Thread: Sparkie advice for tool killer.

I am having difficulty following some of these arguments.

I understand that resistance in the cable due to damaged conductors and/or bad or corroded terminations will cause a voltage drop across the cable. I understand that the voltage drop across the extension plus the voltage at the plug of the machine equals the voltage at the GPO. (Kirchoff's voltage law)

It is the comments about increased current I am having problems understanding.

For example if 240V is applied, the motor might draw say 2 Amps but if only 120V is applied the motor will draw 4 Amps, 60V 8 Amps, 30V 16 Amps? What about if Zero volts was applied?

In other words the power consumed by the motor under identical mechanical loads is constant regardless of the applied voltage, the motor just draws more current to compensate?

Try Ohms law. (I=E/R)
With zero volts there is no differential between + & - (DC) or active & neutral (AC).
No differential = no emf = no current.
The resistance in a cable is affected by its length, the cross section of the conductors, the quality of the connections, the state of the pins etc.
Ernie. (As distinct from Ern)

Please be careful with some of the assumptions being made here. Usually a power tool will draw less current when the voltage is reduced. The exception is a constant power appliance which usually have switch-mode power supplies (as in a computer) - these will draw more current as the incoming voltage is reduced.

Most power tools will behave essentially as a constant impedance - dropping the voltage will drop the current (and power).

Ohm's law can be used to determine the voltage drop across the extension cord. V(drop) = I(drawn by the tool) x R(of the extension cord). The more current the tool draws, the greater the voltage drop across the extension cord and the less voltage at the tool (240 - Vdrop). Power is V x I (ignoring "power factor") - less "V" gives less "I" (from above) and therefore less "Power". Dropping the Voltage by 10% will result in the current also dropping 10% and the power at the tool dropping by a 21%.

As the the 230V - there is a push to adopt international standards. The Australian voltage will be referred to as 230V rather than 240V - but the tolerance of the supply will be such that 240V will be quite acceptable.

Thanks Chrisp,
You are correct. The one thing that stays constant is the resistance or impedance of the motor windings.
A drop in voltage will mean a drop in current and a reduction in tool performance.
Having said that, the normal V drop over an extension lead should not give a noticable decrease in performance. Not enough to damage the tool or stop it from working any way.
It is more likely to be a surge or spike that causes damage to electrical motors.
By all means have the lead and power point checked but also consider what else is on the circuit. Particularly look for large motors like a compressor that starts automatically.

For reference: the Voltage Drop on a HD 1.5mm sq 20m extension lead for a 1000W tool is around 1.92V

Originally Posted by NCArcher

For reference: the Voltage Drop on a HD 1.5mm sq 20m extension lead for a 1000W tool is around 1.92V

And thank you NCArcher,
I didn't mean to imply (or mislead the reader) that the typical voltage drops might be 10%, I was just trying to use easy numbers to illustrate the calculation and show the compounding effect of voltage drop on the power. I do agree with your point that the voltage drop is likely to be minor and unlikely to be the cause of the tool failures.

As to the original question proposed at the start of the thread, my guess is the problem is a bad run of crap tools.

Originally Posted by chrisp

As to the original question proposed at the start of the thread, my guess is the problem is a bad run of crap tools.

I reckon that could be the problem. Bad things happen in 3's don't they?

Also check the mains fuse/ circuit breaker holder to see if it's hot when the problem occurs.
I've seen a few of them, (even turned to carbon,) on household & heavier duty bus bars & boards.
Generally caused by "wrist strength impaired" sparkies not correctly tightening the screws/bolts.
Ernie.

Thanks for the information.

I had no idea if the latest electric motors had constant current or constant power sink circuitry, apparently they haven't.

I agree that the problem with the "blown" motors is probably due to sub standard production and quality control of the tools and a bit of bad luck thrown in too.

It is possible for a faulty extension lead to blow a motor if the resistance of the extension lead is so high that it will not pass enough current to turn the motor but sufficient current to heat the windings and do damage. If this was the case then it would have been quite obvious where the problem lay - nobody is going to leave the switch on when nothing obvious is happening.

Originally Posted by Ernie47

Try Ohms law. (I=E/R)
With zero volts there is no differential between + & - (DC) or active & neutral (AC).
No differential = no emf = no current.
The resistance in a cable is affected by its length, the cross section of the conductors, the quality of the connections, the state of the pins etc.
Ernie. (As distinct from Ern)

Ernie

You win the cupie doll!

As does NC and chrisp

The impedance doesnt change as it is a function of winding resistance frequency ( which doesnt change) and inductance ( also doesnt change)

As mentioned ohms law I = V/Z

If voltage drops and impedence remains constant -Current must reduce.

woodcutta

Gee thanks Woodcutta, I knew those 20 something years fixing (biggish) computer systems (& checking the sparkies work before switching on anything,) would pay off one day!

Ernie.

Carry,

My understanding is that the type of the motors we are talking about here draw minimal current at full speed ie with no load (can't remember the theory, something to to with back EMF or something like that). When loaded up the motor slows and draws more current. Effectively, the slower it runs the more current it draws. If the motor does not get the rated voltage it can also cause it to draw excessive current - a faulty lead could cause this due to voltage drop in the lead.

Usually the big tell tale signs of excessive current are smell, smoke and then you realise something is too hot. You do not mention how hard the tools were being worked, whether they seemed to be running slow or were getting hot or if the lead or plugs were hot.

Check the extension lead for cuts or scrapes which have gone through to bare wire. If you're using it outside it may be earthing out with the ground.

End of Story

(Moderator- if I overstep the mark on putting down a brand I understand that you may delete this post).

I took the Worx brand (Green and silver edition) reciprocal saw back to the HW store for a replacement/refund. As soon as the guys saw it they remarked,
'Oh another one of these. We have had heaps of this brand returned.' They recommended the cheaper Worx brand (orange edition) or the Makita for $100 more. Apparently the green model of the Worx tools has lots of gimmicky things which ultimately affects the working of the machine. It is marketed at the tradesmans level but they are reluctant to touch them.

I'm now thinking that the many-year-old GMC had probably had it's day; the Talon hedge trimmer (cheaper version) was not really up to it - even though it sported Richmond colours; and the Worx green machine had inherent weaknesses.

I have, however, replaced the extension cord that has 'served me many days.

I sincerely appreciate the advice- although I don't understand it all from all and sundry, especially the sparkies who have taken time away from their work to assist this tool destroyer.
Cheers to all, and thank you.

Graham