Motor power ratings ?

[Blister]

Hi

may sound like a strange question but here goes :?

Is there such a thing as input power rating ?

Is there such a thing as output power rating ?

The reason for this question is as follows

The new bandsaw I have is rated at 550w , but the smaller model is rated at 1500w ???

When I asked the question do you do this model bandsaw with a 1500w motor , I was sort of told the 1500w model is input power and the model I have is output power :?
That went straight over my head , so I said " O right Thanks "

Any advice :?

 

[speed]

on the bandsaw you have theres a plate on the back stamped 0.83kw imput and 0.55kw out put
so i would like to know they same question

 

[Jake]

Yes, motors are not 100% efficient, far from it, so the input power is the electrical power drawn from the mains and the output power is the mechanical power that comes out the other side 'at some point' - presumably at the motor shaft. Most manufacturers cite the input power, JET are an exception.

 

[NeilO]

Im sure some of our electrically minded members will put me right,but how I read those figures is:

0.83 kw of input power (electical phase) is needed to produce:
0.55 kw of output energy(kinetic force) req.to turn wheel of BS at rated speed.

the energy burned will always be greater than the energy produced because of other factors..

friction is probably the greatest factor

BUT I could be wrong :shock: :lol: :lol:

 

[speed]

dont seem alot of power now compared to the 1 1/2 hp output that reord power states in there bs350s
but my little 0.55kw motor can cut 7 3/4 mahogany so im not scared

 

[Digit]

If the data is on the motor plate and is in Watts or Kilo Watts that is the power drawn by the motor.
The output POWER will be about 60% of that.
Typical salesman's ploy to make the thing seem more powerful than it is.
Unfortunately there is now a tendency for catalogues etc to state that input figure in horsepower, which strictly speaking should be reserved for the output.

Roy.

 

[Niki]

We are lucky that in EU the tools/machines power is stated usually in Watts or (Kilowatts).

In USA, everything is stated in Horsepower (HP) and, if they are cheating and deceiving the customers....

Just to make it clear

Volt x Ampere = Watt
746 Watts = 1HP

So, in the US of A they are selling a simple vacuum cleaner...that is called also "Shop vac" that has "Peak 6.5 HP" for around $100....I replied "Buy it....and change the motor with your table saw, planer etc".
http://lowes.com/lowes/lkn?action=produ ... lpage=none

Simple calculation: 6.5HP x 746 = 4,849W divided by 120V = 40.4 Ampere...they must have very thick wires in their home circuits.....

But, in small letters below, it's written "12A motor"............well.....

The Voltage in US is 110~120V so 120 x 12 = 1440W..........1440 : 746 = 1.93HP....so, how it became 6.5HP...very simple, they "catch" the motor by the "throat" till it stops and measure the Amperes....but they probably "forgot" to mention that at "6.5HP" the motor does not produce any work just consuming electricity (a few seconds before it starts "smoking" and finito...)

Even the Hitachi M12V router is a "3.25 peak HP" and it's only 15A (1800W) motor that can give only 2.4HP just have a look here
http://www.tool-home.com/products/catal ... cts_id=662
And here (UK) nobody is talking about "horses".....
http://www.amazon.co.uk/Hitachi-Variabl ... B0001P0A22

The efficiency of any device that uses one kind of energy and turns it to another kind of energy, is never 100%.

On the motors that we are usually using on our tools, the efficiency is around 60% as Digit said but, don't be surprised if the Metabo 1000W tool, gives more output power than some "Made in Chiwan" tools.

On big motors or 3 Phase motors the efficiency can reach up to 85%.

Metabo, Festool and maybe some other big makers are giving the input and the output power and those that are not....maybe they have very good reason...

niki

 

[Rich]

Hi Niki, I'm a mechanical maintenance engineer and Iv'e always struggled to understand the electrical side of things, well now, you have explained things in such a way that it becomes very clear even for me, so thank you very much for the education and I have printed a copy for my workshop noteboard.
I wish you a happy new year,Rich.

