.com/forums/viewtopic.php?t=166079

Originally Posted by cba_melbourne

http://www.diynot.com/forums/viewtopic.php?t=166079

Interesting read.. well at least the parts that didn't go over my head.
I hadnt twigged to the fact the that center tap earth is isolated so an RCD on the primary wont do anything you need another rcd on the secondary.

Stuart