Thread: Real Power Ratings of Machinery

Bob, thanks for that clear, succinct explanation - very enlightening.

Yeah thanks Bob - I printed that out so's I can re-read it over and over and get to grips with something I don't understand - electrickery. . !!
Thanks
Cheers
Jedo

Keep the info coming, Bob, it's fascinating. I've always wondered about some of those figures.

Thanks Bob,

Some very useful info.

Do you know if any organisation produces a table of approximate current loadings for various sizes of motors (induction and brushed) at zero, nominal and maximum loading? Or is every calculation specific to the unit at hand, so to speak?

I sort of started a current draw survey myself a while ago but plans went astray before I got far.

I have a peak hold clamp meter.
I fired up the "3Hp" table saw.... peak draw on start up was something like 56amps:eek: cant remember the other figures but a bit of solid ripping and 17.5 amps showed.

I and others who have the MBS300 are suspicious of the 3hp figure quoted.
That enormours lump of iron has to be more than 3Hp.

the "3hp" motor on my moulder is half the size.

I recon there is a magazine article there somewhere.

cheers

Originally Posted by Auld Bassoon

Do you know if any organisation produces a table of approximate current loadings for various sizes of motors (induction and brushed) at zero, nominal and maximum loading? Or is every calculation specific to the unit at hand, so to speak?

I doubt whether any organization other than the motor manufatcurers themselves would have this sort of info which they would normally provide for those making devices that use their motors so they could wire them correctly. I note that some companies publish a fiar bit of their motor specs on the web. The typical currents published are "no load", and "stall", and sometimes current at max torque.

I should add I have deliberately kept things as simple as possible for the forum. The real available working HP at the TS blade requires taking into account things like the motor/belt drive efficiencies and something called a Power Factor.

if you are game you could try reading http://www.faqs.org/faqs/woodworking/motors/ which goes into a bit more detail. There is actually heaps of info available if you search.

Anyway I'm pleased some folks have found it useful and if it stops one more person from repalcing their 15A plug with a 10A plug I guess it has been worthwhile.

Cheers

'Puts on Sparky hat'...

The current drawn by a motor is not always (in fact never) the power output from the motor.

Say you have your 240v motor pulling 15A, so it's consuming 3600w. Multiply that by your mysterious power factor, say 0.85 and you get 3060w that is possible for the motor to deliver.

Problem is that motors are not all that efficient, so in reality, the output is somewhere closer to 2000w in a decent induction motor, or the 3HP it says on the tag. All manner of inefficiencies lurk in that lump of iron, and it's not worth muddying the waters describing and evaluating it all.

Suffice to say, if you compare apples to apples, the current draw is a good means of deciding what kinda power figure you have.


A good example is this.

I have a bunch of very large motors, approximately 300mm x 200mm diameter, with clutches and all other manner of things. They draw 4.6A at 100V here. Their output is the grand total of 200W. Somewhere, 260W of input has gone AWOL, but not really.

These motors are rated at 0.8 pf, and 200% duty, so they are big, ugly, heavy and will NOT quit no matter what you inflict upon them. I have another motor that is the same size, but rated at 700W, but only at 100% duty cycle. I suspect that the motors many of you have in your table saws are less than 100% duty cycle, simply because the saw is not going to be running for an hour at a time. The dust collector is though...

If you are going to compare motors, and you have the means to measure what they are doing, then it would be a good idea to state the current draw at start, run no load, run at load (specify the load), the power factor, duty cycle (if it's on the plate) and the physical size of the motor. That way, it might be possible to build a database where you could theoretically compare x motor to y motor, and know who's telling the truth.

Universal motors (brushes) are seriously inefficient, but pack a big punch for their size. Induction motors also pack a decent punch, but are far more efficient. So the 3HP table saw might well be 3HP, but the 3HP router prolly isn't most of the time, but it can muster it for a short time.

