Motor power ratings ?

[Digit]

Yes Ivan, I agree it was the governments, but still they tend to come up with ridiculous laws, and the trouble with daft laws is that they make the lawmakers look daft.
Take the latest 'Green' agenda to have us all using 20% bio fuel in a few years time. Britain doesn't have enough acreage to grow that and cereal crops as well!
Bent Cucumbers! The shops get round that impossible load of BS by cutting them in half, and are you aware that you now need a licence to produce compost or climb a tree?
They must be short of something to do!

Roy.

 

[TheTiddles]

Cost and power shouldn't be confused. I can by a Ferrari with a 600bhp engine for a lot more than I can buy a tractor with the same. People who mass produce vacuum cleaners will buy the motors at a fraction of the retail price, it's not uncommon for the motor manufacturer to make the entire product for free to sell the motor in it, same for batteries, power connectors etc...

The shaft efficiency of a motor is easily obtained and there is often a website with that information around.

Aidan

 

[roombacurious]

Two other questions on the subject of motors:

1) How do induction motors compare to brush motors with regards to torque and speed (rpm)? Is a 1,500w induction motor more powerful than a 1,500w brush one and why? Which on will be rotating faster (at shaft level, without load and gearbox)?

2) Why are pulley systems used in pillar drills to change speeds, rather than simple electronic variable speed controllers (like in hand-held drills)? It doesn't seem logical (or cheaper) unless you want to keep the motor running at the same speed all the time to take advantage of the maximum torque obtained at a certain speed of rotation.

More food for thought...

 

[newt]

Electronic speed control on induction motors is more complex than a brushed motor.

 

[Anonymous]

Just to make it clear

Volt x Ampere = Watt
746 Watts = 1HP

niki

This is not correct for an AC machine (all mains is AC).

For AC, one has to take into account the power factor which is an indiction of the phase difference between current and voltage and is affected by inductance (all coils of wire such as those in motors have inductance) and capacitance.

Input power (Watts) to an AC machine is V*I*cos(phi) where phi is the phase angle between current and voltage producing it

Output power (Watts) is torque (Nm) * angular velocity (radians per second)

 

[Digit]

All depends on what you mean by power RBC. An induction motor is normally constructed as a low RPM high torque unit as this allows for greater flexibility. They can be constructed for different arrangements but are not mass produced as such.
The generally available brush motors, vacuum cleaners, washing machines etc are generally constructed as high RPM low torque units, but again they don't have to be.
Brush motors are used in a number of low speed high torque situations, such as in bakeries for mixing dough as they will start under load, something that induction motors are not too keen on.
You pays your money and you takes your choice.
Speed control on brush motors using solid state parts is very cheap, for induction motors it's much more complex and correspondingly expensive.

Roy.

 

[Digit]

True Tony, but if the manufacturers would all stick to the same formula people would at least be able to make comparisons.
With differing figures for HP and different methods of measuring them the present situation is ridiculous!

Roy.

 

[9fingers]

Two other questions on the subject of motors:

1) How do induction motors compare to brush motors with regards to torque and speed (rpm)? Is a 1,500w induction motor more powerful than a 1,500w brush one and why? Which on will be rotating faster (at shaft level, without load and gearbox)?

2) Why are pulley systems used in pillar drills to change speeds, rather than simple electronic variable speed controllers (like in hand-held drills)? It doesn't seem logical (or cheaper) unless you want to keep the motor running at the same speed all the time to take advantage of the maximum torque obtained at a certain speed of rotation.

More food for thought...

A universal brush motor will, in theory run infinitely fast with no load. It will in practise run at 20-30 Thousand rpm and at this speed destroy itself by flying apart.
NEVER ever try out one of these motors without a load. Typically they will designed to run at 10-15 kRPM and hence need gearing down for nearly every purpose. They are suitably cheap, nasty and noisy for short term use such as hand held drills.

Motors salvaged from modern washing machines are mainly of this type and have sophisticated electronic feedback controllers dedicated to washing machines. Unless you really know what you are doing, these are only suitable for what they were designed for.