 

[Niki]

Thank you Rich

I was an Aircraft ground engineer and my knowledge in electricity is very basic but, enough to understand when they want to cheat me....

Happy new year
niki

 

[Blister]

Ok , everyone , Thanks

I take it that the input power of my bandsaw would be " enough "

:? :lol: :wink:

 

[Digit]

The problem started with us, the customer. We want power from our machines, so one company starts to use the definiton that seems to indicate higher power, and the rest have to follow suit. It's a Red Queen's Race fuelled by Mr Average not knowing a Watt from a volt.
Take a look at Hi Fi outputs. They used to be RMS, (Root Mean Square) power, now they are rated at peak power so that your 30 Watt amplifier ain't what it use to be.
Car horse power used to Imperial measure, now it can be any one of at least three. Then you get the problem I had with my son as I explained that power at low revs was more 'powerful' than the same power at high revs.
Perhaps some day somebody will come up with some meaningful measure

Roy.

 

[Niki]

Hi Digit

You are so correct about "Mr Average not knowing a Watt from a volt"...and RMS...

I made a mistake with the Hitachi router...and you kindly did not correct me...that what we see on the advertisement as 1800W or 2.4 HP is actually the input power and the output power will be 1.44 HP (60%).

I made a search about the "EU Electric Motors Labeling" and it looks like the EU is going to some standards that the motor efficiency will be indicated on any motor that is produced or imported to EU to promote the high efficiency motors and by that, save energy and cause less environmental damages.

They have some methods that I'm not familiar with, to check the motor efficiency....something like "Carbon test" and "Star test"...well, they know better...

Regards
niki

 

[Digit]

well, they know better...

You kidding? It took 'em 13 yrs to come up with a definition for a lawnmower damn it! Now they are telling us that grade one Bananas must have less than a 10% curve in 'em.
Next they'll be advertising for Banana inspectors!

Roy.

 

[TheTiddles]

Rating the input power is about the most sensible idea there is as...

Electric motors are extremely efficient, like 95%, then what we add on to them takes the toll, such as gear boxes, inertia etc... Also remember that the effect is cumulative so a 90% efficient transmission is loosing 10% of the 95% provided by the motor, add several things together and it can drop quite a lot.

Electric motors provide a fairly constant torque accros the speed range, unlike an internal combustion engine where it peaks then drops. The power output of a rotating machine can be expressed as a product of the torque and speed of the rotor so under no load the machine is at maximum power but once you start cutting then the speed drops so does the power. This assumes a fairly simple speed control (most likely) but a more advanced system will provide more power to the motor under higher loads... This can continue much further so it's easier to express what you put in and obviously there will be some loss, it's not a con by any means,

Aidan

 

[Niki]

Hi Aidan

As I said, my knowledge is very basic...

I understand that there are a few HP measuring methods

But, when an 120V - 12A motor with all the vacuum cleaner is called 6.5HP and sold for $100, while a 2HP motor (for DC) alone costs around $200....it sounds very strange to me...or maybe there are "Imperial HP" and "US HP" (like the Imperial Gallon and the US Gallon) :lol:

Personally, I prefer to see the Input power and the Output Power on the motor label that are measured the same way for all the electric motors

Regards
niki

 

[malb]

Hi, Australian newbie here.

Motor data plates show the design input (rated) power in Watts, KW, Horsepower (746W) or ps (metrified horsepower 750W). This is so the end user or their appointed installer can determine the power needs of the unit, and ensure that adequate power is available from the supply system at the point of use.

I have used the term design to indicate the designers intended maximum power draw. This determines the size of the motor frame for heat dissipation, the size of wire used in the windings, and the size of brushes and commutator if the motor is of the brushed (universal type) commonly found in powered hand tools.