Also, the difference between a 10A plug top/socket and a 15A plug top/socket is the size of the earth pin. The current carrying pins are identical in size, so the change doesn't affect anything there. However, 15A sockets are wired differently to a 10A socket, and a lot of that has to do with not only the current carrying capacity of the power wires, but the earth too, hence the bigger pin.

I'll leave out the specifics of the active/neutral power wires, just know a proper 15A socket can handle it.

The earth however is important. If you have this big machine hooked up to a 10A socket with a 15A plug top with the pin ground down (or a 10A socket), you increase the resitance of the earth circuit. If you increase it by enough, there is a chance that the earth will not work correctly.

What happens is:

Wire comes loose, touches the frame.
Power goes through the active to the earth wire to ground.
Lots of power goes through the earth, causing the fuse/breaker to pop.
Power shuts off, end of story.

Lets take our nominal 15A circuit.

The cable run is 30m long, hooked into a 25A breaker with 32A cable. If the earth is less than 9.6 ohms, then the earth circuit will allow enough current to flow to trip the breaker.

Take our nominal 10A circuit.

The cable run is 50m long, since there are more outlets. It's hooked up to a 20A breaker with 25A cable, and the earth circuit needs to be under 12 ohms.

You plugged your TS into a 10A outlet with a ground down pin, so the resitance is increased. The length of cable is also 20m longer, increasing resistance. The cable size is also smaller on the earth and power cables, increasing resistance. The saw has been running, warming the cable (normal) but also increasing resistance. The active-saw-earth circuit is now 14 ohms.



And the saw's table is now at 240V...

Under the same conditions, the 15A circuit might be only 5 ohms, and the power is off leaving you scratching your head.

In theory, it's ok to hook up a 15A appliance to a 10A circuit, it's only when there's a fault that a problem occurs, and it might just kill you.


Sorry for all that, but I felt the need to put into perspective what Bob said about not plugging 15A gear into a 10A socket. Nothing to do with whether the circuit can handle it, just whether it's safe or not.

Originally Posted by Schtoo

'Puts on Sparky hat'...

Sorry for all that, but I felt the need to put into perspective what Bob said about not plugging 15A gear into a 10A socket. Nothing to do with whether the circuit can handle it, just whether it's safe or not.

It all depends what you mean by "the circuit" and "handling it". If you mean a 10A circuit with nothing else plugged into it and a big enough breaker on the end of it I agree.

However, real 10A circuits are permitted to have a number of sockets and devices plugged into them. Adding 15A gear to that circuit does not necessarily mean "the real 10A circuit will handle it". If the devices on the real circuit are already drawing current close to the breaker limit the chances are high that you will trip the breaker when you use the 15A gear. You don't even need to use 15A gear to do this, sometimes just adding another household device will be enough to trip.

My understanding is that proper 15A circuits should use appropriate wiring and breakers to prevent this from happening?

Originally Posted by BobL

It all depends what you mean by "the circuit" and "handling it". If you mean a 10A circuit with nothing else plugged into it and a big enough breaker on the end of it I agree.

Of course.

Originally Posted by BobL

However, real 10A circuits are permitted to have a number of sockets and devices plugged into them. Adding 15A gear to that circuit does not necessarily mean "the real 10A circuit will handle it".

The regs last time I read them were a 20A breaker with 25A cable and in a domestic installation, unlimited outlets. Run a fuse, well, your guess is as good as mine there.

Originally Posted by BobL

If the devices on the real circuit are already drawing current close to the breaker limit the chances are high that you will trip the breaker when you use the 15A gear.

If. A very big if, since if you note above the outlets in domestic installations are unlimited. This takes into account that most people do not have large numbers of power hungry appliances all running at the same time. The 20A limit using a breaker is enough to let you use the DC, TS and maybe some lights on the same circuit, some of the time. Don't scoff, I have seen it. Also seen melted cable insulation, and the circuit was still running...

Originally Posted by BobL

You don't even need to use 15A gear to do this, sometimes just adding another household device will be enough to trip.

True, but you could also add 20 houshold things and nothing will happen.