Single phase induction motors only run at one speed which is (120 x mains frequency / number of poles) minus a little bit.

eg 2 pole 2850 rpm , 4pole 1425 rpm, 6 pole 960 rpm.

Single phase induction motors cannot be run at other speeds.

Three phase induction motors also run at speeds defined by the above formula but can also be run from an inverter which generates 3 phase power at variable frequency. You cannot run a single phase motor from one of these inverters as single phase motors require special starting arrangements that 3 phase ones do not.

This is a complex subject and I hope I have helped answer without creating too many more questions but feel free to ask anyway.

Bob

 

[Anonymous]

A universal brush motor will, in theory run infinitely fast with no load. It will in practise run at 20-30 Thousand rpm and at this speed destroy itself by flying apart.
NEVER ever try out one of these motors without a load.
Bob

A commonly held misconception. They will not fly apart unless there is a manufacturing or design defect.
I have been a degree qualified electrical engineer for over 20 years and seen hundreds of DC series wound (universal brushed) motors spinning quite happily under 'no load' conditions.

In theory they will fly apart (in theory) due to centripetal forces, in practice their speed will be limited by inertia, bearing frictions and wind resistance and simply reach a very high maximum velocity.

 

[Digit]

This is getting technical now. Anyone want to explain Back EMF to Roombacurious?
No, I'm not volunteering. :lol:

Roy.

 

[Niki]

This is not correct for an AC machine (all mains is AC).

For AC, one has to take into account the power factor which is an indiction of the phase difference between current and voltage and is affected by inductance (all coils of wire such as those in motors have inductance) and capacitance.

Hi Tony

You are correct, I just wanted to simplify the formula for all the guys that are not in the electric "business" (including myself).

As Roy said, I don't care so much on the way that the HP is calculated, I just want all the makers to calculate it the same way so I can compare the power of on maker to the other.

We are lucky that in EU all the tools/appliances power are given in input Watts so, in general we can compare between different makes even though the motor efficiency can vary slightly

If anyone is interested about HP calculation please have a look here
http://www.engineeringtoolbox.com/elect ... d_653.html

And here (for single phase power equation)
http://www.engineeringtoolbox.com/singl ... d_887.html

Regards
niki

 

[andycktm]

Can't beat the old electric's for getting the testosterone flowing :lol: :lol:



ps the Tiddlles A tractor with 600 hp (diesel) would cost at least the same maybe more than a 600hp ferrari

 

[9fingers]

A commonly held misconception. They will not fly apart unless there is a manufacturing or design defect.
I have been a degree qualified electrical engineer for over 20 years and seen hundreds of DC series wound (universal brushed) motors spinning quite happily under 'no load' conditions.

Maybe the one I was playing with a few years ago which threw out its commutator segments under no load had a manufacturing defect then.

In the circumstances now trying to answer a question from someone without the experience you have, I thought it best to err on the side of caution!
(33 years degree qualified electronic engineer :lol: )

Bob

[/quote]

 

[roombacurious]

This forum is an endless source of knowledge - I guess there isn't a question this lot can't answer! :lol:

I've already tried to run a 1,500w brush motor without load but under completely safe conditions: lying on my workbench, entirely unsupported
Once powered, it run for a few milliseconds, jumped about a foot above the workbench and landed on the floor missing my head by a few inches... The shock was enough to stop me from trying it again!

It's still working BTW!

ccasion5:

 

[Digit]

This forum is an endless source of knowledge

I'll say. Great fun as well.

Roy.

 

[TheTiddles]

This seems to be getting very achedemic (not that it's a bad thing)

Coming back to the original question the power rating of a tool isn't really that important beyond 2kW being lots more than 1kW, sharp cutting edges, low friction trailing edges and a correctly matched feed rate will negate several hundred watts of extra power if you're trying to force a blunt cutter too fast through a piece of rock hard maple.

Motors don't self-destruct because there's no load on them, they will quite probably overheat and burn out, but mechanically I've never seen one fall to pieces, though a transmission shaft did once fail at 85,000rpm leaving pieces of itself embedded in another building on the university campus, someone let an electrical engineer try to do a mechanical's job...

Aidan