If operating with no load ( e.g. running but not engaged in cutting, planing drilling etc.), the power drawn from the supply is typically about 10 to 30% of the rated power. This power is lost as heat from friction in the bearings, brushes etc, heating of the copper windings due to current flow, and the magnetizing losses of the motor frame.

A unit such as a router (direct drive, no transmission) would probably be in the 10 to 15% range, while a unit with reduction gears or a belt drive would be in the 20 to 30% range due to the losses incurred in the transmission system.

Worst case would be a motor driving a worm drive gear system where the no load power could be up to 80% of the rated power.

As the tool is worked more heavily, the power draw increases. While some of the additional power is used to do work, the friction losses in bearings increase due to an increase in the load on them, and the heating losses in the windings and commutator increase due to the increasing current through them.

It not that difficult to increase the power being drawn to more than the rated power by overloading the tool. This obviously means that the machine will heat up as it is generating heat faster than it can dissipate it, due to limited case size or airflow. Prolonged overload produces the inevitable outcome of smoke and permanent damage so some manufacturers will include a thermal circuit breaker or other protective device in the system to protect the motor.

A similar situation occurs with other machines as well. A 200KW car engine at idle is probably producing around 15 to 20 KW of power, but is loosing it all to friction and driving ancillaries. Coasting downhill at 60 mph, it is probably generating a similar power, and absorbing power from the driving wheels as well, to overcome transmission losses and increased engine friction. But once it starts to haul the car up a hill at 60MPH, its probably generating about 80KW, to overcome transmision losses and do the work of hauling the car.

Output power is normally determined by using a dynanometer. This is a device that can load the running motor in accurately measured steps while recording RPM, and hence determine the amount of work that the motor can do, and the relationship between input power and output power.

Peak power or locked rotor power is the power that the motor will consume if the shaft is locked and cannot rotate. It can be up to 7 to 8 times the rated power. It corresponds reasonably to the starting current required before the motor starts to spin. It has no real relevance to actual power beyond being a gauge of the suppliers level of exageration.

I hope that this makes sense to those of you interested in the subject.

 

[Digit]

Yes Niki, you are correct.
Britain uses, (used) the Imperial HP, America uses SAE, (Society of Automotive Engineers), Japan, Europe and others use PS.

Roy.

 

[RobertMP]

Just to make it clear

Volt x Ampere = Watt


niki

I've not read the whole thread so apologies if this is already covered but there is an error here for inductive loads like motors...

Volt x Ampere = VA

Voltage x Amps drawn x Power factor = watts

if the power factor were say 0.8 then you can see more current will be needed to get the stated wattage input.

 

[Niki]

You are correct of course Robert

Even in the airplanes the generators are rated as XX KVA and we also measure the KVAR (KVA Reactive...and I don't remember even why... ) but for "normal" people like me that are not dealing with electric engineering, V x A = W is close enough.

And , it tells us that an 12A - 120V motor is even not 1440W as I gave them the credit, but even less so, the deceiving is even bigger...

Anyway, on my Electra Beckum table saw motor, they already indicate the input Watt (2.5KW) and the output Watt (1.9KW) that can be translated to electric HP.

Regards
niki

 

[ivan]

Peak HP does actually exist, so it's not actually a lie, up to you to decide if the power developed just before the windings melt is much of a useful comparison. I suppose it has to be when everyone does it as seems to be the case with vacs etc. in the USA. Even an EU rated motor carries a start up surge, perhaps of 30 or 40 amps for a few milliseconds after switch on. A peak, but not a very useful one.

By the way, bent bannana specs (and all other veg & fruit shape/size/colour standards) existed long before the EU was invented, and exist so importers, wholeslers and retailers can order a naturally variable product without inspecting it and be fairly sure of what they will get. Common EU standards (and not just for fruit and veg) make this possible throughout the EU. Actually most of the reason for the time it took to get a standard was down to the individual governments arguing rather than civil servants drinking tea (coffee in Brussels, maybe)