Originally Posted by BobL

My understanding is that proper 15A circuits should use appropriate wiring and breakers to prevent this from happening?

The best solution is to run sub mains to the shed, with your own little fuse box in the shed.

That way, when you get all the bigger gear, running power is dead easy and getting enough of it there safely is even easier.

What I was trying to say (and I may have failed it seems. ) is that it is very VERY easy to use a regular circuit to run 15A rated gear and get away with it for a very long time, even indefinitely. The fact that it's extremely unsafe is never realized till it kills you...

A properly installed 15A circuit to run a 15A device is safe however, which is why it's always sensible to use the right circuit for the job.

We've had a number of discussions along this vein in the past but in this case I think the horse is far form dead so I'll keep flogging.

All of our wood working machines and workshop equipment will draw far in excess of their rated current at various time for various reasons.

This is normal and to be expected, as a result the electrical system and the regulations are designed to cope with this hapening.

Our equipment especialy the stuff with big motors realy likes a good solid supply.
The better the electrical supply the better your gear will work and last.

If you have a good solid low impedance supply your machines will....
Come up to speed faster with less thermal and other stress
Will handle load variations better and recover speed faster
Arguably run cooler
and generaly be happier
also
In the event of a short circuit your circuit breakers will act much faster minimising secondary damage.

Consider for the most part (almost without exception) your 15 amp outlet will have its own wire going back to the switch board and a circuit breaker to its self. your tool has a beter chance of a low impedance supply and more consistant voltage.

With the current regs ( new purple AS3000 ) there has been a shift in the determinig size of cables throughout the installation.

In the past the wire just had to be adequate to carry the load.
Now in addition "fault loop impedance" requirements will overide curent carrying capacity in most cases.
What we are looking for is the ability of the wiring to carry a fault situation and quickly and effectivly trip the breaker. (if you still have fuses oh dear)
This is good for our gear.

However if you have an old instalation the voltage could be very saggy and baggy and the ability to deliver current will not be real flash.

SO running a 3hp saw bench that may draw..:eek: lots.. off an old 10 amp outlet which may have other things on it with wiring that got there the long way that also may have some....:confused: unhappy connections on the way
is not a real clever choice particularly if it is protected by an old type wire fuse.

cheers

This has nothing to do with the mumbo jumbo above - no offence intended, it IS important, electricity kills - I read some articles some years ago where a high quality furniture manufacturer wanted some way to indicate to his workers when machine blades needed sharpening. He had amp meters installed on his machinery and by trial and error, came up with a system that current drawn was an indicator for blade maintenance. Apparently it worked for him!

It would work, a dull blade increases the load a fair bit, just ask the blokes who resaw aussie hardwoods on their Bandsaws!

Just on the relativities of different types of motor, I am confused (a little) by the implcations of say a 1HP motor. For example it can be

1HP Universal motor

1HP Induction motor
1HP Induction motor with capacitor start
1HP Induction motor with capacitor start and capacitor run

1HP Inducton motor (3 phase) etc. etc.

Now all these are 1HP motors remember. In addition some will be rated only for "intermttent" use while others will be "continuous" rated. A further muddying of the waters is the number of poles in the induction motors. A 2 pole speed is 2950 rpm, 4 pole 1450rpm and 6 pole 920 rpm. These speeds are all approximate based on 50hertz and varying power factors.

More poles, I think, gives more torque, more phases also gives more torque.

But they are all rated a 1HP. Turn on a 1HP single phase bench grinder and then a 1HP three phase grinder and tell me they are producing the same power. I don't think so!!!

So presumeably the power quoted is the input and a more realistic rating would be the power output. Perhaps an analogy would be with motor vehicles to quote power of cars at the drive wheels rather than at the flywheel.

HP just ain't HP.

Regards
Paul

Originally Posted by Bushmiller

HP just ain't HP.

I'm pretty sure that even if I lined up 10 horses, they couldn't match my 2HP dusty no matter how hard they suck...